Characterizing the Oak Ridge Large Volume Active Well Coincidence Counter for Nuclear Safeguards Applications

For decades 3He filled proportional counters have been the gold standard for detection of slow neutrons, offering high efficiency and excellent discrimination against gamma rays. They are extensively used for the nondestructive assay (NDA) of special nuclear materials (SNM) for security and safeguards. NDA instruments used in the field range in design from small portable units used by safeguards inspectors for on-site verification to large bespoke systems installed at nuclear facilities for continuous unattended monitoring. An important class of detector is that of the ‘standard’ well counters currently used by international inspectorates, with various designs for specific sample sizes, shapes, or mass ranges. The Oak Ridge Large Volume Active Well Coincidence Counter (LV AWCC) is a variant of the Los Alamos National Laboratory-designed AWCC but with a larger cavity. In this work, various measurements including a high- voltage characteristic, count rate as a function of predelay time, and dead time were taken with the LV AWCC to characterize this system. Non-ideal behaviors of the counter are discussed. Double pulsing, a form of non-ideal behavior, is an unwanted artifact of the electronics design and operation of neutron detectors. It contributes to an increased count rate when assaying samples and is therefore necessary to identify and characterize before accurate measurements can be taken. By running the LV AWCC at 1900 V (far above the recommended setting of 1720 V), this phenomenon was studied more easily. The authors thank the DOE/NNSA Office of Nonproliferation and Arms Control (NA-24) for the support given to enable this work.