Measurement of /spl gamma/-ray dose in a thermal neutron field by using a /sup 3/He-filtered GM counter

A new type of /sup 3/He-filtered GM counter has been used to measure the dose of /spl gamma/-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 /sup 3/He gas at a pressure up to 6 atmospheres. The contribution of secondary /spl gamma/-rays produced by thermal neutron capture in the GM counter itself can be excluded by increasing the /sup 3/He pressure, because the thermal neutron capture reaction of /sup 3/He(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 /spl gamma/-rays was calculated by EGS4 Monte-Carlo code for a /spl gamma/-ray energy range from 60 keV to 10 MeV and then the response was derived through calibration of the counter at /spl gamma/-ray energies of 60 keV, 662 keV and 1.17 MeV+1.33 MeV. The /spl gamma/-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 /spl gamma/-ray fluence and the effective dose equivalent in the thermal neutron field have been determined by using the GM counter response, the /spl gamma/-ray energy distribution in the field and the conversion factor from fluence to effective dose equivalent.