Revealing neutral bremsstrahlung in two-phase argon electroluminescence

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.

Neutral bremsstrahlung in two-phase argon electroluminescence: further studies and possible applications

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We further study the effect of neutral bremsstrahlung (NBrS) in two-phase argon electroluminescence (EL), revealed recently in [1]. The absolute EL yield due to NBrS effect, in the visible and NIR range, was remeasured in pure gaseous argon in the two-phase mode, using a two-phase detector with EL gap read out directly by cryogenic PMTs and SiPMs.

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Electron transport and electric field simulations in two-phase detectors with THGEM electrodes

“…The results obtained here have been already used in our current studies [3] and will definitely be used in the future ones.…”

“…In particular, E EL is 6541, 6535 and 6470 V/cm for V T HGEM1 = 2200, 2000 and 0 V respectively, while in the infinite plane approximation it is 6513 V/cm. Accordingly, this effect is negligible compared to the systematic errors in the absolute EL measurements [3].…”

“…Two-phase detectors with electroluminescence (EL) gap, based on Ar and Xe, are relevant to experiments for direct search of dark matter particles [1,2]. Several versions of such detectors were developed in our laboratory for the study of EL mechanism in pure argon [3] and argon doped with nitrogen [4,5], for SiPM-matrix readout of two-phase detectors, directly [6] or using combined THGEM/SiPM-matrix multipliers [6,7,8], and for the measurements of ionization yields in liquid argon [9]. One of the main features of two-phase detectors developed in our laboratory is the extensive use of THGEMs (Thick Gas Electron Multipliers [10]).…”

“…To the best of our knowledge, no one has simulated THGEM performance in liquid argon thus far; it has been done for the first time in the present work. Figure 1: Generalized version of the two-phase detector with EL gap used in [3,4,6,9] (not to scale). The 4 MΩ resistance defining the voltage across THGEM0 is referred to as R T HGEM0 .…”