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 (8.8 mm diameter and 31.8 mm height) surrounded by a cylindrical aluminum vessel (220.5 mm diameter and 163 mm height) filled with ³ He gas in the pressure range up to 6 atmospheres. The contribution of secondary γ-rays produced by thermal neutron captures in the GM counter itself can be excluded by increasing the gas pressure of ³ He, 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 deposited energy distribution of the counter to monoenergetic γ-rays was calculated by the EGS4 Monte Carlo code in the γ-ray energy range from 60 keV to 10 MeV and then the response was derived through the calibration of the counter at γ-ray energies of 60 keV, 662 keV and 1.17 MeV +1.33 MeV. On the other hand, the γ-ray energy distribution in the thermal neutron field was calculated by the MCNP Monte Carlo calculation, assuming the three dimensional experimental assembly of a graphite pile including the concrete floor in the experimental room. As a result, 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