Physical Aspects of the Use of Caesium Fission Products in Teletherapy

In the fission of uranium in a nuclear reactor the most abundant elements formed may be arranged into two groups (Plutonium Project Report, 1946). The first group contains 90Sr and elements of comparable mass number, the second contains 137Cs and its neighbouring isotopes. When chemically separated from fission products caesium is found to consist of a mixture of isotopes, including stable 133Cs, and β-active 135Cs with a half-life of the order of 3 × 106 years, as well as the γ-active 137Cs. The relative yields of these isotopes (Brown, 1955) determine the limiting specific activity attainable even with absolute chemical purity. Some of the 133Cs formed by fission is transformed by neutron capture in the reactor to the γ-emitter 134Cs. The yield of 134Cs is determined by the radiation history of the uranium undergoing fission and could be different for each sample of gross fission products. In sources separated from the U.K.A.E.A. reactor at Windscale the 134Cs disintegration rate is less than 5 per cent of that of the 137Cs isotope. The dose-rate from a caesium source can be calculated from the decay schemes of its γ-emitting components (Fig. 1). It is easily shown that the dose-rate at 1 cm from a 1 mc point source of n different γ rays is given by: where xn is the number of γ rays of energy En emitted per 100 disintegrations, E1….En are the energies of the various γ rays in MeV, and (μα)n is the real absorption coefficient of air, at energy En, expressed in cm2/g.