Varian Linatron High-Energy X-ray Applications 2007

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.**