Performance characteristics of a High Efficiency Passive Neutron Assay System using alternative neutron detectors to helium-3

Passive neutron non-destructive assay systems demand high sensitivity in order to be capable of detecting milligram levels of plutonium for safeguards and waste characterization applications. Chamber efficiencies greater than 30% are required for neutron coincidence and multiplicity counting systems. Existing systems are based on
³
He proportional counters and require hundreds of liters of this gas. The severe
³
He shortage has created a driver to evaluate alternative detectors in this application. An alternate design must meet the following performance requirements: 1) high absolute detection efficiency, 2) low gamma ray sensitivity and 3) short die-away times. In addition, practical aspects of the detectors must be considered including: 1) system lifetime costs, 2) backward compatibility with existing protocols, 3) long term stability in industrial environments, 4) low maintenance, 5) technology maturity, 6) production scalability and 7) materials toxicity. Several alternative sensors are currently available that could potentially meet the above requirements. These include Boron trifluoride (BF
3
) filled detectors,
¹⁰
B lined proportional counters (including novel designs that increase surface area) and various scintillators based on
⁶
Li. For each category the most promising candidate has been identified and evaluated for use as a direct replacement for
³
He in a typical High Efficiency Passive Neutron Assay System designed for transuranic / low-level waste sentencing with a lower limit of detection of 3700 Bq/g (alpha activity concentration). Performance of the alternative designs has been modeled and compared to the
³
He baseline with any potential technical improvements also being considered.