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For almost all food irradiation applications, the required kinetic energy lies in the range of 2–10 MeV, while the required average power generally lies in the range of a few kilowatts to a few hundred kilowatts. In addition to these basic requirements, other important considerations in the choice of an accelerator system include size, cost, operating efficiency, ease of maintenance, and reliability. The several electron acceleration approaches that might be considered for food irradiation can be generally grouped into three categories: (1) direct methods, in which the accelerating field results from the direct application of a high potential difference across an insulating column; (2) induction methods, in which the accelerating field results from a time-changing magnetic field; and (3) microwave (or radio-frequency) acceleration methods, in which acceleration results from oscillating electromagnetic fields established in a resonant microwave cavity structure. The direct acceleration methods are generally deemed less favorable for food processing applications because of their large size and the significant downtime required for repair or replacement of the high-voltage insulating column. Of the induction methods, the average power of the betatron is too low, while the induction linac approach is quite expensive. However, several microwave accelerator approaches appear to have the flexibility to process almost every type of food product in an efficient, effective manner in facilities of reasonable size and cost.

In this chapter we have discussed the design and function of the key microwave accelerator components and subsystems that must be successfully integrated for proper accelerator system function. Important topics have included the various microwave tubes, the high-voltage systems used to drive them, and the various microwave engineering components. Important auxiliaries include the vacuum, cooling and pressurized gas subsystems. Finally, an automatic frequency control system is essential to ensure that the microwave source is precisely tuned to the resonant frequency of the accelerator structure.

In this chapter we have considered various problems of radiation safety, with emphasis on the design of the radiation shield. Since electron accelerators for food irradiation are limited to maximum kinetic energies of 10 MeV for direct electron irradiation, and 5 or 7.5 MeV for indirect x-ray irradiation, there is essentially no neutron production and no possibility of induced radioactivity. Consequently, the primary area of concern is providing protection against penetrating x-radiation. The general problem of calculating the radiation dose at a particular location for a given shield design is quite complicated. Fortunately, it has proven to be possible to design adequate radiation shields using a simplified approach based on empirical source strengths and attenuation factors. The results of such analyses lead to three generally conservative guidelines for shield design in food irradiation facilities; these can be summarized as follows: (1) eleven feet of concrete in the forward direction; (2) eight feet of concrete in the side directions; and (3) any unshielded path for reflected x-rays should include at least three “bounces.” These guidelines should be used as the starting points. Once a preliminary facility design has been developed, more detailed calculations can then be performed using the techniques of this chapter, with shielding corrections to be made as necessary. While this chapter has emphasized shield design, it should be noted that several additional precautions and procedures are required to prevent inadvertent or accidental radiation exposure. These include the use of safety interlocks, warning devices, appropriate radiation signs, training programs covering all important aspects of radiation safety, and monitoring of facility personnel.