Commissioning of the COBRA demonstrator and investigation of surface events as its main background

TL;DR

This paper reports on the commissioning of the COBRA demonstrator, a low-background detector setup for neutrinoless double beta decay, and investigates surface events as the main background, exploring discrimination methods and detector improvements.

Contribution

The study provides detailed analysis of surface event backgrounds, compares pulse shape discrimination methods, and tests instrumentation improvements to enhance background rejection in COBRA detectors.

Findings

Surface events dominate the background in COBRA detectors.

Pulse shape analysis A/E outperforms LSE in background discrimination.

Guard ring instrumentation significantly reduces surface event contamination.

Abstract

The COBRA collaboration investigates 0{\nu}\beta\beta-decays (neutrinoless double beta-decays). Therefore, a demonstrator setup using coplanar-grid CdZnTe detectors is operated at the LNGS underground laboratory. In this work, the demonstrator was commissioned and completed, which is discussed extensively. The demonstrator works reliably and collects low-background physics data. One result of the analysis of the data is that surface events are the dominating background component. To better understand and possibly discriminate this background, surface events were studied in detail. This was done mainly using laboratory measurements. For a better interpretation of these measurements, simulations of particle trajectories and ranges were done. The surface sensitivity tests showed large differences between the individual detectors. Often, a dead-layer was determined, especially at the surfaces where the non-collecting anode (NCA) is the outermost anode rail. Due to this, the sensitivity of the surfaces where the collecting anode (CA) is adjacent was typically about a factor of three larger than the NCA sensitivity. A comparison of the pulse shape analysis methods LSE and A/E was done. Laboratory measurements indicate, that the latter performs better. Alpha scanning measurements were done to spatially investigate the surface sensitivity. Plausible variations were measured. However, no hints were found how to improve the surface event recognition. The instrumentation of the guard ring, which surrounds the anode structure, was tested and improved the surface event discrimination significantly. The fraction of surviving alpha events was at a per-mill level. Important steps for a future large-scale COBRA experiment are discussed briefly, mainly the use of an integrated read-out system. Overall, the results indicate a large potential in background reduction for the COBRA experiment.