large diameter of the capsules, which causes considerable self-absorption, only about 70% of the gamma radiation emitted by the 137Cs escapes from the source geometry and is available for irradiation. In contrast, the thinner 60Co source pins make 95% of their gamma radiation available. The lower efficiency of ,37Cs partly offsets the advantage of the lower annual replacement. A 60Co irradiator as shown in Figure 3 may be loaded with 3 MCi of 60Co (or 111 PBq) or even more. How much product can be irradiated with such a facility per hour or per year? As mentioned earlier, each radioactive decay of a 60Co atom produces one gamma photon of 1.17 MeV and another one of 1.33 MeV. Together that is 2.5 MeV. A 3-MCi or 111-PBq source produces 2.5X 111 X 10 MeV/ sec = 2.775 X 10 MeV/sec.