Characterization of spurious-electron signals in the double-phase argon TPC of the DarkSide-50 experiment

TL;DR

This study investigates the origins and characteristics of spurious-electron signals in the DarkSide-50 argon TPC, revealing impurity-related electron capture and release, and potential photo-ionization sources affecting dark matter detection sensitivity.

Contribution

First comprehensive analysis of spurious electrons in argon TPCs, identifying impurity capture, delayed release, and photo-ionization as key mechanisms influencing background signals.

Findings

30-70% of events caused by impurity-captured electrons

Spurious-electron release delays of 5-50 ms observed

Evidence of photo-ionization from steel grid

Abstract

Spurious-electron signals in dual-phase noble-liquid time projection chambers have been observed in both xenon and argon Time Projection Chambers (TPCs). This paper presents the first comprehensive study of spurious electrons in argon, using data collected by the DarkSide-50 experiment at the INFN Laboratori Nazionali del Gran Sasso (LNGS). Understanding these events is a key factor in improving the sensitivity of low-mass dark matter searches exploiting ionization signals in dual-phase noble liquid TPCs. We find that a significant fraction of spurious-electron events, ranging from 30 to 70% across the experiment's lifetime, are caused by electrons captured from impurities and later released with delays of order 5-50 ms. The rate of spurious-electron events is found to correlate with the operational condition of the purification system and the total event rate in the detector. Finally, we present evidence that multi-electron spurious electron events may originate from photo-ionization of the steel grid used to define the electric fields. These observations indicate the possibility of reduction of the background in future experiments and hint at possible spurious electron production mechanisms.