The importance of secondary particle equilibrium for neutron irradiations of cultured cells and intact organisms

At interfaces of tissues of different composition in intact organisms local perturbations of charged particle equilibrium occur, which may be different for irradiations with fast neutrons and electromagnetic radiation. The variations in absorbed dose at the interface of bone and soft tissue have been investigated with a cell culture system. For X-irradlations and 15 MeV neutron irradiations survival curves have been determined for cells irradiated on muscle equivalent plastic and on bone equivalent plastic. These results have been related to the occurrence of the bone-marrow syndrome and the intestinal syndrome in mice after neutron- and X-irradiation. Due to the various types of interactions of fast neutrons with the components of tissue, the distribution of the dose in LET is complex. It is therefore difficult to assess an average LET for the different types of fast neutrons. An exact knowledge of the relevant distributions of dose in LET is required. For a first approximation a distinction has been made between a "high-LET component" and a "proton component". In order to separate these components experimentally, irradiations where carried out in which perturbations of charged particle equilibrium were induced artificially. Survival curves hove been determined for neutron irradiations with either layers of tissue equivalent plastic or layers of pure carbon mounted in front of the cells. These results will be discussed in relation to the OER and RBE values measured for fast neutrons of different energies