A calibration technique for rem counters in a heavy water moderated 252Cf neutron field

Rem counters are widely used to monitor neutrons in the energy range from thermal to 20 MeV, because their energy response is chosen to correspond to the maximum dose equivalent created in a human body. If rem counters are to be used in environments like those around the pressure vessels of nuclear reactors, they should be calibrated in a neutron field of a 30 cm diameter D2O moderated 252Cf source as recommended by the International Organization for Standardization (ISO). For this purpose, we constructed the source assembly and calculated the neutron spectra at ISO recommended calibration distances from the source by using three dimensional MORSE-CG Monte Carlo code. We have investigated the following aspects of the source and shadow cone which are needed for the calibration of rem counters: (1) Contribution of thermal neutrons emitted from the source to the total dose equivalent, (2) determination of the optimum structure and size of a shadow cone by using MCNP Monte Carlo code, and (3) preliminary studies on anisotropic emission for our standard D2O moderated 252Cf neutron source. (author)