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

Abstract

Development of ultra low background gas proportional counters has made the contribution from naturally occurring radioactive isotopes -primarily α and β activity in the uranium and thorium decay chains -inconsequential to instrumental sensitivity levels when measurements are performed in above ground surface laboratories. Simple lead shielding is enough to mitigate against gamma rays as gas proportional counters are already relatively insensitive to naturally occurring gamma radiation. The dominant background in these surface laboratory measurements using ultra low background gas proportional counters is due to cosmic ray generated muons, neutrons, and protons. Studies of measurements with ultra low background gas proportional counters in surface and underground laboratories as well as radiation transport Monte Carlo simulations suggest a preferred conceptual design to achieve the highest possible sensitivity from an array of low background gas proportional counters when operated in a surface laboratory. The basis for a low background gas proportional counter array and the preferred shielding configuration is reported, especially in relation to measurements of radioactive gases having low energy decays such as 37 Ar.

High Throughput Argon-37 Field System

“…The seven gas proportional counters are housed inside an optimized active and passive shield. This design minimizes the reduction in performance seen from added background radiation when it is not possible to employ an underground laboratory with depth (overburden) to provide shielding against cosmic-ray backgrounds (Aalseth et al 2016). The lead shielding is produced from a series of lead rings that stack on top of each other.…”