sorbed dose must be measured by placing dose meters into the carriers, as described in Section V. If all of the gamma radiation coming from the 60Co source were absorbed by the irradiated goods, the radiation facility would have % irradiation efficiency. This is of course impossible because some of the radiation will be absorbed by the concrete shielding, by the carrier metal, and by the water below the source rack. Well-designed gamma facilities have an efficiency of about 30%. Designs differ­ ent from that shown in Figure 3 may use tote boxes on a conveyor belt instead of carriers hanging from a monorail, or the carriers may move around the source in a double loop instead of a single loop. Storage of the radioactive source in air in a concrete-shielded or lead-shielded cask instead of the water pool is also possible but is not often practiced. The outside appearance of a gamma irradiation facility is not much different from that of any other small or medium size industrial plant. A view of the first irradiation plant in the United States designed exclusively for the processing of foods (1) is shown in Figure 4. In principle, a 137Cs irradiator operates exactly like a 60Co irradiator. Some­ what less concrete shielding (1.2 m) is needed because the 0.66 MeV gamma rays of ,37Cs are less penetrating than the 1.33 MeV of Co. The longer half-life of