Background Rejection of n$^+$ Surface Events in GERDA Phase II

TL;DR

This paper presents a method to significantly reduce background noise from surface events in the GERDA Phase II experiment by using pulse shape discrimination, enhancing the search for neutrinoless double beta decay.

Contribution

It demonstrates the effectiveness of pulse shape discrimination in reducing n$^+$ surface event backgrounds by up to a factor of 145 in GERDA Phase II.

Findings

Pulse shape discrimination reduces background by up to 145 times.

Surface events from $^{42}$K $eta$ decays are the main background.

Simulation confirms the effectiveness of pulse shape analysis.

Abstract

The GERDA experiment searches for neutrinoless double beta ($0\nu\beta\beta$) decay in $^{76}$Ge using an array of high purity germanium (HPGe) detectors immersed in liquid argon (LAr). Phase II of the experiment uses 30 new broad energy germanium (BEGe) detectors with superior pulse shape discrimination capabilities compared to the previously used semi-coaxial detector design. By far the largest background component for BEGe detectors in GERDA are n$^+$-surface events from $^{42}$K $\beta$ decays which are intrinsic in LAr. The $\beta$ particles with up to 3.5 MeV can traverse the 0.5 to 0.9 mm thick electrode and deposit energy within the region of interest for the $0\nu\beta\beta$ decay. However, those events have particular pulse shape features allowing for a strong discrimination. The understanding and simulation of this background, showing a reduction by up to a factor 145 with pulse shape discrimination alone, is presented in this work.