Initial characterization of unequal-length, low-background proportional counters for absolute gas-counting applications

Abstract

Characterization of two sets of custom unequal length proportional counters is underway at Pacific Northwest National Laboratory (PNNL). These detectors will be used in measurements to determine the absolute activity concentration of gaseous radionuclides (e.g., 37 Ar). A set of three detectors has been fabricated based on previous PNNL ultra-low-background proportional counter designs and now operate in PNNL's shallow underground counting laboratory. A second set of four counters has also been fabricated using clean assembly of Oxygen-Free High-Conductivity copper components for use in a shielded above-ground counting laboratory. Characterization of both sets of detectors is underway with measurements of background rates, gas gain, and energy resolution. These results will be presented along with a shielding study for the above-ground cave.

Development of a low-level 37Ar calibration standard

“…The length-compensated method has been used as a primary measurement method in the determination of gaseous radionuclides by a variety of national metrology institutes (Makepeace et al, 1998;Picolo et al, 1998). A variant of this method has been developed at PNNL for quantification of low-level gaseous radionuclides by employing materials and methods used in the production of ULBPCs (Mace et al, 2013;Williams et al, 2013). The PNNL absolute gas counting system consists of a gas handling and mixing manifold (used to quantitatively blend samples with P10 counting gas and transfer this mixture to the detectors); a set of three unequal-length ULBPCs, which are filled above ground then taken to PNNL's shallow underground laboratory for counting; and a set of four counters fabricated from oxygen free high conductivity (OFHC) copper.…”

“…Once the pressure and temperature have equilibrated, the detectors are removed from the manifold and taken back to the shallow underground laboratory for extended measurements. Prior to these measurements, the background count rates for these three detectors in the shallow counting laboratory were determined over a range of pressures (Mace et al, 2013) and found to be <10 counts per day in the 37 Ar region of interest (ROI).…”

“…PNNL has developed a low-level absolute gas counting capability based on the proven method known as length-compensated internal source proportional counting (Mace et al, 2013;Williams et al, 2013). The method has been adopted by many national metrology laboratories for determining the specific activity of a number of gaseous radionuclide standards (Unterweger, 2007).…”

Shielding concepts for low-background proportional counter arrays in surface laboratories

“…Above ground proportional counter measurements using a OFHC copper proportional copper were performed (Mace et al (2013)) to determine the affects of lead shielding on non-cosmic ray backgrounds. The shield used in this study was composed of 2 ′′ thick lead in a cave of dimensions 8 ′′ ×8 ′′ ×16 ′′ (W×H×L).…”