Monte Carlo calculation of the response of the portable neutron monitor SNOOPY

The response function of the portable neutron monitor SNOOPY has been calculated for neutrons in the energy range from thermal to 4 MeV using a three-dimensional Monte Carlo program. The results for a broad beam incident perpendicular to the SNOOPY symmetry axis are in agreement with the published response from 1 eV to 1 MeV. Below 1 eV the calculated response falls below the published curve by an amount which increases with decreasing energy until at 0.025 eV the calculation is low by a factor of 5.5. This discrepancy can be explained as being due to the directionality of the sources used in the calculation (broad beam incident perpendicular to the symmetry axis) and experiment (isotropic source). At 4 MeV the calculation yields an over-response of about 25 per cent which may be due to an inadequate description of the carbon cross-section. For a highly collimated 1 MeV source directed perpendicular to the symmetry axis and incident at the center of the detector, the calculation yields a factor of 2 over-response relative to the broad beam results. As the collimated source is moved away from the center of the detector towards its edges, the response falls off and eventually falls below the broad beam results. Since the calibration of SNOOPY at neutron energies above 30 keV is based on a broad beam measurement, care must be taken when applying the detector in situations where a highly collimated source may exist.