An event-triggered coincidence algorithm for fast-neutron multiplicity assay corrected for cross-talk and photon breakthrough

• Published in: Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment Accelerators, spectrometers, detectors and associated equipment, 2018-09, Vol.903 (C), p.152-161
• Main Authors: Sarwar, R., Astromskas, V., Zimmerman, C.H., Nutter, G., Simone, A.T., Croft, S., Joyce, M.J.
• Format: Article
• Language: English
• Subjects: Breakthrough; Cross-talk; ENGINEERING; Gate fraction; INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY; Multiplicity; Neutron; Scintillator
• ISSN: 0168-9002; 1872-9576
• DOI: 10.1016/j.nima.2018.06.056

A model quantifying detector cross-talk and the misidentification of events in fast neutron coincidence distributions is described. This is demonstrated for two experimental arrangements comprising rings of 8 and 15 organic liquid scintillation detectors. Correction terms developed as part of this model are tested with  252Cf and a relationship is developed between the  235U enrichment of U3O8 and the order of correlated, fast neutron multiplets induced by an americium–lithium source. The model is also supported by Geant4 simulations. The results suggest that a typical assay, for experimental arrangements that are similar to the examples investigated in this research, will exhibit cross-talk for less than 1% of all detected fast neutrons but, if not accounted for, this can bias the numerical analysis by a margin of 10% and 35% in second- and third-order coincidences (i.e. couplet and triplet counts), respectively. Further, for the case of  252Cf, it is shown that a relatively low proportion of 4% breakthrough by γ rays (that is, photons misidentified as neutrons by the pulse-shape discrimination process) can lead to an erroneous increase of 20% in total neutron counts in the assay of a mixed-field, in this case of  252Cf. These findings will help direct the developments needed to enable organic scintillation detectors with pulse shape discriminators to be applied reliably to nuclear safeguards and non-proliferation verification tasks. •Two models are presented to correct for cross-talk and photon breakthrough.•The models yield consistent corrections to the gate fraction for californium-252.•The associated biases are estimated for couplet and triplet event counts.•Data are presented for 235U enrichment in U3O8 and induced fast neutron multiplets.