Large volume organic liquid scintillation detectors as a vehicle radiation portal monitor prototype at the 3rd SCINTILLA benchmark campaign

摘要

The Detection for Nuclear Nonproliferation Group (DNNG) previously tested a pedestrian radiation portal monitor (RPM) incorporating eight EJ309 liquid scintillation detectors at the 2nd SCINTILLA benchmark for ~3He-free RPMs at the Joint Research Centre in Ispra, Italy, in February 2014. Our goal for the 3rd SCINTILLA (November 2014) benchmark was to develop a liquids-based vehicle RPM. Achieving a similar detection success rate with the vehicle RPM (larger source-RPM-distance, higher source speed than pedestrian RPM), required an increased total detector volume. Adding additional detectors was the simplest solution. However, photomultiplier tubes (PMTs) are the predominant cost of liquid scintillation detectors. Therefore we focused on whether larger liquid cells coupled to 7.62 cm diameter PMTs could still provide increased gamma ray and neutron counting efficiency without any major degradation in the pulse shape discrimination (PSD) performance. The vehicle RPM consisted of four 12.7 cm diameter by 12.7 cm length cylindrical EJ309 liquids and one 25 cm by 25 cm by 8 cm paddle shaped BC501-A liquid. The SCINTILLA testing facility consists of an electric rail-cart system capable of transporting sources past the RPMs at variable source heights and speeds. In a false alarm test using the cart with no source, the DNNG system recorded zero false alarms in 1,781 occupancies. The system struggled to detect low energy gamma-ray sources, like ~(57)Co and ~(241)Am, but performed well for higher energy gamma-ray sources, such as ~(133)Ba, ~(60)Co and ~(137)Cs. The system also performed exceptionally well on neutron alarm tests with a 20,000 n/s ~(252)Cf source. This paper compares the performance of the three different liquid cell volumes for RPM applications as well as a new method for choosing PSD lines.