Calibration sources for the LEGEND-200 experiment

TL;DR

This paper details the production, activity measurement, and neutron emission characterization of calibration sources used in the LEGEND-200 neutrinoless double beta decay experiment, ensuring minimal background interference.

Contribution

It introduces a custom production process for low-neutron emission $^{228}$Th calibration sources and quantifies their activities and neutron flux, reducing background noise in the experiment.

Findings

17 sources produced and characterized for LEGEND-200

Neutron emission rate significantly below commercial sources

Neutron-induced background from sources is negligible

Abstract

In the search for a monochromatic peak as the signature of neutrinoless double beta decay an excellent energy resolution and an ultra-low background around the $Q$-value of the decay are essential. The LEGEND-200 experiment performs such a search with high-purity germanium detectors enriched in $^{76}$Ge immersed in liquid argon. To determine and monitor the stability of the energy scale and resolution of the germanium diodes, custom-made, low-neutron emission $^{228}$Th sources are regularly deployed in the vicinity of the crystals. Here we describe the production process of the 17 sources available for installation in the experiment, the measurements of their alpha- and gamma-activities, as well as the determination of the neutron emission rates with a low-background LiI(Eu) detector operated deep underground. With a flux of $\left( 4.27 \pm 0.60_{\rm stat} \pm 0.92_{\rm syst} \right) \times 10^{-4} ~\text{n / (kBq$\cdot$s)}$, approximately one order of magnitude below that of commercial sources, the neutron-induced background rate, mainly from the activation of $^{76}$Ge, is negligible compared to other background sources in LEGEND-200.