Neutral bremsstrahlung and excimer electroluminescence in noble gases and its relevance to two-phase dark matter detectors

E Borisova,A Buzulutskov

doi:10.1140/epjc/s10052-021-09913-z

Abstract

Proportional electroluminescence (EL) is the physical effect used in two-phase detectors for dark matter searches, to optically record (in the gas phase) the ionization signal produced by particle scattering in the liquid phase. In our previous work the presence of a new EL mechanism, namely that of neutral bremsstrahlung (NBrS), was demonstrated in two-phase argon detectors both theoretically and experimentally, in addition to the ordinary EL mechanism due to excimer emission. In this work the similar theoretical approach is applied to all noble gases, i.e. overall to helium, neon, argon, krypton and xenon, to calculate the EL yields and spectra both for NBrS and excimer EL. The relevance of the results obtained to the development of two-phase dark matter detectors is discussed.

Highlights

In two-phase detectors for dark matter searches and lowenergy neutrino experiments, the scattered particle produces two types of signals [1,2]: that of primary scintillation, produced in the liquid and recorded promptly (“S1”), and that of primary ionization, produced in the liquid and recorded with a delay in the gas phase (“S2”)
Proportional electroluminescence (EL) is the physical effect used in two-phase detectors for dark matter searches, to optically record the ionization signal produced by particle scattering in the liquid phase
We evaluate here the absolute EL yields, in terms of the number of photons produced by a drifting electron, and absolute electric fields and voltages needed to provide given reduced electric fields: these are presented in Table 1

Summary

Introduction

In two-phase detectors for dark matter searches and lowenergy neutrino experiments, the scattered particle produces two types of signals [1,2]: that of primary scintillation, produced in the liquid and recorded promptly (“S1”), and that of primary ionization, produced in the liquid and recorded with a delay in the gas phase (“S2”). Excimer EL in noble gases has a threshold in the electric field (of about 4 Td for Ar), defined by the lowest atomic excitation levels Ar∗(3 p54s) In addition it is characterized by emission continuum in the VUV (so-called “second continuum” [11]): Fig. 3 shows this for all noble gases. Below 4 Td corresponding to Ar excitation threshold, the NBrS theory developed in [4] correctly predicts the absolute value of the EL yield This is seen when comparing the experimental and theoretical EL yields in Fig. 2 and when comparing the experimental and theoretical photon emission spectra [4,20,24]. We apply the theoretical method developed in [4] for Ar to all other noble gases, i.e. overall to He, Ne, Ar, Kr and Xe, both for NBrS and excimer EL

Theoretical formulas

Cross sections and electron energy distribution functions

EL spectra and yields

Relevance to two-phase dark matter detectors

Findings

Conclusions