Cross-Talk Characterization in Passive Neutron Coincidence Counting of Radioactive Waste Drums With Plastic Scintillators

Study of gamma-ray background noise for radioactive waste drum characterization with plastic scintillators

“…In view to limit parasitic non-fission coincidences in plutonium measurements with plastic scintillators, we plan to use low-and high-energy thresholds to cut as many gamma rays as possible. Uncorrelated gamma rays from alpha and beta disintegrations, and from (α,x) reactions do not create coincidences, except from cross talk between close detectors but such events can be discarded by time rejection as explained in [3]. However, as mentioned above, prompt gamma rays accompanying (α,n) and neutron inelastic scattering reactions, as well as cascade gamma rays following disintegrations or reactions, can create non-fission coincidences.…”

“…In the context of passive neutron measurements for nuclear material characterization, conventional coincidence collars based on helium 3 gas counters permits a temporal discrimination of spontaneous fissions neutrons and parasitic neutrons from (α, n) reactions. 3 He proportional counters are the golden standard because they show a very high capture cross section for thermal neutrons, leading to high detection efficiency when surrounding detectors with polyethylene moderator, while being practically insensitive to gamma rays. However, the global shortage on 3 He gas [1] has greatly increased their cost in the past 15 years, leading to worldwide initiatives to look for cheaper alternatives with equivalent performances [2].…”

“…3 He proportional counters are the golden standard because they show a very high capture cross section for thermal neutrons, leading to high detection efficiency when surrounding detectors with polyethylene moderator, while being practically insensitive to gamma rays. However, the global shortage on 3 He gas [1] has greatly increased their cost in the past 15 years, leading to worldwide initiatives to look for cheaper alternatives with equivalent performances [2]. The Nuclear Measurements Laboratory of CEA, DEN, Cadarache, is currently studying the use of lowcost Polyvinyl Toluene (PVT) plastic scintillators as a potential alternative [3] [4] that have the advantage of being sensitive to fast neutrons, thus avoiding thermalizing materials and offering a nanosecond response time.…”

“…However, the global shortage on 3 He gas [1] has greatly increased their cost in the past 15 years, leading to worldwide initiatives to look for cheaper alternatives with equivalent performances [2]. The Nuclear Measurements Laboratory of CEA, DEN, Cadarache, is currently studying the use of lowcost Polyvinyl Toluene (PVT) plastic scintillators as a potential alternative [3] [4] that have the advantage of being sensitive to fast neutrons, thus avoiding thermalizing materials and offering a nanosecond response time. Consequently, very short coincidence windows of a few tens of nanoseconds allow greatly reducing accidental coincidences compared to three order of magnitude slower 3 He detection systems.…”

“…Other correlated gamma emissions may also occur in neutron-induced reactions, such as (n,n') inelastic scattering. Pulse Shape Discrimination (PSD) liquid organic scintillators are commonly used to parry gamma sensitivity, but they would not constitute a cost-effective alternative to 3 He counters for radioactive waste package characterization. Indeed, large detection volumes are needed to preserve coincidence counting statistics and therefore, a large number of PSD scintillators should be implemented as their size is limited.…”