Monte-Carlo Calculation of the Response Functions for Two Prototype Cosmic Neutron Metrology Instruments

Abstract

The response functions for two cosmic neutron detection systems have been calculated using Monte-Carlo computational methods. The detection systems that form the focus of this research are modified Leake detector designs in which a central thermal neutron detector is surrounded by a sphere of high-density polyethylene. In this arrangement, the surrounding polyethylene moderates the incident fast neutrons that are then detected by the central detector; in this case a 3He-filled proportional counter. In order to extend the response of these detector systems to cater for cosmic neutron environments, a shell of high-Z material has been included in each to promote (n, xn) reactions in the polyethylene moderator. We have used shells of lead and copper for this purpose to bring the high-energy component of the cosmic field, extending up to several Ge V, within the capability of the detector systems. In particular, copper has been used in comparison with lead since the former is easier and safer to machine and handle. The overall diameter of the instruments studied in this work is 208 mm. Calculations of the neutron response have been performed with MCNP4C, for the thermal-20 MeV energy range, and with MCNPX 2.1.5 / LA150N neutron libraries for the higher-energy cosmic region of the spectrum beyond 20 MeV. The results of these calculations are compared with experimental data that have been recorded with the instruments at the CERN Cosmic Reference Field Facility (CERF), Geneva, Switzerland. This comparison is discussed in respect of the likely applications of these detector systems to high-energy neutron field measurement on-board aircraft and in the vicinity of high-energy particle accelerators. The former application is gaining considerable research attention following the revised estimates of relative biological effectiveness of cosmic neutron fields and the related recommendation that aircrew be regarded occupationally-exposed radiation workers, on behalf of the International Commission on Radiological Protection (lCRP) and the European Union. The latter application is of interest to accelerator operators and users who can be exposed to neutron fields with 1 00 MeV components as a result of scattering reactions from the materials comprising the accelerator and associated shielding. This is of growing concern to the medical field where the use of cyclotrons is often a key aspect of radiation treatment and therapy, in which the control and estimation of administered doses is a key objective.