Neutrons for Capture Therapy Produced by 72 MeV Protons

Abstract

Neutrons (and protons) move in nuclear matter with an energy which is roughly the same in all nuclei, and which can be thought of as a temperature (of about 10¹¹K). Neutrons from the fission process therefore emerge with an energy corresponding to this temperature; so do most but not all neutrons from the ‘spallation’ process, in which fast protons strike nuclei and liberate neutrons. The use of a proton beam or a reactor for the production of neutrons for capture therapy (NCT) thus present basically similar problems: to reduce the energy of the neutrons from the MeV level at which they emerge to the few keV required for NCT. There are however significant differences: the total intensity of a spallation source is much lower – some four orders of magnitude between the layout planned at the Paul Scherrer Institute (PSI) and the Petten reactor, for example. On the other hand, the source is much smaller: of the order of a cubic centimetre as against the cubic metre of a typical reactor. This means that it can be made mobile, as in the PSI layout, and in practice one can work closer to it. Again, less γ radiation is produced by a spallation source.