Non-destructive actinide assay of neutron sources

Transuranic neutron sources were assayed using γ-spectrometry and neutron counting, with respect to revealing illicit trafficking of radioactive and nuclear materials. Source type and constituents were identified by γ-spectrometry. For determining the source strength of 244Cm, 252Cf, AmBe, AmLi and 238PuBe sources, a passive neutron coincidence collar was used with 3He counters in two moderator configurations. The electronics consist of independent channels of pulse amplifiers and discriminators as well as a shift register for coincidence counting. From the neutron output of the sources measured by gross neutron counting, the actinide content (or activity) can be assessed by adopting specific spontaneous fission and (α, n) reaction yields of individual isotopes from the literature. Various types of the sources can be characterized by the ratio of the real coincidence counts to total (singles) counts, R/T. This quantity is a specific constant for different types of spontaneously fissioning sources, not depending on their strength, while exhibits a power function increasing with the strength of (α, n) sources. An individual source of the latter type can additionally be characterized by the ratio of R/T to the product of detection efficiency and a factor taking into account coincidence loss, irrespective of the instrumental details (detector type and size, moderator thickness, etc.).