Rossi-alpha measurements of fast plutonium metal assemblies using organic scintillators

Journal Article · 23 January 2020 · Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment

DOI: https://doi.org/10.1016/j.nima.2020.163507 · OSTI ID:1630879

Hua, Michael Yeungjun  ^[1]; Bravo, Caiser A. ^[2]; MacDonald, Aaron T. ^[2]; Hutchinson, Jesson D.  ^[3]; McKenzie, George Espy IV  ^[3]; Kiedrowski, Brian C. ^[2]; Clarke, Shaun D. ^[2]; Pozzi, Sara A. ^[2]

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1. Univ. of Michigan, Ann Arbor, MI (United States); Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
2. Univ. of Michigan, Ann Arbor, MI (United States)
3. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)

The reactivity and the k-effective multiplication factor (k_eff) of a fissionable assembly are quantities of widespread interest. These values can be inferred from Rossi-alpha measurements of the prompt neutron decay constant (or the inverse:α^-1). It has been shown that ³He-gas proportional counter-based detection systems are insensitive to α^-1 of fast assemblies (much faster than tens of microseconds). Therefore, it is of interest to investigate fast detection systems such as those based on organic scintillation detectors. In this work, an array of 12 cylindrical, 5.08 cm x 5.08 cm diameter trans-stilbene organic scintillators was used to measure five subcritical assemblies. One assembly was a sphere of approximately 4.5 kg of alpha-phase, weapons-grade plutonium (k_eff = 0.773, α^-1 = 11.84 ns) known as the BeRP ball encased in a thin stainless-steel clad. The other assemblies used the same encased sphere and 7.62 cm of iron, nickel, copper, or tungsten reflectors (k_eff = 0.884, 0.916, 0.924, 0.939, respectively and α^-1 = 36.60, 41.56, 49.60, 70.32 ns, respectively). Finally, this work (1) validates Rossi-alpha measurements with organic scintillators by demonstrating good agreement between measurements and simulations; and (2), demonstrates that organic scintillator-based systems are sensitive to α^-1 on the order of 10–100 ns.

• Research Organization: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States); Univ. of Michigan, Ann Arbor, MI (United States)
• Sponsoring Organization: National Science Foundation (NSF); USDOE National Nuclear Security Administration (NNSA); USDOE National Nuclear Security Administration (NNSA), Nuclear Criticality Safety Program (NCSP); USDOE National Nuclear Security Administration (NNSA), Office of Nonproliferation and Verification Research and Development (NA-22)
• Grant/Contract Number: 89233218CNA000001; NA0002534; NA0003920
• OSTI ID: 1630879
• Report Number(s): LA-UR--19-32637
• Journal Information: Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment, Journal Name: Nuclear Instruments and Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors and Associated Equipment Journal Issue: C Vol. 959; ISSN 0168-9002
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English