A novel method of quantitative assay of PuBe neutron sources by neutron coincidence technique

Using a simple model for PuBe neutron sources, in which the neutrons were assumed to be randomly emitted from the centre of a spherical source, and induce neutron-producing secondary reactions in the material, calculation shows that the ratio of the real coincidence count rate R to the total count rate T, measured by neutron coincidence technique, is proportional to the Pu content of the sample. Therefore, measuring the ratio R/T can be considered a method for determining the Pu content of PuBe neutron sources. The effect of the moderator of a neutron coincidence collar on the count rate of doubles was investigated experimentally using uranium samples of different enrichment (0.7-2.7% 235U). An important conclusion from the data of the experiments is that the influence of the neutrons backscattered from the moderator for the secondary reaction rate can be neglected during measuring PuBe neutron sources. This result leads to an empirical formula for determining the Pu content of PuBe neutron sources, which is independent of the type of the detector. The accuracy of the method is about 15%