instruments Review Electroluminescence and Electron Avalanching in Two-Phase Detectors Alexey Buzulutskov 1,2 1 Budker Institute of Nuclear Physics, 630090 Novosibirsk, Russia; [email protected] 2 Novosibirsk State University, 630090 Novosibirsk, Russia Received: 7 May 2020; Accepted: 10 June 2020; Published: 18 June 2020 Abstract: Electroluminescence and electron avalanching are the physical effects used in two-phase argon and xenon detectors for dark matter searches and neutrino detection, to amplify the primary ionization signal directly in cryogenic noble-gas media. We review the concepts of such light and charge signal amplification, including a combination thereof, both in the gas and in the liquid phase. Puzzling aspects of the physics of electroluminescence and electron avalanching in two-phase detectors are explained, and detection techniques based on these effects are described. Keywords: two-phase detectors for dark matter search; liquid noble-gas detectors; proportional electroluminescence in noble gases; electron avalanching at cryogenic temperatures 1. Introduction The ultimate goal for liquid noble-gas detectors is the development of large-volume detectors of superior sensitivity for rare-event experiments. A typical deposited energy in such experiments might be rather low: of the order of 0.1 keV in coherent neutrino-nucleus scattering experiments, 1–100 keV in those of dark matter search and 100 keV in those of astrophysical and accelerator neutrino detection. ≥ Accordingly, the primary ionization and scintillation signals produced by a particle in noble-gas liquid have to be amplified in dense noble-gas media at cryogenic temperatures. The idea of two-phase detectors [1] is based on the requirement of amplification of such weak signals (see reviews [2,3]). In particular, the scattered particle produces in the liquid phase two types of signals: that of primary scintillation recorded promptly (S1 signal) and that of primary ionization recorded with delay (S2 signal). While for the readout of the S1 signal it is sufficient to use optical devices such as photomultiplier tubes (PMTs) and silicon photomultipliers (SiPMs) having their own electronic amplification, to record the S2 signal, one has to amplify it in the detection medium directly (in case of the low deposited energy). In two-phase detectors this is done in the following way: the primary ionization electrons, after drifting in the liquid phase, are emitted into the gas phase where their (S2) signal is amplified. In “classic” two-phase detectors, the way to amplify the S2 signal is provided by electroluminescence (secondary scintillation) produced by drifting electrons under high electric fields. The second possible way to amplify the S2 signal is that of electron avalanching (electron multiplication) at high electric fields. In the following, we refer to these two ways of amplification as those of light and charge signal amplification, respectively. This review is dedicated to the issue of S2 signal amplification. We review all known concepts of light and charge signal amplification (i.e., those based on electroluminescence and electron avalanching in cryogenic noble-gas media), including a combination thereof, both in the gas and in the liquid phase. We explain puzzling aspects of the physics of electroluminescence and electron avalanching in two-phase detectors and describe detection techniques based on these effects. Instruments 2020, 4, 16; doi:10.3390/instruments4020016 www.mdpi.com/journal/instruments Instruments 2020, 4, 16 2 of 31 Instruments 2020, 4, x FOR PEER REVIEW 2 of 31 Instruments 2020, 4, x FOR PEER REVIEW 2 of 31 2. BasicMany Concepts methods of Signalfor light Amplification and charge insignal Two-Phase amplification Detectors in two-phase detectors have been proposed,Many andmethods some offor these light are and described charge signalin review amplifications [2–6]. Among in two-phase this variety, detectors two basic have concepts been proposed,can beMany distinguished, and methods some of forthat these light most are and other described charge concepts in signal review come amplification sfrom. [2–6]. Among in two-phase this variety, detectors two basic have concepts been canproposed, beThe distinguished, basic and concept some ofthat of these light most are signal other described amplificationconcepts in reviewscome is from.that [2– of6 ]. the Among “classic” this two-phase variety, two detector basic concepts(see [3]); canit is beprovidedThe distinguished, basic byconcept the effect that of light mostof proportional signal other amplification concepts electr comeoluminescence is that from. of the “classic”(EL): see two-phaseFigure 1. Here detector each (see drifting [3]); itelectron is providedThe of basic the by concept ionization the effect of light of(S2) proportionalsignal signal amplification emits electr lightoluminescence in is thatthe EL of thegap (EL): “classic” under see highFigure two-phase electric 1. Here detector field, each the (seedrifting light [3]); electronitintensity is provided of being the by ionizationroughly the effect proportional (S2) of proportional signal toemits the electroluminescence electriclight in field.the EL This gap concept (EL): under see splitshigh Figure electricinto1. Heretwo field, sub-concepts: each the drifting light intensityelectronwith indirect of being the and ionization roughly direct proportional (S2)optical signal readout. emits to the lightFor electric the in the former, ELfield. gap theThis under optical concept high readoutelectric splits intogoes field, two via the sub-concepts:a light wavelength intensity withbeingshifter indirect roughly (WLS). and proportional direct optical to the readout. electric field.For the This former, concept the splits optical into readout two sub-concepts: goes via a withwavelength indirect shifterand direct (WLS). optical readout. For the former, the optical readout goes via a wavelength shifter (WLS). Figure 1. Basic concepts of light signal amplification in two-phase detectors, using proportional Figureelectroluminescence 1. BasicBasic concepts concepts (EL) ofin of lightthe light EL signal gap. amplification amplificationLeft: with indirect in two-phase optical readout detectors, via using wavelength proportional shifter electroluminescence(WLS), in two-phase (EL) (EL)Ar using in the excimer EL gap. emission Left: withwith in theindirect indirect vacuum optical optical ultraviolet readout readout (VUV) via wavelength at 128 nm. shifterRight: (WLS),with direct in two-phase optical readout, Ar using either excimer in two-phase emission Xe in usingthe vacuum excimer ultraviolet emission in(VUV) the VUV at 128 at nm.175 nm,Right: or within two-phase direct optical Ar using readout,readout, neutral eithereither bremsstrahlung inin two-phasetwo-phase Xe Xe(N using BrS)using emission excimer excimer emissionin emission the non-VUV in in the the VUV VUVrange at at 175(at 175 nm,200–1000 nm, or or in intwo-phasenm). two-phase Ar Ar using using neutral neutral bremsstrahlung bremsstrahlung (NBrS) (NBrS) emission emission in the in non-VUVthe non-VUV range range (at 200–1000 (at 200–1000 nm). nm). The basic concept of charge signal amplificationamplification in two-phase detectors is that of using a gas electronThe multipliermultiplierbasic concept (GEM)-like(GEM)-like of charge structure structure signal in amplificationin the the gas gas phase phase in that two-phasethat provides provides detectors electron electron avalanchingis avalanchingthat of using within within a gas the electronGEMthe GEM holes multiplier holes at cryogenic at cryogenic (GEM)-like temperatures: temperatures: structure see in Figureseethe Figure gas2 andphase 2 reviewand that review provides [4]. [4]. electron avalanching within the GEM holes at cryogenic temperatures: see Figure 2 and review [4]. Figure 2.2. BasicBasic concept of charge signal amplificationamplification in two-phase detectors, using gas electron Figuremultiplier 2. Basic (GEM)-like concept structures of charge [[4].4 ].signal amplification in two-phase detectors, using gas electron multiplier (GEM)-like structures [4]. Instruments 2020, 4, x FOR PEER REVIEW 3 of 31 Instruments 2020, 4, 16 3 of 31 In Sections 3–6, we describe in detail the physics and state-of-the-art of these basic concepts, as well as consider other (most developed) concepts of signal amplification in cryogenic noble-gas In Sections3–6, we describe in detail the physics and state-of-the-art of these basic concepts, as well media. as consider other (most developed) concepts of signal amplification in cryogenic noble-gas media. 3.3. LightLight SignalSignal Amplification Amplification in in the the Gas Gas Phase Phase of a of Two-Phase a Two-Phase Detector, Detector, Using Using Proportional Proportional ElectroluminescenceElectroluminescence 3.1.3.1. Three ELEL MechanismsMechanisms AccordingAccording toto modernmodern concepts,concepts, therethere areare threethree ELEL mechanismsmechanisms responsibleresponsible forfor proportionalproportional ELEL ∗ ∗ inin gaseousgaseous ArAr andand Xe:Xe: that of excimer ( Ar2∗ or Xe2∗)) emissionemission inin thethe VUVVUV (“ordinary”(“ordinary” EL),EL), thatthat ofof emissionemission duedue toto atomicatomic transitionstransitions inin thethe nearnear infraredinfrared (NIR),(NIR), andand thatthat ofof neutralneutral bremsstrahlungbremsstrahlung (NBrS)(NBrS) emissionemission inin thethe UV,UV, visiblevisible andand NIRNIR range.range. TheThe energyenergy levellevel diagrams,diagrams, appropriateappropriate reactionsreactions