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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 ( 𝐴𝑟Ar2∗ or Xe𝑋𝑒2∗)) 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 andand theirtheir constants,constants, relevant to the former two mechanisms,mechanisms, are presented in Figure 33 andand TableTable1 .1. PhotonPhoton emissionemission spectraspectra forfor all all three three mechanisms mechanisms and and their their absolute absolute EL EL yields yields are are shown shown in in Figures Figures4 and4 and5 respectively 5 respectively for Arfor [Ar7]. [7] For. For other other noble noble gases gases the emissionthe emission spectra spectra and ELand yields EL yields look alikelook [alike4,5]. For[4,5]. Ar, For the Ar, EL the mechanisms EL mechanisms are described are described below below in detail. in detail.

FigureFigure 3.3. EnergyEnergy levels levels of of the the lower lower excited excited and and ionized ionized states states relevant relevant to tothe the ternary ternary mixture mixture of Ar of Ardoped doped with with Xe Xeand and N N2 in2 in the the two-phase two-phase mode mode [7]. [7]. These These are are shown shown for for gaseous Ar, gaseousgaseous NN22,, gaseousgaseous Xe,Xe, liquidliquid ArAr andand XeXe inin liquid Ar. The solid arrows indicate the radiative transitions observedobserved inin experimentsexperiments andand relevantrelevant toto thethe presentpresent reviewreview (i.e.,(i.e., whenwhen thethe excitationexcitation isis inducedinduced byby ionizationionization oror electroluminescence).electroluminescence). The numbers next to each arrow show the photonphoton emissionemission bandband ofof thethe transition,transition, defineddefined byby majormajor emissionemission lineslines oror byby fullfull widthwidth ofofthe the emission emission continuum.continuum. TheThe dasheddashed arrowsarrows indicateindicate the the most most probable probable non-radiative non-radiative transitions transitions induced induced by atomicby atomic collisions collisions for Ar,for XeAr, and Xe

Nand2 species N2 species and theirand their pair combinationspair combinations in the in gas the and gas liquidand liquid phase. phase.

ProportionalProportional ELEL isis characterizedcharacterized byby aa reducedreduced ELEL yieldyield ((YY/N/N),), whichwhich byby definitiondefinition isis thethe numbernumber ofof photonsphotons producedproduced perper driftingdrifting electronelectron perper atomicatomic densitydensity andand perper unitunit driftdrift path.path. ItIt isis typicallytypically plottedplotted as a a function function of of the the reduced reduced electric electric field field (E/N (E),/N the), the latter latter being being expressed expressed in Townsends: in Townsends: 1 Td 17 2 1 1= Td10−=17 V10 cm− 2V atom cm −1atom, corresponding− , corresponding to 0.87 tokV/cm 0.87 kVin /gaseouscm in gaseous Ar at 1.00 Ar atatm 1.00 and atm 87.3 and K. 87.3 K.

Table 1. Basic reactions of excited species relevant to the performance in the two-phase mode, namely in Ar in the gas and liquid phase, doped with Xe (1000 ppm in the liquid and 40 ppb in the gas phase) and N2 (50 ppm in the liquid and 135 ppm in the gas phase), their rate (k) or time (τ) constants reported

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Table 1. Basic reactions of excited species relevant to the performance in the two-phase mode, namely Instrumentsin Ar in 2020 the, 4 gas, x FOR and PEER liquid REVIEW phase, doped with Xe (1000 ppm in the liquid and 40 ppb in the gas phase)4 of 31

and N2 (50 ppm in the liquid and 135 ppm in the gas phase), their rate (k) or time (τ) constants reported in the literature and their time constants reduced to given atomic densities at 87 K (τTP), in particular in the literature and their time constants reduced to given atomic densities at 87 K (τTP), in particular 19 −3 22 −3 forfor Ar Ar to to that that of of 8.63 8.63 × 1019 cmcm and3 and 2.11 2.11 × 10 10 cm22 cm in the3 in gas the and gas liquid and liquid phase, phase, respectively respectively [7]. The [ 7]. × − × − Thereferences references presented presented in th ine thetable table are those are those of [7]. of [7].

3.2. Electroluminescence due to Excimer Emission 3.2. Electroluminescence Due to Excimer Emission In ordinary (excimer) EL, the electrons accelerated by the electric field excite the atoms, which in turnIn ordinary produce (excimer)the excimers EL, (excited the electrons molecules) accelerated in three-body by the electriccollisions. field The excite excimers the atoms, are produced which in ∗ ∗ turnin either produce a singlet the excimers 𝐴𝑟 (∑ (excited) or a triplet molecules) 𝐴𝑟 (∑ in ) three-body state. These collisions. states decay The with excimers photon areemission produced in the VUV, peaked at 128 nm for Ar and 175 nm for Xe (see Figures 3 and 4 and reactions (1) and (2) in Table 1):

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1P + 3P + in either a singlet Ar2∗ u or a triplet Ar2∗ u state. These states decay with photon emission in theInstruments VUV, peaked2020, 4, x atFOR 128 PEER nm REVIEW for Ar and 175 nm for Xe (see Figures3 and4 and reactions (1) and (2)5 of in 31 TableInstruments1): 2020, 4, x FOR PEER REVIEW 5 of 31 e− + Ar e− + Ar∗ (1) 𝑒 + 𝐴𝑟→𝑒→ + 𝐴𝑟∗ (1) 𝑒 + 𝐴𝑟→𝑒 + 𝐴𝑟∗ (1) Ar∗∗+ 2Ar Ar2∗∗+ Ar (2) 𝐴𝑟 +2𝐴𝑟→→ 𝐴𝑟 + 𝐴𝑟 (2) 𝐴𝑟∗ +2𝐴𝑟→𝐴𝑟∗ + 𝐴𝑟 (2) + Ar𝐴𝑟2∗∗ →22Ar𝐴𝑟+ℎ𝜈hν (3) ∗ → (3) 𝐴𝑟 →2𝐴𝑟+ℎ𝜈 (3)

FigureFigureFigure 4.4. 4. Photon Photon emission emission spectraspectra in gaseousgaseousseous Ar Ar due due to to ordinary ordinary scintillations scintillationsscintillations in inthe thethe VUV VUVVUV measured measuredmeasured in [8], NBrS EL at 8.3 Td theoretically calculated in [9] and avalanche scintillations in the near infrared inin [[8],8], NBrSNBrS ELEL atat 8.3 Td theoretically calculatedcalculated inin [[9]9] and avalanche scintillationsscintillations inin the near infrared (NIR) measured in [10,11]. Also shown are the photon detection efficiency (PDE) of SiPM (MPPC (NIR)(NIR) measuredmeasured inin [[10,11].10,11]. Also shown are thethe photonphoton detectiondetection eefficiencyfficiency (PDE)(PDE) ofof SiPMSiPM (MPPC(MPPC 13360-6050PE (Hamamatsu)) at overvoltage of 5.6 V and the transmittance of the acrylic plate (1.5 mm 13360-6050PE13360-6050PE (Hamamatsu))(Hamamatsu)) atat overvoltageovervoltage ofof 5.65.6 VV andand thethe transmittancetransmittance ofof thethe acrylicacrylic plateplate (1.5(1.5 mmmm thick) [12]. thick)thick) [[12].12].

FigureFigure 5. 5.Summary Summary of of experimentalexperimental data on reduced reduced electroluminescence electroluminescence (EL) (EL) yield yield in ingaseous gaseous Ar Arfor for allall known known (EL) (EL) mechanisms: mechanisms: forfor NBrS EL below 1000 1000 nm nm,, measured measured in in [9,13] [9,13 at] at8787 K; K;for for ordinary ordinary EL EL Figure 5. Summary of experimental data on reduced electroluminescence (EL) yield in gaseous Ar for inin the the VUV, VUV, due due to to excimer excimer emission emission goinggoing viavia ArAr*(3p(3p554s4s1)1 )excited excited states, states, measured measured in [9,13] in [9,13 at] 87 at K 87 K all known (EL) mechanisms: for NBrS EL below 1000 nm, measured in [9,13]* at 875 K;1 for ordinary EL andand in in [14 [14]] at at 293 293 K; K; for for EL EL in in the the NIR NIR duedue toto atomicatomic transitionstransitions going going via via Ar Ar(3p*(3p4p54p) 1excited) excited states states in the VUV, due to excimer emission going via Ar*(3p54s1) excited states, measured in [9,13] at 87 K measuredmeasured in in [15 [15]] at at 163 163 K. K. and in [14] at 293 K; for EL in the NIR due to atomic transitions going via Ar*(3p54p1) excited states measuredThe singlet in [15] and at triplet 163 K. excimer states are responsible respectively for the fast and slow emission components observed in proportional EL. Their time constants are of 4.2 ns and 3.1 μs respectively: The singlet and triplet excimer states are responsible respectively for the fast and slow emission components observed in proportional EL. Their time constants are of 4.2 ns and 3.1 μs respectively:

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Instruments 2020, 4, x FOR PEER REVIEW 6 of 31 The singlet and triplet excimer states are responsible respectively for the fast and slow emission seecomponentsInstruments Table 1 and2020 observed, references4, x FORin PEER proportional therein. REVIEW Ordinary EL. Their EL timehas a constants threshold are in ofthe 4.2 electric ns and field, 3.1 µ ofs respectively: about 4 6Td of 31(see see FigureTable1 5),and defined references by the therein. energy Ordinary threshold EL of has11.5 a5 thresholdeV of Ar atom in the excita electriction field,in Equation of about (1). 4 Td (see see Table 1 and references therein. Ordinary EL has a threshold in the electric field, of about 4 Td (see FigureOrdinary5), defined proportional by the energy EL threshold has two ofcharacteri 11.55 eVstic of Ar properties: atom excitation a threshold in Equation and (1).linear field Figure 5), defined by the energy threshold of 11.55 eV of Ar atom excitation in Equation (1). dependenceOrdinary (far proportional enough from EL has the two threshold). characteristic For properties:Xe, this can a threshold be seen andfrom linear Figure field 6 dependencepresenting Ordinary proportional EL has two characteristic properties: a threshold and linear field compilation(far enough fromof the the experimental threshold). data For Xe,on EL this yield can by be seen2007: fromthe results Figure of6 presentingthe different compilation groups are ofclose, the experimentaldependence data (far onenough EL yield from by the 2007: threshold). the results For of Xe, the this different can be groups seen arefrom close, Figure with 6 thepresenting exception withcompilation the exception of the of experimental early works. data on EL yield by 2007: the results of the different groups are close, of early works. withThe the absolute exception photon of early yields works. of ordinary (excimer) EL were theoretically calculated for Ne, Ar, Kr The absolute photon yields of ordinary (excimer) EL were theoretically calculated for Ne, Ar, and Xe Thein [16] absolute using photon Monte yieldsCarlo ofsimulation ordinary (excimerin microscopic) EL were approach: theoretically see Figure calculated 7. One for canNe, seeAr, Krfrom Kr and Xe in [16] using Monte Carlo simulation in microscopic approach: see Figure7. One can theand figure Xe in that [16] well using above Monte the Carlo EL threshold, simulation the in microscopic theoretical predictionsapproach: see are Figure in rather 7. One good can agreementsee from see from the figure that well above the EL threshold, the theoretical predictions are in rather good withthe thefigure yields that wellmeasured above thein experiment EL threshold, at the room theoretical temperature, predictions for Arare inand rather Xe. goodExtrapolating agreement the agreement with the yields measured in experiment at room temperature, for Ar and Xe. Extrapolating theoreticalwith the ELyields yield, measured the EL threshold in experiment was determined at room temperature, as 1.5, 4.1, 2.6for andAr and2.9 Td Xe. for Extrapolating Ne, Ar, Kr and the Xe respectivelythetheoretical theoretical [16].EL EL yield, yield, the the EL EL threshold threshold was was determined determined as 1.5, as 4.1, 1.5, 2.6 4.1, and 2.6 2.9 and Td 2.9 for Td Ne, for Ar, Ne, Kr Ar, and Kr Xe and Xerespectively respectively [16]. [16 ].

FigureFigure 6.6. 6.ReducedReduced Reduced ELEL EL yields yields yields for for for ordinary ordinaryordinary (excimer) (excimer)(excimer) EL inEL Xe inin measured XeXe measuredmeasured by di byff byerent different different groups, groups, groups, as a function as asa a functionoffunction the reduced of ofthe the electricreduced reduced field electric electric [17]. field field The [17]. references[17]. ThThee referencesreferences in the figure inin thethe are figure figure those are are of those [those17]. of of [17]. [17].

Figure 7. Measured reduced EL yield for ordinary (excimer) EL at room temperature for Ar and Xe Figure(data 7. points), MeasuredMeasured as a reducedfunction reduced ELof EL th yield yielde reduced for for ordinary electric (excimer)field, (excimer) compared EL EL at at toroom Monte temperature Carlo simulation for for Ar and data Xe (data(curves), points), for as asNe, a Ar,function Kr and of Xe th the (frome reduced [16], with electric permission field, field, compared from Elsevier). to Monte Carlo simulation data (curves), for Ne, Ar, Kr and XeXe (from(from [[1616],], withwith permissionpermission fromfrom Elsevier).Elsevier). Far enough from the threshold, the EL yield can be parametrized by a linear function, indicating theFar proportionality enough from of the the threshold, EL yield to the the EL electric yield canfield: be parametrized by a linear function, indicating the proportionality of the EL yield to the electric field:

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Far enough from the threshold, the EL yield can be parametrized by a linear function, indicating the proportionality of the EL yield to the electric field:

h 17 1 2 1i Y/N 10− photon electron− cm atom− = a E/N b (4) · − 17 2 1 where reduced electric field E/N is given in Td (10− V cm atom− ). This equation is universally valid for all temperatures and gas densities. Parameter “b” defines the electric field threshold for proportional EL, while parameter “a” describes the light amplification factor, namely the number of photons emitted by drifting electron per applied voltage across the EL gap. For Ar, Kr and Xe this parameter was measured to be 81 [14], 120 [18] and 140 [17] photon/kV respectively. The parametrization of the experimental data by linear field dependence was presented in [14] and [17] for Ar and Xe respectively:

Y/N = 0.081 E/N 0.190, for Ar; (5) · − Y/N = 0.140 E/N 0.474, for Xe. · − However, near the EL threshold this parametrization gives overestimated EL yield values, due to contribution of neutral bremsstrahlung electroluminescence (see Section 3.3). In this case the experimental data relevant to ordinary EL should be used directly. In particular, in two-phase Ar at 87 K and 1.0 atm, the number of photons in the VUV emitted by drifting electron per 1 cm, deduced from experimental data of Figure5, is 345 and 43 at electric field of 8 and 4.6 Td respectively, the latter corresponding to the nominal operation field of the DarkSide experiment.

3.3. Electroluminescence Due Neutral Bremsstrahlung Effect In addition to the ordinary EL mechanism, there is a concurrent EL mechanism, based on bremsstrahlung of drifting electrons scattered on neutral atoms, so-called neutral bremsstrahlung (NBrS) [9,13,19]: e− + Ar e− + Ar + hν (6) → The development of the mechanism is described in detail in [9]. The NBrS differential cross section is proportional to elastic cross section (σel(E)) of electron-atom scattering [9]: !1/2 dσ 8 re 1 E hν = − [(E hν) σ (E) + E σ (E hν)] (7) dν 3 c hν E − · el · el −

2 2 Here re = e /mec is the classical electron radius, c = νλ is the speed of light, E is the initial electron energy. For electron-atom scattering cross sections in Ar, see Figure8. Consequently, the NBrS EL yield is predicted to be maximal for electron energies of the order of 10 eV where the elastic cross section has a maximum, thus corresponding to the electric fields of 4–5 Td. This is confirmed in Figure9 showing the absolute yield of NBrS EL theoretically calculated in [9], in comparison with that of ordinary EL. NBrS EL has a continuous emission spectrum, extending from the UV (200 nm) to the visible and NIR range (1000 nm) and even to the radio frequency range [20]: see Figure4. From Figures4 and9 one can see that NBrS EL, albeit being significantly weaker than ordinary EL above the Ar excitation threshold, has no threshold in electric field, in contrast to ordinary EL, and thus dominates below the threshold. Accordingly, the NBrS effect can explain two remarkable properties of proportional EL observed in experiment [9,13,19]: that of the photon emission below the Ar excitation threshold and that of the substantial contribution of the non-VUV spectral component above the threshold. Instruments 2020, 4, 16 8 of 31 InstrumentsInstruments 20202020,, 44,, xx FORFOR PEERPEER REVIEWREVIEW 88 ofof 3131

Figure 8. Electron scattering cross sections in Ar: elastic (𝜎), momentum-transfer (𝜎), excitation Figure 8. Electron scattering cross sections in Ar: elastic ( 𝜎σel), momentum-transfer (σ𝜎trans ),), excitationexcitation (𝜎 ) and ionization (𝜎 ) (from [9], with permission from Elsevier). ((σ𝜎exc) and ionization (σ𝜎ion)) (from (from [9], [9], with permi permissionssion from Elsevier).

Figure 9. Reduced EL yield in gaseous Ar as a function of the reduced electric field, for neutral Figure 9. Reduced EL yield in gaseous Ar as a function of the reduced electric field, for neutral bremsstrahlung EL (below 1000 nm) calculated theoretically in [9], compared to ordinary (excimer) EL bremsstrahlung EL (below 1000 nm) calculated theoreticallycally inin [9],[9], comparedcompared toto ordinaryordinary (excimer)(excimer) calculated in [16] (from [9], with permission from Elsevier). EL calculated in [16] (from [9], with permission from Elsevier). At lower electric fields, below 4 Td corresponding to Ar excitation threshold, the NBrS theory At lower electric fields, below 4 Td corresponding to Ar excitation threshold, the NBrS theory developed in [9] correctly predicts the absolute value of the EL yield. This is seen when comparing the developed in [9] correctly predicts the absolute value of the EL yield. This is seen when comparing experimental data on EL yields in Figure5 to those of the theory in Figure9 and when comparing the thethe experimentalexperimental datadata onon ELEL yieldsyields inin FigureFigure 55 toto thosethose ofof thethe theorytheory inin FigureFigure 99 andand whenwhen comparingcomparing experimental and theoretical photon emission spectra in Figure 10. On the other hand, at higher fields thethe experimentalexperimental andand theoreticaltheoretical photonphoton emissionemission spspectra in Figure 10. On the other hand, at higher (above 5 Td), the experimental EL yields quickly diverge from those of the theory, exceeding those in fieldsfields (above(above 55 Td),Td), thethe experimentalexperimental ELEL yieldsyields quickly diverge from those of the theory, exceeding the UV spectral range (below 400 nm). In [9] this discrepancy was hypothesized to be explained by thosethose inin thethe UVUV spectralspectral rangerange (below(below 400400 nm).nm). InIn [9][9] thisthis discrepancydiscrepancy waswas hypothesizedhypothesized toto bebe theexplained contribution by the of contribution electron scattering of electron on sub-excitationscattering on sub-excitation Feshbach resonance Feshbach (going resonance via intermediate (going via explained by the contribution5 2 of electron scattering on sub-excitation Feshbach resonance (going via negative ion state Ar (3p 4s )), which might be accompanied by the enhanced photon emission [21]. intermediateintermediate negativenegative− ionion statestate 𝐴𝑟(3𝑝(3𝑝4𝑠)),)), whichwhich mightmight bebe accompaniedaccompanied byby thethe enhancedenhanced photonphoton emission [21].

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Figure 10.10. PhotonPhoton emission emission spectra spectra of proportionalof proportional EL in EL Ar in in theAr UVin th ande UV visible and range visible [19 ],range measured [19], atmeasured room temperature at room temperature in [19] (data in points)[19] (data in comparisonpoints) in comparison with those with of the those NBrS of EL the theory NBrS EL [9] (lines),theory at[9] reduced (lines), at electric reduced field electr of 4.6icic fieldfield and 8.3 ofof 4.64.6 Td. andand 8.38.3 Td.Td.

Near the threshold of ordinary EL, namely in thethe fieldfield rangerange ofof 4–64–6 Td,Td, itit becomesbecomes importantimportant to taketake intointo accountaccount thethe possiblepossible contributioncontribution of NBrS EL, EL, to to correctly correctly measure measure the EL EL yield yield of of ordinary ordinary EL. Note that just inin thisthis fieldfield region,region, namelynamely nominallynominally atat 4.64.6 Td,Td, thethe DarkSideDarkSide darkdark mattermatter searchsearch experiment operates [[22].22]. In In [9,13] [9,13] the NBrS contributioncontribution was measured,measured, usingusing dedicateddedicated spectralspectral devices insensitiveinsensitive to to the the VUV,and VUV, and appropriately appropriately subtracted subtracted from thefrom data the obtained data obtained with VUV-sensitive with VUV- devices.sensitive Thedevices. resulted The ELresulted yield EL of ordinaryyield of ordinary EL, shown EL, in shown Figure in 11 Figure, is in good11, is agreementin good agreement with the theory.with the On theory. the other On hand,the other the hand, experimental the experimental data of [14 data] in Figure of [14] 11 in have Figure an 11 excess have over an excess those ofover [9] andthose theory of [9] and [16], theory below [16], 6 Td. below It would 6 Td. be It logical would tobe explain logical to this explain discrepancy this discrepancy by considering by considering the NBrS contribution,the NBrS contribution, which was which not taken was not into taken account into in account [14]. in [14].

Figure 11. ReducedReduced EL EL yield yield in in gaseous gaseous Ar Ar as as a function a function of ofthe the reduced reduced electric electric field, field, for forordinary ordinary EL EL(due (due to excimer to excimer emission emission in the in VUV) the VUV) near nearits thresh its threshold,old, measured measured at 87 atK [9] 87 and K [9 ]at and 293 atK 293[14] K(from [14] (from[9], with [9], permission with permission from Else fromvier). Elsevier). The theoretical The theoretical calculation calculation of [16] of is [also16] is shown. also shown. 3.4. Electroluminescence Due to Atomic Transitions in the NIR 3.4. Electroluminescence due to Atomic Transitions in the NIR At higher electric fields, above 8 Td, another “non-standard” EL mechanism comes into force, At higher electric fields, above 8 Td, another “non-standard” EL mechanism comes into force, namely thatthat ofof ELEL in in the the NIR NIR due due to to transitions transitions between between excited excited atomic atomic states states [4,7 [4,7,10,11,15,23].,10,11,15,23]. Here Here the namely that of EL in the NIR5 1due to transitions between excited atomic states [4,7,10,11,15,23]. Here higher atomic levels Ar 3p ∗4p are excited; the transitions between those and the lower excited levels the higher atomic levels∗ 𝐴𝑟 ∗(3𝑝4𝑝) are are excited;excited; thethe transitionstransitions betweenbetween thosethose andand thethe lowerlower excitedexcited are responsible for the fast emission in the NIR, at 700–850 nm (see Figure3 and reactions (3) in Table1): levels are responsible for the fast emission in the NIR, at 700–850 nm (see Figure 3 and reactions (3) in Table 1):  5 1 e− + Ar e− + Ar∗ 3p 4p (8) → ∗ 𝑒 + 𝐴𝑟→𝑒 + 𝐴𝑟 ∗(3𝑝4𝑝) (8)  5 1  5 1 Ar∗∗ 3p4p Ar∗∗ 3p 4s + hν (9) 𝐴𝑟 ∗(3𝑝4𝑝)→→ 𝐴𝑟 ∗(3𝑝4𝑠) +ℎ𝜈 (9)

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Atomic EL in the NIR has a line emission spectrum: see Figure 4. In addition, it has higher thresholdAtomic in electric EL in the field NIR than has ordinary a line emissionEL (see Figure spectrum: 12). The see absolute Figure4 yield. In addition,of atomic itEL has in the higher NIR thresholdwas measured in electric in [15,24]; field than it was ordinary well described EL (see Figure by the 12 theory). The absoluteusing microscopic yield of atomic approach EL in [23]: the NIR see wasFigure measured 12. It should in [15 be,24 remarked]; it was well that described atomic emissi by theon theoryin the usingNIR is microscopicparticularly approachnoticeable [ 23at]: even see Figurehigher 12fields,. It should above be 30 remarked Td, where that the atomic avalanche emission multiplication in the NIR is particularlyof the electrons noticeable takes at place, even higheraccompanied fields, aboveby corresponding 30 Td, where the secondary avalanche scintillations: multiplication by of theso-called electrons “avalanche takes place, scintillations” accompanied by[11,25]. corresponding secondary scintillations: by so-called “avalanche scintillations” [11,25].

Figure 12. Reduced ELEL yieldyield inin gaseous gaseous Ar Ar as as a a function function of of the the reduced reduced electric electric field, field, for for atomic atomic EL inEL the in NIRthe NIR due due to atomic to atomic transitions transitions going going via Ar via*(3p Ar54p*(3p1)5 excited4p1) excited states, states measured , measured in [15] in at [15] 163 Kat (data 163 K points) (data andpoints) theoretically and theoretically calculated calculated in [23] (hatched in [23] area (hat betweenched area two curves)between (from two [23curves)], with (from permission [23], fromwith Elsevier).permission For from comparison, Elsevier). that For for comparison, ordinary EL that in thefor VUVordinary going EL via in Ar the*(3p VUV54s1 )going (solid via curve) Ar* and(3p54s the1) number(solid curve) of secondary and the electrons number due of tosecondary electron avalanchingelectrons due in 2to mm electron gap (dashed-dot avalanching curve, in right2 mm scale), gap theoretically(dashed-dot curve, calculated right in scale), [23], aretheoretically shown. calculated in [23], are shown. 3.5. Concepts of Light Signal Amplification 3.5. Concepts of Light Signal Amplification Ordinary EL in Ar and Xe results in two “standard” concepts of light signal amplification in Ordinary EL in Ar and Xe results in two “standard” concepts of light signal amplification in two-phase detectors, depicted in Figure1. In the first concept used in Ar, the EL gap is optically two-phase detectors, depicted in Figure 1. In the first concept used in Ar, the EL gap is optically readout by the SiPM or PMT matrix indirectly, via a wavelength shifter (WLS), typically TPB. This is readout by the SiPM or PMT matrix indirectly, via a wavelength shifter (WLS), typically TPB. This is because the typical cryogenic PMTs and SiPMs are not sensitive to ordinary EL of Ar, at 128 nm. because the typical cryogenic PMTs and SiPMs are not sensitive to ordinary EL of Ar, at 128 nm. In the second concept used in Xe, the EL gap is optically read out directly by the PMT matrix, In the second concept used in Xe, the EL gap is optically read out directly by the PMT matrix, provided that the typical cryogenic PMTs has quartz windows and thus become sensitive to ordinary provided that the typical cryogenic PMTs has quartz windows and thus become sensitive to ordinary EL of Xe, at 175 nm. EL of Xe, at 175 nm. The concept of direct optical readout (Figure1 (right)), used for Xe, can also apply to Ar to detect The concept of direct optical readout (Figure 1 (right)), used for Xe, can also apply to Ar to detect NBrS EL in the non-VUV range. Such an alternative readout concept has the advantage of doing NBrS EL in the non-VUV range. Such an alternative readout concept has the advantage of doing without WLS, in two-phase Ar detectors. This may lead to more stable operation, due to avoiding without WLS, in two-phase Ar detectors. This may lead to more stable operation, due to avoiding the the problems of WLS degradation and its dissolving in liquid [26] as well as that of WLS peeling off problems of WLS degradation and its dissolving in liquid [26] as well as that of WLS peeling off from from the substrate. Such a concept was realized in [12]. It might be considered as backup solutions for the substrate. Such a concept was realized in [12]. It might be considered as backup solutions for large-scale two-phase Ar detectors, in case the problem of WLS instability cannot be resolved. large-scale two-phase Ar detectors, in case the problem of WLS instability cannot be resolved. 4. Charge Signal Amplification in the Gas Phase of Two-Phase Detector, Using Electron Avalanching 4. Charge Signal Amplification in the Gas Phase of Two-Phase Detector, Using Electron 4.1.Avalanching Charge Signal Amplification Concepts at Cryogenic Temperatures

4.1. ChargeOver theSignal past Amplification two decades, Co therencepts has at beenCryogenic a growing Temperatures interest in so-called “Cryogenic Avalanche Detectors”: see review [4]. In the wide sense these define a class of noble-gas detectors operated at cryogenicOver temperaturesthe past two decades, with electron there avalanchinghas been a grow (electroning interest multiplication) in so-called performed “Cryogenic directly Avalanche in the detectionDetectors”: medium, see review the latter[4]. In being the wide in a gaseous, sense these liquid define or two-phase a class of state, noble-gas provided detectors by electron operated impact at ionizationcryogenic temperatures reaction: with electron avalanching (electron multiplication) performed directly in the detection medium, the latter being in a gaseous, liquid or+ two-phase state, provided by electron e− + Ar 2e− + Ar (10) impact ionization reaction: →

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Earlier attempts to obtain high and stable electron avalanching directly in noble gases and liquids at cryogenic temperatures, using “open-geometry” multipliers, have not been very successful: rather low gains ( 10) were observed in liquid Xe [27–30] and Ar [31] and low gains ( 100) in gaseous ≤ ≤ Ar [32] and He [33] at cryogenic temperatures, using wire, needle or micro-strip proportional counters. Moreover, two-phase detectors with wire chamber readout, which initially seemed to solve the problem, turned out to have unstable operation in the avalanche mode due to vapor condensation on wire electrodes [34]. The problem of electron avalanching in cryogenic noble-gas detectors was solved in 2003 [35] after introduction of cryogenic gaseous and two-phase detectors with gas electron multiplier (GEM) readout. GEM [36] and thick GEM (THGEM, [37]) belongs to the class of micro-pattern gas detectors (MPGDs). GEM is a thin insulating film, metal clad on both sides, perforated by a matrix of micro-holes, in which gas amplification occurs under the voltage applied across the film. The typical GEM geometrical parameters are the following: dielectric (Kapton) thickness is 50 µm, hole pitch—140 µm, hole diameter on metal—70 µm. THGEM is a similar, though more robust structure with about ten-fold expanded dimensions. In particular, the typical THGEM geometrical parameters are the following: t/p/d/h = 0.4/0.9/0.5/0.1 mm. Here t/p/d/h denotes “dielectric thickness/hole pitch/hole diameter/hole rim width”. Contrary to wire chambers and other “open geometry” gaseous multipliers, cascaded GEMs and THGEMs have a unique ability to operate in dense noble gases at high gains [38], including at cryogenic temperatures and in the two-phase mode [39]. Consequently, at present, the basic idea of cryogenic avalanche detectors (CRADs) in the narrow sense is that of the combination of MPGDs with cryogenic noble-gas detectors, operated in a gaseous, liquid or two-phase mode. The original CRAD concept [35] is the following (see Figure2): electron avalanching at cryogenic temperatures is performed in pure noble gases using GEM multiplier, either in a gaseous or two-phase mode. In the latter case the conventional two-phase electron-emission detector is provided with charge signal amplification using electron avalanching in the gas phase: the primary ionization electrons produced in the liquid are emitted into the gas phase by an electric field, where they are multiplied in saturated vapor using cascaded GEM multiplier. The proof of principle of this concept was demonstrated in 2003–2006 in two-phase Ar, Kr and Xe [35,40] at appropriate cryogenic temperatures (at around 87 K, 120 K and 165 K, respectively) and in gaseous He and Ne at lower temperatures, down to 2.6 K [41,42]. Later, the original CRAD concept was elaborated: it was suggested to provide CRADs with new features. There are more than a dozen different concepts with charge and light amplification in two-phase and liquid detectors developed by different groups since 2003: their gallery by 2012 is shown in Figure 13. For their detailed description, we refer to review [4] and appropriate references listed in the figure: [15,25,35,40,43–54]. The references to such concepts and their realization after 2012 are as follows: [24,55–62]. In Sections4–6, we confine ourselves to the discussion of the most developed and successfully realized concepts, as well as of the physical effects and specific technology underlying them. Instruments 2020, 4, 16 12 of 31

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FigureFigure 13. 13.Gallery Gallery of of concepts concepts ofof chargecharge signalsignal amplificationamplification in in two-phase two-phase detectors, detectors, using using electron electron avalanchingavalanching in in the the gas gas phase phase and and charge charge oror opticaloptical readout,readout, by 2012 in in order order of of introduction. introduction.

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4.2. GEM Operation in Pure Noble Gases at Cryogenic Temperatures InIn mostmost CRADCRAD concepts,concepts, thethe MPGDMPGD multipliermultiplier shouldshould be able toto operateoperate atat highhigh gainsgains inin densedense purepure noblenoble gases,gases, particularlyparticularly inin saturatedsaturated vaporvapor andand atat cryogeniccryogenic temperatures. Fortunately, unlike open-geometry gaseous multipliers (e.g., wire chambers), cascaded GEMs and THGEMsTHGEMs permitpermit 3 attaining high ( 103 ) charge gains in “pure” noble gases at atmospheric pressure, presumably due attaining high (≥10 ) charge gains in “pure” noble gases at atmospheric pressure, presumably due to toconsiderably considerably reduced reduced photon-feedback photon-feedback effects. effects. At room At room temperature, temperature, this thisremarkable remarkable property property was wasdiscovered discovered first firstfor Ar for [38,63] Ar [38 and,63] andthen then for other for other noble noble gases gases [4,39,64,65], [4,39,64 ,using65], using GEM GEM multipliers. multipliers. For Foroperation operation of other of other MPGD MPGD multipliers multipliers in “pure” in “pure” noble noblegases gasesat room at temperature, room temperature, we refer we to referreview to review[4]. [4]. ItIt should be be remarked remarked that that the the stable stable operatio operationn of GEMs of GEMs at cryogenic at cryogenic temperatures temperatures is a non- is a non-trivialtrivial fact, since fact, sincethe electrical the electrical resistance resistance of their of dielectric their dielectric materials materials considerably considerably increases increases with the withtemperature the temperature decrease, decrease, i.e., by an i.e., order by an oforder magnit ofude magnitude per 35 degrees per 35 degrees for Kapton for Kapton GEMs [66], GEMs which [66], whichmight result might in result strong in strongcharging-up charging-up effects within effects the within holes. the Fortunately, holes. Fortunately, the latter the did latter not happen: did not happen:the charging-up the charging-up effects for eff ectsGEMs for GEMshave not have been not beenobserved. observed. The TheGEM GEM performance performance in gases in gases at atcryogenic cryogenic temperatures temperatures was was found found to be to generally be generally independent independent of temperature of temperature down down to ~100 to ~100 K, at K, a atgiven a given gas gasdensity density [66]: [66 stable]: stable and and high-gain high-gain GEM GEM op operationeration was was observed observed in in all all noble noble gases gases and in theirtheir mixturesmixtures withwith selectedselected molecularmolecular additivesadditives thatthat dodo not freeze in a wide temperature range (CH44, N22 and H 2).). This This is is seen seen from from Figure Figure 1414 (left):(left): rather rather high high triple-GEM triple-GEM gains gains were were reached at cryogenic 4 temperatures,temperatures, exceedingexceeding 10104 in Ar, KrKr andand XeXe+CH+CH4..

Figure 14. Gain characteristics of GEM multipliers atat cryogeniccryogenic temperaturestemperatures [[4].4]. Left:Left: in gaseous He,

ArAr andand Kr,Kr, inin thethe PenningPenning mixturemixture NeNe+0.1%H+0.1%H22 (its densitydensity correspondingcorresponding to saturatedsaturated NeNe vaporvapor inin thethe two-phasetwo-phase mode)mode) andand inin XeXe+2%CH+2%CH4.. Right:Right: inin gaseous gaseous He He at at low low temperatures temperatures (down (down to to 4.2 4.2 K). K). TheThe appropriateappropriate temperatures temperatures and and densities densities are are indicated. indicated. In HeIn He at 39 at K 39 and K 4.2and K, 4.2 the K, maximum the maximum gains weregains limitedwere limited by discharges, by discharges, while atwhile other at temperatures other temperatures and in otherand in gases other the gases discharge the discharge limit was limit not reached.was not reached. The multiplier The multiplier active area active was area 2.8 was2.8 2.8 cm 2×. 2.8 cm2. × ItIt isis amazingamazing thatthat GEMsGEMs werewere ableable toto operateoperate inin electronelectron avalanchingavalanching modemode atat eveneven lowerlower temperatures,temperatures, downdown toto 2.62.6 KK inin gaseousgaseous HeHe [[41,42].41,42]. On the otherother hand,hand, highhigh GEMGEM gainsgains observedobserved inin 5 He and Ne above 77 K were reported to be due to the Penning effect in uncontrolled ( 10 −5) impurities He and Ne above 77 K were reported to be due to the Penning effect in uncontrolled ≥(≥10− ) impurities (i.e.,(i.e., NN2)2) which which froze froze out atout lower at lower temperatures, temperatures, resulting resu in thelting considerable in the considerable gain drop at gain temperatures drop at belowtemperatures 40 K: see below Figure 40 14 K: (right). see Figure A solution 14 (right). to the A gain-drop solution problemto the gain-drop at lower temperaturesproblem at lower was foundtemperatures in [41]: Newas and found He can in form[41]: high-gain Ne and PenningHe can mixturesform high-gain with H2 Penningat temperatures mixtures down with to ~10H2 K.at 4 Thistemperatures is seen from down Figure to ~10 14 (left):K. This triple-GEM is seen from gains Figure exceeding 14 (left): 10 triple-GEMwere obtained gains at 57exceeding K in the 10 Penning4 were mixtureobtained Ne at+ 570.1%H K in2 the, its Penning density correspondingmixture Ne+0.1%H to that2, its of saturateddensity corresponding Ne vapor in the to two-phasethat of saturated mode. Unfortunately,Ne vapor in the this two-phase does not mode. work for Unfortunately, two-phase He, th dueis does to the not very work low for H two-phase2 vapor pressure He, due at 4.2to the K. very low H2 vapor pressure at 4.2 K.

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4.3. Two-Phase Two-Phase Detectors with GEM Multipliers Regarding GEM operation in two-phase detectors, most most promising results results were obtained for two-phasetwo-phase Ar Ar detectors (see Figure 15):15): the the char chargege gain gain of of 5000 5000 with triple-GEM readout was routinelyroutinely attained attained [40] [40] and and the the stable stable operation operation for for tens tens of of hours hours at at this this gain gain was was demonstrated demonstrated [67]. [67]. This maximum gaingain value value should should be be compared compared to thatto that of 600of 600 and and 200 obtained200 obtained in GEM-based in GEM-based two-phase two- phaseKr and Kr Xe and detectors Xe detectors respectively: respectively: see Figure see Figure 15. 15. InIn this review the the charge gain gain is is defined defined as as th thee ratio of the output anode charge of of the GEM or THGEM multipliermultiplier incorporatedincorporated into into the the two-phase two-phase detector detector to to the the input input “primary” “primary” charge, charge, i.e., toi.e., that to thatof prior of prior to multiplication to multiplication measured measured in special in special calibration calibration runs. runs.

Figure 15. GainGain characteristicscharacteristics in in two-phase two-phase Ar, Ar, Kr andKr an Xed detectors Xe detectors with triple-GEMwith triple-GEM multiplier multiplier charge chargereadout readout [4,40]. The[4,40]. appropriate The appropriate temperatures, temperatures, pressures pressures and electric and fields electric in thefields liquids in the are liquids indicated. are indicated.The multiplier The multiplier active area active was2.8 area was2.8 cm 2.82 ;× the 2.8 maximumcm2; the maximum gains were gains limited were bylimited discharges. by discharges. × The successfulsuccessful performance performance of two-phaseof two-phase Ar detectors Ar detectors with triple-GEM with triple-GEM charge readout charge isreadout illustrated is illustratedin Figures 16in Figuresand 17. 16 In particular,and 17. In particular, the high triple-GEM the high triple-GEM gain provided gain the provided operation the ofoperation the two-phase of the two-phasedetector in detector single-electron in single-electron counting mode counting (Figure mode 16)[ (Figure68]. It should16) [68]. be It remarkedshould be that remarked though that the thoughsingle-electron the single-electron spectra are spectra well separated are well separated from electronic from electronic noise at gains noise exceeding at gains exceeding 5000, they 5000, are theydescribed are described by an exponential by an exponential function (rather function than (r byather a peaked than function).by a peaked The function). latter is generally The latter a rule is generallyfor gaseous a multipliersrule for gaseous operated multipliers in proportional operated mode, duein proportional to intrinsically mode, considerable due to fluctuations intrinsically of considerablethe avalanche fluctuations size. Consequently, of the avalanche the single- size. and Consequently, double-electron the eventssingle- canand hardly double-electron be distinguished events canin two-phase hardly be Ardistinguished detectors with in two-phase GEM (or THGEM)Ar detectors charge with readout. GEM (or For THGEM) that, the charge combination readout. with For that,PMT- the or SiPM-basedcombination optical with readoutPMT- or should SiPM-based be used, op astical suggested readout in should [15,24]. be Accordingly, used, as suggested the function in [15,24].of GEM- Accordingly, or THGEM-based the function charge readoutof GEM- would or THGEM-based be to provide charge superior readout spatial would resolution. be to provide superiorFigure spatial 17 demonstrates resolution. the unique ability of the two-phase Ar detector with GEM charge readout to observeFigure directly17 demonstrates and simultaneously the unique the ability fast and of slowthe two-phase components Ar ofdetector electron with emission GEM through charge readoutthe liquid-gas to observe interface directly [69 ].and The simultaneously fast component the is fast due and to theslow hot co electrons,mponents heatedof electron by an emission electric throughfield, while the theliquid-gas slow component interface [69]. is due The to thefast electronscomponent reflected is due fromto the the hot potential electrons, barrier heated and by then an electricthermalized field, [ 4while]. Note the that slow the component two-phase Ar is detectordue to the with electrons optical readoutreflected of from the EL the gap potential cannot providebarrier andsuch then an ability, thermalized due interference [4]. Note that with the the two-phase slow component Ar detector of excimer with optical (VUV) readout emission. of Itthe should EL gap be cannotalso emphasized provide such that an the ability, slow due component interference has never with the been slow observed component in two-phase of excimer Kr (VUV) and Xe emission. systems, Itpresumably should be also due emphasized to higher potential that the barrierslow comp at theonent interface has never as compared been observed to Ar. in Vice two-phase versa, theKr and fast Xecomponent systems, presumably has never been due observed to higher inpotential two-phase barri Heer at and the Ne interface systems as sincecompared the electrons to Ar. Vice in versa, liquid theHe andfast component Ne are localized has never in the been bubbles observed and thus in tw cannoto-phase be heatedHe and by Ne the systems electric since field. the Consequently, electrons in liquidthe two-phase He and ArNe system are localized is unique in in the this bubbles sense. and thus cannot be heated by the electric field. Consequently, the two-phase Ar system is unique in this sense.

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Figure 16. Pulse-height spectra in in two-phase two-phase Ar Ar detector detector withwith triple-GEMtriple-GEM multipliermultiplier chargecharge readout,readout, operated inin singlesingle electron electron counting counting mode mode with with external external trigger, trigger, at charge at charge gains gains of 6000 of and6000 17,000 and 17,000 [4,68]. [4,68]. The electronic noise spectrum (dashed line), corresponding to the equivalent noise charge of σ The[4,68]. electronic The electronic noise spectrum noise spectrum (dashed (dashed line), corresponding line), corresponding to the equivalent to the equi noisevalent charge noise of chargeσ = 940 of e −σ, − 2 =is 940 also e shown.−,, isis alsoalso The shown.shown. multiplier TheThe multipliermultiplier active area activeactive was areaarea 2.8 waswas2.8 cm2.82.82 .×× The2.82.8 cmcm results2.. TheThe were resultsresults obtained werewere obtained forobtained specially forfor × speciallyselected GEMselected foils GEM resistant foils toresistant discharges, to discharges, to reach the toto highestreachreach thethe gain. highesthighest Note gain.gain. that theNoteNote stable thatthat maximum thethe stablestable maximumgain of typical gain triple-GEM of typical triple-GEM multipliers multipliers in two-phase in two-phasetwo-phase Ar was somewhat ArAr waswas somewhatsomewhat lower: of aboutlower:lower:5000. ofof aboutabout 5000.5000.

Figure 17. TypicalTypical anode signals in two-ph two-phasease Ar detector with triple-GEM multiplier charge readout at emission electric field field in the liquidliquid ofof 1.71.7 kVkV/c/cmm [[4,69].4,69]. The fast and slowslow componentscomponents of electron emission through liquid-gasliquid-gas interfaceinterfaceterface areare distinctlydistinctly seen.seen.seen.

It shouldshould be be remarked remarked that that all theall the gain gain data data of this of section this section were obtained were obtained with GEMs with with GEMs relatively with It should be remarked that2 all the gain data of this section were obtained with GEMs with small active area, of 2.8 2.8 cm . Unfortunately,2 as we will see later, the maximum gain decreases relatively small active area,× of 2.8 × 2.8 cm2.. Unfortunately,Unfortunately, asas wewe willwill see later, the maximum gain with the multiplier active area. decreases with the multiplier active area. 4.4. Two-Phase Detectors with THGEM Multipliers 4.4. Two-Phase Detectors with THGEM Multipliers In two-phase detectors with THGEM charge readout, the maximum gains are comparable to those In two-phase detectors with THGEM charge readout, the maximum gains are comparable to with GEM readout: see Figures 18 and 19 (left). Gains as high as 3000 [48] and 600 [70] were obtained thosethose withwith GEMGEM readout:readout: seesee FiguresFigures 1818 andand 1919 (left).(left). GainsGains asas highhigh asas 30003000 [48][48] andand 600600 [70][70] werewere obtainedin two-phase in two-phase detectors indetectors Ar and in Xe Ar respectively, and Xe respectively, with double-THGEM with double-THGEM multipliers multipliers having active having area obtained in two-phase2 detectors in Ar and Xe respectively, with double-THGEM multipliers having of 2.5 2.5 cm . 2 active ×area of 2.5 × 2.5 cm2..

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FigureFigure 18. 18. GainGain characteristics characteristics of of double-THGEM double-THGEM multipliersmultipliers inin two-phasetwo-phase XeXe detector detector for for THGEM THGEM 2 activeactive area area of ofof 2.5 2.5 ×× 2.52.5 cm cm2 [4,70].[[4,70].4,70]. GainGain Gain characteristicscharacteristics characteristics ofof of triple-GEMtriple-GEM triple-GEM [40][40] [40 ] andand single-GEM single-GEM [71] [[71]71] × multipliersmultipliers (of (of similar similar active active area) area) are are shown shown for for comparison. comparison. HereHere thethe maximummaximum gainsgains werewere limitedlimited byby discharges. discharges.

HoweverHowever for for larger larger active active area, area, of of 10 × 1010 cm cm22, ,the the maximum maximum gain gain in in two-phase two-phase Ar Ar detector detector However for larger active area, of 10 ×× 10 cm2, the maximum gain in two-phase Ar detector decreaseddecreased three-fold:three-fold: three-fold: downdown down toto about toabout about 10001000 1000 forfor doubdoub for double-THGEMle-THGEMle-THGEM multipliermultiplier multiplier [70][70] andand [70 100–200100–200] and 100–200 forfor single-single- for THGEMsingle-THGEMTHGEM multipliermultiplier multiplier [72,73]:[72,73]: [72 seesee,73 ]:FiguresFigures see Figure 1919 (left) (left)19 (left) andand and 2020 Figure(right).(right). 20 ThisThis (right). couldcould This resultresult could fromfrom result thethe from largerlarger the numberlargernumber number ofof holes,holes, of implyingimplying holes, implying aa largerlarger a dischargedischarge larger discharge probabilityprobability probability onon defects.defects. on defects. NoteNote thatthat Note thethe thatnominalnominal the nominal electricelectric fieldelectricfield inin Figure fieldFigure in 20,20, Figure defineddefined 20, defined asas thethe THGEMTHGEM as the THGEM voltagevoltage voltage dividividedded divided byby itsits dielectric bydielectric its dielectric thickness,thickness, thickness, isis higherhigher is higher thanthan thethanthe realreal the electricelectric real electric fieldfield in fieldin thethe in THGEMTHGEM the THGEM hole.hole. hole.

Figure 19. Left: gain characteristic of double-THGEM multipliers in a two-phase Ar detector for a Figure 19. Left: gaingain characteristic characteristic of of double-THGEM double-THGEM multipliers multipliers in in a a two-phase Ar detector for a THGEM active area of 10 × 10 cm2 (maximum gain was limited by discharges) compared to that of 2.5 THGEM active area area of of 10 10 × 1010 cm cm2 (maximum2 (maximum gain gain was was limited limited by bydisch discharges)arges) compared compared to that to that of 2.5 of × 2.5 cm2 (maximum gain was× not reached) [72]; a comparison to that of a single-THGEM multiplier 2.5× 2.5 cm2.52 cm(maximum2 (maximum gain gain was was not not reached) reached) [72]; [72 a]; comparison a comparison to to that that of of a a single-THGEM single-THGEM multiplier multiplier × (maximum(maximum gain gaingain was waswas limited limitedlimited by byby discharges) discharges)discharges) is is also also sh shown.shown.own. Right: Right: gain gaingain characteristics characteristics characteristics of of hybrid hybrid 3-stage 3-stage 2 2THGEM/GEM2THGEM2THGEM/GEM/GEM multipliermultiplier multiplier inin in two-phasetwo-phase two-phase ArAr Ar detector detector detector forfor for activeactive active areaarea area ofof of 1010 10 ×× 10 10 cm10cm cm2 [4,72].[4,72].2 [4,72 ShownShown]. Shown isis thethe is × overalltheoverall overall multipliermultiplier multiplier gaingain gain asas aa as functionfunction a function ofof the ofthe the voltagvoltag voltageee acrossacross across GEM,GEM, GEM, atat at twotwo two fixedfixed fixed voltagesvoltages voltages across across each each THGEM,THGEM, i.e., i.e., at at two two values values of of 2THGEM 2THGEM gain gain indi indicatedindicatedcated in in the the figure. figure.figure. HereHere the the maximum maximum gains gains were were limitedlimited by by discharges. discharges.

TheThe maximummaximum gains gainsgains of ofof 100–1000 100–1000100–1000 are areare obviously obviouslyobviously not not sunotffi sufficientcientsufficient for charge forfor chargecharge readout readoutreadout in a single-electron inin aa single-single- electroncountingelectron countingcounting mode. mode.mode. Nevertheless, Nevertheless,Nevertheless, the gain thethe gaingain of about ofof aboutabout 80, 80,80, obtained obtainedobtained in inin two-phase two-phasetwo-phase Ar ArAr detectordetector with with single-THGEMsingle-THGEM having having 2D 2D readout readout [73], [[73],73], permitted permitted to to obtain obtain track track images, images, demonstrating demonstrating the the excellent excellent imagingimaging capability capability of of two-phase two-phase Ar Ar detectors detectors with with THGEMTHGEM multipliermultiplier chargecharge readout readout even even at at such such a a moderatemoderate gain.gain. ThisThis techniquetechnique isis ofof particularparticular importanceimportance forfor giantgiant liquidliquid ArAr detectorsdetectors forfor long-long- baselinebaseline neutrinoneutrino experiments.experiments. AtAt present,present, thethe R&DR&D ofof thisthis techniquetechnique hashas continuedcontinued asas partpart ofof thethe

Instruments 2020, 4, 16 17 of 31 moderate gain. This technique is of particular importance for giant liquid Ar detectors for long-baseline neutrinoInstruments experiments. 2020, 4, x FOR At PEER present, REVIEW the R&D of this technique has continued as part of the ArDM17 of 31 and DUNEArDM experiments and DUNE [74 experiments–76]. It should [74–76]. be remarked It should thatbe remarked in that communitythat in that anothercommunity terminology another is used,terminology namely “dual-phase is used, namely TPC” “dual-phase and “Large TPC” Electron and “Large Multiplier” Electron (LEM). Multiplier” (LEM).

FigureFigure 20. Left: 20. Left:electric electric field field configuration configuration in two-phasein two-phase detector detector with with THGEM THGEM (LEM) (LEM) 2D 2D readout readout [4 ,73]. [4,73]. Right: “Effective” gain characteristic for single-THGEM multiplier charge 2D readout in two- Right: “Effective” gain characteristic for single-THGEM multiplier charge 2D readout in two-phase Ar phase Ar detector, of an active area of 10 × 10 cm2 [4,73], as a function of the nominal electric field in detector, of an active area of 10 10 cm2 [4,73], as a function of the nominal electric field in THGEM. THGEM. The maximum gain× was limited by discharges. The “effective” gain value should be The maximummultiplied by gain about was a limitedfactor of by3, to discharges. be normalized The to “e theffective” gain definition gain value of the should present be review multiplied (see by aboutdetails a factor in [4]). of 3, to be normalized to the gain definition of the present review (see details in [4]).

The furtherThe further reduction reduction of the of the maximum maximum charge charge gain gain in in two-phase two-phaseAr Ar detectordetector when increasing increasing the 2 activethe area active up area to 40 up to40 40 and × 40 50and 5050 × cm50 cm, was2, was reported reported inin [74] [74] and and [76] [76 respectively.] respectively. For Forthe latter, the latter, × × the THGEMthe THGEM multiplier multiplier could could not operatenot operate at nominal at nominal electric electric fields fields higher higher than than 28 28 kV kV/cm,/cm, i.e., i.e., at chargeat gain highercharge gain than higher 3 according than 3 according to Figure to 20 Figure (right). 20 (right). This is This the is real the problem;real problem; it is it discussedis discussed below. below. An interestingAn interesting way toway increase to increase the maximum the maximum charge charge gain ingain two-phase in two-phase Ar CRADs Ar CRADs was suggested was in [72suggested], namely thatin [72], using namely a hybrid that using 3-stage a hybrid 2THGEM 3-stage/GEM 2THGEM/GEM multiplier: multiplier: see Figure see19 (right).Figure 19 The (right). idea was The idea was that the avalanche charge from a THGEM hole would be shared between several GEM that the avalanche charge from a THGEM hole would be shared between several GEM holes. This might holes. This might reduce the overall avalanche charge density, believed to be responsible for the reduce the overall avalanche charge density, believed to be responsible for the discharge development, discharge development, and thus might increase the discharge voltage. The maximum gain as high and thusas 5000 might was increaseattained for the 10 discharge × 10 cm2 voltage.active area, The in maximumpractical configuration gain as high when as 5000the last was multiplier attained for 10 10 cm2 active area, in practical configuration when the last multiplier stage was followed by a × stage was followed by a printed circuit board (PCB) anode, convenient for readout electronics printedcoupling. circuit boardThis is (PCB)almost anode, enough convenient for stable foroper readoutation in electronics single-electron coupling. counting This mode is almost with enough2D for stablereadout, operation for which in single-electron charge gains of counting 104 are needed. mode with It should 2D readout, be noted for that which the chargeidea of gainshybrid of 104 are needed.multiplier, It shouldcombining be hole noted multipliers that the with idea different of hybrid hole multiplier, densities, can combining be realized hole without multipliers “fragile” with differentGEMs: hole by densities, cascading canmore be robust realized THGEMs without of “fragile”different geometry, GEMs: by with cascading sequential more increasing robust THGEMshole of diffdensitieserent geometry, and decreasing with sequential thicknesses. increasing hole densities and decreasing thicknesses.

4.5. Gain4.5. Gain Limit, Limit, Gain Gain Stability Stability and and Discharge-Resistance Discharge-Resistance Problems in in Two-Phas Two-Phasee Detectors Detectors with with GEM GEM and and THGEMTHGEM Multipliers Multipliers Several problems of the performance of two-phase Ar detectors with GEM and THGEM charge Several problems of the performance of two-phase Ar detectors with GEM and THGEM charge readout remain unsolved. The first problem is that of the gain limit due to discharges. The obvious readout remain unsolved. The first problem is that of the gain limit due to discharges. The obvious way way to increase the gain would be adding an additional multiplication stage, i.e., up to overall 4 to increasestages thein the gain case would of GEMs be adding and 3 stages an additional in the ca multiplicationse of THGEMs. stage,Additionally, i.e., up toit looks overall attractive 4 stages to in the case ofcombine GEMs GEMs and 3 and stages THGEMs in the in case a hybrid of THGEMs. multiplier Additionally, such as that discussed it looks attractiveabove. Another to combine way is an GEMs and THGEMsoptical readout in a hybrid from THGEMs. multiplier Indeed, such the as thatTHGEM discussed charge above.gain of the Another order of way 100 isis anstill optical significant readout fromif THGEMs. combined Indeed,with optical the THGEMreadout using charge SiPM gain matrices of the order[55] orof CCD 100 cameras is still significant [60], and might if combined be with optical readout using SiPM matrices [55] or CCD cameras [60], and might be sufficient for track imaging and even for single-electron counting mode. These will be discussed in the following section. Instruments 2020, 4, 16 18 of 31

The second problem is that of the resistance to electrical discharges of “standard” (thin Kapton) GEMs. It was observed [77] that when operating two-phase Ar detectors with triple-GEM multiplier at maximum gains (approaching 104), the triple-GEM was not able to withstand electrical discharges on a long term: after several series of measurements, the maximum reachable gain decreased by Instruments 2020, 4, x FOR PEER REVIEW 18 of 31 several times. Obviously, this is due the low resistance of thin GEMs to discharges because of metal and/or carbonsufficient evaporation for track imaging and their and depositioneven for single-electron on the insulator counting inmode. the These GEM will holes. be discussed The solution in of the problem mightthe following be switching section. to thicker THGEMs that were found to behave in more reliable way under The second problem is that of the resistance to electrical discharges of “standard” (thin Kapton) discharges,GEMs. or else It was combining observed [77] THGEM that when and operating GEM tw multiplierso-phase Ar detectors or THGEM with triple-GEM multipliers multiplier with increasing hole density,at maximum as discussed gains (approaching above. 104), the triple-GEM was not able to withstand electrical discharges Thereon is a another long term: unsolved after several problem series of in measurements, two-phase CRAD the maximum performances. reachable Namely,gain decreased the performanceby of MPGD multipliersseveral times. inObviously, two-phase this Aris due and the Xe low is resistance not fully of understood: thin GEMs to discharges not all multiplier because oftypes metal were able and/or carbon evaporation and their deposition on the insulator in the GEM holes. The solution of to operatethe with problem electron might multiplication be switching to thicker in saturated THGEMs vapor. that were In found two-phase to behave Ar, in while more reliable G10-based way THGEM multipliersunder successfully discharges, operated or else combining for tens ofTHGEM hours and with GEM gains multipliers reaching or aTHGEM few thousands, multipliers others,with namely RETHGEMincreasing and Kapton hole density, THGEMs, as discussed didnot above. show stable multiplication in the two-phase mode [4,72]: they either didThere not multiplyis another at unsolved all (with problem gain below in two-phase 1) or showed CRAD unstable performances. operation Namely, due the to large gain performance of MPGD multipliers in two-phase Ar and Xe is not fully understood: not all multiplier variations.types Inaddition, were able to in operate [78] it with was electron reported multipli oncation the unsuccessful in saturated vapor. performance In two-phase of Ar, the while Micromegas multiplierG10-based in two-phase THGEM Xe: multipliers in half an successfully hour the multiplicationoperated for tens collapsed. of hours with The gains most reaching rational a few explanation of these instabilitiesthousands, others, is the namely effect ofRETHGE vaporM condensation and Kapton THGEMs, within did the not THGEM show stable holes multiplication or Micromegas in mesh that preventsthe two-phase electron mode multiplication. [4,72]: they either The did criteria not multiply for such at all a (with condensation gain below 1) are or notshowed yet unstable clear. It could be operation due to large gain variations. In addition, in [78] it was reported on the unsuccessful that the specificperformance properties of the Micromegas of the holes multiplier and electrodes in two-phase might Xe: in play half aan role, hour i.e., the themultiplication wetting capability dependingcollapsed. on the electric-fieldThe most rational non-uniformity explanation of these and instabilities consequently is the on effect the of MPGD vapor condensation geometry, as well as on the temperaturewithin the THGEM gradients, holes theor Micromegas latter in turn mesh depending that prevents on electron the electrode’s multiplication. heat The conductivity.criteria for such a condensation are not yet clear. It could be that the specific properties of the holes and 4.6. THGEMelectrodes as Interface might Gridplay ina Two-Phaserole, i.e., the Detectors wetting capability depending on the electric-field non- uniformity and consequently on the MPGD geometry, as well as on the temperature gradients, the In additionlatter in turn to the depending performance on the electrode’s as charge heat multiplier, conductivity. THGEM can be used as an interface grid immersed in the liquid (see Figure1), acting as an e ffective electron transmission electrode defining the 4.6. THGEM as Interface Grid in Two-Phase Detectors electron emission and EL regions of the two-phase detector [79]. Examples of such THGEM electrodes In addition to the performance as charge multiplier, THGEM can be used as an interface grid used in practice [9,12] are shown in Figure 21, left. The feature of such a THGEM interface grid is that immersed in the liquid (see Figure 1), acting as an effective electron transmission electrode defining one must applythe electron a high emission enough and voltage EL regions across of the it totwo- havephase the detector 100% [79]. electron Examples transmittance. of such THGEM In particular, in liquid Ar,electrodes for the used THGEM in practice electrode [9,12] are with shown 28% in Figure optical 21, transparencyleft. The feature theof such nominal a THGEM electric interface field should exceed 5 kVgrid/cm is that to one provide must apply the 100%a high enough electron voltage transmission across it to have [79]: the see 100% Figure electron 21 transmittance., right. On the other In particular, in liquid Ar, for the THGEM electrode with 28% optical transparency the nominal hand, it was shown there that the THGEM with 75% optical transparency provides 100% electron electric field should exceed 5 kV/cm to provide the 100% electron transmission [79]: see Figure 21, transmissionright. at On already the other 2 hand, kV/cm it was of shown nominal there electric that the THGEM field. The with advantage 75% optical transparency of such THGEM provides electrodes compared100% to wire electron electrodes transmission is that at theyalready are 2 quitekV/cm rigid of nominal which electric avoids field. the The problem advantage of wire of such grid sagging under highTHGEM electric electrodes field in compared large-area to wire two-phase electrodes detectors.is that they are quite rigid which avoids the problem of wire grid sagging under high electric field in large-area two-phase detectors.

Figure 21. Left: microscope images of two THGEM electrodes used as interface grids in two-phase Ar Figure 21. Left: microscope images of two THGEM electrodes used as interface grids in two-phase Ar detectors in [9,12]: with 28% (a) and 75% (b) optical transparency (from [79], with permission from detectors inElsevier). [9,12]: Right: with calculated 28% (a) electron and 75% transmission (b) optical throug transparencyh 28% THGEM (from acting [ 79as], an with interface permission grid from Elsevier). Right: calculated electron transmission through 28% THGEM acting as an interface grid

immersed in liquid Ar as a function of the voltage across THGEM (bottom axis) and nominal electric field in THGEM (top axis) (from [79], with permission from Elsevier). The electric fields below and above the THGEM were 0.56 and 4.3 kV/cm respectively. Instruments 2020, 4, x FOR PEER REVIEW 19 of 31

immersed in liquid Ar as a function of the voltage across THGEM (bottom axis) and nominal electric field in THGEM (top axis) (from [79], with permission from Elsevier). The electric fields below and Instruments 2020, 4, 16 19 of 31 above the THGEM were 0.56 and 4.3 kV/cm respectively.

5.5. Combined Combined Charge/Light Charge/Light Signal AmplificationAmplification inin thethe GasGas PhasePhase of of Two-Phase Two-Phase Detector, Detector, Using AvalancheUsing Avalanche Scintillations Scintillations

5.1.5.1. Two-Phase Two-Phase Ar Detector with Co Combinedmbined THGEM/SiPM THGEM/SiPM-Matrix-Matrix Multiplier FigureFigure 2222 showsshows the the concept concept of of combined combined charge/light charge/light signal signal amplification amplification in in two-phase two-phase detector detector with EL gap, using avalancheavalanche scintillations.scintillations. In In this this concept, the combined THGEM/SiPM-matrix THGEM/SiPM-matrix multipliermultiplier isis coupledcoupled to to the the EL EL gap: gap: the THGEMthe THGEM provides provides the charge the signalcharge amplification signal amplification by operating by operatingin electron in avalanching electron avalanching mode, while mode, the SiPMwhile matrix the SiPM optically matrix records optically avalanche records scintillations avalanche scintillationsproduced in theproduced THGEM in holes the thusTHGEM providing holes thethus light providing signal amplification the light signal with amplification position resolution. with positionSince ordinary resolution. SiPMs Since produced ordinary by SiPMs industry produced are typically by industry sensitive are typically to the NIR sensitive but not to the sensitive NIR but to notthe VUV,sensitive the to avalanche the VUV, scintillations the avalanche can scintillations be recorded can in two be recorded ways: either in two directly, ways: using either avalanche directly, usingscintillations avalanche due scintillations to atomic transitions due to atomic in the transiti NIRons (see in Figures the NIR4 and (see5 ),Figures or indirectly 4 and 5), via or WLS indirectly film viain front WLS of film the in SiPM front matrix, of the using SiPM excimer matrix, avalancheusing excimer scintillations avalanche in scintillations the VUV. The in earlier the VUV. works The on earlierrealizing works this concepton realizing in two-phase this concept Ar [55 in] andtwo-phase Xe [56] detectorsAr [55] and should Xe be[56] mentioned, detectors albeitshould with be mentioned,smaller active albeit area with and smaller SiPM matrix active size. area and SiPM matrix size.

FigureFigure 22. 22. ConceptConcept of of combined combined charge/light charge/light signal signal amplification amplification in in two-phase two-phase detector detector with with EL EL gap, gap, usingusing avalanche avalanche scintillations and combined THGEM/SiPM-matrix THGEM/SiPM-matrix multiplier.

InIn the the most most elaborated elaborated way, way, the concept was real realizedized in two-phase Ar detector with combined 2 THGEM/SiPM-matrixTHGEM/SiPM-matrix multiplier of of 10 10 × 1010 cm cm2 activeactive area area in in [12], [12], using using a a5 5× 5 SiPM5 SiPM matrix matrix having having 1 × × cm1 cm pitch pitch of of SiPM SiPM elements. elements. The The amplitude amplitude and and posi positiontion resolution resolution properties properties of of the the detector detector are are illustratedillustrated in in Figure Figure 23.23. As As large large as as about about 500 500 phot photoelectronsoelectrons were were recorded recorded by by the the SiPM matrix for for 109 8888 kev gamma-rays from 109CdCd source, source, at at rather rather moderate moderate THGEM THGEM charge charge gain, of 37. This would correspondcorrespond to thethe detectordetector yield yield of of about about 1–2 1–2 photoelectrons photoelectrons per per drifting drifting electron electron inthe in the gas phasegas phase [12]. [12].Moreover, Moreover, the highest the highest position position resolution resolution was was obtained obtained in this in this detector detector than than ever ever was was measured measured for σ = √ fortwo-phase two-phase detectors detectors with ELwith gap: EL gap:26 mm σ =/ 26mm/NPE, where𝑁 N, PEwhereis the totalNPE numberis the total of photoelectrons number of photoelectronsrecorded by the recorded SiPM matrix. by the Such SiPM photoelectron matrix. Su yieldch photoelectron and positions yield resolution and positions would be resolution sufficient wouldfor applications be sufficient in neutrino for applications and dark in matter neutrino search and experiments.dark matter search experiments.

Instruments 2020, 4, 16 20 of 31 Instruments 2020, 4, x FOR PEER REVIEW 20 of 31 Instruments 2020, 4, x FOR PEER REVIEW 20 of 31

109 Figure 23. Total SiPM-matrix amplitude spectrum for gamma-rays of 109Cd source (left) and its Figure 23. TotalTotal SiPM-matrix amplitude spectrum for gamma-rays of 109CdCd source (left) (left) and its position resolution (right), in two-phase Ar detector with EL gap and combined THGEM/SiPM-matrix position resolution ((right),right), in two-phase two-phase Ar dete detectorctor with with EL EL gap gap and and combined combined THGEM/SiPM-matrix THGEM/SiPM-matrix multiplier, shown in Figure 22 [12]. The THGEM charge gain was 37. The two characteristic peaks of multiplier, shown in Figure 22[ [12].12]. The THGEM chargecharge gain was 37. The two characteristic peaks of low (22–25 keV) and high energy (60–70 and 88 keV) lines of 109Cd source on W substrate are well low (22–25 keV) and high energy (60–70 andand 8888 keV)keV) lineslines ofof 109109CdCd source on W substrate are well separated in the amplitude spectrum. The position resolution (standard deviation) is shown as a separated in the amplitude spectrum. The The position position resolution resolution (standard (standard deviation) deviation) is is shown shown as a function of the total number of photoelectrons recorded by the SiPM matrix. The curve is the fit by function of the total number of photoelectrons reco recordedrded by the SiPM matrix. The The curve curve is is the the fit fit by inverse root function. inverse root function. 5.2. Two-Phase Ar Detector with Combined THGEM/CCD-Camera Multiplier 5.2. Two-Phase Two-Phase Ar Detector with Combined THGEMTHGEM/CCD/CCD-Camera-Camera MultiplierMultiplier Figure 24 (left) shows the concept of combined charge/light signal amplification in a two-phase Figure 24 (left) shows the concept of combined charge/light signal amplification in a two-phase Ar ArFigure detector, 24 using(left) showsavalanche the scintillationsconcept of combined in a combined charge/light THGEM/CCD-camera signal amplification multiplier in a [60].two-phase It is detector, using avalanche scintillations in a combined THGEM/CCD-camera multiplier [60]. It is similar to Arsimilar detector, to thatusing considered avalanche above, scintillations with the in differe a combinednce that THGEM/CCD-camerathe SiPM matrix is replaced multiplier by the [60]. CCD It is that considered above, with the difference that the SiPM matrix is replaced by the CCD cameras. Here the similarcameras. to that Here considered the avalanche above, scintillations with the were differe recordednce that indirectly the SiPM via matrix WLS film is replacedbehind the by THGEM the CCD avalanche scintillations were recorded indirectly via WLS film behind the THGEM plate, using excimer cameras.plate, using Here excimer the avalanche avalanche scintillations scintillations were in the recorded VUV. In indirectly the most elaboratedvia WLS film way, behind the concept the THGEM was avalanche scintillations in the VUV. In the most elaborated way, the concept was realized in [61] in plate,realized using in excimer [61] in two-phase avalanche Ar scintillations detector with in 54the × VUV.54 cm 2In active the most area elaboratedand overall way,liquid the Ar concept mass of was1 two-phase Ar detector with 54 54 cm2 active area and overall liquid Ar mass of 1 t. The advanced CCD realizedt. The inadvanced [61] in two-phase CCD cameras, Ar× detector with additional with 54 ×electronic 54 cm2 active amplificat area andion ofoverall the signal liquid (an Ar electron mass of 1 t.cameras, Themultiplying advanced with additionalcharge CCD coupled cameras, electronic device with amplification (EMCCD)) additional were of electronic the used, signal whichamplificat (an electron allowedion multiplying offor the single signal photoelectron charge (an electron coupled multiplyingdevicecounting. (EMCCD)) charge were coupled used, whichdevice allowed(EMCCD)) for singlewere photoelectronused, which allowed counting. for single photoelectron counting.

FigureFigure 24. 24. Left: Left:concept concept ofof combinedcombined chargecharge/light/light signal signal amplification amplification in in two-phase two-phase detector detector [61], [61 ],

usingusing avalanche avalanche scintillations scintillations in combined in combined THGEM TH/GEM/CCD-cameraCCD-camera multiplier multiplier [60], in[60], the mostin the elaborated most 2 wayFigureelaborated realized 24. Left: way in concept [ 61realized], with of in 54 combined[61], 54with cm 542charge/light active× 54 cm area active signal and area overall amplification and overall liquid liqui Ar in masstwo-phased Ar mass of 1 t. ofdetector Right:1 t. Right: light[61], light signal amplitude on the CCD-c× ameras as a function of the nominal electric field in THGEM holes usingsignal amplitudeavalanche onscintillations the CCD-cameras in combined as a function THGEM/CCD-camera of the nominal electric multiplier field in THGEM[60], in holesthe (datamost (data points) [61]. points)elaborated [61]. way realized in [61], with 54 × 54 cm2 active area and overall liquid Ar mass of 1 t. Right: light signal amplitude on the CCD-cameras as a function of the nominal electric field in THGEM holes (data points) [61].

Instruments 2020, 4, 16 21 of 31

InstrumentsUsing 2020 such, 4, detector, x FOR PEER 3D REVIEW reconstruction of cosmic muon tracks and beamline interactions21 has of 31 been successfully performed. The detector was proposed to be employed in future large scale two-phase liquidUsing Ar neutrino such detector, experiments 3D reconstruction as an alternative of cosm readoutic muon method, tracks replacingand beamline the highlyinteractions segmented has been successfully performed. The detector was proposed to be employed in future large scale two- anode planes. phase liquid Ar neutrino experiments as an alternative readout method, replacing the highly One can see from Figure 24 (right), showing the light signal amplitude dependence on the THGEM segmented anode planes. electric field, that two operation modes were observed: that of proportional EL (linear part) and that One can see from Figure 24 (right), showing the light signal amplitude dependence on the of electron avalanching (exponential rise part). The optical readout allows to measure the THGEM THGEM electric field, that two operation modes were observed: that of proportional EL (linear part) chargeand that gain of ad electron hoc: using avalanching the dependence (exponential of the rise light part). signal The intensity optical readout on the electric allows field.to measure In particular, the atTHGEM the maximum charge fieldgain ofad 32hoc: kV using/cm, thethe avalanchedependence light of the gain, light defined signal asintensity the measured on the electric light intensity field. normalizedIn particular, to that at the of themaximum linearextrapolation field of 32 kV/cm, from the the avalanche lower electric light fields, gain, isdefined close toas 4. the This measured avalanche lightlight gain intensity corresponds normalized to larger to that charge of the gain, linear because extrapolation secondary from (avalanche) the lower electric electrons fields, produce is close less to EL light4. This per driftingavalanche electron light gain compared corresponds to initial to electron,larger charge since gain, they because drift over secondary smaller distance(avalanche) in the hole.electrons If to approximate produce less theEL light THGEM per drifting hole by electron a parallel-plate compared gap, to initial it can electron, be shown since that they the drift light over gain of 4 correspondssmaller distance to the in chargethe hole. gain If to of approximate 8. the THGEM hole by a parallel-plate gap, it can be shown that the light gain of 4 corresponds to the charge gain of 8. 6. Charge and Light Signal Amplification in the Liquid Phase 6. Charge and Light Signal Amplification in the Liquid Phase In the liquid, the excited, ground and ionized atomic states transform to the exciton, valence   In the liquid, the excited, ground and ionized atomic states transform to the exciton,Ar valencen = and2P and conduction bands, respectively (see Figure3 and [ 7]), in particular in Ar, to the ∗ 1, 3/2    ∗  conduction bands,2 respectively (see Figure 3 and3 [7]), in particular1 in Ar, to the 𝐴𝑟5 (𝑛1 = 1, 𝑃 ) and Ar∗ n = 1, P excitons, which reflect the P and P levels of the Ar∗ 3p 4s atomic/ states. ∗ 1/2 1 1 ∗ Accordingand 𝐴𝑟 (𝑛 to reaction = 1,/ 𝑃 (15)) excitons, of Table which1, the excitonsreflect the are 𝑃 immediately and 𝑃 trapped levels of in the singlet 𝐴𝑟 (3𝑝 or triplet4𝑠 ) atomic excimer   states. According1,3P + to reaction (15)1P of+ Table 1, the ex3citonsP + are immediately trapped in singlet or triplet states Ar∗ u . The singlet u and triplet u excimers provide the fast and slow emission 2 ∗ , componentsexcimer states in the𝐴𝑟 VUV(∑ for). The scintillations singlet ∑ in and liquid triplet Ar (reaction∑ excimers (16) of provide Table1 ))the with fasta and lifetime slow of 7 nsemission and 1.6 componentsµs respectively in the (seeVUV [7 for] and scintillations references in therein). liquid Ar This (reaction mechanism (16) of isTable the basic1)) with one a for μ scintillationslifetime of 7 in ns liquid and 1.6 Ar ands respectively is similar to (see that [7] of and ordinary references electroluminescence therein). This mechanism in gaseous is Ar the described basic one for scintillations in liquid Ar and is similar to that of ordinary electroluminescence in gaseous Ar in Section 3.2. As for the other scintillation mechanism considered in that section, namely that of described in section 3.2. As for the other scintillation mechanism considered in that section, namely atomic transitions in the NIR in gaseous Ar, it does not work in liquid Ar due to the disappearance of that of atomic transitions in the NIR in gaseous Ar, it does not work in liquid Ar due to the analogues of the higher excited levels. On the other hand, there are some indications that the neutral disappearance of analogues of the higher excited levels. On the other hand, there are some indications bremsstrahlung emission may still exist in liquid Ar: see Sections 6.1 and 6.2. that the neutral bremsstrahlung emission may still exist in liquid Ar: see Sections 6.1 and 6.2. 6.1. Charge and Light Signal Amplification in Liquid Xe and Liquid Ar Using wires, Strips and Needles 6.1. Charge and Light Signal Amplification in Liquid Xe and Liquid Ar Using wires, Strips and Needles The attempts to obtain high and stable charge amplification directly in noble-gas liquids, The attempts to obtain high and stable charge amplification directly in noble-gas liquids, using using “open-geometry” multipliers, have not been very successful: rather low charge gains ( 10) were “open-geometry” multipliers, have not been very successful: rather low charge gains (≤10)≤ were observedobserved in in liquid liquid Xe Xe using using wirewire proportionalproportional counters counters [27,28,30] [27,28,30 ]and and micro-strip micro-strip counter counter [29] [29 and] and in in liquidliquid Ar Ar using using needle needle counter counter [[31].31]. This is is clear clear from from Figure Figure 25 25 showing showing charge charge gain gain in inliquid liquid Xe Xefor for didifferentfferent wire wire diameters diameters as as a a function function ofof thethe applied voltage. voltage.

FigureFigure 25. 25.Charge Charge gaingain asas aa functionfunction of applied voltage voltage for for different different diameters diameters of ofthe the anode anode wire wire in in liquidliquid Xe Xe [28 [28].]. The The maximum maximum gainsgains are limited by by discharges. discharges.

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On the other hand, proportional electroluminescence (EL) observed in liquid Xe using thin wires and OnVUV-sensitive the other hand, PMTs proportional [28,30] looks electroluminescence more promising (EL) for observedapplications in liquid [62]: Xe see using Figure thin 26. wires In particular,and VUV-sensitive the maximum PMTs [28 proporti,30] looksonal more EL promising yield of forabout applications 300 phot [ons62]: per see Figuredrifting 26 .electron In particular, was obtainedthe maximum using proportional 10 μm diameter EL yield wire of[30]. about In a 300 wide photons voltag pere range, drifting proportional electron was EL obtained gain was using well 10describedµm diameter by a linear wire function, [30]. In defining a wide voltage a nominal range, EL threshold proportional at a certain EL gain electric was well field. described In particular, by a thelinearthe nominalnominal function, thresholdsthresholds defining forfor a nominal proportionalproportional EL threshold ELEL andand elecelec at atrontron certain multiplicationmultiplication electric field. werewere In particular,measuredmeasured asas the aboutabout nominal 400400 andthresholds 700 kV/cm, for proportional respectively. EL The and linear electron dependence multiplication of proportional were measured EL gain as about on the 400 electric and 700 field kV and/cm, therespectively.the existenceexistence Theofof thethe linear thresholdthreshold dependence forfor ELEL of inin proportional liquidliquid XeXe werewere EL gainconfirmedconfirmed on the alsoalso electric inin aa field needle-typeneedle-type and the existencedevicedevice [80]:[80]: of seethesee Figure thresholdFigure 27.27. for EL in liquid Xe were confirmed also in a needle-type device [80]: see Figure 27.

Figure 26. Left: Left: proportionalproportionalproportional ELEL EL gaingain gain expressedexpressed expressed inin photoelephotoele photoelectronsctronsctrons (PE)(PE) recordedrecorded by by PMT PMT as as a a function function of applied voltage for 10 μµm anode wire in liquid Xe [30]. [30]. The The curve curve is is the the linear linear fit fit to to the the data data points. points. Notice thatthat the the experimental experimental data data points points diverge dive fromrgerge fromfrom thisfit thisthis near fitfit and nearnear below andand the belowbelow nominal thethe ELnominalnominal threshold. ELEL threshold.Right:threshold.the ELRight: pulse thethe width ELEL pulsepulse at 10% widthwidth of pulse atat 10%10% maximum ofof pulsepulse as amaximaxi functionmum ofas applied a function voltage. of applied Notice voltage. that the NoticeEL pulse that width the EL decreases pulse width below decreases the nominal below EL the threshold, nominal presumably EL threshold, indicating presumably on alternative indicating ELon alternativemechanism EL below mechanism the threshold. below the threshold.

Figure 27. 27. ProportionalProportional EL EL gain gain of needle-type of needle-type device device in liquidliquid in liquid XeXe asas Xe aa functionfunction as a function ofof thethe appliedapplied of the applied voltagevoltage [80].voltage[80]. TheThe [80 electricelectric]. The fieldfield electric nearnear field thethe near tiptip ofof the needleneedle tip of variesvaries needle fromfrom varies 3030 from ×× 101033 30kV/cmkV/cm10 3totokV 22 ×/×cm 101033 to kV/cmkV/cm 2 10 overover3 kV /distancedistancecm over × × ofdistance 0.1 μm. of If 0.1 to µextrapolatem. If to extrapolate to zero, ligh to zero,tt emissionemission light emission hashas aa thresholdthreshold has a threshold atat 4444 V.V. at 44 V.

ItIt is is interestinginteresting however, however, thatthat belowbelow thethe nominalnominal ELEL thresholdthreshold thethe ELEL signalsignal waswas stillstill observedobserved inin [30]:[30]: [30]: seesee see FigureFigure Figure 2626 26(left).(left). (left). Moreover,Moreover, Moreover, thethe pulsepulse the pulsewidthwidth widthforfor suchsuch for “alternative”“alternative” such “alternative” ELEL dramaticallydramatically EL dramatically droppeddropped downdropped compared down compared to “ordinary” to “ordinary” EL (see ELFigure (see 26 Figure (right)), 26 (right)), indicating indicating that alternative that alternative EL is fast. EL is This fast. Thisunder-threshold under-threshold and fast and EL fast in liquid EL in liquidXe looks Xe very looks similarsimilar very similar toto under-thresholdunder-threshold to under-threshold andand fastfastand ELEL inin fast gaseousgaseous EL in Argaseous described Ar described in Section in Section 4.2, where 4.2, where it was it wasconvincingly convincingly explained explained by by the the neutral neutral bremsstrahlung bremsstrahlung (NBrS)(NBrS) eeffect:effect:ffect: compare compare Figure Figure 2626 (left)(left) toto FiguresFigures5 55 and and9 .9. This This similarity similarity has has lead leadlead us usus to toto a aa conclusion conclusionconclusion thatthat proportional proportional EL EL in in liquid liquid Xe Xe,Xe,, observed observed underunder thethe nominalnominal ELEL threshold,threshold, ofof aboutabout ofof of 400400 kV/cm,kV/cm, kV/cm, withwith highhigh probabilityprobability isis duedue toto thethe NBrSNBrS eeffectffect asas well.well.well. Note Note that thethe similarsimilar hypothesishypothesis has has beenbeen proposed inin [[9]9] toto explainexplain proportional proportional EL EL in in liquid liquid Ar, Ar, observed observed in in immersed immersed THGEMs THGEMs and and GEMs GEMs at atrelatively relatively low low fields: fields: see see below. below.

6.2. Light Signal Amplification in Liquid Ar Using THGEMs and GEMs

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In liquid Ar, proportional EL, with its characteristiccharacteristic properties of havi havingng a threshold and linear fieldfield dependence, was observed in immersed THGE THGEMM and and GEM GEM structures structures in in [4,49]: [4,49]: see see Figure Figure 28.28. The mystery of the results obtained is that liquid Ar EL started started at at much much lower lower electric electric fields fields compared compared to liquid liquid Xe, Xe, at at about about 60 60 kV/cm, kV/cm, which which almost almost an an or orderder of ofmagnitude magnitude lower lower than than that that of liquid of liquid Xe. Xe.In addition,In addition, these these electric electric fields fields are are far far less less than than those those of of 3 3 × 101033 kV/cmkV/cm expected expected from theoretical × calculations for EL in liquid Ar due to atomic excitation mechanisms [[81].81].

Figure 28.28. RelativeRelative amplitudeamplitude of of the the proportional proportional EL EL signal signal generated generated in THGEM in THGEM (left )[(left)4,49 ][4,49] and GEM and (GEMright )[(right)4] plate [4] immersed plate immersed in liquid in Ar liquid as a functionAr as a offunction the voltage of the across voltag thee plate.across The the appropriate plate. The appropriateelectric fields electric in the THGEMfields in the and THGEM GEM hole and centers GEM arehole shown centers on are the shown top axes. on the top axes.

It was was supposed supposed in in [9] [9 ]that that the the NBrS NBrS effect effect could could be be responsible responsible for for propor proportionaltional EL ELin liquid in liquid Ar Arobserved observed in immersed in immersed GEM-like GEM-like structures. structures. Indeed, Indeed, the the electric electric fields fields in in the the center center of of GEM GEM or THGEM holes used in liquid Ar, of 60–140 kVkV/cm,/cm, correspond to EE/N/N == 0.3–0.7 Td that are not that small. For For such such reduced reduced electric electric fi fields,elds, the the theory theory predicts predicts that that NBrS NBrS EL EL already already exists: exists: see see Figure Figure 99.. It alsoalso predictspredicts the the linear linear dependence dependence of theof ELthe yield EL yield observed observed in experiment. in experiment. Thus, oneThus, cannot one excludecannot excludethe NBrS the origin NBrS of origin proportional of proportional EL in noble-gas EL in noble-gas liquids at liquids lower at electric lower fields. electric fields. Another possiblepossible explanation explanation might might the presencethe presen (force some (for reason)some reason of gaseous) of bubblesgaseous associated bubbles associatedto the THGEM to the and THGEM GEM and holes, GEM within holes, which within proportional which proportional EL in the EL gas in phase the gas could phase develop. could Thisdevelop. effect This will beeffect discussed will be below. discussed This hypothesis,below. This of bubble-assistedhypothesis, of EL,bubble-assisted could be confidently EL, could tested be confidentlyby measuring tested the ELby emissionmeasuring spectrum the EL emission in the NIR: spectrum it would in be the confirmed NIR: it would if the be emission confirmed spectrum if the wouldemission consist spectrum of atomic would lines consist corresponding of atomic to gaseouslines corresponding Ar. Otherwise, to the gaseous continuous Ar. spectrumOtherwise, would the continuousindicate on thespectrum real liquid would Ar emission.indicate on This the method real liquid has beenAr emission. realized This in [82 method] when studyinghas been electricalrealized inbreakdown [82] when in studying liquid Ar; electrical the results breakdown will be discussedin liquid Ar; in the results following. will be discussed in the following.

6.3. Liquid-Hole Liquid-Hole Multipliers in Liquid Ar and Liquid Xe The liquid-hole multiplier (LHM) concept [57,58] [57,58] has emerged after observation of proportional EL in THGEMs immersed in liquid Xe: see Figure 29 [[83].83]. Notice Notice the the linear linear voltage voltage dependence dependence of EL yield and rather low voltage threshold for EL, corresponding to the electric field field in THGEM holes of about 1010 kV kV/cm./cm. This This field field value value is one is andone two and orders two oforders magnitude of magnitude smaller than smaller that for than proportional that for proportionalEL in liquid ArEL usingin liquid THGEMs Ar using and THGEMs in liquid and Xe in using liquid wires, Xe using respectively. wires, respectively. Therefore, suchTherefore, a low suchthreshold a low EL threshold can hardly EL becan explained hardly be by explained ordinary by EL ordinary or NBrS EL EL. or This NBrS effect EL. was This proved effect was to be proved due to proportionalto be due to proportional EL in the bubbles EL in formedthe bubbles in the formed holes ofin thethe THGEMholes of the and THGEM under it and [58,84 under]. it [58,84]. In thethe LHMLHM concept, concept, shown shown in in Figure Figure 30 ,30, the the noble-gas noble-gas bubble bubble is formed is formed in noble-gas in noble-gas liquid liquid under underthe THGEM the THGEM or GEM or plate GEM in aplate controlled in a controlled way using way heating using wires. heating Radiation-induced wires. Radiation-induced ionization ionization electrons and scintillation (S1)-induced photoelectrons from a CsI photocathode (optionally deposited on top of the THGEM) are focused into the THGHEM holes; both cross the liquid-vapor interface and create EL signals (S2 and S1′, respectively) in the bubble.

Instruments 2020, 4, 16 24 of 31 electrons and scintillation (S1)-induced photoelectrons from a CsI photocathode (optionally deposited on top of the THGEM) are focused into the THGHEM holes; both cross the liquid-vapor interface and createInstruments EL 2020 signals, 4, x (S2FORand PEER S1 REVIEW0, respectively) in the bubble. 24 of 31

Figure 29. Amplitude of proportional EL signal (pulse-area)(pulse-area) and its EL yield (number of photons per drifting electron) producedproduced byby THGEMTHGEM inin liquidliquid XeXe asas aa functionfunction ofof thethe THGEMTHGEM voltagevoltage [[83].83].

Figure 30. Concept of liquid-hole multiplier (LHM) detector [[57–59].57–59].

In some sense, the LHM bubble resembles the DarkSide scheme [85], where a gas pocket is formed In some sense, the LHM bubble resembles the DarkSide scheme [85], where a gas pocket is using heating wires, confined from above by a metallized quartz-plate anode. However, there is a formed using heating wires, confined from above by a metallized quartz-plate anode. However, there fundamental difference: the gas bubbles in LHM are kept from coming up through the THGEM holes is a fundamental difference: the gas bubbles in LHM are kept from coming up through the THGEM due to surface tension. This allows ionization to enter the gas phase from both sides of the liquid: from holes due to surface tension. This allows ionization to enter the gas phase from both sides of the below and above the bubble. liquid: from below and above the bubble. Figure 31 illustrates the LHM performance in liquid Xe and liquid Ar: the EL (S2) signals are Figure 31 illustrates the LHM performance in liquid Xe and liquid Ar: the EL (S2) signals are distinctly seen along with the primary scintillation (S1) signals. In liquid Xe, the EL yield of as high as distinctly seen along with the primary scintillation (S1) signals. In liquid Xe, the EL yield of as high 400 photons per drifting electron was reached, which is comparable with that obtained in traditional as 400 photons per drifting electron was reached, which is comparable with that obtained in two-phase Xe detector with 1 cm thick EL gap. Nevertheless, further research is needed before this traditional two-phase Xe detector with 1 cm thick EL gap. Nevertheless, further research is needed technique can be applied in real rare-event experiments. before this technique can be applied in real rare-event experiments.

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Figure 29. Amplitude of proportional EL signal (pulse-area) and its EL yield (number of photons per drifting electron) produced by THGEM in liquid Xe as a function of the THGEM voltage [83].

Figure 30. Concept of liquid-hole multiplier (LHM) detector [57–59].

In some sense, the LHM bubble resembles the DarkSide scheme [85], where a gas pocket is formed using heating wires, confined from above by a metallized quartz-plate anode. However, there is a fundamental difference: the gas bubbles in LHM are kept from coming up through the THGEM holes due to surface tension. This allows ionization to enter the gas phase from both sides of the liquid: from below and above the bubble. Figure 31 illustrates the LHM performance in liquid Xe and liquid Ar: the EL (S2) signals are distinctly seen along with the primary scintillation (S1) signals. In liquid Xe, the EL yield of as high as 400 photons per drifting electron was reached, which is comparable with that obtained in traditionalInstruments 2020 two-phase, 4, 16 Xe detector with 1 cm thick EL gap. Nevertheless, further research is needed25 of 31 before this technique can be applied in real rare-event experiments.

Instruments 2020, 4, x FOR PEER REVIEW 25 of 31 Figure 31. Example of EL signals (S2 signals) induced by alpha particles, recorded from a LHM Figuredetector 31. [59 Example]: (left) in of liquid EL signals Xe and (S2 bare-PMT signals) and induced (right )by in liquidalpha Arparticles, and TPB-coated recorded PMT.from a LHM detector [59]: (left) in liquid Xe and bare-PMT and (right) in liquid Ar and TPB-coated PMT. 6.4. Breakdowns in Noble-Gas Liquids 6.4. BreakdownsBreakdowns in Noble-Gas in noble-gas Liquids liquids are relevant to the subject of this review because they are accompaniedBreakdowns by bothin noble-gas charge multiplication liquids are relevant and electroluminescence. to the subject of this Their review study because is important they are for accompaniedpractical implementation by both charge of large multiplication liquid Ar TPCs and forelectroluminescence. neutrino experiments, Their where study the is highimportant electrical for practicalpotentials implementation are envisioned, of from large 100 liquid kV to Ar 1 MV.TPCs for neutrino experiments, where the high electrical potentialsThe breakdown are envisioned, mechanism from 100 is kV not to fully 1 MV. understood; it includes several physical effects such as electronThe fieldbreakdown emission, mechanism streamer propagationis not fully understood; and electroluminescence. it includes several Nevertheless, physical effects the universal such as electronempirical field rule emission, was revealed streamer in breakdown propagation measurements and electroluminescence. at cm scale [ 82Nevertheless,,86–88]: the the threshold universal for empiricaldielectric breakdownrule was revealed in any in noble-gas breakdown liquid measurem decreasesents with at cm the scale increase [82,86–88]: of the surface the threshold area of thefor dielectricelectrodes breakdown according to in inverse any noble-gas power law: liquid decreases with the increase of the surface area of the electrodes according to inverse power law: p Emax = C A (11) 𝐸 =𝐶∙· 𝐴 (11) where C is a material-dependent constant, A isis the stressed area (defined (defined as the area with an electric field intensity above 90% of its maximum), and p 0.25. See also Figure 32 showing the breakdown field intensity above 90% of its maximum), and≈ − 𝑝≈−0.25. See also Figure 32 showing the breakdownfield in liquid field Ar in as liquid a function Ar as of a function stressed area.of stre Theressed area. does There not appear does not to beappear a significant to be a significant difference differencebetween breakdown between breakdown behavior in behavior liquid Ar in and liquid liquid Ar and Xe [ 88liquid]. Xe [88].

Figure 32. Breakdown field field versus stressed area of the cathode in liquid Ar [82]. [82]. The stressed area is defineddefined as the areaarea withwith electricelectric fieldfield greater greater than than 90% 90% of of the the maximum maximum electric electric field field in in the the gap. gap. The The fit p fitline line represents represents the the dependence dependenceEmax 𝐸= C=𝐶∙𝐴A with withC = 139𝐶 and = 139p and= 0.22.𝑝 = −0.22. · − It is remarkable that the breakdown in liquid Ar was proposed to have a two-phase nature [82]: while at its first stage electron electroluminescence and avalanching occurred directly in the liquid (induced by field emission of the electrons from the cathode), at the second stage the streamer propagated in the elongated gas bubble. This hypothesis is supported by Figure 33. It shows photon emission spectra in the visible range for a breakdown in liquid Ar at the first (field emission) and the second (streamer) stage. The field emission stage was observed in the form of a glowing cone. These spectra feature the presence of both a liquid and a gas phase in the breakdown. Indeed, the curve with a broad continuum resembles the scintillation continuous spectrum observed in the visible range in liquid Ar [89], while the curve featuring distinct peaks around 700 and 750 nm is attributed to the line spectrum in the NIR due to 𝐴𝑟∗(3𝑝4𝑝) →𝐴𝑟∗(3𝑝4𝑠) transitions in gaseous Ar (see Figure 4 and Equation (9)).

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It is remarkable that the breakdown in liquid Ar was proposed to have a two-phase nature [82]: while at its first stage electron electroluminescence and avalanching occurred directly in the liquid (induced by field emission of the electrons from the cathode), at the second stage the streamer propagated in the elongated gas bubble. This hypothesis is supported by Figure 33. It shows photon emission spectra in the visible range for a breakdown in liquid Ar at the first (field emission) and the second (streamer) stage. The field emission stage was observed in the form of a glowing cone. These spectra feature the presence of both a liquid and a gas phase in the breakdown. Indeed, the curve with a broad continuum resembles the scintillation continuous spectrum observed in the visible range in liquid Ar [89], while the curve featuring distinct peaks around 700 and 750 nm is attributed to the     line spectrum in the NIR due to Ar 3p54p1 Ar 3p54s1 transitions in gaseous Ar (see Figure4 and ∗ → ∗ EquationInstruments (9)).2020, 4, x FOR PEER REVIEW 26 of 31

Figure 33. PhotonPhoton emission emission spectra spectra of of electroluminescence electroluminescence at the at the first first (field (field emission emission cone) cone) stage stage and andthe second the second (streamer) (streamer) stage stage for a forbreakdown a breakdown in liquid in liquid Ar [82]: Ar the [82 ]:curves the curves are that are of that blue of with blue a withbroad a broadcontinuum continuum and that and of that red of with red with distinct distinct peaks, peaks, respectively. respectively. The The first first st stageage curve, curve, with with a broad continuum,continuum, isis similarsimilar toto thethe scintillation continuouscontinuous spectrumspectrum ofof liquidliquid ArAr observedobserved inin [[89],89], whilewhile thethe secondsecond stagestage curve,curve, featuringfeaturing distinctdistinct peakspeaks aroundaround 700700 andand 750750 nm,nm, isis attributedattributed toto thethe line spectrum     of Ar𝐴𝑟∗(33𝑝p544𝑝p1 ) →𝐴𝑟Ar ∗3(p3𝑝54s4𝑠1 transitions) transitions in in gaseous gaseous Ar. Ar. ∗ → ∗ 7. Conclusions InIn thisthis workwork wewe havehave reviewedreviewed physicsphysics andand techniquestechniques ofof directdirect signalsignal amplificationamplification inin thethe detection media of two-phasetwo-phase argon andand xenonxenon detectors.detectors. Originally given rise from the idea of thethe two-phasetwo-phase detectordetector itself, the conceptconcept ofof amplifyingamplifying thethe primaryprimary ionizationionization signalsignal (S2(S2 signal)signal) hadhad triggeredtriggered intenseintense andand didifficultfficult R&DR&D workwork overover thethe lastlast 2020 years.years. This resulted in a large varietyvariety ofof advanced conceptsconcepts ofof S2 S2 signal signal amplification amplification developed developed in in this this period. period. All All these these concepts concepts are are based based on twoon two physical physical effects: effects: electroluminescence electroluminescence and and electron electron avalanching avalanching under under high high electric electric fields, fields, both both in thein the gas gas and and in thein the liquid liquid phase. phase. For thethe timetime beingbeing thethe mostmost intensivelyintensively usedused conceptconcept (particularly(particularly in darkdark mattermatter searchsearch experiments) is that ofof thethe “light“light signalsignal amplification”amplification” utilizingutilizing proportionalproportional electroluminescenceelectroluminescence inin thethe gasgas phase.phase. Another most intensively studied concept concept is is that of the “charge signal amplification” amplification” using electronelectron avalanchingavalanching inin THGEMTHGEM multipliers.multipliers. This however encounters didifficultiesfficulties with large active areas neededneeded forfor large-scalelarge-scale liquidliquid ArAr detectors.detectors. The solution of the problem might be to useuse combinedcombined chargecharge/light/light signalsignal amplification.amplification. InIn addition,addition, thisthis R&DR&D workwork hashas significantlysignificantly advancedadvanced understandingunderstanding of thethe physicalphysical eeffectsffects relatedrelated toto amplificationamplification processesprocesses inin two-phasetwo-phase detectors.detectors. Among them are new electroluminescence mechanisms, includingincluding thatthat duedue toto neutralneutral bremsstrahlungbremsstrahlung eeffect.ffect. Finally, these kinds of techniques may come to be in great demand in rare-event experiments, such as dark matter searches, coherent neutrino-nucleus scattering and giant liquid Ar detectors for neutrino physics. Further studies in this field are in progress.

Funding: The minor part of the work (Section 4), was supported by Russian Foundation for Basic Research (project No. 18-02-00117). The major part of the work (Sections 1–3 and 5–7) was supported by Russian Science Foundation (project No. 20-12-00008).

Acknowledgments: This review was triggered by author’s participation in preparing a forthcoming book “Two- phase emission detectors”, in collaboration with D. Akimov, A. Bolozdynya and V. Chepel, to be published by World Scientific Publishing Co. Pte. Ltd.

Conflicts of Interest: The author declares no conflict of interest

References

1. Dolgoshein, B.A.; Lebedenko, V.N.; Rodionov, B.U. New method of registration of ionizing-particle tracks in condensed matter. JETP Lett. 1970, 11, 351.

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Finally, these kinds of techniques may come to be in great demand in rare-event experiments, such as dark matter searches, coherent neutrino-nucleus scattering and giant liquid Ar detectors for neutrino physics. Further studies in this field are in progress.

Funding: The minor part of the work (Section4), was supported by Russian Foundation for Basic Research (project No. 18-02-00117). The major part of the work (Sections1–3 and5–7) was supported by Russian Science Foundation (project No. 20-12-00008). Acknowledgments: This review was triggered by author’s participation in preparing a forthcoming book “Two-phase emission detectors”, in collaboration with D. Akimov, A. Bolozdynya and V. Chepel, to be published by World Scientific Publishing Co. Pte. Ltd. Conflicts of Interest: The author declares no conflict of interest

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