Linear Accelerator
As explained in X-ray theory, 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 Linear Accelerators when high energy electrons are
accelerated and strike a high density metal target.
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 high energy X-ray sources creating
focused bremsstrahlung radiation such as 1MeV, through 9MeV,
11 MeV and 15 MeV, have been developed for the detection of flaws
in heavy metal sections. More recently, these x-ray sources that have
found important uses in High Energy CT inspection 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.
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.
In 1968, Varian Medical Systems introduced a line of industrial linear accelerators with energy ranges from 1 to 15 MeV. This represented a significant advance in reliability and ease of operation and handling.
Advantages of High-Energy X-Rays include:
• Making computed tomography of very dense object sections economically feasible.
• Making it possible to achieve large focal spot to object and object to detector distance ratios to minimize object distortion.
• Allowing short exposure times for faster projection images.
• Combining high resolution detectors and high detail resolution in CT Scans of large complex assemblies.
• Making it possible to achieve high bit counts for much better dynamic range through thicker and more dense materials and objects.
Generation of High Energy X-Rays are created when 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 linear accelerator electron beam produces a higher efficiency conversion of electrons into 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 the Gray, often abbreviated "Gy''**. Gy is defined as the amount of energy imparted co matter per unit mass of irradiated material and is equal to 1 joule per kilogram.
In practice, the radiation output of a linear accelerator is measured by first measuring exposure, the charge produced by the X-ray 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. Linear accelerator outputs are described in units of Gy per minute at one meter.
'The term "exposure" is used primarily to describe the fact that digital detector has received X-ray radiation during radiography of an object. Exposure refers to the effect of the X-ray beam on the detector in this context. This text uses Gy values for absorbed energy dose values. 1 Gy (Gray) = 100 rad.
Target Characteristics
The target is a component in the linear accelerator, 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 many linear accelerator targets. It has a thickness slightly greater than the range of the electrons in the target material. Linear accelerator 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 linear accelerators. 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 linear accelerator target form a beam which passes through the object under inspection. The X-ray 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-ener1:,')' 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.
"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. Varian Medical Systems, Linatron Applications ( SIP 2010 )