Varian Linatron High-Energy X-ray Applications 2007

Varian Linatron High-Energy X-ray Applications
2007

Table of Contents
Page
I Introduction 3
II Characteristics of High-Energy Radiation 5
Advantages of High-Energy X-rays 5
Generation of High-Energy X-rays 5
Measurement of High-Energy X-rays 5
Target Characteristics 5
Radiation Absorption and Scattering in the Object 6
Half-Value Layer and Energy Selection 7
Half-Value Thickness versus Energy Spectrum 8
III Linatron Radiographic Characteristics 9
General Description 9
X-ray Quality 9
Field Coverage, Beaming, and Field Flatness 9
• Field Coverage
• Beaming and Field Flatness
IV Theory, Variables, and Practices in
Linatron Radiography 13
Radiographic Setup 13
• Collimation
• Source-to-Film Distance
• Object Placement and Image Formation
• D/T Ratio, Magnification, and Image Sharpness
• Reducing Scattered Radiation
• Blocking
• Lead Intensifying Screens as Filters
• Filters
• Film Holders
Intensifying Screens 16
• Metal Foil Screens
• Metal-Phosphor Screens
X-Ray Film Characteristics 18
• Film Properties and Classification
• Speed
• Contrast and the Film Characteristic Curve
• Multifilm Techniques
Definition and Unsharpness 22
Sensitivity and Image Quality 24
• Thickness Sensitivity
• Wire Penetrameter Sensitivity
• Drilled Hole Plaque Penetrameter Sensitivity
• Radiographic Sensitivity
Exposure Curves 25
• Method for Generating Exposure Curves
• Latitude
• Exposure Times and Material Densities
• Using Film Response Curves to Generate
New Exposure Curves
• Increasing Latitude with Multiple Film Use
Page
V Radiographic Procedures 40
General Considerations 40
• Records and Logs
• Exposure Room Equipment
Castings Radiography 41
• Radiographic Coverage
• Layout and Marking
• Casting Radiographic Procedures
- Radiography of Flanges
- Radiography of Valve Bodies
Radiography of Welds 44
• Radiographic Coverage
- Circumferential Butt Welds
- T-Sections
• Design and Placement of Image Quality Indicators
• Defect Location
Radiography of Rocket Motors 47
• Motor Attitude and Support Equipment
• Rocket Motor Radiographic Procedures
• Grain Radiography Coverage Requirements
• Tangential Radiography
• Tangential Radiography Coverage Requirements
Assembly Radiography 51
VI Bibliography 52
VII High-Energy Real-Time Radiographic Imaging 53
VIII Glossary 59

Preface
This manual presents theory, principles, and practical
application of radiography. It is intended to provide
general guidance only in performing radiography with a
Linatron system. It is not intended to supersede or preempt
the requirements of applicable inspection procedures,
specifications, standards, safety procedures, or regulations.
Where statements in this manual conflict with local, state,
national or international regulations, the regulations must
take precedence.
Although this document is intended primarily for Linatron
users, the principles of high-energy radiography presented
here are generally applicable to other high-energy X-ray
equipment. Not all questions concerning Linatron
applications can be covered here. Any specific questions
may be directed to Varian Security and Inspection Products
(SIP) representatives. A list of Varian offices is given on
the back cover.
page 1
The principle objectives of this manual are to
provide:
1) Applications information for prospective
industrial radiographic uses of a Linatron
2) Technical information concerning Linatron
radiography not readily available elsewhere.
Conventions and Terminology
This manual uses the convention of designating an X-ray
energy distribution as MeV or KeV, i.e., 2 MeV designates
a distribution of X-ray energies up to 2 MeV.
Varian Linatron applications

Introduction
X-rays are a type of electromagnetic energy that occupies a
particular place in the electromagnetic spectrum. What we
refer to as X-rays is a type of energy that can be understood
as both particles and waves because they have characteristics
of both. Photons are the particle aspect of X-rays, which
are mass-less and travel at the speed of light. These are
produced by Linatrons when high energy electrons are
accelerated and strike a high density metal target. This
releases photons in the X-ray frequency band as shown in
Fig. 1-1.
FIGURE 1-1. Spectrum
Nondestructive testing using high energy (greater than 1
MeV) radiographic techniques has been in use for more
than 60 years. During this period, a number of highenergy
X-ray sources have been developed for the detection
of flaws in heavy metal sections. More recently, these x-ray
sources have found important uses in cargo screening and
related security inspections applications. Please consult
other Varian documentation for information on cargo
screening applications.
The first commercial high-energy X-ray source was the 1
MeV resonant transformer, introduced by General Electric
in 1939. A few years later there appeared 2 MeV versions
of the resonant transformer, Van de Graaff generators with
energies of 1 MeV and 2 MeV, and Betatrons with electron
beam energies ranging from 15 MeV to 25 MeV.
page 3
The maximum X-ray output of these early machines was
limited. Electron linear accelerators, which became available
commercially about 1956, offered a way to substantially
increase the X-ray output and made practical the
radiography of steel sections greater than 2 feet thick. This
met the need of modern nuclear technology which required
radiographic examination of assemblies containing relatively
thick sections of very dense material, such as uranium and
tungsten alloys. Linear accelerators proved capable of
penetrating and recording flaws or other anomalies on x-ray
film through many inches of such materials.
Varian SIP (Security and Inspection Products) first
extended its linear accelerator technology into the highenergy
radiography field in December 1959 with the
delivery to the U.S. Navy of a 10 MeV machine designed
to produce high-quality radiographs of the Polaris missile
and other solid rocket motors. In the years that followed,
Varian SIP continued to supply similar machines to
government and industrial customers.
In 1968, Varian SIP introduced the Linatron®, a line of
industrial linear accelerators with energy ranges from 1 to
15 MeV. The Linatron represented a significant advance
in reliability and ease of operation and handling. A typical
Linatron installation includes an X-ray head (Figs. 1-2 and
FIGURE 1-2. Linatron M9 X-ray head.
Varian Linatron applications

FIGURE 1-3. Linatron M System Diagram
1-3), a Control Console (Fig. 1-4), a modulator cabinet
(Fig. 1-5). Among the more important characteristics of
the Linatrons are:
• Small focal spot size
• Appropriate energy selection for the material to be
radiographed
• Ease of handling and beam positioning
• High radiation output
• High reliability
• Low maintenance cost
FIGURE 1-4. The standard operator interface to the Linatron is a
Touchscreen Control Console.
page 4
FIGURE 1-5. Linatron modulator cabinet contains main power
supplies, pulse modulator, and power distribution electronics.
Varian Linatron applications

Characteristics of High-Energy Radiation
Advantages of High-Energy X-Rays
The high energy x-rays produced by Linatrons provide a
number of advantages over traditional x-ray radiography.
These include:
• Making the radiography of very thick sections
economically feasible.
• Making it possible to achieve large source-toobject/object-to-film
distance (D/T) ratios to
minimize object distortion.
• Allowing short exposure times for high
throughput rates.
• Combining high film latitude with reduced
scatter allowing high detail resolution in
radiographs of large complex assemblies.
• Making it possible to use slower fine-grained Xray
film and real time radiography systems.
Generation of High Energy X-Rays
Electrons are injected at moderately high energies into a
tuned resonant waveguide structure and accelerated
toward a target by high electric fields. When these
electrons strike the target, they rapidly decelerate. This
deceleration creates high-energy bremsstrahlung X-ray
spectrum. The spectrum is characteristic of the target
material, the target design, and the energy spectrum of
the incident electron beam. The same process takes place
in conventional X-ray equipment, but the higher energy
Linatron electron beam produces a higher efficiency
conversion of electrons into X-rays.
Measurement of High Energy X-Rays
The “Roentgen” is the standard unit of measure for x-rays,
which quantifies exposure to a source of ionizing radiation.
“Exposure”* is fundamentally a property of the beam
rather than a measure of the effect of the beam on the
object to be irradiated. The basic quantity that
characterizes the energy imparted to matter by ionizing
particles is the absorbed dose. The unit of absorbed dose is
page 5
the Gray, often abbreviated “Gy”**. Gy is defined as the
amount of energy imparted to matter per unit mass of
irradiated material and is equal to 1 joule per kilogram.
In practice, the radiation output of a Linatron is measured
by first measuring exposure, the charge produced by the xray
beam in a given volume of air using an ionization
chamber dosimeter. Correction factors are then used to
calculate the absorbed dose in a material. Ion chamber
measurements are normally made at a given depth in a
water phantom or with the ion chamber surrounded by a
plastic cylinder or equilibrium cap in order to achieve
electronic equilibrium. For low-atomic-number materials,
a Roentgen measured in air is approximately equivalent to
one rad of absorbed dose. Linatron outputs are described
in units of Gy per minute at one meter.
*The term “exposure” is used primarily to describe the fact that
film has received X-ray radiation during radiography of an
object under test in subsequent sections of this manual.
Exposure refers to the effect of the X-ray beam on the film in
this context.
**This manual uses Gy values for absorbed energy dose values.
1 Gy (Gray) = 100 rad.
Target Characteristics
The target is a component in the Linatron, which absorbs
high energy electrons and produces x-rays. The intensity
of the X-rays produced at the target is a function of the
electron beam intensity and the X-ray production
efficiency of the target. Target efficiency is defined as the
ratio of the total X-ray radiation power produced to the
total power of the impinging electron beam. This
efficiency depends on both target composition and
geometry. The most efficient targets are made of materials
with a high atomic number (high Z elements). Tungsten
(Z=74) offers the best combined efficiency and physical
properties. This is the primary material used in the
Linatron target. It has a thickness slightly greater than the
range of the electrons in the target material.
Varian Linatron applications

Linatron targets are specially designed to produce the
minimum focal spot size consistent with their high
radiation output. Focal spot sizes of less than 2 mm are
achieved in operating Linatrons. Focal spot size and
uniformity are routinely determined by using a special spot
size camera.
Radiation Absorption and Scattering in the
Object
The x-rays produced in a Linatron target form a beam
which passes through the object under inspection. The xray
beam consists of photons at varying energy levels,
which are attenuated because the photons interact with
nuclei and atomic electrons of the object as it enters and
passes through material. Depending on the energy of
individual photons, three distinct processes contribute to
the aggregate attenuation of the beam.
In the “photoelectric absorption” process, the photon
loses all of its energy to an atomic electron; that electron
then leaves the orbit of the atom and continues to move
through the material at high speed. This process occurs in
steel with most of the low-energy photons (0.1 MeV and
less). As photon energy increases above 0.1 MeV,
probability of photoelectric absorption decreases and rarely
happens to photons with energies of 1 MeV and higher.
In the “Compton scattering” process, the primary photon
is deflected from its initial line of travel. It loses some of
its energy due to interaction, and continues passing
through the material in the new direction as a lower energy
photon. The atomic electron involved in the interaction is
ejected from its bound position. Compton scattering is
the major attenuation process for photons with energies
between 0.1 and 10 MeV. A high intensity of scattered
radiation can emanate from an object being radiographed
because many of the photons in a high-energy X-ray beam
are in this energy range. For example, the intensity of
scatter from a wall or hardware bracket behind the film can
easily reach values that nearly equal the intensity of the
transmitted primary beam
page 6
“Pair production” is the third process. This occurs
when the photon is completely absorbed and an
electron-positron particle pair is created. Pair
production has a threshold energy of 1.02 MeV, and
becomes significant when enough photons with energies
above 4 MeV are present.
The total attenuation of a high-energy X-ray beam is a
combination of all three processes, plus other processes
such as the generation of secondary X-rays within an object
by the slowing-down process of the scattered electrons.
The amount of absorption and the total attenuation
depends on the atomic number(s) of the material, the
density and thickness of the object, and the X-ray energies
of the photons that make up the beam. Figure 2-1 is a
plot of absorber atomic numbers versus electron beam
energy. This shows the range of absorber atomic number
and photon beam energy where the photoelectric process,
Compton scattering, and pair production dominate the
attenuation process.
FIGURE 2-1. Dominant x-ray attenuation process for each
element (atomic number = Z) at each x-ray energy.
Varian Linatron applications

Half-Value Layer and Energy Selection
An X-ray beam that enters an absorber is attenuated as it
passes through, and is then measured by a detector at some
fixed point behind the absorber material. The relationship
between the intensity of the X-rays at the detector and the
absorber thickness is expressed by the familiar exponential
law of attenuation:
Ix = Ioe -μx
where:
e = 2.71828 (natural log base).
I o = measured intensity at the detector with no
absorber in the beam.
I x = measured intensity at the detector with the
absorber of thickness x in the beam.
μ = total linear attenuation coefficient.
x = absorber thickness.
There is a single value for the linear attenuation coefficient
when the absorber is a fixed composition and density for
X-rays of a single wavelength or energy.
X-rays emitted by the Linatron are not of a single energy,
but contain all energies up to the nominal energy rating of
the unit. Therefore, the actual linear attenuation
coefficient for a substance does not take on a single value
for all conditions of irradiation because of this wide energy
spectrum. For a thin absorber, the attenuation is mostly
caused by the lower energy X-rays. Only the more
energetic X-rays pass through a thick absorber while nearly
all of the lower energy X-rays are absorbed completely.
The thicker absorber will appear to have an attenuation
coefficient value that corresponds to a higher X-ray energy.
The attenuation reflects a constant value attenuation
coefficient for thickness over one or two half-value layers
(HVLs). HVL is defined as the thickness of material that
reduces the intensity of the transmitted X-rays by a factor
of two.
In general, the linear attenuation coefficient is proportional
to the density of the absorber material, and it is sometimes
necessary to use the mass attenuation coefficient (μ m )
which is independent of density:
where:
μ m = μ d (cm2 /gm)
page 7
d = density (gm/cm 3 )
If the mass attenuation coefficient is used, the exponential
attenuation equation becomes:
Ix = loe (-μmdx) It is often very useful to identify the X-ray attenuation of
material with the HVL. HVL is related to the linear
attenuation coefficient by:
HVL = 0.693/μ
The linear attenuation coefficient and the HVL are also
used to express the quality, or energy makeup, of the
beam from an X-ray generator since they have definite
values for each material and for each photon energy.
Practical radiography set-ups use broad-beam radiation,
which produces a scatter effect in the exposure. The
amount of scatter received by a detector will affect the
measured HVL.
The slope of the exposure curve and the contrast and
latitude achieved in a step block exposure are indicators of
the HVL and the effect of scatter. The selection of
generator energy should be governed by the type and
thickness of materials that are required to be radiographed
in a specific application. The broadbeam HVL is the most
useful material index for the radiographer to use for energy
selection since it is related directly to exposure time.
Figures 2-2 through 2-5 illustrate HVL as a function of
incident electron energy for steel, rocket propellant, lead,
and concrete.
FIGURE 2-2. Half-value layer for steel as a function of energy.
Varian Linatron applications

FIGURE 2-3. Half-value layer for propellant as a function of energy.
FIGURE 2-4. Half-value layer for lead as a function of energy.
I
Z
0
a
a
I
6
0
0
0 5 10 15 20 25 30
TARGET INCIDENT ELECTRON ENERGY(MeV)
FIGURE 2-5. Half-value and tenth-value layer for concrete as a
function of energy.
page 8
19.8
13.2
6.6
E
0
z
a
a
I-
Z
w
Half-Value Thickness Versus Energy Spectrum
The unfiltered X-rays emitted by the Linatron target
contain photons varying in energy, with the highest being
equal to the incident electron energy. This collection of
photons makes up the energy spectrum of the Linatron Xray
beam. This continuous spectrum of energies is often
termed “white” radiation.
The emerging Linatron X-ray beam becomes filtered and
modified by the obstacles in its path. If an absorber with a
thickness that reduces beam intensity to one-half is placed
in the beam, that thickness can be identified as the first
HVL. That first layer will be thinner than subsequent
HVLs because less material is needed to reduce the
intensity of a beam containing large numbers of lower
energy photons. The first two layers modify the beam,
filter it, and make it “harder”. After about two HVLs,
additional absorber material no longer changes the
distribution of X-ray energies and a constant value of the
HVL results. This is described as the equilibrium state.
The values plotted in Figures 2-2 to 2-5 are equilibrium
values.
Varian Linatron applications

Linatron Radiographic Characteristics
General Description
The Linatron is an RF-powered electron linear accelerator
system that produces a very intense beam of high-energy
X-rays for industrial radiography. The system consists of
an RF unit, an X-ray head, a modulator and a control
console. In addition, an optional temperature control unit
is available to provide the required 30° C. liquid coolant
supply to the Linatron. Table 3-1 lists the important
performance characteristics of Linatron Models M3, M6,
M9 and K15.
X-ray Quality
As noted in Section 2, the most useful measurement of Xray
beam quality is the Half-Value Layer (HVL), which is
derived from the broadbeam attenuation curve or from the
attenuation coefficient. Table 3-2 lists the broadbeam
HVL for several common materials for the LINATR0N
energies.
Linatron Model
M3
M3A
M6
M6A
M9
M9A
K15A
Table 3-1 Linatron Performance Characteristics
Energy (Nominal Peak)
(MeV)
3
1 - 3
6
3.5 -6
9
5 - 9
9 - 15
page 9
Field Coverage, Beaming and Field Flatness
Field Coverage. X-rays emitted from an X-ray machine
create secondary radiation (scatter) that can blur and
distort the image on the radiograph when they strike
objects located near the item under inspection. Scatter can
also fog the film and cause a corresponding loss in image
contrast and sensitivity. Therefore, the primary X-ray
beam should be restriced to the approximate size and area
of the actual inspection. This is accomplished by a device
known as a source collimator, which is a shielding device
located at the source of the x-ray beam. These vary in
geometry and density to control the shape of the beam.
Frequently, custom collimators are specified by the
customer to meet the needs of a specific application. The
standard collimator opening for the Model M9 Linatron is
a 30° cone and the standard collimator opening for the
K15 Linatron is a 15° x 20° pyramid.
Maximum Dose Rate
(Gy/min-m)
3.0
0.2 - 3.0
8.0
2.0 - 8.0
30.0
6.0 - 30.0
60.0 - 150.0
Varian Linatron applications
Focal Spot Size (mm)
≤2
≤2
≤2
≤2
≤2
≤2
≤2

Beaming and Field Flatness. Electrons reach velocities
approaching the speed of light in the Linatron accelerator.
Most of the electrons continue to travel in the forward
direction after their initial interaction with the X-ray
target. The deflection angle of scatter tends to be small,
and decreases further as the energy of the incident electrons
increases. X-ray radiation across the cone of the Linatron
is not uniform or “flat”. There is a “beaming” effect which
increases with increased energy. Both X-ray energy and
intensity (dose rate) are lower at off-center beam angles.
Material
Tungsten (18 gm/cc)
HVL (cm)
HVL (in.)
Lead (11.3 gm/cc)
HVL (cm)
HVL (in.)
Steel (7.85 gm/cc)
HVL (cm)
HVL (in.)
Aluminum (2.70 gm/cc)
HVL (cm)
HVL (in.)
Concrete (2.35 gm/cc)
HVL (cm)
HVL (in.)
Solid Propellant (1.7 gm/cc)
HVL (cm
HVL (in.)
Lucite (1.2 gm/cc)
HVL (cm)
HVL (in.)
Rubber (1.11 gm/cc)
HVL (cm)
HVL (in.)
1MV
0.55
0.21
0.75
0.30
1.60
0.63
3.90
1.50
4.50
1.80
6.10
2.40
10.50
4.10
11.18
4.40
Table 3-2 Typical Broadbeam Half-Value Layer at Characteristic Linatron Energies
page 10
2MV
0.90
0.36
1.25
0.49
2.00
0.79
5.40
2.10
6.20
2.40
8.40
3.30
12.10
4.80
12.70
5.00
Because Linatron targets are slightly thicker than the
electron path length of the most energetic electron, a
proportionately larger number of lower energy photons are
produced, when compared with a thin target. This reduces
the beaming effect, broadens field coverage, and increases
sensitivity when inspecting thin and low-density materials.
Typical Half-Value Layer*
* Values measured by film techniques may vary somewhat depending upon actual material characteristics, scatter control and other factors.
4MV
1.15
0.45
1.60
0.63
2.50
1.00
7.50
2.90
8.60
3.40
11.60
4.60
16.80
6.60
19.30
7.60
Varian Linatron applications
6MV
1.20
0.48
1.57
0.62
2.80
1 .10
8.90
3.50
10.20
4.00
13.80
5.40
19.90
7.80
21.00
8.30
9MV
1.20
0.48
1.52
0.60
3.00
1.20
9.60
3.80
11.00
4.30
14.90
5.90
21.50
8.50
24.40
9.60
15MV
1.15
0.45
1.37
0.54
3.30
11.30
11.0
4.30
12.70
5.00
20.40
8.00
29.50
11.60
29.80
11.75

With high-energy Linatrons, the intensity of the X-ray
beam is so much greater at its centerline than at small
angles off-center that a compensator or beam flattener may
be used to reduce the centerline intensity and produce a
more uniform intensity across the field.
In radiography, accurate film exposures require uniform xray
beam intensity. Lower energy x-ray sources have a
characteristically more uniform beam and can be employed
if exposure times are not excessively long. Some
conditions, such as x-raying thicker or denser objects,
covering larger areas, or utilizing lengthy source to film
distances, require higher beam power. For these
conditions, a higher energy source may be combined with
a beam compensator.
FIGURE 3-1. Linatron x-ray intensity distribution, models M3 and M6
page 11
Figure 3-1 shows X-ray intensity distribution across
uncompensated beams of 1 MeV, 2 MeV, and 6 MeV
Linatrons. Figure 3-2 gives beam profiles for compensated
and uncompensated 9 MeV Linatrons. Figure 3-3 shows
plots for the uncompensated and compensated 15 MeV
Linatron. These plots, which illustrate the beaming effect
and the degree of flatness of the field, can determine
variation in final film density from the center to the edge
of a film.
Varian Linatron applications

FIGURE 3-2. Linatron x-ray intensity distribution at 9 MV.
page 12
Varian Linatron applications

Theory, Variables, and Practices in Linatron Radiography
Radioagraphic Setup
Collimation. Collimation is the term used to describe the
methods to eliminate the unwanted portion of the X-ray
beam cone. Collimation is accomplished in three stages:
a. Source (Primary) Collimation. Each Linatron has a builtin
collimator which limits the maximum field size the unit
will produce. The collimators are purposely designed to
cover wide areas so that multiple detectors can be exposed
simultaneously. However, for maximum scatter control,
additional inserts must be placed in the collimators or in
front of them to narrow the beam to the detector area. A
variable jaw collimator is set to the proper size by electric
motor adjustment. Some variable collimator jaw openings
are set from the control console by remote control. This is
very useful when performing real time imaging.
b. Pre-Collimation. This is the term used when shielding is
placed between the X-ray source and the object. Precollimation
is used to restrict the beam to just the area of
the projected size of the detector.
c. Post-Collimation. This is similar to pre-collimation except
the shielding is placed between the object and the
recording media. Post-collimation is one of the most
important and least used practices in achieving an
optimum X-ray image. Although most radiographic
techniques do not require the extensive collimation
described above, it can be of valuable assistance where high
quality radiographs are required or when performing real
time imaging.
Source-to-Film Distance (SFD). Radiation intensity
varies inversely as the square of the distance from the
source (target) to the object. If d1 and d2 are the sourceto-film
distances (SFD), and t1, and t2 are the exposure
times (or dose) respectively, then the terms are related by:
t1 /(d1 ) 2 = t2 /(d2 ) 2
page 13
An exposure technique plan will specify an SFD. The
exposure time and dose must change with distance square
relationship if that distance is changed for a particular
application.
Example
If the original exposure was obtained for an SFD of 4 feet,
and the new SFD must be 6 feet, then the new exposure
time for each thickness is obtained by multiplying the old
values by the factor (f):
f = (6) 2 /(4) 2 = 2.25
Object Placement and Image Formation. Unlike some
other nondestructive testing methods, high-energy x-ray
radiography can be performed with objects and assemblies
of every conceivable size, shape, and makeup. The
inspections can be done with the object in any one of
many orientations. The exterior and interior of the object
can have any configuration, and the front and back
surfaces need not be flat or parallel.
Regardless of their location or orientation in the object,
internal conditions such as voids and inclusions usually
project interpretable images. Cracks and gaps form the
best images when they are aligned with the X-rays, i.e.
when the X-rays are permitted to stream through the open
space of the gap to reach the film. This also applies to the
inspection of assemblies for internal adjacent solid surfaces
and for internal parts. The best image will require a
particular orientation of the object because of the
relationship of the internal surfaces to the X-ray beam.
D/T Ratio, Magnification, and Image Sharpness. The
D/T ratio is the source-to-object distance, D, divided by
the object-to-film distance, T. The value of T is obtained
by measuring the distance from the source side of the
object to the film. T is the object thickness when the film
is directly behind the object.
Varian Linatron applications

It is most convenient to have exposure curves and
technique plans for materials based on the SFD (source to
film distance) for which the work will usually be
performed. Normally, in Linatron radiography, the SFD
D/T ratio is about 4 or more. The optimum D/T ratio for
a particular application depends on the same factors that
affect other variables in the radiography procedure.
Among these are object thickness, screen material and
thickness, type of X-ray film, filters used, and amount of
magnification that can be tolerated.
Because the X-ray film is located in back of the object, the
images of all internal conditions are projected and
magnified. Images of cracks and other defects that occur
on the back and adjacent to the film have very little
projection and magnification. These images are usually
sharper than others for this reason. Because radiography
utilizes a non-point source, the sharpness of all images in
high-energy X-ray radiography depends on the projection
involved and the distance to the film.
Images of small voids, gaps, etc., that are located deep
within the object (i.e., not at the back surface) may have
very limited sharpness. They may still be visible, however,
because of high contrast. Many penetrameter images are
have minimal sharpness, yet are still visible and
interpretable because of adequate contrast.
Within the practical limits permitted by focal spot size and
other factors, moving the film away from the object will
reduce the intensity of scatter originating in the object.
This may improve sensitivity where the film-screen unsharpness
greatly exceeds geometrical un-sharpness.
Projection radiography will benefit most from image
magnification because of the inherently larger system unsharpness
in real time imaging.
Reducing Scattered Radiation. Scattered radiation is
present in almost every high-energy radiography
application. Scatter control during Linatron use is
important because the scatter may be equally intense as
(and sometimes greater than) the primary rays that reach
the film. If not eliminated or minimized, scatter can
reduce film contrast, obscure and distort image visibility,
and cause false images to appear on the film. A
radiographer must deal with two types of scatter:
page 14
(1.) OBJECT SCATTER is secondary X-ray and electron
radiation emitted in all directions from the object, and is
mainly due to the Compton scattering process. For
example, when radiographing 10 HVLs of material, only
0.1% of the primary beam intensity reaches the film, and
99.9% of the incident primary beam is absorbed or
scattered. A large amount of that scatter may reach the
film holder. It can arrive at the film at all angles other
than the primary beam to blur and diffuse images.
(2.) EXTRANEOUS SCATTER is scattered radiation from
material and structures around the object, or from areas in
back of the film (“backscatter”). Another source is the
leakage radiation from the X-ray head itself which also
contributes to this scatter.
The radiographer can do the following to eliminate and
minimize scatter:
• Limit the primary beam to the area of interest by
shielding, blocking, or using special collimators.
• Avoid irradiating nearby structures.
• Protect the film from extraneous scatter by suitable
blocking.
• Prevent backscattered radiation from reaching the film
with back lead filters and absorbers.
• Increase the object-to-film distance.
• Use suitable object-film filtration to reduce forward
scatter.
Blocking. Blocking eliminates unwanted portions of the
X-ray beam at the object through the use of lead bricks,
shot-filled bags, and other shielding material. Blocking is
needed when large thickness differences in an object
produces high-intensity scatter, or when the object
thickness is many HVLs and the object is smaller than the
radiation field.
A radiographer can judge the effectiveness of the blocking
in a particular setup by observing the film density in the
area of the radiograph over which the blockimg projects. If
that area is absolutely clear (or of very low density), it is a
good indication the desired imaging area also received little
or no scatter.
Varian Linatron applications

Comparing the exposure and density for the particular
application with standard exposures and densities from an
exposure curve or other reference is another way to
determine blocking effectiveness. If a shorter exposure of
the material corresponds to a higher film density, when
compared with other well-blocked exposures, it is very
likely that scatter has contributed to the exposure.
Lead Intensifying Screens as Filters. The thin metal foil
(usually lead) intensifying screens that are placed in front
and back of the film to intensify the primary beam and
shorten the exposure also function as filters to reduce the
amount of scattered radiation that reaches the film. Much
of the scatter can be stopped by the lead screen since the
average energy of the scattered radiation is lower than the
energy of the primary beam.
Example
At 10 MeV, a front lead foil 0.01 inch (0.25 mm) thick
will reduce scatter reaching the film by 35% when
exposing about 5 HVLs of low-Z material.
It is customary to select the front lead screen thickness so
the screen provides both intensification and filtration.
However, this practice may not always give the optimum
image quality results. The thickness of a front lead screen
should never exceed 0.050 inch (1.3 mm). The
radiographer should consider using a composite filter
placed in front of the film holder if additional filtration is
needed beyond that. The quality of the image should be
the basis for deciding whether filtration is effective. Some
radiographers eliminate backscatter by using external
(back) lead shielding rather than using lead foils.
Filters. Absorbing plates or assemblies placed between the
object and the film holder may be necessary to achieve the
best image quality when:
• The object shape, position, or composition produces
extensive scatter.
• Large object thickness produces scatter greatly in excess
of the primary radiation.
• The object is smaller than the radiation field.
• Large thickness differences exist in the object.
• High-speed photoelectrons produced in the object may
reach the film holder.
page 15
In most cases, the front metal foil intensifying screen will
provide enough filtration to obtain good image quality.
However, in applications (e.g., assembly radiography),
where image sharpness is paramount and a number of the
conditions listed above may exist, an object-film filter of
suitable design is recommended. The filter should be
placed as close behind the object as practicable. The ideal
filter can be layers of high-Z material (such as lead) closer
to the source with subsequent layers of decreasing Z
materials since the filter itself may be a source of scatter. In
some applications a simple filter of lead and brass may be
sufficient.
Screen choices depend on the application, the details of the
radiographic procedure, the energy level, and the degree of
blocking used. For an application in which the
collimation, blocking, and shielding have not minimized
the amount of scatter reaching the film holder, and the
setup geometry must not be modified, the use of external
filters may provide an effective solution. A radiographer
can judge the effectiveness of a filter by comparing image
quality with and without the filter.
Film Holders. The film holder, or cassette, has two
functions. In addition to securing the film from light, it
also maintains firm contact between the film and screens.
Loss of good contact between film and screens results in a
loss of contrast and sharpness in the image.
Three types of film holders or cassettes are frequently used:
semirigid, rigid, and vacuum. The best choice of holder
depends on the requirements of the application.
The Semirigid cardboard or plastic holder is the most
common type. It offers low cost, ease of handling, and
flexibility. Soft plastic, semirigid film holders should be
used when the film must be curved to fit the shape of the
object being inspected.
Rigid holders with mounted screens and spring-loaded
backs are also common. The rigid holder provides more
positive film-screen contact than the semi-rigid holder,
especially when the film must be held in a vertical
orientation.
Varian Linatron applications

Vacuum cassettes are recommended where high detail
resolution is needed and where good film-screen contact
cannot be assured by any other means. Vacuum cassettes
may also be required if precise measurements must be
made from the radiograph.
Intensifying Screens
Metal Foil Screens. The previous section described the
filtering action of lead foil screens placed inside the film
holder in front and back of the film. These screens prevent
the scatter from reaching the film by acting as filters.
Contrast is not reduced, and image quality is enhanced as
when other filters are used outside of the film holder.
A second and equally important function of the lead foil is
to increase the speed of exposure and image formation by
intensifying the photographic effect of the primary beam.
Most of this intensification is from the emission of
photoelectrons and Compton electrons from the screen
into the films. Intimate contact of the film and screens is
always required in applied radiography. This sometimes
necessitates using vacuum cassettes or spring-loaded
holders. Intensification will be slightly reduced for the
situation when emulsion that does not touch the screen
because two or more films are placed in the same holder
for the same exposure.
Metal foil screens may be used either un-mounted or
mounted on flexible plastic or cardboard. Mounting
Energy
(MeV)
1
2
4
6
8
9
11
15
Table 4-1
Front Lead Thickness for Maximum
Intensification
inches mm
0.005 0.13
0.01 0.25
0.02 0.51
0.03 0.76
0.04 1.02
0.04 1.02
0.05 1.02
0.05 1.27
page 16
does not effect the intensification action. Lead is the
screen material most commonly used because of its low
cost, high intensification capabilities, and filtering
effect. However, the lead must have a uniform surface
finish and purity required to characterize the screen as
“radiographic quality”.
Lead screens have a few significant disadvantages. For
instance, they are easily damaged and produce “lead marks”
and phantom images on the film. Also, lead may oxidize
over years of use and require abrasive cleaning to restore its
surface. Some screen suppliers furnish the lead foil either
with the surface chemically treated to resist deterioration or
with a thin Mylar film for protection. Before removing the
protective film, it is advisable to perform tests to determine
how the image quality is affected.
In most X-ray procedures, it is routine to use a back lead
screen for intensification and filtering. The thickness of
the back screen is not critical for intensification, since
maximum effect is achieved at about 0.01 inch at all
Linatron energies.
The front screen provides a somewhat larger range of
variability of intensification and image quality. As
shown in Figure 4-1, maximum intensification at each
energy is obtained from the approximate thicknesses
listed in Table 4-1.
Varian Linatron applications
Screen Thickness for Optimum
Image
inches mm
0.002-0.005 0.051-0.13
0.005-0.010 0.13-0.25
0.010 0.025
0.010 0.025
0.020-0.030 0.51-0.76
0.030 0.76
0.030 0.76
0.030-0.050 0.76-1.27

The intensification with a specific screen and for a given
application can be determined experimentally by making
exposures with and without the screen. With “narrow”
beam geometry (i.e., where the collimation and spacing
between the absorber and the film is less than 0.01
steradian of angle), the increase in film density with the
thicker screens is entirely due to the increased intensifying
effect of the screen.
In “broad beam” geometry, a normal amount of scatter is
present, and the filtering of the lead screen determines the
relative amount of scatter that exposes the film. As
indicated in Figure 4-1, intensification for narrow beam,
low front-scatter conditions reach maximum values for
each energy.
Example
At 2 MeV a film density increase of about 0.8 density units
is observed for a 0.01 inch- lead screen compared to an
exposure without a front screen. This corresponds to a
film exposure increase of about 110%. If normal amounts
of front scatter impinge on the film holder and screen, an
apparent intensification of from 20% to 40% may occur
when compared with an exposure without a front screen.
High sensitivity in Linatron radiography usually requires
sharp images and high contrast. Image contrast is affected
by the screen and film responses. Screens must be selected
and used carefully in order to obtain optimum results at
each energy level and for each application. Unlike kilovoltrange
radiography, where the radiographer can improve
contrast by lowering the voltage, Linatron energies are
fixed. Thus, for a Linatron system, after a specific type of
X-ray film and screens are selected, and after optimum
scatter reduction is accomplished, the radiographer can do
little to improve contrast without changing the imaging
system. Table 4-2 suggests lead screen and filter
thicknesses that typically achieve optimum image quality.
page 17
FIGURE 4-1. Front lead screen intensification: 1 to 15 MV X-rays.
Metal-Phosphor Screens. Composite screens consist of a
layer of fluorescent salt phosphor over a metal foil screen.
The metal foil is a source of electrons that strike the
phosphor, causing visible light photons to radiate the film.
They can sometimes be used in Linatron radiography to
shorten exposure times beyond that used with lead screens.
This can allow radiographers to obtain higher contrast
images when compared with the use of lead screens. These
composite screens can be used with ordinary x-ray film, and
do not require special “screen” film.
Composite screens may also be constructed of lead or
calcium tungstate. Rare earth phosphors such as
gadolinium oxysulfide are also available.The shorter
exposure times obtained with these screens make it possible
to use thin phosphors to obtain sharper images. The
higher speed screens usually have a thicker phosphor layer.
Typically, metal foil thickness ranging from 0.01 to 0.06
inch (0.25 to 1.5 mm) provide the optimum range of
speed, contrast, and image sharpness.
Varian Linatron applications

Table 4-3 lists some relative speeds for the various types of
fluorescent screens currently available. Fluorescent screens
must be well maintained, carefully handled, and kept
clean. Dust particles leave white spots as images on the
film and screens deteriorate with age.
X-Ray Film Characteristics
Film Properties and Classification. Commercial X-ray
films that have been used for years in kilovoltage and
megavoltage X-ray and gamma-ray radiography also meet
the needs of high-energy Linatron radiography.
Speed. The relative speed of commercial X-ray film can be
established with the Linatron by exposing each film in a
real radiography arrangement until it will produce a
developed film darkening of a specified density.
Energy (MeV)
1-4
6-9
15
Radiographic Conditions
Flat object, low scatter, up
to 4 inches steel or equal.
Complex object, high
scatter, or thick object
over 4 in. of steel or equal.
Flat object, low scatter, up
to 5 inches steel or equal.
Complex object, high
scatter, or thick object
over 5 in. of steel or equal.
Flat objects, low scatter,
up to 6 inches steel or
equal.
Complex object, high
scatter, or thick object
over 6 in. steel.
Table 4-2 Suggested Screen and Filter Thickness Speed
page 18
Example
A comparison chart can be created for various film types
by exposing each film to produce a specified density, such
as 2.0 on the H. & D. scale. If all other variables are held
constant, a relative film speed index can be assigned to
each film, with a numerical value that is proportional to
the exposure level needed to obtain the 2.0 film density
The data presented in Table 4-3 was obtained using this
procedure.
A change in screen material or thickness will cause
shorter or longer exposures. Therefore, the film speed
index will depend on the specific screen used. The
specific developing technique can change the film speed
index by up to 100%. Therefore, it is as important to
quantify and control the developing process as carefully
as the exposure itself.
Screen Thickness
Front Back
0.010 in 0.010 in
(0.25 mm) (0.25 mm)
0.020 in 0.010 in
(0.51 mm) (0.25 mm)
0.020 in 0.010 in
(0.51 mm) (0.25 mm)
0.030 in 0.010 in
(0.76 mm) (0.25 mm)
0.030 in 0.010 in
(0.76 mm) (0.25 mm)
0.050 in 0.010 in
(1.27 mm) (0.25 mm)
* Back lead filters may be placed behind the film holder if there is a large amount of backscatter present. 0.250-inch (6.35 mm) thickness
should be adequate for all cases.
** Object filters are placed between the object and the film holder to reduce the effect of secondary or scatter radiation, or scatter generated by
the object.
Varian Linatron applications
Remarks
Backing lead as needed.*
0.030 in. lead or composite
object filter may improve
sensitivity.**
Backing lead as needed.*
0.030 in. lead or composite
object filter may improve
sensitivity.**
Backing lead as needed.*
0.125 in. lead or composite
filter may improve
sensitivity.**

A film’s response to scatter may be related to the primary
radiation in conjunction with the relative film speed index.
Therefore, changing to a different film type should not
alleviate a scatter problem in applied radiography. The
intensification index is an indicator that designates
decreasing numbers for faster screens, corresponding to the
exposure level needed to produce a specified film density.
Table 4-4 lists the exposure factors (the index number that
is proportional to the exposure rads) at different film
densities for 12 film varieties in common use. This table is
primarily used to produce a constant film density when
changing film types. To accomplish this, a radiographer
will multiply the film speed index of the new film times
the initial film exposure level to get the new exposure level.
In this way, the new density should be the same as the first
film density.
Contrast and Film Characteristic Curve. Each structural
element, inclusion, void, and crack in the object being
radiographed uniquely alters the radiation passing through
Film Type
DuPont NDT 75
Kodak AA
Agfa Gevaert D-7
DuPont NDT 70
DuPont NDT 65
DuPont NDT 55
Kodak T
Agfa Gevaert 0-4
Kodak M
DuPont NDT 45
Agfa Gevaert D-2
Kodak R
Table 4-3 Relative Intensification Index of Screens*
page 19
Film Type
the object. These variations affect the attenuation in each
area, which results in different image densities on the
exposed film.
FILM CONTRAST is the magnitude of the density
difference between two areas that received different
exposures. A radiographic image with large differences is
said to have “high contrast”. A “low contrast” image
occurs where film density differences are small across the
image. Each type of film has a characteristic response curve
that indicates its sensitivity to incremental exposure. For a
particular film, a FILM RESPONSE CURVE can be made
by plotting density versus exposure level. To create the
data for a film response cure, a radiographer will make a
series of radiographs using a single solid object with
uniform thickness. Each film in the series will subject to a
incrementally longer exposure, creating a range of film
densities. Film response curves can be made for all the
film used in a Linatron facility, and can be used to
compare film speeds. Figure 4-2 shows such plots.
Film Density (H. & D. Units)
1.5 2.0 2.5 3.0 3.5
Film Speed Index
0.35 0.5 0.7 0.9 1.2
0.7 1 1.3 1.6 2
0.7 1 1.3 1.6 2
0.7 1 1.3 1.6 2
0.8 1 . I 1.4 1.7 2.1
1.2 1.8 2.4 2.8 3.4
1.4 2 2.5 3 3.7
2.2 3 4 5 5.6
2.7 4 5 6 7
5 7 8.7 10 11
5.5 7 8.7 10 11
7 10 12.6 15 17
Varian Linatron applications

Manufacturers publish density versus log relative exposure
plots for their films, and identify them as FILM
CHARACTERISTIC CURVES. Both response curves and
characteristic curves illustrate the relationships between the
gradient or contrast, which is the slope of the curve,
exposure rads, density, and speed. In Figure 4-2, the steep
rise in the curves above a density of 2.0 shows how much
speed and contrast increase at higher densities. The curves
also show that fine-grained (slower) films have higher
gradients and contrast than faster films. If the exposure
level for an object with a specific thickness is known for
one type of film, the radiographer can find the:
• Exposure time that would be required using another
type of film.
Screen, Front and Back
Lead, optimum thickness
Lead and Tungstate Salt Composites
Very High Speed
High Speed
High Definition
Lead and Gadolinium Oxysulfide
Composites
High Speed
High Definition
page 20
X-Ray Energy (MV)
• Film density that would occur with the second film,
with the same exposure as the first film.
Figure 4-3 shows a plot of film gradient, which is the slope
of the film density versus log relative exposure
(“characteristic”) curve, for three speeds of film.
The exposure gradient shows a continuous increase with an
increase in density, and the slower film shows the higher
film gradients.
The maximum usable density is determined by the
limitations of the viewing equipment. Commercial variable
brightness illuminators are adequate at densities up to about
3.5. For films darker than 3.5, a very high-intensity spot
illuminator with a variable iris should be used.
1-3 4-6 9 15
1 1 1 1
0.14 0.20 0.33 0.50
0.33 0.50 0.67 0.77
0.60 0.70 0.80 1.00
0.20 0.33 0.50 0.67
0.55 0.67 0.80 1.00
Table 4-4 Relative Speeds of Industrial X-Ray FilmAt Linatron X-Ray Energies With Lead Screens and Automatic Processing
(With 2.5” Steel Absorber)
* The intensification index is arranged with decreasing numbers for faster screens, corresponding to the exposure level needed to produce a
specified film density.
Varian Linatron applications

FIGURE 4-2. Film response curves for three industrial X-ray films
with relative speeds (exposure factors, EF): 1 = Fast, 3 = Medium,
7 = Slow.
If an object with varying thicknesses (steps) is radiographed,
the densities of the steps on each film, when plotted, will
appear as shown in Figure 4-4. These points also show that
the density differences, and therefore the contrast, are larger
with slower film. This further illustrates the general rule
that contrast increases with increased film density.
A numerical value for contrast can be taken from this plot.
Example
For the 1/8th-inch steel steps, the slope of the curve is
approximately 0.20 density units per 1 /8-inch step at the
2.0 density point. This value corresponds to the contrast
achieved with lead intensifying screens. It shows the
increased contrast achieved with phosphor screens.
page 21
FIGURE 4-3. Curves of gradient versus film density for three Xray
films shown in Figure 4-2.
FIGURE 4-4. Film density plot for step exposures of steel for lead
screens at 2 MeV. *
Varian Linatron applications

FIGURE 4-5. Plot of density versus steel steps for lead and leadphosphor
at 9 MeV. *
Figure 4-5 is a plot of density versus steel steps for lead and
metalphosphor screens. It shows the increased contrast
achieved with phosphor screens.
Multifilm Techniques. It has been shown that a single film
will exhibit a range for film densities that bears a relationship
to the range of thicknesses in the object. If there are
thicknesses in the object being radiographed that exceed the
corresponding limits of a single film, the thicknesses may
still be covered in a single exposure by placing two or more
films in the film holder. This is true with the additional film
having the same or a different speed.
Example
It may be desired to maintain a minimum density of 1.5 to
achieve the required radiographic quality for an exposure
where the film density ranges from 1.0 to 1.7.
If a second film of the same type is used and the two
films are exposed simultaneously, the resultant exposure
is about the same. The density of each film will again
range from about 1.0 to 1.7. The two densities will sum
and the density range will be 2.0 to 3.4 when they are
viewed together.
page 22
For this example, there wasn’t an increase in exposure time
to get a very desirable density range. Using two films of
the same type also permits the interpreter to distinguish
between an artifact (blemish or flaw) in the film and a
defect in the object. This procedure can also be used with
two films of different speeds. The first is exposed to a
density ranging from 1.0 to 1.7, as before. If the second is
a faster film, its density may range from 2.0 to 3.5. The
interpreter may then view the darker portion of the lower
density film and the low density portion of the higher
density film, and thus obtain coverage of the entire
thickness range in one exposure.
Double film loads can be used between a single pair of
screens. There is also mutual intensification of the
touching film using this technique. Also, many electrons
from metallic screens are sufficiently energetic to pass
through multiple layers of film and intensify the
succeeding layers. Using interleaved lead screens will
ensure better resolution in each film. Interleaving is also
employed to adjust the speed relationships. This makes the
second layer darker. The interleaved lead foil screens must
be without backing material and be between 0.005 and
0.010 inch thick.
Definition and Unsharpness
It is often desirable to obtain the very best image available
in radiography, i.e., one with high contrast, minimal
distortion, low graininess, and optimum definition. To
accomplish this, an intermediate exposure time is required,
since it allows improved density variations to occur on the
film. Therefore, compromises are required where there is a
need for the shortest possible exposure times. Some
approaches to this include:
• Sharpness is increased by using thin screens (and
sometimes eliminating the back screen), by using single
emulsion film and fine-grained film, and by using
effective filters between the object and the screen.
HOWEVER,
Exposure times are reduced by using thick screens,
double-emulsion films, coarser- grained films, and by
deleting a separate filter between the object and screen.
Varian Linatron applications

• Moving the film farther from the object (increasing “TI’)
reduces scatter, which improves image quality.
HOWEVER,
Exposure time and geometric unsharpness are increased.
• Increasing the source-to-object distance provides greater
area coverage and reduces geometric unsharpness.
HOWEVER,
Exposure time increases by the inverse square law.
• Using composite metal-phosphor screens shortens
exposure times and provides higher contrast in the
image.
HOWEVER,
Composite metal-phosphor screens result in increased
graininess and a loss of sharpness and detail resolution.
Unsharpness in a radiograph can be defined as blurring of
image edges. This causes a loss of fine crack definition,
image detail, and penetrameter (Image Quality Indicator)
detail. Unsharpness and lack of contrast are not
synonymous. Film contrast is the difference in film
density between two areas of a radiograph. A film may
have high contrast but lack sharpness because of image
edge blurring, and vice versa. Good image quality needs
sharpness and high contrast for good radiographic
sensitivity.
Three sources of unsharpness have been identified in
general radiography. These are geometric, inherent, and
scatter (see Figure 4-6). U F (inherent unsharpness of the
film and screens), and U G (geometric unsharpness due to
focal spot size and object thickness and arrangement)
contribute mostly to unsharpness. U F is usually the major
element of unsharpness in high-energy X-ray radiography.
It increases with increasing radiation energy and film grain
size. It is a function of screen material and thickness, and
is affected by the film processing technique. We can expect
the following values of U F with Linatron radiography
using lead screens, fine-grained film (i.e., speed index = 4,
at density = 2.0), and automatic processing:
page 23
FIGURE 4-6. Sources of unsharpness.
Energy, MeV UF, mm
1 0.15
2 0.3
4 0.4
6 0.5
9 0.6
15 1 .0
U G is a linear unsharpness, described by the adjacent
diagram and the expression:
U G = S/(D/T)
Where S = source spot size,
D = distance from the source spot to the
front surface of the object and
T = distance from the front surface of the
object to the film, or usually the thickness of
the object.
The total unsharpness (UTOT ) results from the combined
effect of the inherent, scatter and geometric unsharpnesses,
and can be expressed as the square root of the sum of the
square of each.
UTOT = (U 2
F + UG2 + US2 ) 1/2
Varian Linatron applications

The total unsharpness is reduced to include only inherent
and geometrical unsharpness because they contribute much
more than spot size to unsharpness. This reduces the
expression the expression above to the following.
UTOT = (U 2
F + UG2 ) 1/2
Example
Let energy equal 2 MeV and focal spot equal 2 mm. If we
use double emulsion film and automatic development,
with U F = 0.3 mm, and a radiography arrangement where
6 inches of steel are exposed with a D/T = 12, U TOT is:
U TOT = (0.3 2 + (2/12) 2 ) 1/2 = 0.34 mm
From this example, it can be seen that with a film
unsharpness of 0.3, the geometric value only has a slight
effect on the total. Figure 4-7 shows total unsharpness
plotted for the case where inherent unsharpness is 1 mm.
The plots show the effect of various values of D/T on the
total unsharpness.
FIGURE 4-7. Total unsharpness as a function of source spotsize.
page 24
Sensitivity and Image Quality
A standard requirement in most high-energy radiography
applications with Linatron sources is that the inspection
process demonstrates 2% sensitivity using penetrameter
wires or holes. Linatron radiography regularly produces
sensitivities better than 1% through a wide range of
material thicknesses. Many factors determine image
quality in all high-energy Linatron radiography
applications. Thickness, wire penetrameter, plaque
penetrameter, and radiographic sensitivity are four kinds of
sensitivity commonly evaluated determine Linatron
capability.
Thickness Sensitivity. Thickness sensitivity refers to the
ability of the radiographic inspection to demonstrate a
thickness step by seeing the density difference produced on
the film. Under good viewing conditions a trained eye can
reliably perceive 0.006 density units with a reasonably
sharp step edge in the image. This minimum perceptible
density difference varies with energy and object thickness
depending on the energy of the source and the contrast
achieved by the film and screens. All Linatron sources can
provide approximately 1% thickness sensitivity from 1 inch
(25.4 mm) of steel up to 10 HVL thicknesses using finegrained
film, lead screens and good scatter control.
Wire Penetrameter Sensitivity. Wire penetrameters are
generally used in Europe, and in several special applications
in the United States. The German D.I.N. wires come in
16 sizes from 3.2 mm to 0.10 mm as follows: 3.2, 2.64,
2.0, 1.6, 1.32, 1.0, 0.8, 0.66, 0.5, 0.4, 0.33, 0.25, 0.20,
0.165, 0.15, and 0.10 mm.
Data shows that all Linatron sources should demonstrate
better than 1% sensitivity above 1 inch (24.5 mm) of steel,
and should achieve better than 0.5% for steel thicknesses at
about 6 inches (152.4 mm).
Varian Linatron applications

Drilled Hole Plaque Penetrameter Sensitivity. The
drilled hole plaque is the standard American penetrameter
for all radiography. It consists of a small rectangular strip
of material with three drilled holes. Thickness of the
penetrameter is specified as 1%, 2%, or more, of the object
thickness. The three hole diameters are equal to 1 x T, 2 x
T, and 4 x T where the thickness of the penetrameter “T”.
Sensitivity is described in terms of the thickness percent (1,
2, 4) and the required hole size that must be discerned.
Example
2-2T is the standard way to describe a penetrameter whose
thickness must be 2% of the object thickness and whose 2
x T hole must be seen on the film.
Drilled hole sensitivity data for each Linatron show that all
sources should demonstrate better than 1% sensitivity
above 2 inches (50.8 mm) of steel, up to 10 HVLs, and
better than 2% for thicknesses down to 1 inch (25.4 mm).
Radiographic Sensitivity. Radiographic sensitivity refers
to the ability to see and recognize a variety of defects and
other internal conditions in the radiograph in this manual.
Standard penetrameters provide an indicator of the level of
image quality that is achieved with a particular
radiographic technique used. Penetrameters do not,
however, give defect size. Penetrameters must be placed on
the source side of an object since the image improves as the
defect occurs closer to the film.
Exposure Curves
The plot of exposure level that corresponds to a specific
film density (2.0 for example) for each thickness of
material is called an exposure curve. Exposure curves
should be made for the Linatron after it is completely
operational for all applications. Such curves represent
operational data that are indicative of the operating
characteristics of the setup. Subsequent exposure curves for
the same setup should provide consistent results.
page 25
Established exposure curves also allow the radiographer to
simplify the exposure calculation process. With these
curves, the radiographer only needs to look up the required
exposure level for that thickness and make the exposure. If
the source-to-film distance must be changed, using the
inverse square law provides the new exposure level. A
change to a different film can lead to an additional
adjustment of exposure level using the relative speed table.
Method for Making Exposure Curves. This process
involves making a series of test exposures of an object with
uniform density but varying thickness. Typically, a wedge
or stair-step shaped block is used in conjunction with
additional slabs of uniform thickness. Because exposure
time can be represented as a straight line on semi-log graph
paper, at least two measurements are needed to establish
this line.
FIGURE 4-8. Wedge-shaped block used for experimentally
obtaining exposure curves. Standardized to a film density of 2.0.
To make an exposure curve for a new material, a step or
wedge-shape block (see Figure 4-8) of the same material
that is being radiographed, and a slab of that material, are
placed together to form the object for the first test exposure.
It is most helpful if the slab thickness and the wedge center
thickness are approximately 1 HVL. Note, the material
used in this work should first be radiographed to assure that
it is uniform and free of defects.
Varian Linatron applications

This wedge block is then radiographed with substantially
the same film, screens, focal-film distance, and
arrangements for the same conditions as future routine
inspection work. It should be noted that good collimation
and shielding must be used for the test exposures to
achieve a low amount of scatter at the film holder. The
film is then inspected to determine where the film density
is 2.0 on the step or wedge image. The total thickness of
the wedge-slab combination corresponding to this spot is
measured and the exposure level is recorded. This data
provides the first point for the exposure curve. The
assembly is then made thicker by adding several slabs, and
the process is repeated to obtain the second point. These
two points should then be plotted on semilog graph paper
and connected by a straight line.
The first check on the accuracy of this exposure curve is to
determine from the slope of the line if the HVL equals the
handbook values or other predicted value for the material.
An accuracy check can be obtained by selecting a third
exposure that should correspond to another point along
the line. The line can be extended down to 1 HVL, and up
to 12 HVLs or greater. The curve provides a good
approximation of exposure times for the entire range of
thickness the Linatron can inspect. Since the same wedge
exposure will have a region where the film density is about
3.0, it could provide points for an exposure curve for the
density 3.0. These data also provide important
information about contrast and latitude.
Latitude. A common task in high-energy Linatron
installations is to radiograph objects that have varying
shapes and thicknesses. A single film exposure can cover a
range of useful film densities where sensitivity and
interpretability are accurate and valid. The thickness range
that corresponds to the range of useful densities, usually
1.5 to 3, is called the latitude of the exposure. Latitude
depends on the film gradient, or contrast, and the
attenuation of the material, or HVL. By noting the points
on the wedge image that correspond to densities 1.5 and 3,
the wedge exposure used to generate the exposure curves
can also be used to provide data for determining latitude.
Figure 4-9 shows a plot of latitude for each energy.
page 26
FIGURE 4-9. Latitude ranges for Linatron exposures of steel.
Hint:
While a single film has a limited latitude, two films of
different speed can be combined in one exposure to
increase the total latitude of the exposure.
Exposure Times and Material Densities. When all other
variables remain constant the correct radiographic exposure
time and level for an object depend on its thickness and
density. Two objects with different densities (d 1 and d 2 )
can have approximately the same exposure time, when
their thickness (X 1 and X 2 ) are related by the expression
Example
d 1 X 1 = d 2 X 2
A 1.00-inch-thick (25.4 mm) slab of copper alloy with a
density of 8.8 gm/cc will have approximately the same
exposure time and rads as a 1.13-inch (28.7 mm) thick
slab of steel with a density of 7.87 gm/cc.
Likewise, an object with unknown radiographic exposure
characteristics may be compared with one whose exposure
values are known. To accomplish this, first approximate the
objects linear attenuation coefficient (μ 2 ) from its density
(d2) by
Varian Linatron applications
μ 1 /d 1 = μ 2 /d 2

and then determining the approximate exposure level from
the known value of exposure for object number 1 (Rl ), by
R2e -μ2X2= R1e -μ1X1 Example
The exposure of a 4-inch-thick (101.6 mm) bronze alloy
(density = 9) may be estimated from the steel exposure
curve at 2 MV by:
μbronze = 0.693/0.8 X 9/7.85 = 1 inch-1 For 4 inches (101.6 mm) of steel, the exposure rads:
R l = 150 rads.
Therefore for bronze: R2 = 150 e-(.866)(4)
e
= 256 rads
-(1)(4)
Thus, an exposure curve for the bronze alloy can be
constructed by calculating the exposure rads at two
thicknesses, plotting these points, and connecting them
with a straight line.
Table 4-4 lists the physical density ratio conversion factors
for converting steel or solid propellant exposure curve into
curves for a wide variety of materials.
• Physical density may be used for a first approximation,
but a more accurate value results from t using linear
attenuation coefficients.
Multiply by factors belowto convert steel
exposures to approximate exposures for these
materials:
Zirconium 0.83
Copper 1.13
Molybdenum 1.3
Silver 1.33
Lead 1.44
Uranium or Tungsten 2.38
Gold 2.45
page 27
Using Film Response Curves to Generate New Exposure
Curves. When an exposure curve is obtained and verified
for one film and at one value of film density, new exposure
curves for other films and for other film densities can be
generated without remaking the entire series of exposures.
To determine response curves for new films, refer to the
film response curves that plot film density versus the
exposure level for X-rays, in the Linatron energy ranges.
Example
Table 4-4 Physical Density Ratios for Common Materials When Using High Energy X-Rays
If an exposure curve for steel is made with AGFA-Gevaert
Type D-2 x-ray film for a density of 2.0, and it is desired
to have a similar curve for Type D-4 film at density 2.0,
the film response data of Table 4-3 (presented earlier) and
the characteristic curve plots for x-ray film in the AGFA-
Gevaert Handbook supply the data necessary for the new
plot. Table 4-3 shows that at a film density of 2.0, D-2 has
a film speed index of 7 and D-4 has a film speed index of
3. These indices are in proportion to the exposure rads.
Therefore, at each thickness, the factor of the D-4 film
speed index relative to the D-2 film index (or 3/7 when
multiplied by the exposure rads from the D-2 exposure
curve) will determine the correct rad exposure at each
thickness for the D-4 film. the Agfa-Gevaert Handbook
indicates the same relative speed indices for these films, but
with another series of index numbers.
Multiply by factors below to convert propellant
exposures to approximate exposures for these
materials:
Sodium 0.57
Lucite 0.69
Magnesium 1.02
Carbon 1.3
Concrete 1.38
Aluminum 1.6
Titanium 2.67
Varian Linatron applications

Increasing Latitude With Multiple Film Use. The
exposure curves for steel and solid propellant are shown in
Figures 4-10 through 4-21. Figures 4-10 and 4-16 are for
two typical films and a single film speed index (Exposure
Factor) of 1. The remaining figures are for three film
types, with film speed indices of 1,2, and 4. Note the
overlapping of film densities. The densities in the three
films are not equal at any one particular value of thickness
and exposure, since the film speed indices and responses
are unequal. Because the response characteristics of each
film at any thickness and exposure, when the fast film (low
film speed index) shows a density of 3.0, the slower film
exposed to the same level and thickness may show a
density of 1.6 or 1.7.
FIGURE 4-10. Linatron tvpical exposure curves for steel.
page 28
Example
The curves for the 9 MeV Linatron show a latitude of 2.8
inches (71.12 mm) of steel between film densities of 1.0 to
3.0. This means if an object is made of various
thicknesses, and the range exceeds about 2.5 inches (63.5
mm), the latitude limits of one film are exceeded, and a
second film, either slower or faster (for the thinner or
thicker section), is needed to extend the exposure range.
Varian Linatron applications

FIGURE 4-11. Linatron 1 MeV typical exposure curves for steel.
page 29
Varian Linatron applications

FIGURE 4-12. Linatron 2 MeV typical exposure curves for steel.
page 30
Varian Linatron applications

FIGURE 4-13. Linatron 4 MeV typical exposure curves for steel.
page 31
Varian Linatron applications

FIGURE 4-14. Linatron 9 MeV typical exposure curves for steel.
page 32
Varian Linatron applications

FIGURE 4-15. Linatron 15 MV typical exposure curves for steel.
page 33
Varian Linatron applications

FIGURE 4-16. Linatron typical exposure curves for solid propellant.
page 34
Varian Linatron applications

FIGURE 4-17. Linatron 1 MeV typical exposure curves for solid propellant.
page 35
Varian Linatron applications

FIGURE 4-18. Linatron 2 MeV typical exposure curves for solid propellant.
page 36
Varian Linatron applications

FIGURE 4-19. Linatron 4 MeV typical exposure curves for solid propellant.
page 37
Varian Linatron applications

FIGURE 4-20. Linatron 9 MeV typical exposure curves for solid propellant.
page 38
Varian Linatron applications

FIGURE 4-21. Linatron 15 MeV typical exposure curves for solid propellant.
page 39
Varian Linatron applications

Radiographic Procedures
General Considerations
Practices in radiography of castings, welds, rocket motors,
and assemblies are described in this section. The
information presented here is intended as a general
description only. Specific radiographic facilityoperating
procedures are usually determined by unique facility
requirements. These guidelines must not change, alter,
supercede or take precedence over any controlling
procedures, specifications or standards.
Records and Logs. It is extremely important to keep
careful records of all radiographic work. The daily log
should show the names of the radiographers and the
identification of each part radiographed.
In addition to the job information, a daily log should be
maintained containing information concerning radiation
hazards and operation of all equipment, including all safety
devices, interlocks, warnings. All malfunctions should be
dated and identified. Checks of the equipment after
repairs or adjustments should be logged.
Exposure Room Equipment. In addition to the Linatron
and its manipulation system, an x-ray facility should have
the following:
• Materials handling crane, forklift, or other means to
move and set up the parts that require x-ray
examination.
• Electrical power outlets.
• Grounding system leading to an outside earth ground.
• Film cassette support and holders.
page 40
A facility designed to examine objects such as large rocket
motors or similar assemblies should include yokes, belt
slings, roller support frames, motorized turntables, and
tangential shielding supports. Other accessories and
supplies for radiography are listed below.
• Image Quality Indicators (IQI) for the applicable
standards and material being radiographed
• Deep block lead numbers and letters
• Film holders (all sizes), rigid cardboard, soft plastic, and
vacuum cassettes
• Lead screens from 0.010 inch (0.25 mm) to 0.030 inch
(0.76 mm) thick, assorted sizes, some unbacked and
some with cardboard backing
• Lead sheets 1/8 inch (3.18 mm) to 1/2 inch (12.7 mm)
thick, in all sizes
• Lead or copper shot in canvas bags
• Lead bricks
• Calipers and other wall-thickness gages
• Survey meters for radiation protection surveys; also Rmeters,
pocket dosimeters, film badges, warning lights,
signs, and other safety equipment
• Automatic temperature-controlled film processing
equipment
• Film viewing equipment with variable intensity viewers
• Magnifying glass, 3x to 5x, for film reading
• Densitometer with capabilities up to density 4.0
Varian Linatron applications

Casting Radiography
Radiography of castings can reveal defects which can be
repaired before expensive machining operations are
performed. Therefore, it can be a very effective tool to
determine the suitability of the casting. Radiography also
reveals how well casting techniques are working.
Most castings inspected by high-energy radiography are
produced in sand mold flasks or in metal-framed
containers. The upper section of the mold is referred to as
the cope, the bottom section as the drag, and the junction
of the two as the parting line. The mold of a large,
complex casting may be assembled from a number of parts
and cores. Radiography may also be required for large
ingots that are cast in permanent molds and for parts
suitably shaped for casting by the centrifugal process.
Listed below are common casting defects that radiography
can reveal.
• COLD SHUTS are areas between two quantities of cast
material that have cooled below the molten state before
coming into contact with each other, thereby causing an
unfused interface.
• HOT TEARS show as crackline indications usually
appearing singly near the surface and at hot spots or
where temperature gradients are high.
• CRACKS show as sharply defined indications, sometimes
with penumbral shadows, caused by metal parting from
itself. These are frequently caused by improper stress
and temperature gradients. Cracks are often created
when a core or mold resists the compression of the
casting as it shrinks on solidification.
• PIPING shows as elongated, cylindrical-shaped
indications located at or near the center of the casting.
These are caused by risers and feeders not being kept hot
and liquefied, which causes a lack of metal flow to the
center of the casting mold.
• SHRINKAGE is generally associated with improper
feeding of metal into the casting mold. Shrinkage
manifests itself in the following three forms:
- Sponge Shrinkage, found in heavier sections, appears
on the radiograph as a dark, lacy area with a diffused
outline.
- Feather Shrinkage, found in thinner sections, appears
on the radiograph as a sponge, with a feathery outline.
page 41
Varian Linatron applications
- Linear Shrinkage, a continuous structure of connected
lines with branches of variable lengths, widths, and
densities.
• GAS HOLES AND VOIDS are caused by trapped air or
mold gases. They appear on the radiograph as round or
elongated, smooth edged, dark spots and may appear
individually, in clusters, or distributed throughout the
casting.
• DROSS represents oxidized metals. This appears as a
series of lines in a swirl pattern sometimes combined
with agglomerated irregular indications.
• SEGREGATION is the result of certain elements of a
metal to concentrate in the last liquid portion of the
metal. This results in an uneven distribution of the
chemical constituents. These can appear as uneven,
irregularly shaped, dark areas on the radiograph.
• MOLD MATERIAL, SAND, AND OTHER
INCLU SIONS
• UNFUSED CHAPLETS AND CHILLS
• INCOMPLETE DEFECT REMOVAL
• DEFECTIVE REPAIR WELDS
Radiographic Coverage. The coverage on castings may be
specified by the customer or it may be given by casting
specifications. The radiographer is required to select
optimum projections and techniques in the latter case.
Visual inspection of rough-cleaned castings reveals where
risers and gates have been removed. Pattern numbers are
usually on the drag side. Areas typically radiographed
include heavy sections in the drag, the flanges, the
junctions of diaphragms and casting walls in the valve
areas. Among the critical areas are valve seats, steam chest
areas, and other parts where extensive machining is
performed.
Shrinkage defects usually lie in the centerline of cast
sections and, unless they are extensive or in areas to be
machined, are less critical than hot tears or cracks. Both
hot tears and cracks tend to be perpendicular to cooling
stress directions and are often difficult to examine unless
the radiographic projection is parallel to the crack or tear
direction.

When radiographing curved sections, the concave side
should face the source and, when possible, flexible film
holders should be formed to the casting contour. While
this practice tends to improve film/screen contact, it may
increase the image distortion at the edges of the curved
film. To control for this, image quality indicators should
be placed at the extreme widths of the film, and there
should be adequate overlap in adjacent exposures to
ensure complete coverage. While an oblique angle may
be required in some regions, when possible, the X-ray
beam should enter the casting at right angles to the front
surface, and the film should be set up perpendicular to
the central ray.
When the casting thickness varies more than 1/2-inch
(12.7 mm) equivalent steel, the multiple-film technique
can be employed to cope with the varying section
thicknesses. These variations may also create significant
scatter, which can be minimized using special procedures
(see the discussion on scatter control).
Layout and Marking. Radiographic assignments can
involve fairly simple techniques. This can be as simple as
placing the film cassette under or behind the casting and
making the exposure. But, for large, heavy castings such as
turbine bodies, the film placement and radiographic
projections can be complex and may require careful
planning and fixturing.
Crane mounted Linatron used to examine large objects.
page 42
The radiographer should be furnished with either a marked
print of the casting, showing the projections and areas to
be covered, or a rough sketch showing the film locations,
markers and other relevant information. The marked print
or sketch should remain with the casting radiographs. The
radiographer should retain a copy to add information
about the film and screens used, image quality indicator
selection and placement, filtration, exposure and resulting
film density, and other pertinent facts about the job.
Casting Radiographic Procedures. When compared with
lower energy systems, Linatron radiography of castings can
often produce significantly more readable film images.
The increased latitude and reduced wide angle scatter
provide major advantages not found in lower energy
radiography. However, high-energy radiographs will have
less contrast, and the imaging of fine cracks and
filamentary porosity can be difficult. Cracks usually do
not progress in a straight line and do not present welldefined
edges to the X-ray path. This usually causes
images to have penumbral edges, which makes detection
difficult. Cracks which lie at angles to the X-ray beam can
be seen if their width exceeds the limit imposed by the
overall radiographic sensitivity, as determined from the
equation;
Varian Linatron applications
Δx = W/sin ø (see Figure 5-1)
where Δx = the crack width in the direction of the radiation
W = the crack width
ø = the angle of the crack to the x-ray beam

FIGURE 5-1. Relationships between crack width (W), projection
angle (ø), and contrast sensitivity (Δx/x).
Almost all castings contain discontinuities of various types
(inclusions, microshrinkage, etc.) that have no effect on the
strength or serviceability of the casting. Repairing these
discontinuities is expensive and sometimes can do more
harm than good. It is the radiographer’s task to determine
the criticality of the various areas and to produce the best
possible radiographs.
• RADIOGRAPHY OF FLANGES. Whenever flanged
areas of a casting are radiographed, the radiograph
should cover the flanges, the cylindrical wall, and the
corner between them. Film should be placed under the
flange and inside the casting, as shown in Figure 5-2.
Two or three projections may be required since an
exposure that shows shrinkage defects might not show
tears. It is important the exposures provide a clear
picture of the junction between the flange and the wall.
The radiographs must show shrinkage completely if
there is shrinkage in the flange and wall areas. Cracks
and tears must be found if they are present in the fillet.
Flange exposures should be made for the average
thickness of the section being radiographed. Multiple
film techniques should be used as necessary if multiple
thicknesses are present in the radiographed section.
page 43
• RADIOGRAPHY OF VALVE BODIES. High-energy Xrays
can penetrate both outside walls and provide the
necessary sensitivity for demonstrating internal section
defect when applied to small and some large valve
bodies. This is especially true in the critical valve seat
areas. Exposures are made by placing the film behind
the valve body and beaming into the object at the best
angle to project the critical internal section onto the
film. Blocking, filtering, or other scatter reduction
techniques should be used to ensure maximum
sensitivity. A large D/T ratio is needed to minimize
distortion and undesirable enlargement.
FIGURE 5-2. Typical flanged casting exposure plan showing 12
exposures at 30˚ increments for three beam orientations (cr =
central ray).
Varian Linatron applications

Radiography of Welds
Radiography is one of the most widely used inspection
techniques and still remains the most reliable choice for
weld inspection. Because radiographic weld inspections are
often controlled by mandatory procedures with codified
accept-reject criteria, the radiographer should know the
specified requirements, and as much as possible about the
specific fabrication before deciding to apply a certain
procedure. To assure the appropriate inspection is
conducted, the radiographer must know the ability of each
radiographic procedure and technique to form images of
defects. For example, the orientation of weld defects is
often known and should determine the optimum
radiographic coverage and angulation to provide the best
inspection.
Frequently, the defect of most concern is the crack, and
small cracks are hardest to detect. The probability of crack
detection by radiography is a function of crack width,
orientation, and X-ray machine sensitivity. A
supplementary test such as ultrasonic examination may be
required since it is often not practical to exhaust all
possible angulations of an X-ray beam in order to increase
the crack detection probability. The following list describes
defects that are common to most welding and can be
revealed by radiography.
• CRACKS can occur in the weld metal (centerline
shrinkage cracks, root cracks, etc.), in the heat-affected
zone and in the base metal. They appear on the
radiograph as dark lines with sharp distinct edge, or
edges that are fuzzy and diffused.
page 44
• LACK OF PENETRATION is failure to melt the base
metal during the stringer pass weld. It appears on the
radiograph as a straight dark line at the center of the
weld This defect is usually caused by improper heat or
choice of weld rod.
• INCOMPLETE FUSION is failure of the weld metal to
fuse to the base metal. This appears as slightly wavy
lines running parallel to the weld length and off the
centerline of the weld.
• SLAG, TUNGSTEN, AND OTHER INCLUSIONS are
caused by entrapment of foreign material in the weld
metal. This defect appears on the radiograph as
irregularly shaped high or low-density areas depending
upon the material entrapped.
• POROSITY is gas entrapped in the weld metal. This
appears on the radiograph as small, globular shaped dark
voids. This can be linear in nature or can be scattered
throughout the weld metal.
• UNDERCUT is base metal sucked into the cover pass
causing a reduction of thickness at the juncture. This
appears on the radiograph as darker lines along the edges
of the weld.
• CONCAVITY occurs on joints that are welded from one
side only, where excessive melting of the underside
occurs, causing it to be pulled into the joint.
• CENTERLINE CREVICE.
• JOINT DISTORTION, MISMATCH, AND WELD
MISPLACEMENT.
Varian Linatron applications

Radiographic Coverage
* There are special requirements for the radiography of all
welds that may not be found in other radiographic
applications. These requirements are as follows.
(1) Film with the highest contrast gradient consistent with
acceptable exposure times should be used.
2) The exposure should obtain the highest film densities
that can be viewed conveniently.
Full Length Of Film
Inches cm
6 15.2
8 20.3
10 25.4
12 30.5
14 35.6
16 40.6
18 45.7
19 48.3
Angle from Central Ray to
Film Edge (degrees)
2.4
3.2
4.0
4.8
5.5
6.3
7.1
7.5
Table 5-3 Angle From Central Ray to Film Edge at 72 Inch (1.83 m) SF
page 45
(3) To demonstrate transverse cracks, longitudinal
coverage per exposure should not exceed the
maximum permissible projection angles that the
radiation source makes with the film ends, unless
otherwise specified. This is shown in Table 5-3 for a
1.83 meter SFD distance as an example.
The sensitivity required to demonstrate cracks at a
projection angle of 3˚ for various crack separation widths
and material thicknesses is shown in Table 5.4. These data
are calculated from the relationship Δx = W/sin ø
(See Figure 5-1).
Varian Linatron applications
Ratio of Crack Thickness & Width
(T/W) that Presents an Open
Channel For the x-rays
24 to 1
18 to 1
14 to 1
12 to 1
10 to 1
9 to 1
8 to 1
7.6 to 1

The width of separation required for sensitivity
demonstration is increased proportionately if the
projection angle is increased. The demonstrated crack
width is decreased proportionally if the sensitivity is
improved, i.e. from 1% to 0.5%.
The data given in Table 5-4 provide general guidelines
only. Cracks do not follow a single plane and sometimes
portions of the crack edges will not be parallel to the
radiation beam in actual practice.
Weld Thickness
Inches cm
2 5.1
4 10.2
6 15.2
8 20.3
10 25.4
10 25.4
1 30.5
12 30.5
15 38.1
15 38.1
Crack Separations
Mils mm
1.0 0.03
2.0 0.05
1.5 0.04
2.0 0.05
2.5 0.06
5.0 0.13
3.0 0.08
6.0 0.15
3.3 0.08
7.5 0.19
Table 5-4 Sensitivity Needed to Show Cracks at Angles 3˚ to the Beam
page 46
• CIRCUMFERENTIAL BUTT WELDS. Radiographs of
circumferential butt welds in heavy sections usually are
obtained with the source outside the circumference. Care
must be exercised when radiographing curved sections
with this technique. Be aware that small transverse
cracks may be obscured if the projection angles are too
large. Therefore, the film coverage must be reduced to
control the projection angle through the curved section.
Image quality indicator placement should be the same as
linear welds. Distortion from projection is increased
when film holders are made to conform to the inside
curvature. Consequently, the use of rigid cassettes with a
suitable front filter is recommended.
Varian Linatron applications
Sensitivity Required (%)
1.0
1.0
0.5
1.0
0.5
1.0
0.5
1.0
0.5
1.0

• T-SECTIONS. T-Section welds present problems that
are solved only by the radiographer ingenuity. It may be
necessary to take two projections to ensure adequate
examination. For example, when a dome is welded to a
heavy flange (with a double-J bevel), orient one
projection parallel to the flange surface and the other at a
projection angle near the angle of the bevel. A
projection parallel to the surface should be made if the
flange material is rolled stock with the weld attachment
perpendicular to the rolling direction. Many highly
stressed T-weld failures occur because of cracking in the
parent material beneath the weld.
Design and Placement of Image Quality Indicators
(IQI). A typical practice is to require at least two IQls per
exposure area. These are placed at the limits of film
coverage adjacent to the weld. If weld reinforcement is not
removed, the IQI must be shimmed up so that the total
thickness under the IQI is equal to the total thickness
through the weld. This includes backing strips that were
not removed. In the United States, ASTM plaque-type IQI
are usually required. In Europe, DIN, International
lnsitute of Welding (IIW), or AFNOR (French) IQls may
be used. Reports correlating sensitivities of various IQls
are listed in the Bibliography.
FIGURE 5-3. Typical rocket motor configuration.
page 47
Defect Location. The linear extent of defects can be
determined by comparing radiographs with markers placed
adjacent to the welds. It is sometimes necessary to locate
the depth of defects to determine which side of the weld
should be excavated for repair. This is done by making a
double exposure with a shift of the source across the length
of the defect. Defect depth is a simple ratio of the defect
shift compared to the shift of the marker on the surface,
relative to the known weld thickness.
Radiography of Rocket Motors
Solid propellant rocket motors are made with a wide range
of diameters, and with a variety of bore configurations as
shown in Figure 5-3. A solid propellant rocket motor
consists of a rigid case, an internally bonded insulator and
liner, and solid propellant. These are shown in Figure 5-4.
These propellant configurations have a burning surface
area that normally produces a predictable pressure/flight
curve. If this area is increased by the presence of a crack,
void, or separation, overpressurization of the case can
occur, causing a malfunction. One or more nozzles at the
back end complete the basic motor.
FIGURE 5-4. Cross section of front end of solid rocket motor,
showing construction details for radiographic examination.
Varian Linatron applications

The propellant in large motors is often adhesively bonded
to the liner to provide structural support and to restrict the
burning to the bore surfaces and typically provide low
subject contrast when radiographed. Therefore, every
precaution must be taken to increase the radiographic
contrast. This contrast can be greatly enhanced by
selecting the proper X-ray energy.
MeV
1
2
6
9
15
Propellant Thickness Range
inches cm
2-25 5.1 - 63.5
6-33 15.2 - 83.8
8-48 20.3 - 121.9
12-65 30.5 - 165.1
16-100+ 40.6 - 254.0+
Table 5-5 Thickness Range for Routine Radiography with
Linatron Sources
Table 5-5 lists practical radiography ranges for solid
propellant. The table shows that a Linatron providing Xray
energies of 1, 2, 6, 9, and 15 MeV, covers the
radiographic range for solid propellant from 2 inches (5.1
cm) to over 100 inches (2.54 m) in thickness. Radiography
of the propellant grain and the peripheral bonded regions,
including the front and back domes, is normally required.
Radiographs of the grain should reveal the presence of
cracks, voids, non-uniform mixing, lack of bond between
internal interfaces, bore deformation, foreign material,
excessive porosity, low-density volumes, and other defects
that are characteristic of the propellant type and the
manufacturing process.
Tangential radiography of peripheral areas reveals interpropellant
bond failures, deterioration or separation of the
liner-propellant bond, insulation failure, peripheral
porosity, and other substandard conditions. Radiographs
are also used to judge the quality of repair work in rocket
motors.
page 48
FIGURE 5-5. Typical roll stand to support rocket motor for horizontal
radiography.
Motor Attitude and Support Equipment. Radiography
of many solid propellant rocket motors is best
accomplished with the motor horizontal on a roller
support stand as illustrated in Figure 5-5. The structural
strength of the motor must allow it to be placed in the
horizontal position, and a method used for support during
rotation. An effective arrangement is to position the X-ray
source above the motor with film for the grain exposures
placed on a shelf below the area to be covered. It is easily
changed to accommodate larger or smaller motors, it
makes the bore accessible, and it allows tangential shields
and film holders to be placed along both sides of the
motor for tangential exposures. The vertical position is
used when the structural strength of the motor case will
not support the weight of the motor in the horizontal
position. This type of radiography requires mounting
equipment such as a turntable (see Figure 5-7). Adapter
rings, handling yokes, belt slings and other specialized
devices.
Varian Linatron applications

Rocket Motor Radiographic Procedures. Radiography
of the grain of a solid propellant rocket motor is usually
performed in accordance with an established layout and
exposure plan designed to give complete coverage of the
entire grain. The cylinder is divided into a number of
segments that correspond to the film centers and range.
These segments may be exposed individually, or they may
be grouped into exposure regions, with several films
exposed simultaneously. In addition to specifying the film
locations, the plans should show the exact location and
direction of the central ray. A significant amount of film
overlap is used in most plans to ensure complete coverage.
The overlap also allows some of the propellant to be
exposed a second time at a different angle. This offers an
added opportunity to show cracks that may be present but
at an angle that would not show detection in the first
exposure.
FIGURE 5-6. Typical turntable for vertical rocket motor radiography.
page 49
The dome and tangential areas require separate exposures.
Plaque or wire-type image quality indicators should be
placed on the front (source side) of the motor, so that at
least one such image appears on a film in each exposure.
Plaque-type IQI thickness need not be more than 1% of
the average propellant thickness penetrated. The source-tofilm
distance should be as large as practical.
The angle from the central ray to the edge of the film
should not exceed 15 degrees when calculating the film
coverage for a grain exposure layout. Lead front and back
screens (or equivalent screens made of other materials such
as tungsten, copper, tantalum, or fluorometallics), at a
thickness of 20 mils (0.51 mm), should be used. In some
instances, cassette filters may be required to control the
effects of this scatter. The film holders should be placed as
close to the back of the motor as possible for the through
body exposures. Ideally they should be enclosed in an
evacuated envelope to ensure film-screen contact. The
film-screen contact should be maintained by springpressure
cassettes or other mechanical means if a vacuum
system is not available.
For x-raying propellants, ideal film densities are from 1.5
to 3.0. When thicknesses are greater than those that will
render these densities, the multiple (two-speed) film
technique should be used. Although film exposure curves
will normally indicate film combinations that can be used
for various thick- nesses, some experimental exposures are
usually required to finalize film selection and exposure.
Occasionally, a thin lead foil is used as a separator between
films when optimum intensification and definition are
required.
Varian Linatron applications

Grain Radiography Coverage Requirements. Contrast
sensitivities of better than 1% can be achieved routinely
with the Linatron using optimum radiographic production
techniques. Defects such as voids and gross density
variations in the solid propellant are readily detected at that
sensitivity. The simplest radiographic procedure (e.g., one
exposure through the grain or two exposures, one at 0˚ and
the second at 90˚ may be used when these are the only
concern. However, cracks and separations are also of great
concern in rocket motors.
Cracks with widths of 1 to 5 mils (0.03 - 0.13 mm) are
detected only when they are aligned with the radiation
source. Most propellants with cracks will open to a width
of 10 to 30 mils (0.25 - 0.76 mm) minimum. At these
widths cracks can readily be detected at almost any angle to
the radiation, and through large thicknesses of propellant.
FIGURE 5-7. Visualization of crack-like defects, as a function of
propellant thickness and crack angle, for x/x = 0.2%.
Figure 5-7 illustrates the width of crack-like defects that
can be demonstrated through various thicknesses of
propellant, at various projection angles, with an assumed
contrast sensitivity of 0.2%. Refer to the approximate
relationship Δx = W/ø. It can be seen from Figure 5-7, as
propellant thickness increases, the ratio W/ø increases,
which means that the same width crack will be shown at
smaller angles. For example,
page 50
Propellant Thickness Ratio of Crack Width
to Projection Angle
inches cm
Varian Linatron applications
30-36 76.2 - 91.4 W/ø ≈ 1.0
40-50 101.6 - 127.0 W/ø ≈ 1.5
50-60 127.0 - 152.4 W/ø ≈ 2.0
To observe the required rocket grain cracks, the number of
exposures must be increased as the diameter increases. If
the technique being used produces contrast sensitivity
other thanAx/x = 0.2%, the procedures must be modified
accordingly to obtain an equivalent defect detectability.
Tangential Radiography. Propellant-to-insulation (liner
separations) in the domes are considered critical defects
wherever they occur in a rocket motor. Separations as small
as 2 mils (0.05 mm) can be shown on the radiograph,
when the separation occurs at the tangential point.
FIGURE 5-8. Probability (P) of detecting propellant to insulation
separation versus number of exposures (N) for various circumferential
lengths of separation (Lc , for 54-inch (137 cm) diameter (D),
rocket motor, P = Lc/n - D N.

Tangential radiography is performed by placing the area of
inspection at nearly right angles to the x-ray beam. The xray
forming the outermost part of the cone of radiation
will pass through the interfaces formed by the successive
diameters of the propellant, insulator, and case. By placing
the film against the motor case, below the inspection area,
a radiographic image of the tangential points of the
successive interfaces is formed. The detectability of a
separation depends upon the number of tangential
exposures, as illustrated in Figure 5-8.
Example
12 tangential exposures evenly distributed around a 54inch-diameter
(1 37.2 cm) motor have a 50% probability
of showing an area of separation that extends 7 inches (1
7.8 cm) around the periphery and 100% probability of
showing an area extended 15 inches (38.1 cm) or more
around the periphery.
Tangential Radiography Coverage Requirements.
Radiography of the peripheral areas of the dome and
cylindrical areas of a solid propellant rocket motor requires
an exposure plan similar to the plan for the grain. In fact,
the same layout and marking may be used for the
tangential exposures as for the grain. The central ray
positions depend on the size of the motor and on the
particular source to be used. The grain exposure with
simultaneous bilateral tangential exposures can be made by
directing the central ray radially because of the Linatron’s
high output intensities and large radiation cones. With a
less powerful source or one with a small cone of radiation,
only one side of the motor can be exposed at a time. In
general there is no need to use the two-film technique,
since one film may have sufficient latitude to show the
critical areas inside the case. Some experimental exposure
tests may be needed to finalize the optimum technique.
Tangential shielding may be required to reduce the forward
scatter that is characteristic tangential radiographs. This
occurs because of the very small angle between the primary
beam and the object during tangential radiography. A lead
shield can be added to the outer surface of the motor at
the tangential point when exposures with very high
contrast sensitivity are required. The attenuation of the
shield should be approximately the same as the average
attenuation of the chord at the propellant interface.
page 51
Wires and slits with thicknesses often less than 1% of the
total equivalent chord at the propellant interface have been
employed in tangential radiography. Lead screen and filters
should be used like grain exposures. The interface image is
a projection since the curvature of a motor prevents the
placement of film close to the tangent point. For this
reason the largest source-to-film distance possible should
be used.
Assembly Radiography
Assemblies such as jet engines, gas turbines, valves, nuclear
fuel elements, and explosive devices (e.g. bombs and fuzes)
are frequently radiographed with high-energy x-rays to
show internal conditions or dimensions. These assemblies
may have material thicknesses that vary by several HVLs at
adjacent regions. Also, many assemblies can have material
and assembly characteristics that produce forward scatter,
which obscures the sharpness of the radiographic image.
The following good radiographic practices should be
observed when establishing the techniques:
• Use the highest practical x-ray energy to minimize the
effect of forward scatter.
• Use object-film filters and screen-film combinations that
produce the highest contrast and best sharpness.
• Use large D/T ratios to avoid distortion.
• For radiography of a radioactive object, place the film at a
distance from the object sufficient to reduce fogging of
the film by the object’s radiation. Also, use an object-film
filter in back of the object to further reduce object effects.
• Use multifilm techniques where thickness variations
exceed the range of a single film.
• Exposure times for each film may be obtained from
established exposure curves when the average thickness of
the objects in the area of penetration is known.
Varian Linatron applications

Bibliography
Books
Practical Radiography for Industry
H. R. Clauser
Reinhold Publishing Co., N.Y. (1 952)
Applied X- Rays
G. L. Clark
McGraw Hill Book Co., N.Y.
4th Edition, (1 955)
Handbook of Radiology
Editors-R. H. Morgan & K. E. Corrigan
The Year Book Publishers, Chicago (1 955)
A Further Handbook of Industrial Radiology
Editor-W. J. Wilshire
Edward Arnold & Co., London (1957)
Handbook of Nondestructive Testing
Editor-R. C. McMaster
Roland Press, N.Y. (1 960)
Techniques of Nondestructive Testing
Editors-C. A. Hogarth & J. Blitz
Butterworth & Co., London (1 960)
Nondestructive Testing
W. J. McGonnagle
McGraw Hill Book Co., N.Y. (1961)
An Introduction to Industrial Radiology
J. C. Rockley
Butterworth & Co., London (1 964)
Physics of Industrial Radiology
Editor-R. Halmshaw
American Elsevier Publishing Co., N.Y. (1966)
Industrial X-Ray Interpretation
Justin G. Schneeman
lntex Publishing Co., IL (1968)
Radiography in Modern Industry
Eastman Kodak Company, Rochester, N.Y.
4th Edition (1 979)
Industrial Radiology Techniques
R. Halmshaw
Wykeham Publishing Co., London (1 971 )
page 52
Sections of Books and Technical Reports
Radiography with High-Energy Radiation
C. C. Pollitt
Journal of the British Steel Castings
Research Association, No. 65, Feb. 1962
10-MeV X-Ray Technique
D. T. O’Connor, E. L. Criscuolo, & A. L. Pace
Special Technical Publication No. 96
American Society for Testing & Materials
Radiology with High-Energy X-Rays
R. Halmshaw & C. C. Pollitt
Progress i n Non- Des t ruct ive Testing
Vol. 2, Heywood & Co., London (1959)
10- MeV Rotating Target Linear Accelerator for
Radiography of Large Rocket Motors
J. H. Cusick & J. Haimson
Proceedings, Missiles & Rockets Symposium,
U.S. Naval Ammunition Depot, Concord, Ca.(1961)
Radiography of Large Missiles
With the Linear Electron Accelerator
J. Haimson
Nondestructive Testing, Mar-Apr 1963
High - Energy Radiography in the 6- to 30-MeV Range
J. H. Bly & E. A. Burrill
Special Technical Publication No. 278
American Society for Testing & Materials (1959)
High Voltage Radiography
Section No. 23 of the “Handbook of Nondestructive
Testing”, Editor R. C. McMaster,
Roland Press, N.Y. (1 960)
Radiographic lnspection
Section in “Metals Handbook”, Vol. 11, 8th
Edition, American Society for Metals, 1978.
Nondestructive Testing Handbook
Editors - Paul Mclntire and Lawrence E. Bryant.
American Society for Nondestructive Testing.
Varian Linatron applications

High Energy Real-Time Radiographic Imaging
Real Time Radiography can be defined as nondestructive
radiographic testing where an image of the test object is
viewed concurrently with the irradiation. This method of
inspection is relatively new to nondestructive testing but is
an outgrowth of fluoroscopy, which has been used in the
industrial field for many years. The technology has made
great leaps, with improvements in image processing,
development of new fluors and camera designs.
FIGURE 6-1. Typical real-time x-ray imaging system.
page 53
Imaging Heads - In general, high energy real time
radiography imaging heads (cameras) have three main
components, the imaging screen, lens system and low-light
video camera. The imaging screen directly converts x-rays
into low-level light, usually in the yellow/green part of the
visible spectrum. The lens system contains one or more
large aperture lenses that couple the light to the low-light
video camera.
Varian Linatron applications

Normally, the light generated in the imaging screen is
coupled to the lens-camera system by a front surface mirror
mounted at 45 degrees to the screen. This positions the
camera and lenses outside the cone of direct radiation,
minimizing damage to these components.
In general, two types of low-light video cameras are used in
real-time radiography, Silicon Intensified Target (SIT) and
Image lsocon video tube cameras. SIT cameras have
excellent low-light sensitivity and are compact, rugged and
require no special cooling. Image lsocons have more
dynamic range and spatial resolution but are very position
sensitive and require special cooling.
Because Image lsocons are physically larger tubes, these
cameras are generally larger and vibration sensitive. SIT
tubes are small, low cost and more adaptable to special
applications.
Signal Processing - Signals from real-time cameras are
digitized and stored in digital memory for further
processing or archival storage. The digital memory is part of
an image processor, which is a specialized computer system
designed to process image data, most often in real time.
FIGURE 6-2. Schematic of ER real-time system functional elements.
page 54
Image processors can vary considerably from systems that
only have the ability to acquire individual video frames to
systems that can perform complicated image filtering
and/or transforms. Images stored and processed by the
image processor are displayed on a high resolution video
monitor for interpretation. This is illustrated in Figure 6-2.
Archival Storage - Archival storage is necessary to keep an
image record of inspection procedures performed. Two
basic methods are used to store real-time radiography
images, analog and digital. The most popular, and
presently least expensive, method to store real-time images
is high bandwidth analog video tape. Any image presented
on the video monitor will be recorded on video tape.
Additionally, images can be annotated verbally on the
video tape’s audio track.
Images may also be stored digitally on hard disk, floppy
diskette, magnetic tape and optical laser disk. Magnetic
tape and optical laser disk have the most capacity, but
magnetic tape is classified as a volatile medium with a
definite possibility for the data to be corrupted by
magnetic fields, erased or being overwritten.
Varian Linatron applications

Optical disks are classified as non-volatile because they can
be written to once, read as many times as desired and
cannot be erased. Therefore, optical disks are the digital
equivalent to standard film. Additionally, optical disks have
a very high capacity. Present optical disks can store up to
8000 images per disk pack. Since disk packs are removable,
virtually unlimited storage is possible.
Hard disk storage is usually an intermediate digital storage
before transferring the images to optical disk, magnetic
tape or floppy diskette. This is because the hard disk has a
definite limit on storage capacity. Therefore, image data
must be regularly erased to make room for more data.
Operator Control - Early real-time radiography system
operator controls were a collection of the individual
component controls mounted on an operators console.
The trend for present systems is to integrate all of the
controls from the separate components that make up a
real-time system into one centralized control system. This
is most often accomplished by controlling all system
components with a centralized computer system.
Today it is possible to use a real-time radiography system as
the control core of an automated radiographic inspection
system. This requires communication and control
protocols between the imaging system and the parts
handling device (manipulator) - and possibly other
peripherals - and the ability for the imaging system to
perform programmable inspection sequences. Then, after
the part is loaded and the x-ray source and camera are
aligned to an initial position, the control computer can
control the rest of the inspection process.
Applications
Real-time radiography applications can be generally
divided into two subsections, actual real-time imaging
where processes are examined for dynamic function and
automated test sequences where high test throughput and
low-cost is the primary function.
Test procedures such as observing the pattern of metal
filling molds and observing the function of motor
components while the motor is actually running are typical
dynamic function applications. It is also often necessary to
use real-time systems to properly position test objects
where the inner mechanism is unknown or obscure.
page 55
Automated testing has become more important in recent
times because of an emphasis upon 100% testing of critical
components. Real time systems, in conjunction with parts
handling systems (manipulators) and integrated system
control, allow automated test sequencing with higher
throughputs and more coverage than previous film systems.
Figure 6-3 illustrated a real-time automated inspection
system.
The following characteristics should be considered when
planning a real-time radiography application:
X-ray energy - Selection of an x-ray energy for real-time
imaging should be based upon several considerations.
Selection of an energy that is too great for the part being
inspected can lead to difficulties. When the energy is so
high that the material does not present at least 1.5 half
value layers, small changes in attenuation may be lost.
Additionally, radiation buildup can cause tremendous
scatter problems when x-raying thin sections. This is
stabilized after approximately 1.5 HVLs. This thickness is
called the “equilibrium” thickness.
Selecting too low an energy also presents problems,
especially when actual real-time images are important. In
general, dynamic real-time radiography is like taking
pictures at a thirtieth of a second per image. If the
radiation energy or output is too low, little detail will be
seen in the image. This imaging time can be lengthened by
averaging two or more frames together. However, enough
radiation must be reaching the imaging screen to provide
the amount of light on the conversion screen needed for
proper image evaluation.
Ideally, x-ray energy and output is chosen so that 4 to 7
half value layers are being examined and the radiation to
the imaging screen exceeds 15 to 25 rads/min. However,
real-time systems can be useful at extremes outside of this
ideal envelope.
Varian Linatron applications

FIGURE 6-3. Real-time automated inspection system.
page 56
Varian Linatron applications

Real-time inspection applications, using proper technique,
have been successful for up to twelve half value layers.
However, throughput suffers when the long image
exposures for this type of imaging are used.
Collimators - When real-time imaging is being performed,
it is extremely important to restrict the x-ray beam to the
area that is being inspected. Remote controlled collimators
are very useful in real-time radiography because the size
and shape of the collimated beam can be seen on the
imaging screen in the control room in “real-time”, allowing
the operator to select the exact area desired for viewing.
Additional collimation may be needed at the object being
inspected to control scattered radiation. This can be
located either on the x-ray side of the object or on the
screen side of the object, depending upon the needs of the
situation.
These are termed “pre-collimators” and “postcollimators”,
respectively, and are similar to the practice of “blocking”
except that these collimators restrict the beam to the size of
the imaging screen, while blocking is only shielding placed
around the item being inspected.
Manipulators - A key factor in real-time radiography is the
ability to manipulate the item under inspection
automatically or by remote control.
To inspect complicated items, manipulators (or parts
handling systems) must be accurately controllable and
repeatable. For automated imaging applications,
manipulators are programmed to follow predictable,
planned paths during the inspection.
The design of manipulators is very applications specific
and the system integration needed for automated test
sequences is challenging. In general, the manipulator must
have sufficient flexibility to position the inspected object
under inspection properly for the total x-ray inspection.
Control of the manipulator can be a remote manual
console or the controls can be integrated into the imaging
system. Integrating manipulator control with the imaging
system allows completely autamatic inspection sequences.
page 57
Screens/Filters - A distinction should be made between
conversion screens used in real-time radiography, which
directly convert x-rays to visible light, and intensifying
screens, which filter low energy x-rays and emit
intensifying photoelectrons to increase light output.
Real-time radiography uses several different types of
conversion screens. For low energy, these screens can be
used without intensifying filters. However, for higher
energies, intensifying filters (screens) are used to provide
better image contrast. The combination of conversion
screen and filter depend upon the application.
As higher energies are used, image degradation results from
high energy photons passing through the conversion screen
without sufficient interaction with the phosphors in the
screen. By adding a heavy metal intensifying screen on the
x-ray side of the fluorescent (conversion) screen, “knock
out” electrons are emitted which interact with the
phosphors of the screen. These intensifying screens are
usually made from tungsten or tantalum. Their thickness
requirement is a function of the x-ray energy, but in
practice 0.020 inch (0.50 mm) thick tantalum is used for
energies from 2 to 6 MV and 0.040 inch (1 mm) thick
tantalum for higher energies.
Fused scintillating fiber optic conversion screens are thicker
than conventional fluoroscopic screens and do not
generally need intensifying screens. Fiber optic screens
exhibit excellent conversion efficiency, which is a function
of the screen thickness, and low energy scatter rejection,
which is a function of the collimating effect of the
individual fibers. Fiber optic screens are especially effective
in high scatter, high energy applications.
Field of View - Real-time cameras generally provide more
than one field of view by using two or more optical lenses
to provide magnification. A magnified image results in
higher resolution but with a smaller field of view and lower
throughput. The application will determine which field of
view is appropriate.
Image Acquisition - There are generally three basic
methods of image acquisition used in real-time
radiography, real time, recursive averaging and integration
(summation).
Varian Linatron applications

Real-time image acquisition displays a new image at the
basic frame rate. This is normally an image every thirtieth
of a second. This mode of obtaining images is useful for
observing the proper positioning of a part, collimator setup
and screening applications where plenty of radiation
output is available. Quantum fluctuation can have a large
effect upon the detail that can be observed in an image.
Recursive Averaging is a frame summing procedure that
provides for a moving average image. The result of this
procedure is to accumulate in memory an exponentially
weighted sum of previously input frames. Because the
memory is constantly being refreshed with a new image, it
is possible to conduct motion imaging with an improved
signal to noise ratio.
The random noise will be reduced. However, as more
frames are averaged, the time lag of the image also
increases, making the use of this averaging technique less
useful for motion imaging. Therefore, a balance must be
found between noise improvement and image lag.
Considering random noise only, integration (summation)
increases the signal to noise ratio of the image by a factor
of the square root of the number of integrated frames-
(N) 1/2 . This method of image acquisition compares with
film exposure methods and is widely used to improve
image quality. Integration is used most often for static
imaging rather than dynamic processes. For any given time
of image acquisition, summation improves the image signal
to noise ratio of the image more than recursive averaging.
Image Processing - Real-time radiography provides several
different types of image processing to improve the
detectability of defects in the test object.
page 58
Contrast Enhancement or Stretching is the most common
form of image processing. Often the important
information in an image is contained in a small part of the
contrast range available. Contrast enhancement allows
using the total dynamic contrast range over a small
statistical range. Contrast enhancement does not modifiy
the original image data. It only modifies the way the data is
displayed. Subtraction techniques subtract one image from
another after moving either the x-ray source, the test object
(or part of the test object) or the camera between images.
Any data, such as systemic noise, that are common
between the two images is subtracted out of the image.
Several different subtraction methods can be used (shift
subtract, mask subtract, etc.) depending upon the
application. Care must be taken during subtract processes
not to introduce false information into the resultant image.
Additionally, the direction that the x-ray source, the object
or the camera is moved is critical.
Digital Filtering can remove high or low spatial frequency
data from an image or Edge Enhancement can sharpen the
representation of internal structure in a test object.
Conclusion
Real-time radiography is a nondestructive testing technique
that is useful for dynamic imaging and automated highthroughput
radiographic testing. Present real-time systems
include an x-ray source, test-object manipulator, (partshandling
system) and an imaging system for image
acquisition, processing and archival.
High energy applications of real-time radiography are
rocket motor, ordnance and heavy casting inspection.
Varian Linatron applications

Glossary
ABSORPTION
Process in which X-ray photons, as they pass through a
material, are absorbed in the material. (See
PHOTOELECTRIC ABSORPTION and PAIR
PRODUCTION).
ATTENUATION
Combination of absorption and scattering processes in
which X-rays, as they pass through material, are either
stopped, or diverted from straight and forward travel. Total
Attenuation, depending upon the X-ray energy, consists of:
PHOTOELECTRIC ABSORPTION, COMPTON
SCATTERING and PAIR PRODUCTION.
BACKSCATTER
Secondary radiation produced by scattering of X-rays from
their original (forward) direction through angles greater
than 90 degrees.
BEAM FLATTENER
Cone shaped X-ray absorber placed centrally in the beam
of a high-energy X-ray source, to absorb a relatively greater
proportion of the high-intensity central rays than of the
rays at the edges of the beam, to produce a more uniform
X-ray intensity at the plane of the X-ray film. Also referred
to as a BEAM COMPENSATOR.
BEAMING
The condition that higher radiation intensity occurs in the
forward, central X-rays from a high-energy X-ray source
(target) than is emitted in all other directions. Caused by
the large forward momentum of the accelerated electrons
which strike the target to produce the X-rays.
BLOCKING
Use of lead or other shielding material around the edges of
the object being radiographed, to absorb scattered
radiation that would otherwise expose and excessively
darken areas of the X-ray film under the object.
page 59
BROAD BEAM
Arrangement of the source, object and x-ray film (or other
radiation detector) in which scattered radiation from the
object contributes to the total exposure (at a given point)
on the film; specifically, scattered radiation at solid angles
greater than 0.01 steradian. The solid angle is formed by
the point on the film and the perimeter of the exposed
cross- sectioned area of the object. NARROW BEAM
arrangements are those where scatter reaching the x-ray
film or detector is only that scatter radiation within angles
less than 0.01 steradian.
CENTRAL RAY
Line within the x-ray beam, coincident with the direction
of peak x-ray intensity, the (usually) central axis of the
electron accelerator, and with the axis of the collimator. (It
also coincides with the laser beam indicator in a Linatron.)
COLLIMATOR
High-density metal absorber with a conical (sometimes
pyramid-shaped) opening for passage of x-rays to produce
a well-defined beam of x-rays from the target.
COMPENSATOR
(See BEAM FLATTENER)
COMPTON SCATIERING
X-ray attenuation process in which a photon transfers
energy and momentum to an orbital electron of the
attenuating material, and continues to travel through the
material at an angle to the original photon direction.
CONTRAST
RADIOGRAPHIC CONTRAST is the magnitude of the
difference in density from one area to another on a
radiograph resulting from variation in x-ray intensity
transmitted through the corresponding sections
(thicknesses) of the object being radiographed. SUBJECT
CONTRAST refers to the ratio of radiation intensities
transmitted through selected sections of the object, and is
therefore a function of the thicknesses of those sections.
(Also see: FILM CONTRAST)
Varian Linatron applications

D/T RATIO
Ratio of the distance between the source (Linatron target)
and the front surface of the object being radiographed to
the distance from the front surface of the object to the film
(usually the object thickness).
DEFECTS
Usually discontinuities and imperfections in the object
being examined, considered deleterious to the service of the
object, and which are revealed in Linatron radiography.
Depending on the object and service requirements, Voids,
Gaps, Separations, Cracks, Misassemblies, and other
homogeneities may be identified as DEFECTS in a
specification. (See Table 5-2, p. , also Table 5-3, p. )
DEFINITION
Fidelity in x-ray film images of sharp edges of the object,
and the degree with which the detail of small defects and
discontinuities are clearly shown. (See also:
UNSHARPNESS)
DENSITY (Film)
Degree of blackening of the exposed and developed x-ray
film; DENSITY is usually expressed in terms of the H&D
scale. (See H&D UNIT)
DENSITY (Film) GRADIENT
Amount of density change in the exposed and developed
film per unit of change in the logarithm of the exposures.
Film manufacturers also refer to density Gradient as the
GAMMA ( y ) of the film at the particular density
specified.
DIFFERENTIAL ABSORPTION
Differences in the x-ray absorption (and therefore the
transmission through) various adjacent sections of an
object being radiographed, which result in differences in
the exposure to the film behind those areas.
EDGE ENHANCEMENT
TV scanning and electronic processing of radiographic
(Real-Time or Film) images, that produce highlighting of
edges of the object, defects, etc., in the final image. The
resulting processed (edge-enhanced) images are displayed
on a TV monitor.
page 60
ELECTRON EQUILIBRIUM
Occurs in an x-ray measurement arrangement when, for
electrons (set in motion by the x-rays) leaving the
measuring cavity, electrons of equal total energy enter.
ELECTRON LINEAR ACCELERATOR
A system containing an electron source, power supply,
accelerator section, and metal target (when x-rays are to be
produced). The system generates high-energy x-rays by
accelerating the electrons to high energies and by directing
the electrons in a linear path to collide with the metal target
which results in producing x-rays.
ENERGY SPECTRUM (Photon)
Distribution of x-ray intensity (or quantity of x-ray
photons) as a function of the x-ray energy in an x-ray
beam.
EXPOSURE CURVE
Plot of the exposure Rads vs material thickness, that gives
the exposure conditions to obtain a radiograph with
specific film density.
FIELD COVERAGE
Size of the area that can be exposed in a single radiographic
exposure, for a particular source, collimator, and
distance from the x-ray target.
FIELD FLATNESS
Degree of constancy of radiation intensity level across the
field of an x-ray beam usually measured in a plane normal
to the beam central axis.
FIELD HALF-WIDTH
Distance measured at the film plane outward from the
central ray of the x-ray beam, to the outer edge of the x-ray
beam.
FILM CHARACTERISTIC CURVE
Plot of the relationship between exposure time and image
density for a specific film and exposure strength. This term,
in practice, is used only by the film manufacturer as one
specification of the film’s properties. Film users who make
measurements to determine this relationship with their xray
source, exposure procedure, and film processing, refer
to their results as: FILM RESPONSE CURVES.
Varian Linatron applications

FILM CONTRAST
Refers to the slope (steepness) of the FILM RESPONSE
CURVE, and the resulting differences in film density
achievable for a particular source, exposure technique, and
processing; same as FILM GRADIENT. (See also
CONTRAST)
FILM GRADIENT
(See FILM CONTRAST)
FILM GRAIN
The size, in general, of the original silver salt particles in
the emulsion of the x-ray film. “Fine Grained Film” has
very small silver salt particles. Finer film equates to smaller
particle size and greater resolution. (Also see:
GRAININESS)
FILM RESPONSE CURVE
(See FILM CHARACTERISTIC CURVE)
FILM SPEED
Sensitivity of the emulsion type to an x-ray exposure,
expressed as the inverse of the amount of exposure needed
to attain a specified FILM DENSITY.
FILM SPEED INDEX
As used in this Manual, it is a number assigned to each
film type which is proportional to the rad exposure needed
to attain a specified film density. (See Table 4.3, p.23)
FLUORESCENTSCREEN
A thin sheet of material, coated with a fluorescent salt
(Calcium Tungstate, Gadolinium Oxysulfide, etc.), placed
in contact with the front and back of the x-ray film in the
film holder, to intensify the effect of the radiation arriving
at the film holder.
FOCAL SPOT SIZE
Size of that area of the target in the electron accelerator
from which x-rays emanate for useful radiography.
FORWARD SCATTER
Secondary x-rays emitted from the object being radiographed
at small angles from the forward direction of the
primary x-ray beam.
page 61
GRAININESS
Refers to the “clumping” of the grains of silver that form in
the exposed and developed emulsion of an x-ray film
visible when viewing the film.
H&D UNIT
Unit for expressing FILM DENSITY and the scale for
measuring film blackening; named after Hurter and
Driffield who (in 1890) developed it. The film density at a
point on an x-ray-exposed and developed film is
determined from:
DENSITY = Log 10 /lt H&D UNITS
Where lo = Visible light intensity incident on the measured
point of the film.
L t = Visible light intensity transmitted through the
measured point of the film.
HALF-VALUE LAYER
That thickness of material required to attenuate the x-ray
intensity by one-half.
HARD PLATE X-RAY
X-ray radiography which produces the radiographic image
on an x-ray film.
IMAGE SHARPNESS
Fidelity attained in an x-ray image of a step or straight edge
of the imaged object. (See UNSHARPNESS)
INTENSIFYING SCREENS
Thin sheet of lead or other material placed in contact with
the x-ray film inside the film holder to increase the effect
of the radiation resulting in a shorter exposure time. (See
FLUORESCENT SCREEN)
IQI
An acronym for IMAGE QUALITY INDICATOR. A
device, consisting of small wires, narrow slits, or a drilled
hole strip of material with the same composition of the
object being radiographed, and whose thickness or
dimensions are a small percentage of the object’s thickness.
When radiographed with the object, the device provides a
measure of the contrast and resolution attained by the xray
procedure.
Varian Linatron applications

LINEAR ATTENUATION COEFFICIENT
Constant used to describe the degree of x-ray attenuation
per unit path length in a specified material. It is a function
of x-ray energy and is usually expressed in units of
reciprocal inch or centimeter.
MAGNIFICATION
Degree of projection and enlargement of regions of the
object being radiographed in the developed film that result
from the choice of distances between the source, object,
and film in the radiography setup.
NARROW BEAM
(See BROAD BEAM)
NDT
An acronym of NONDESTRUCTIVE TESTING, and
the name given to a test, inspection, or examination of
materials to determine specific properties or characteristics,
or to locate defects without causing damage to the object
under test.
PENETRAMETER
Same as IQI.
PAIR PRODUCTION
An absorption process for photons of energies greater than
1.02 MeV, in which the photon transforms into a pair of
particles (an electron and a positron).
PHOTOELECTRIC ABSORPTION
An absorption process for photons of energies under
1 MeV in which the photon loses all of its energy to an
atomic electron. The electron leaves its atomic orbit and
continues to move though the material.
RAD
Unit of radiation dose; 1 rad equals the absorption of 100
ergs of (any form of) radiation per gram of absorbing
material.
RADIOGRAPHIC COVERAGE
That section and area of the object that is radiographed,
exposed and imaged on the x-ray film. The
RADIOGRAPHIC COVERAGE is usually stated as part
of the specifications for the radiography work.
page 62
REAL-TIME IMAGING
X-ray radiography that produces instantaneous and
continuous images on a TV monitor. The images obtained
can be electronically processed to enhance their quality,
and can be recorded (e.g., on videotape) for later playback.
ROENTGEN
Unit of radiation exposure (the ability of an x-ray beam to
ionize air) corresponding to producing 1.610 x 1012 ion
pairs per gram air.
SCATTER mRADIATION
Secondary x-rays and electrons produced whenever a
primary x-ray beam irradiates an object. Also includes
objects not being examined such as the floor, walls, tables,
etc. Secondary radiations are emitted in all directions, and
are of lower energy than the primary rays.
SENSlTIVITY
Refers to the degree to which images of wires (Wire
Sensitivity), drilled holes in plaques (Plaque Sensitivity),
steps (Contrast or Thickness Sensitivity), and actual defects
in materials (Radiographic Sensitivity) are displayed and
capable of being discerned in an x-ray film or other
imaging device. Sensitivity is usually expressed as a
percentage of the thickness of the object being examined.
SOURCE-TO-FILM DISTANCE
The actual distance between the focal spot of the x-ray
source and the plane of the x-ray film in the specific
exposure. This distance affects the amount of exposure
used in every application of radiography.
22˚ PYRAMID
Refers to the size and shape of the collimator used in the
Linatron Model 200A, 400 and 1000. The collimator
provides a cone (four-sided pyramid) of radiation with the
focal spot as apex, the base a square, and with the sides
forming a 22˚ interior angle.
Varian Linatron applications

UNSHARPNESS
Degree of loss in image fidelity (in the x-ray film) of edges
and other features of the object being examined. Appears
as regions of fuzziness or of indefinite location in the image
of the edge. Major contributor to unsharpness are the
geometry of the setup (Geometric Unsharpness), and the
inherent effect of the screens and film used (Film-Screen
Unsharpness).
WHITE RADIATION
The presence and distribution of x-ray photons at all
energies up to the maximum emitted by an x-ray source.
X-RAY LEAKAGE
Refers to x-ray radiation that originates within an x-ray
machine, and which penetrates the materials of the x-ray
head in directions other than the opening provided by the
collimator.
X-RAY OUTPUT
The intensity of the useful x-ray beam, usually expressed as
rads per minute at l-meter from the x-ray source on the
beam CENTRAL ray.
page 63
Varian Linatron applications

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RAD 1936E © 2007 Varian Medical Systems, Inc. Printed in USA 7/07(1K)