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

TL;DR

This paper discusses shielding strategies for low-background proportional counters in surface labs, emphasizing cosmic ray mitigation to enhance sensitivity for detecting low-energy radioactive gases.

Contribution

It presents a conceptual design and shielding configuration for low-background gas counters optimized for surface laboratory conditions, based on experimental and simulation studies.

Findings

Lead shielding effectively reduces gamma ray background.

Cosmic rays are the dominant background in surface measurements.

Optimal shielding design enhances sensitivity for low-energy radioactive gases.

Abstract

Development of ultra low background gas proportional counters has made the contribution from naturally occurring radioactive isotopes -- primarily $\alpha$ and $\beta$ 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.