Design of a multi-layered shield for low-background HPGe spectrometry

Abstract

High-purity germanium (HPGe) spectrometers are indispensable tools for detecting and analyzing samples with low radioactivity levels, particularly for material screening in rare event experiments. Traditional HPGe detector shielding typically requires at least a 10 cm layer of lead or additional graded-Z liners to attenuate external radiation. However, the cost and toxicity associated with low-background lead motivate the exploration of alternative primary shielding materials. This study proposes a multi-layered, steel-based shield to address these challenges. Through Monte Carlo simulations and calculations, we determined that the optimal structure consists of a 15 cm thick steel outer layer, a 5 mm low-background lead liner, and a 1 cm inner steel liner. Experimental results demonstrated that the introduction of the 5 mm lead and 1 cm steel liner combination significantly reduced the background counting rate of the BE5030 HPGe detector below 600 keV by 40% compared to a steel-only shield. Additionally, the implementation of a boil-off nitrogen purging device effectively reduced peaks associated with radon progeny, resulting in a background counting rate of 1.5 cps for the BE5030 HPGe detector in the energy range below 3000 keV. The proposed multi-layer shield for low-background HPGe spectrometry substantially decreases the use of low-background lead while maintaining background levels consistent with those of a 10 cm low-background lead chamber through the optimization of the liner, which serves as the low-energy radiation absorber.