2006 Volume 55 Issue 2 Pages 61-70
The authors studied angular neutron fluence distributions in the neutron calibration field produced with a 252Cf standard source. The neutron calibration field of Japan Nuclear Cycle Development Institute, Tokai Works, has utilized 50μg of 252Cf doubly-encapsulated in a welded stainless steel (AEA Technology QSA, Xl capsule) . The neutron emission from source material itself is essentially isotropic, however, the source support structures, such as a source holder and an irradiation apparatus, significantly deviate neutron angular distributions from isotropy. To evaluate the departure from isotropy, the neutron angular fluence distributions for Xl capsule was calculated using the MCNP Monte Carlo code. Calculations were made of the fluence rate at various polar angle θ and azimuth angle α. Supplemental fluence distribution measurements were performed to verify computational results. As a result, an isotropy correction factor, Fs (θ=90°, α=0°), was evaluated to be 1.11±2% (k=2) . In addition, neutron energy spectral changes due to scattering was studied, and differences of spectrum-averaged dose conversion coefficients between values of this computation work and ISO recommendations were also discussed.