Comparison of NMC estimates with trans-stilbene, EJ-309, and He-3 detection systems
- Univ. of Michigan, Ann Arbor, MI (United States); Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
- Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
- Univ. of Michigan, Ann Arbor, MI (United States)
Neutron multiplicity counting (NMC) is a technique for the assay of fissile material. In this work, three detection systems are utilized for active interrogation assay of shells of the Rocky Flats shells (highly enriched uranium, 93% 235U) stacked from 13.25-54.92 kg assemblies. The singles ($$R_1$$) and doubles ($$R_2$$) rates are calculated with each system to estimate two sample parameters: $$M_L$$- the leakage multiplication and F - the sample fission rate. The estimated mass, m, is found by dividing F by a constant activity per unit mass. Since we are interrogating HEU, the α ratio of (α; n) to fission neutrons is taken to be zero. The system of equations to calculate these quantities was originally derived. The Neutron Multiplicity 3He Array Detector (NOMAD) consists of 15 3He tubes inside polyethylene and is the traditional, capture-based detection system for NMC. The polyethylene moderates incoming neutrons for thermal capture in individual tubes. The low gamma background, discrete capture signals, and high efficiency of the NOMAD are beneficial for NMC. However, the time to slow down neutrons to thermal energies leads to a system die away time on the scale of microseconds. Comparatively, the Rossi-alpha Measurements – Rapid Organic (n, γ) Discrimination Detector (RAMRODD) and the Organic Scintillator Array (OSCAR) are scatter-based detection systems. RAMRODD consists of 8, 5.08 by 5.08 cm EJ-309 liquid scintillators in 4 pairs, evenly-spaced with 90 degrees separation about the center of each assembly. OSCAR consists of a single array of 12, 5.08 by 5.08 cm trans-stilbene crystals aligned with the center of the assembly. Scatter-based systems detect fast neutrons without any moderation, leading to system die away times on the scale of tens of nanoseconds. This allows much shorter timing gate widths compared to thermal systems, thus increasing counting statistics of true correlated fission events. However, scatter-based systems are susceptible to neutron cross-talk when an incident neutron scatters off one detector and interacts in an adjacent detector, causing two seemingly correlated detection signals. The equations developed adjust for cross-talk to conduct NMC with scatter-based systems.
- Research Organization:
- Los Alamos National Lab. (LANL), Los Alamos, NM (United States); Univ. of Michigan, Ann Arbor, MI (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA), Nuclear Criticality Safety Program (NCSP); USDOE Laboratory Directed Research and Development (LDRD) Program
- DOE Contract Number:
- 89233218CNA000001; NA0003920
- OSTI ID:
- 1899311
- Journal Information:
- Transactions of the American Nuclear Society, Vol. 125, Issue 1; Conference: 2021 ANS Winter Meeting and Technology Expo, Critical and Subcritical Experiments - I, Washington, DC (United States), 30 Nov - Dec 3 2021; Related Information: https://www.ans.org/meetings/wm2021/session/view-837/; ISSN 0003-018X
- Publisher:
- American Nuclear Society
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
97 MATHEMATICS AND COMPUTING
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY
Nuclear Criticality Safety Program (NCSP)
Integral Experiments
Neutron Multiplicity Counting (NMC)
Neutron Multiplicity 3He Array Detector (NOMAD)
Rossi-alpha Measurements – Rapid Organic (n
γ) Discrimination Detector (RAMRODD)
Organic Scintillator Array (OSCAR)