Measurement of γ-ray dose in a thermal neutron field by using a 3He-filtered GM counter

A new type of ³He-filtered GM counter has been used to measure the dose of γ-rays existing in a thermal neutron field. The detector assembly consists of a commercially available GM counter surrounded by a cylindrical aluminum vessel filled with ³He gas at a pressure up to 6 atmospheres. The contribution of secondary γ-rays produced by thermal neutron capture in the GM counter itself can be excluded by increasing the ³He pressure, because the thermal neutron capture reaction of ³He(n, p)T results in the emission of charged particles of a proton and a triton. In order to determine the response of the GM counter, the energy spectrum deposited in the counter gas to monoenergetic γ-rays was calculated by EGS4 Monte-Carlo code for a γ-ray energy range from 60 keV to 10 MeV and then the response was derived through calibration of the counter at γ-ray energies of 60 keV, 662 keV and 1.17 MeV+1.33 MeV. The γ-ray energy distribution in the thermal neutron field was obtained by an MCNP Monte-Carlo calculation, assuming a three dimensional experimental assembly of a graphite pile, which also included the concrete floor in the experimental room. The γ-ray fluence and the effective dose equivalent in the thermal neutron field have been determined by using the GM counter response, the γ-ray energy distribution in the field and the conversion factor from fluence to effective dose equivalent