Evaluation of influence of intermediate and fast neutrons mixed in a thermal neutron calibration field. Evaluation by using simulation

The Facility of Radiation Standard of the Japan Atomic Energy Research Institute (FRS-JAERI) is equipped with a thermal neutron calibration field in order to calibrate instruments for neutron dose measurement such as personnel dosemeter and surveymeter. The thermal neutron field is produced by using a neutron source which is placed in a graphite pile moderator. In case of calibrating a personnel dosemeter in this thermal neutron field, intermediate and fast neutrons (called as 'higher energy component') are mixed in the field, and they can accordingly affect the calibration result. Thus, by using Monte Carlo calculation techniques, neutron energy spectra in the calibration field were calculated, and the influence of the higher energy component to the calibration result was evaluated. The higher energy component in 'outside-pile thermal neutron irradiation field' was estimated as amount of 2% of the thermal neutron fluence, and it would be 10-14% of the dose equivalent due to thermal neutrons. The influence of the higher energy component depends on the energy response of personnel dosemeter. Especially in a case of neutron personnel dosemeters having an energy response even in the high energy region, the higher energy component should be taken into account to obtain a correct calibration result. In addition, in a case of on-phantom calibration of thermal neutron dosemeter, it can be concluded that the higher energy component have substantially no significant effect to the result. (author)