Measurement of the 85Kr specific activity in the GERDA liquid argon

Abstract

The radioactive isotope ⁸⁵Kr is found in significant quantities in the atmosphere largely due to nuclear industry. Its $\beta$ -decay with a half-life of 10.7 years and a Q-value of 687 keV is a dangerous background source for low-threshold noble gas and liquid detectors, which distill their detector medium from air. The Gerda experiment was operating high-purity germanium detectors immersed in a clean liquid argon bath deep underground to search for neutrinoless double beta decay with unprecedented sensitivity. The ⁸⁵Kr specific activity in the liquid argon at the start of the second phase of the experiment has been determined to be $(0.36\pm 0.03)$ mBq/kg through an analysis of the full subsequent data set that exploits the excellent $\gamma$ -ray spectroscopic capabilities of Gerda.

Fig. 1

Simplified ⁸⁵Kr decay scheme [13]. The decay channel studied in this work is highlighted in bold

Fig. 2

The energy spectrum of the Gerda data around the region of interest at the ⁸⁵Kr FEP (514 keV), with labels indicating the prominent spectral features and the expected contribution from the 2$\mathrm{\nu }\mathrm{\beta }\mathrm{\beta }$ decay in orange. The inset shows a zoom around the region of interest with a finer binning (0.2 keV), in linear y-scale

Fig. 3

The best-fit model superimposed to data. The highlighted top-left panel shows a combination of the entire Gerda Phase II data set. The remaining panels separately show data from different detector types (BEGe, Coax and IC) and before or after the May 2018 hardware upgrade. The continuum, the 511 keV peak and the ⁸⁵Kr FEP are plotted separately with dashed lines. The difference between data and best fit model for each bin, normalized by standard deviation expected from Poisson statistics, is shown in a panel below each spectrum

Fig. 4

Top panel: horizontal spatial distribution of a sample of simulated ⁸⁵Kr decays associated with an energy deposition of 514 keV in the HPGe array. Key structural components (the radon shroud, the WLS fiber shroud and the detectors) are pictorially shown. Bottom panel: expected signature of the decay in the Gerda energy spectrum, for each detector type, normalized by ⁸⁵Kr specific activity at the start of Gerda Phase II. 10¹⁰ total decays have been simulated in a LAr cylinder (see text). The energy windows used to calculate the full-energy peak efficiency are shown in gray (see text)

Fig. 5

Visualization of the exponential decrease of the ⁸⁵Kr activity (continuous blue line) during the Gerda data taking. Gerda Phase II data taking (blue areas) started at $t_0=\text{25 December 2015}$. The time of LAr cryostat filling, 17 December 2009, and the Phase I data taking period (dashed area), not considered in this work, are indicated. The activity $A_0$ is obtained by a fit to the Gerda Phase II data (see Sect. 7)