Safety of Irradiated Foods
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crease the proportion of machine sources in the near future. If radiation process ing continues to grow, the shortage of Co, which has caused some delays in deliveries in the past, will become more acute. This also points to an increasingly important role for electron accelerators. Generalizing conclusions about the relative economics of different types of irradiation may be misleading because the relative costs of different radiation facilities are considerably affected by local conditions such as costs of electricity, labor, transportation, and construction. The economics of operation also depends on the use level of a facility. Where operations can be continued day and night for months a year a radionuclide source may be more economic, however, where intermittent operations are more likely a machine source may be more advanta geous. Sociopolitical considerations relate to the observation that in some countries it is getting more and more difficult to overcome local opposition to the installation of new radioisotope sources. Fears for the safety of the environment in shipping and storing large inventories of 60Co or 137Cs are often cited as the main reason for this opposition. Regardless of whether these fears are justified, planners cannot disregard them. As an example, the National Food Processors Association (NFPA), with support from the U.S. Department of Energy, negotiated in the summer of 1985 for a site in Dublin, California, to build a demonstration and training facility for food irradiation, using 3 million Ci of ,Cs. The opposition
Some of these could also be operated in the energy range above lOMeV for experiments designed to determine at which energy level radioactivity can be induced in the irradiated medium. A linac with a maximum energy of 25 MeV was commissioned for the U.S. Army Natick Research and Development Labora tories in 1963. Its beam power was 6.5 kW at an electron energy of 10 MeV, 18 kW at 24 MeV. Assuming 100% efficiency, a 1-kW beam can irradiate 360 kg of product with a dose of 10 kGy/h. The efficiency of electron accelerators is higher than that of gamma sources because the electron beam can be directed at the product, whereas the gamma sources emit radiation in all directions. An efficiency of 50% is a realistic assumption for accelerator facilities. With that and 6.5 kW beam power an accelerator of the type built for the Natick laboratories can process about 1.2t/h at 10 kGy. In Odessa in the former Soviet Union, now in the Ukraine, two 20-kW accelerators with an energy of 1.4 MeV installed next to a grain elevator went into operation in 1983. Each accelerator has the capacity to irradiate 200 t of wheat per hour with a dose of 200 Gy for insect disinfestation. This corresponds to a beam utilization of 56% (9). In France, a facility for electron irradiation of frozen deboned chicken meat commenced operation at Berric near Vannes (Brittany) in late 1986. The purpose of irradiation is to improve the hygienic quality of the meat by destroying salmonella and other disease-causing (pathogenic) microorganisms. The electron beam accelerator is a 7 MeV/10 kW Cassitron built by CGR-MeV (10). An irradiation facility of this type is shown in Figure . Because of their relatively low depth of penetration electron beams cannot be used for the irradiation of animal carcasses, large packages, or other thick materials. However, this difficulty can be overcome by converting the electrons to x-rays. As indicated in Figure 9, this can be done by fitting a water-cooled metal plate to the scanner. Whereas in conventional x-ray tubes the conversion of electron energy to x-ray energy occurs only with an efficiency of about %, much higher efficiencies can be achieved in electron accelerators. The conversion efficiency depends on the material of the converter plate (target) and on the electron energy. Copper converts 5-MeV electrons with about 7% efficiency, 10-MeV electrons with 12% efficiency. A tungsten target can convert 5-MeV electrons with about 20%, 10-MeV electrons with 30% efficiency. (Exact values depend on target thickness.) In contrast to the distinct gamma radiation energy emitted from radionuclides and to the monoenergetic electrons produced by accelerators, the energy spectrum of x-rays is continuous from the value equivalent to the energy of the bombarding electrons to zero. The intensity of this spectrum peaks at about one-tenth of the maximum energy value. The exact location of the intensity peak depends on the thickness of the converter plate and on some other factors. As indicated in Figure
