Active fast neutron singles assay of U-235 enrichment in small samples of triuranium octoxide

摘要

The sensitivity of an active fast neutron assay system for the measurement of U-235 enrichment in small samples of low-enriched triuranium octoxide (U3O8) is described. The system comprises four organic liquid scintillation detectors in a polyethylene container in which a stimulating americium-lithium neutron source is placed below the sample to be interrogated. The sensitivities of both the singles and doubles assay have been corrected for the contribution from the stimulating source and for spontaneous fission in U-238 and found to be (0.116 +/- 0.008) cps per detector per % wt. (7%) and (0.0006 +/- 0.0002) cps per detector per % wt. (33%), respectively. The singles approach with the 4-detector arrangement has been compared via Monte Carlo (MCNP-5) simulations with a fast neutron counting system based on 12 detectors in which the source is placed adjacent to the sample, similar to the arrangement referred to as the liquid scintillator uranium neutron collar. This comparison confirms that whilst singles assay with the 4-detector arrangement is feasible, the collar arrangement does not yield a singles yield that can be correlated with enrichment due to the perturbative scattering by the U3O8 sample. The 4-detector arrangement is particularly suitable for small samples of fresh, low-enriched material that might arise in forensic applications and the analysis of un-irradiated, orphan wastes. In these cases the efficiency of coincidence methods may be too low to yield a practical route to enrichment assessment. Conversely, the use of many detectors and/or a high-intensity source by which sensitivity might be increased might restrict the portability and ease of use of the apparatus. A correction for the contribution by (alpha, n) reactions, the stimulating source, spontaneous fission in U-235 and self-multiplication in U3O8 is made via preparatory passive measurements. (C) 2016 The Authors. Published by Elsevier Ltd.