The development of high-energy electron accelerators with very high-power electron beams has made X-ray processing a practical alternative to gamma-ray processing for applications, such as the sterilization of packaged medical devices and the preservation of foods, which require greater penetration than can be provided by energetic electron beams. The feasibility of radiation processing with high-energy X-rays has been demonstrated in various industrial facilities in several countries.
Recent comparisons have shown that the capital costs and electric power costs for accelerators with electron energies of 5.0 MeV to 7.0 MeV can be lower than the capital costs and source replenishment costs for cobalt-60 source loadings greater than 2.0 MCi. The capability to turn the radiation source on and off and to control the X-ray intensity are attractive features of an accelerator facility.
When an accelerated electron impinges upon any material it generates X-radiation or X-rays. Characteristic monoenergetic X-ray photons are produced by the electron interaction with orbital electrons; bremsstrahlung photons are produced by the interaction with the nucleus of an atom. High energy bremsstrahlung X-rays are a penetrating form of ionizing radiation. Such X-ray intensities from high power, high energy industrial X-ray generators exceed by far those of common medical X-ray equipment.
X-rays are produced by interposing a metal target between the electron beam and the product to be treated. To enhance electron-to-photon conversion, these X-ray targets are made of high atomic number (high Z) metals. Water cooled tantalum is preferred for large area targets.
X-radiation has a forward peaked emission and the rate at which a material receives X-radiation photons, the dose-rate, can be controlled by a combination of the distance from the target, the beam current and under-beam transport speed.
The forward peaked emission of X-rays is significantly different from the panoramic emission of gamma-ray sources. This property facilitates the treatment of single pallet loads of product. X-ray penetration is much greater than E-Beam systems and is even better than gamma ray penetration. X-ray dose-rates are at least one order of magnitude higher than gamma rays, but significantly less than EB.
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