Varian Linatron High-Energy X-ray Applications 2007

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
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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