GeMSE: a Low-Background Facility for Gamma-Spectrometry at Moderate Rock Overburden

TL;DR

The GeMSE facility is a low-background gamma spectrometry setup in Switzerland optimized for continuous remote operation, achieving a significant background reduction and enhanced detection efficiency for complex samples.

Contribution

This work details the optimization and background reduction of the GeMSE detector, including precise efficiency calibration and muon-dominated background analysis.

Findings

Background rate decreased by 33% over five years

Background is now predominantly muon-induced

Efficiency calculation now adaptable to complex sample geometries

Abstract

The GeMSE (Germanium Material and meteorite Screening Experiment) facility operates a low-background HPGe crystal in an underground laboratory with a moderate rock overburden of 620$\,$m.w.e. in Switzerland. It has been optimized for continuous remote operation. A multi-layer passive shielding, a muon veto, and a boil-off nitrogen purge line inside the measurement cavity minimize the instrument's background rate, which decreased by 33% to (164$\pm$2)$\,$counts/day (100-2700$\,$keV) after five years of underground operation. This agrees with the prediction based on the expected decay of short-lived isotopes. A fit to the known background components, modeled via a precise simulation of the detector, shows that the GeMSE background is now muon-dominated. We also present updates towards a more accurate detection efficiency calculation for the screened samples: the thickness of the crystal's outer dead-layer is precisely determined and the efficiency can now be easily calculated for any sample geometry. The advantage of this feature is showcased via the determination of the $^{40}$K content in the screening of a complex-shaped object: a banana.