EXAMINATION OF DIGITAL-DELAY ROSSI-α FOR 252CF-DRIVEN HIGHLY ENRICHED URANIUM USING ORGANIC SCINTILLATORS

Year

2023

Author(s)

F.B. Darby - Department of Nuclear Engineering and Radiological Sciences, University of Michigan & NEN-2: Advanced Nuclear Technology, Los Alamos National Laboratory

M. Y. Hua - Department of Nuclear Engineering & Radiological Sciences, University of Michigan

R.A. Weldon - NEN-2: Advanced Nuclear Technology, Los Alamos National Laboratory

J.D. Hutchinson - NEN-2: Advanced Nuclear Technology, Los Alamos National Laboratory

G.E. McKenzie - NEN-2: Advanced Nuclear Technology, Los Alamos National Laboratory

J.R. Lamproe - Department of Nuclear Engineering and Radiological Sciences, University of Michigan & NEN-2: Advanced Nuclear Technology, Los Alamos National Laboratory

S.D. Clarke - Department of Nuclear Engineering & Radiological Sciences, University of Michigan

S. A. Pozzi - Department of Nuclear Engineering & Radiological Sciences, University of Michigan

File Attachment

finalpaper_437_0508071512.pdf633.56 KB

Abstract

Neutron noise techniques constitute several analysis methods applicable to non-destructive assay. One technique is the Rossi-α method to calculate the prompt neutron decay constant (α) or its inverse, the prompt neutron period (1/α), for assemblies of fissionable material. This work evaluates a high-data-throughput measurement at the National Criticality Experiments Research Center (NCERC) with organic scintillators measuring a subcritical assembly of highly enriched uranium (HEU) metal (93% 235U). The assembly comprises hemi shells stacked together to form fully closed shells and is driven by a 252Cf source at the center. The assembly is a total of 59.85 kg HEU and a keff of 0.98, calculated with MCNP6.2 KCODE. Measurements were acquired with a three-by-four array of 5.08-cm-diameter by 5.08-cm-length trans-stilbene crystals 166 cm from the assembly center. Two types of coincident binning methods are used to build the Rossi-α distribution of coincident neutron detections: type 1 binning, also known as any-and-all forward time differences, and type 1 binning with a digital-delay technique that is analogous to the use of delay cabling in 1950-60’s Rossi-alpha experiments. The digital-delay technique disregards same-detector coincidences and once all nearest time coincidences are collected between all detectors after a trigger, a time delay of 0.75 µs is implemented. The measured prompt neutron decay constants for both techniques are calculated from single exponential fits and the two methods are compared. Any-and-all forward time differences shows an apparent timing discontinuity near 500 ns time differences. This timing discontinuity interferes with the fitting method used to calculate the prompt neutron decay constant. The digital-delay technique mitigates the artificial timing discontinuity and removes the disagreement of the fit. Future work will model the time-dependent detector response to discern why this timing discontinuity occurs, discern how the digital-delay technique reduces the amplitude of this timing discontinuity, and apply this method to critical assembly measurements to ultimately confirm the recommendation to use the digital-delay technique for high-data-throughput measurements.