Calculations of anisotropy factors for radionuclide neutron sources due to scattering from source encapsulation and support structures

Tsujimura, Norio   ; Yoshida, Tadayoshi ; Momose, Takumaro  

The authors developed a model calculating angular neutron fluence distributions for radionuclide neutron sources which are heavily encapsulated or surrounded with some source support structures such as a source holder and a source movement system. These structures cause a highly anisotropic neutron fluence distribution, and hence it should be properly corrected for calibration of neutron measuring instruments. The calculations were made for two types of widely used neutron sources, Am-Be and Cf, by a combination of the in-house code simulating Be(,n) reactions and the Monte Carlo code MCNP4C. 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 1.035 for Cf in a standard Amersham X1 capsule, Am-Be in a X3 capsule and Am-Be in a X4 capsule, respectively. These values are in reasonable agreements with published data. In contrast, anisotropy factors for these neutron sources including support structures increased by approximately 10% due to scattering of neutrons.