Anisotropy of emmision from radionuclide neutron sources.

Radionuclide neutron sources provide a convenient means of producing standard neutron calibration fields for a wide variety of neutron measuring devices. The properties of a source which are required to be known for the characterisation of the field at a point are: the total neutron emission rate, the neutron energy spectrum, and the variation of the intensity of the emission with angle. It is assumed that the variation of the spectrum with angle is negligible for most applications. The total emission rate from radionuclide neutron sources can be measured absolutely at the National Physical Laboratory, NPL, by the manganese sulphate bath technique, or comparatively via a moderating detector. The neutron energy spectra for a variety of commonly used sources are available in the open literature. This report describes the method used at NPL for the measurement of the anisotropic emission from radionuclide neutron sources. The measured neutron angular distributions relative to the cylindrical axes of a variety of source types and encapsulations are given. Also presented are the distributions as calculated using the Monte Carlo transport code MCNP, which in general show good agreement with the measured distributions.