Model design of a compact delayed gamma-ray moderator system using 252Cf for safeguards verification measurements. (June 2019)

• Record Type: Journal Article
• Authors: Rodriguez, Douglas C.; Rossi, Fabiana; Takahashi, Tohn; Seya, Michio; Koizumi, Mitsuo
• Is Part Of: Applied radiation and isotopes. Volume 148(2019:Jun.)
• Page Start: 114
• Page End: 125
• Publication Date: 2019-06
• Subjects: Neutron moderation -- Delayed gamma rays -- 252Cf -- Cf-shuffler -- Neutron interrogation -- Gamma ray spectroscopy -- Nuclear safeguards; Radiology -- Periodicals; Radiation -- Industrial applications -- Periodicals; Nuclear chemistry -- Periodicals; Internet resource; Periodical; 660.298
• DOI: 10.1016/j.apradiso.2019.01.002
• Languages: English
• ISSNs: 0969-8043

Abstract

Abstract: Delayed gamma-ray spectroscopy (DGS) is an active-interrogation nondestructive assay technique that can be used to determine the composition of a nuclear material sample by comparing the ratios of fission product gamma-ray peak intensities. However, high-radioactivity nuclear material (HRNM) contains long-lived fission products that can overwhelm a detector and shielding must be used to reduce the count rate, while minimally affecting those gamma rays from short-lived fission products with energy above 3-MeV. To compensate for the signal loss through the shielding, low-energy neutrons are required to induce more fission events from the high thermal cross-section of fissile nuclides. To improve practical safeguards DGS capabilities, we are developing a compact interrogation system to moderate ∼ 2-MeV neutrons that are easier to moderate than 14-MeV neutrons from standard deuterium-tritium generators. This work describes the optimization of an ideal moderator system for a 252 Cf neutron point source that results in a neutron fluence of 25.9 × 10 − 4 cm −2 n source − 1 passing through the sample space with ≳ 70% of those below 1-eV. Modifications for practical fabrication resulted in ⩽ 20 % reductions of the flux compared to the optimized ideal design. Finally, evaluations made of HRNM DGS signals and backgrounds conclude that a 252 Cf source intensity of 8.9 × 10 7 neutrons per second is required for a single-pass interrogation within this ideal moderator system. However, this can be as low as 3.6 × 10 6 neutrons per second using smaller samples that require less shielding. Abstract : Highlights: Moderator optimized to increase the flux by a factor of 20–30 in the sample space. Direct 70 to > 100 % of the source neutrons into the sample space. Increase the thermal-to-fast neutron ratio by a factor of > 1000 . 20% reductions resulting from practical modifications for fabrication potential. Need ⩽ 6 % of the source neutrons compared to a DT generator under similar conditions.