Year
2006
Abstract
Five 252Cf sources have been cross calibrated to a sixth which had recently been absolutely calibrated at NIST. The inter-comparisons were performed using a passive neutron correlation counter fitted with a shift register so that Singles, Doubles and Triples counting rates could be extracted. This approach provides a fast and convenient means of calibrating test sources with a common basis directly traceable to national standards. The sources are all of similar strength so that relative dead-time corrections between them are small, and can be checked by using the sources in combination. Using multiplicity counting principles allows additional and powerful self-consistency checks to be brought to bear. The correlated neutron rates, particularly the Doubles rate, confer a suitably high precision with a low ambient background rate. The D/S and T/S ratios should be independent of source if the deadtime and background corrections have been applied correctly and the isotopic composition and emission spectra of the sources are equivalent. Similarly the ratio of counting rates between the different rings of 3He proportional counters should be consistent. Once established these performance indicators may also be used to confirm correct operation at a future time. The accuracy of the inter-comparisons was at the 1% level, and the source supplier’s certifications were found to be 7-9% high.