Revealing neutral bremsstrahlung in two-phase argon electroluminescence

TL;DR

This paper investigates neutral bremsstrahlung as a new mechanism contributing to electroluminescence in two-phase argon detectors, combining theoretical analysis with experimental measurements to explain previously puzzling spectral features.

Contribution

It introduces and experimentally validates neutral bremsstrahlung as a significant source of electroluminescence in two-phase argon, expanding understanding beyond excimer-based emission.

Findings

Neutral bremsstrahlung explains non-VUV spectral components.

First absolute EL yield measurement in two-phase argon.

EL observed at electric fields below excitation threshold.

Abstract

Proportional electroluminescence (EL) in noble gases has long been used in two-phase detectors for dark matter search, to record ionization signals induced by particle scattering in the noble-gas liquid (S2 signals). Until recently, it was believed that proportional electroluminescence was fully due to VUV emission of noble gas excimers produced in atomic collisions with excited atoms, the latter being in turn produced by drifting electrons. In this work we consider an additional mechanism of proportional electroluminescence, namely that of bremsstrahlung of drifting electrons scattered on neutral atoms (so-called neutral bremsstrahlung); it is systemically studied here both theoretically and experimentally. In particular, the absolute EL yield has for the first time been measured in pure gaseous argon in the two-phase mode, using a dedicated two-phase detector with EL gap optically read out by cryogenic PMTs and SiPMs. We show that the neutral bremsstrahlung effect can explain two intriguing observations in EL radiation: that of the substantial contribution of the non-VUV spectral component, extending from the UV to NIR, and that of the photon emission at lower electric fields, below the Ar excitation threshold. Possible applications of neutral bremsstrahlung effect in two-phase dark matter detectors are discussed.