Spectral and dosimetric characteristics of a D2O-moderated 252Cf calibration facility

Calculations have been made for a D2O-moderated 252Cf assembly like that being used for the calibration of neutron dosimeters at the U.S. National Bureau of Standards and being proposed by the International Standards Organization. Leakage spectra at various distances from the assembly are given along with variations in dose-equivalent rate, average neutron energy and 235U/237Np fission ratio. The spectral shape changes rapidly near the spherical assembly and the dose-equivalent rate changes more rapidly than would be expected on the basis of the inverse-square dependence. Calibration of neutron dosimeters should therefore be made at distances greater than 15 cm from the surface. At large distances from the source, the dose equivalent per unit fluence for neutrons above 1 eV is 9.3 X 10(-9) rem cm2. The effects of the structural material, recent revisions to nuclear data files and changes in the spectrum of the source neutrons on the external field were investigated. These changes produce only about a 5% change in the neutron-dose equivalent rate. The structural material introduces negligible anisotropy in the radiation field.