Re-evaluation of Neutron Source Emission Rates Measured at the National Physical Laboratory

The neutron source measurement service was established in 1960, based on absolute measurements in a flow-type manganese sulphate bath system, the hydrogen/manganese cross-section ratio having been determined in a dilution experiment. As a result of improvements in chemical analysis of trace elements in the bath solution and in computerized data fitting techniques over this 20-year period, it is possible to re-evaluate the emission rates of neutron sources which have been measured during this period. The re-evaluation demonstrates that the emission rates should be increased by 0.5% mainly due to neutron absorption in traces of boron, cadmium and lithium. The manganese sulphate solution has now been replaced by purer material, and the re-measurement of the cross-section ratio is in progress.