CALCULATIONS OF ANISOTROPY FACTORS FOR RADIONUCLIDE NEUTRON SOURCES DUE TO SCATTERING FROM SOURCE ENCAPSULATION AND SUPPORT STRUCTURES

摘要

A model has been developed for calculating the angular neutron fluence distributions for radionuclide neutron sources that are heavily encapsulated or surrounded by source support structures as a source holder and a source movement system. These structures may cause an anisotropic neutron fluence distribution. This should be taken into account in the neutron-measuring instruments calibration procedure. The calculations were made for two types of widely used neutron sources, ~(241)Am-Be and ~(252)Cf, by combining an in-house code simulating the ~9Be(α,n) reactions and the Monte Carlo code MCNP-4C. As a result, anisotropy factors in the direction perpendicular to the source capsule axis for bare neutron sources were evaluated to be 1.012, 1.030 and 1.039 for ~(252)Cf in a standard Amersham XI capsule, ~(241)Am-Be in a X3 capsule and ~(241)Am-Be in a X4 capsule, respectively. These values are in reasonable agreement with the published data. If the support structures are included in the MCNP simulation, the anisotropy factors for these neutron sources increase by ～10%.