Californium 252 as an external source of neutrons for radiotherapy treatment

Californium 252 is a neutron emitter releasing 2·34×1012 n sec−1 g−1 by spontaneous fission. The half-life for neutron production is 2·65 years. Several authors (Oliver, 1968; Schlea and Stoddard 1965; Wright, Boulogne, Reinig and Evans, 1967) have discussed the application of 252Cf needles to intracavitary and interstitial radiotherapy in place of the more conventional radium needles. 1 μg of californium should give about the same dose-rate as 1 mg of radium in this treatment. There is now some interest in the possibility that 252Cf might also be suitable as an external source of radiation for radiotherapy in the form of a “tele-neutron unit”, analogous to the conventional cobalt machines. The purpose of this report is to examine if this is in fact a feasible proposition. It is assumed that 252Cf neutrons have been accepted as suitable from considerations of OER, RBE and depth dose. The question is, can an adequate treatment dose-rate be obtained at a reasonable cost while providing sufficient shielding round the source to conform with protection regulations? Since at present the Code of Practice (1964) does not contain any definite recommendations for protection from a tele-neutron unit, it has been assumed that the dose-rate at the surface of the shield must not exceed 2 mrem/hour.