Tissue equivalence of materials for possible use in area monitoring of neutron-enriched radiation fields around high-energy accelerators

Nuclear-model calculations are at present the only way to solve principal and practical issues of high-energy neutron dosimetry. The choice of a model is always a controversial problem. In practice, however, computing possibilities and software availability are basic determinants in this matter. The method selected for this work yielded ample variety of numerical results which do not seem to be entirely unreasonable. Monoenergetic neutrons of thirteen selected energies in the range 20-1100 MeV have been considered. It has been demonstrated that with increasing incident neutron energy the quality factor of the neutron-induced radiation field significantly decreases and the problem of tissue equivalence gradually disappears. This is due to the increasing contribution of high-energy cascade emission to the total energy release. Plexiglas seems to be a sufficiently tissue-equivalent material at high incident neutron energies. The choice of conductive material for the thin inner wall is not of primary importance due to the long pathlengths of charged secondaries. Using A-150 plastic for this purpose would ensure applicability of an instrument to low-energy neutron fields also. Therefore, a combination of A-150 with plexi seems to be a reasonable choice for microdosimetry-based area monitoring of neutron enriched radiation fields around high-energy accelerators. (author)