Lopes and Chepel: Liquid Rare Gas Detectors: Recent De©elopments and Applications994

Liquid Rare Gas Detectors: Recent Developmentsand ApplicationsM. I. Lopes and V. Chepel

LIP�Laboratorio de Instrumentaçao e Fısica Experimental de Partıculas´ ˜ ´ ´C.F.R.M. de Departamento de Fısica, Universidade de Coimbra´

3004-516 Coimbra, Portugal

ABSTRACT( )In past years, liquid rare gas detectors LRGD have made a considerable progress

and found application in various fields of research, such as high energy experi-ments, astrophysics, search for rare processes and medical imaging. This reviewpaper starts by a short summary and discussion of the properties of liquid rare

( )gases LRG that make them attractive as radiation detector media. Next, the twobest-established technologies based on liquid rare gases, electromagnetic calorime-ters and time projection chambers, are briefly reviewed along with their main ap-plications. The bulk of the paper is focused on the challenging development ofLRGD for the search of rare processes in low background experiments with an em-phasis on those dealing with the direct detection of dark matter. The challengesposed by those experiments, the detectors proposed and their present status of de-velopment are described. A brief review of the proposals of using LRGD for medi-cal imaging is outlined and the present status of some of the more significant de-velopments under way is presented.

Index Terms — Liquid xenon, liquid argon, liquid krypton, particle detectors,liquid rare gases, gamma-ray detectors, radiation detectors, liquid detectors, di-electric liquids.

1 INTRODUCTIONGREAT progress has been achieved in the de-A Ž .velopment of liquid rare gas detectors LRGD since

liquid argon was suggested as detector medium for ioniza-Žw x .tion chambers 1 and references therein and, especially,

since a liquid argon sampling calorimeter for a high en-w xergy physics experiment was built in 1974 2 .

Systematic studies of the fundamental properties of liq-Ž .uid rare gases LRG have been carried out allowing a

better understanding of the potentialities and limiting fac-Žw xtors of the detectors based on such liquids 3,4 and refer-

.ences therein . Technical difficulties, like the reduction ofthe concentration of electronegative and scintillation pho-ton absorbing impurities down to the level of �1 ppbŽ .parts per billion and the maintenance of this level ofpurity for long periods of time, were overcome after more

w xthan two decades of intensive efforts 5�10 .

Different types of LRGD have been proposed and de-veloped for a great variety of applications. It is beyond thescope of this paper to review all these devices and appli-

Manuscript recei®ed on 1 December 2002, in final form 28 July 2003.

cations. Therefore, we had to restrict ourselves to somerepresentative examples and no claim of completeness ismade. As for the liquids, it was limited to those that so far

Žhave found more use in the field i.e., argon, krypton and.xenon , although liquid helium and neon have also been

w xproposed for detectors 11,12 . Concerning the applica-tions, the emphasis has been put, somewhat arbitrarily, onthe development of detectors for the observation of rareevents under low background conditions and for medicalimaging. However, electromagnetic calorimeters and time

Ž .projection chambers TPCs are also briefly mentioned, asthey probably constitute the most established detectiontechnologies.

This review paper is organised as follows. In Section 2,we present a short compilation and a few comments onthe properties of liquid rare gases in view of their use asdetector media. In Section 3, electromagnetic calorime-ters and time projection chambers are briefly describedand representative examples of each of them, as well as oftheir applications, are presented to illustrate the progressand accomplishments achieved. The reader is referred tothe literature for a more complete coverage of the subject.Section 4 is devoted to the challenging development of

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LRGD for the search of rare processes in low backgroundexperiments with special emphasis on the direct detectionof dark matter. Finally, developments of LRGD for medi-cal imaging are described in Section 5.

2 LIQUID RARE GASES ASDETECTOR MEDIA

Some of the most relevant properties of liquid argon,krypton and xenon in view of their use as radiation detec-tor media are summarised in Table 1. For a more com-plete compilation, as well as a discussion of the spread ofthe data existing in the literature, the reader is referred to

w xthe review papers 3, 4, 13 and references therein .

When a charged particle interacts with a liquefied raregas it gives rise to electron-ion pairs and excited species.Under an applied electric field, a fraction of those pairsescapes recombination and the electrons drift with veloci-ties of the order of a few mmr�s while the ions remainalmost stationary during the drift time of electrons, sincetheir drift velocities are 3 to 5 orders of magnitude lowerthan those of electrons. As the diffusion rate of charges is

Ž .low see Table 1 , by using suitable electrode designs,charge signals induced by the motion of electrons can pro-vide excellent position resolution. In a liquid argon multi-strip ionization chamber, a resolution of 20 �m, r.m.s.,has been measured with minimum ionizing particles in the

w xplane perpendicular to the electron drift direction 14 . Inliquid xenon, by measuring the electron drift time, 20 �m

w xwas obtained with �-particles 15 and about 300 �m withŽ�-rays of �500 keV along the drift direction both values

. w xr.m.s. 16�18 . In the case of �-rays, the resolution waslargely limited by the range of photoelectrons. It is worthto stress that the resolution in the transversal plane de-pends in a great extent on the arrangement of the readoutelectrodes. Furthermore, the contribution of the electrondiffusion to the resolution is expected to be larger in the

transversal plane than along the electric field direction dueto the difference between the transversal and longitudinaldiffusion coefficients.

As liquid rare gases have high free electron yield andŽ .low Fano factor Table 1 , good energy resolution can be

w xobtained in ionization mode 19�25 . The Fano factor, F,is the ratio of the observed variance of the number of ion-izations to that predicted by the Poisson statistics. As onecan see from Table 1, F is much smaller than 1 in liquidrare gases. The resolution improves with increasing elec-tric field due to the higher probability for electrons to es-cape recombination. That improvement is very remark-able up to fields of the order of a few kVrcm, becomingless pronounced at higher fields.

In liquid xenon, the energy resolution for gamma-rayshas been measured by several authors for different ener-gies and applied electric fields. Some of those results are

w x y1r2compiled in 24 showing good agreement with a Edependence on the �-ray energy. Being scaled to 662 keV,most of the results are within the interval from 6.5% to

Ž . w x7.5% FWHM 19�21,24,25 . FWHM refers to Full Widthat Half Maximum of the distribution.

In liquid argon, a slightly worse energy resolution hasw xbeen obtained with �-rays 21 . For 976 keV conversion

Ž .electrons, a resolution of about 3.5% FWHM has beenw xreported 22,23 .

In addition, liquefied rare gases are very good scintilla-tors emitting in the ultraviolet. As scintillator, the best

Ž .LRG is liquid xenon LXe , as it emits at longer wave-Žlengths, is the fastest and has the highest light yield very

Ž .similar to that of NaI Tl , one of the best known scintilla-.tors . On the other hand, the scintillation characteristics

Ž . Ž .of liquid argon LAr and liquid krypton LKr can be im-proved by, for instance, doping with a few % of xenonw x26�28 . Hence, in LRGD, light can provide a very fastsignal for coincidences, trigger for the data acquisition

Table 1. Some properties of liquid argon, krypton and xenon. For more details on the properties of LRG, the reader is referred to the revieww x w xpapers 3,13 , as well as 4 , and references therein.

LAr LKr LXe

Atomic number 18 36 54Ž .Physical Boiling point at 1 bar, T K 87.3 119.8 165.0b

3Ž .properties Density at T grcm 1.40 2.41 2.94b1Ž .W eV 23.6 20.5 15.6

Fano factor 0.11 �0.06 0.041Ž .Ionisation Drift velocity cmr�s at 3 kVrcm 0.30 0.33 0.26

Transversal diffusion coefficient2Ž .at 1 kVrcm cm rs �20 �80

2 Ž .Decay time , fast ns 5 2.1 2.2Ž .slow ns 1000 80 27r45

Ž .Scintillation Emission peak nm 127 150 1752 Ž .Light yield phot.rMev 40000 25000 42000

Ž .Radiation length cm 14 4.7 2.8Ž .Moliere radius cm 10.0 6.6 5.7

Ž1. W is the mean energy required to create a electron-ion pair.Ž2. For 1 MeV electrons and in the absence of electric field.

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Lopes and Chepel: Liquid Rare Gas Detectors: Recent De©elopments and Applications996

system or any other purpose requiring a fast signalpromptly related to the particle interaction in the detec-tor. In liquid xenon, time resolutions of �400 ps, r.m.s.,

w xhave been obtained for gamma-rays of 511 keV 29�31 .Under optimal conditions of light collection the time reso-lution tends to vary as Ey1r2. Hence, very good resolu-

Žtion is expected for high energy particles approximately20 w x.30 ps was reported for 2.7 GeV Ne ions 32 . Concern-

ing the use of scintillation for spectrometric purposes, theprincipal difficulty is to provide high and uniform ultravio-let light collection, which is mostly due to the low reflec-tivity of most materials at these wavelengths. Hence, theenergy resolution, as well as the minimum energy de-tectable and the uniformity, are strongly dependent on thedesign and materials of the detector. It is difficult to givea representative number for the energy resolution of ascintillation spectrometer. Therefore, we refer to a dedi-

w xcated review on this subject 13 and only add some recentresults as examples: about 17%, FWHM, for 662 keV �-

w x w xrays 33,34 , 26%, FWHM, for 122 keV �-rays 35 andw x10%, FWHM, for 5 MeV �-particles 30 .

Compared to the gas phase, the liquid has the addi-tional obvious advantage of providing larger uniform sen-sitive masses. This fact, together with the characteristicsmentioned above, makes liquid rare gases particularlysuitable for building large detectors with tracking andcalorimetric capabilities for a wide range of applications.

Taking into account the properties listed in Table 1,liquid xenon is, in principle, the most attractive LRG de-tector medium for almost all applications: it has the high-

Žest Z and density which implies the highest detection ef-ficiency for gamma-rays and the smallest track length for

.charged particles , it is the best scintillator and presents avery good set of properties concerning ionization andcharge transport. Liquid argon, on the other hand, be-sides having very small diffusion coefficient, has muchlower price and is easier to purify and maintain at thenecessary purity during long periods of time, which is at-tributed mostly to the much lower operating temperatureof LAr detectors compared with those employing LXe.This fact also imposes rather severe restrictions on thematerials that can be used in contact with LXe which arenot so severe in the case of LAr and LKr. Krypton occu-pies an intermediate position in almost all aspects.

Concerning applications, LXe is so far the only LRGused for medical imaging detectors, where the detectionof gamma-rays of a few hundreds keV with high efficiencyis at a premium. It is also the best choice, apart from theprice, for low energy, low background experiments, whereradiopurity is of prime importance. In fact, xenon is freeof long living isotopes whereas natural argon has 39Ar and42Ar �-emitting radioisotopes with half lives of 269 and 33years, respectively. Although the concentration of theseisotopes in natural argon is of the order of 10y21grgw x36,37 , it can become a disturbing source of background

for experiments searching for very rare events, especiallyif these events involve a small deposition of energy in thedetector medium, as it is the case of the direct detectionof dark matter. Krypton is practically ruled out for lowbackground experiments: 85Kr is a �-emitter with anhalf-life of 10.8 years, the endpoint of � spectrum is 0.69

85 y11 w xMeV and the concentration ratio KrrKr is �10 38 .Ž .In high energy physics with and without accelerators ,

in applications requiring very large detector mass, as forcalorimetry andror tracking of high energy particles, liq-uid argon is the most frequent choice, mostly due to itsmuch lower price and ease in achieving and maintainingpurity. Krypton, which is between argon and xenon in thesetwo aspects, is also used in calorimetry, but not so widelyas argon. In spite of the price, there are projects of build-

Žw x .ing liquid xenon calorimeters 13 and references therein ,especially for applications where high efficiency detection

Ž .of medium energy tens of MeV �-rays is of prime impor-tance.

3 ESTABLISHED TECHNOLOGIESAND THEIR APPLICATIONS

3.1 ELECTROMAGNETICCALORIMETERS

Ž .The interaction of high energy typically �1 GeV pho-tons and electrons with matter is dominated by the cre-ation of electron-positron pairs and bremsstrahlung lead-ing to the formation of a cascade of secondary electrons,positrons and photons, which is usually referred as anelectromagnetic shower. An electromagnetic calorimetermeasures the energy of a high energy photon or electronby containing the shower within the device and providinga signal proportional to the energy of the incoming parti-cle.

Electromagnetic calorimeters for high energy experi-ments are so far the most well established technology andthe most widely spread application of LRG. A commoncalorimeter configuration is the transverse sampling typewhere LAr or LKr ionization chambers are interleaved byuranium, iron or lead absorbing plates placed perpendicu-

Žlarly to the incident particle direction an electron or. w xgamma-ray typically in the GeV range 39 . The dimen-

sions are such that the detector fully contains the electro-magnetic shower and the energy of the incident particle isdetermined from the energy deposited in the liquid layers.This type of device has found wide application in experi-

w xments of high-energy physics 39�42 . The disadvantageof this type of calorimeter lies in the long collection time

Ž .for the charge �1 �s . To overcome this drawback andcope with the high rates in some of new generation accel-

Ž .erator experiments with colliders or fixed target , innova-tive calorimeter concepts, different electrode geometriesand ionization readout methods have been developed andimplemented. A discussion of these technical aspects isoutside the scope of the present paper and can be found

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Ž w xin the literature for instance, 39,43 and references.therein . Examples of new developments are the LAr

sampling calorimeter with zigzag longitudinal electrodew xstructure for the ATLAS experiment at CERN 44 , the

LKr quasi-homogeneous calorimeter with longitudinalw xelectrode structure for the NA48 experiment at CERN 45

and the LKr, as well as LXe, quasi-homogeneouscalorimeters with transverse electrodes for the KEDR and

w xCMD-2M experiments in Russia 46 . The construction oflarge detectors based on these developments are inprogress.

Apart from those with ionization readout, calorimetersbased on the scintillation of liquid xenon or LXerLKr have

Žw x .also been proposed 13 and references therein . Al-though several prototypes have been constructed andtested, this type of device has not, so far, been used in areal experiment. Among recent developments is a liquidxenon homogenous calorimeter proposed as the � detec-tor for the experiment MUEGAMMA which searches forthe decay of a positive muon into a positron and a

w xgamma-ray, which is forbidden in the standard model 47 .In this experiment, the signature of the process is theback-to-back simultaneous emission of a positron and a52.8 MeV gamma-ray. To discriminate from the compet-ing processes and other sources of background, a detector

Ž . Žwith good energy better than 2%, FWHM , position a. Ž .few mm and time resolutions f50 ps for 52.8 MeV

gamma-rays is needed. To meet these requirements, a liq-uid xenon homogenous scintillation detector with an ac-tive volume of 800 litres surrounded by 800 photomultipli-

Ž .ers PMTs was proposed. The position of the gamma-rayinteraction is determined from the amplitude distributionof the PMT signals and the energy is obtained from theirsum. A 2 litre prototype with 32 PMTs has been testedusing radioactive �-sources. Position and energy resolu-

Žtion of f3 mm and 4% both corresponding to one stan-

Figure 1. Example of events recorded by ICARUS liquid argonTPC, showing the excellent reconstruction of tracks of high energyparticles. 1, an electromagnetic shower; 2, a muon that stops and

Ž .decays in one electron; 3, electromagnetic shower a , and part of aŽ .14 meter long muon track b .

.dard deviation have been achieved for �-rays of 1.8 MeVw x34 . A 70 litre prototype equipped with 228 PMTs im-mersed in the liquid has also been constructed and is be-

w xing tested under 40 MeV �-ray irradiation 47 .

Another proposal for using a scintillation calorimeterŽ .based on liquid xenon or liquid argon doped with Xe

was made for the detection of gamma cascades producedin radiative neutron capture processes in the framework

w xof the n-TOF experiments 48 that aims at measuring thecross section of neutron induced processes. In this case,the main requirements of the detector are very high de-tection efficiency, in 4� , for �-rays of energy up to f10MeV, good energy resolution and insensitivity to neu-trons. The feasibility and design studies have been donew x49 and a first prototype is under construction.

3.2 TIME PROJECTION CHAMBERS( )TPC

Ž .The Time Projection Chamber TPC , based on liquidargon or liquid xenon, is another type of liquid rare gasdetector that has been proven to work reliably and is be-ing increasingly proposed and used for a variety of parti-cle physics experiments.

The general principle of operation of the device is asfollows. A charged particle produces along its path in theliquid free electrons, which drift under an applied electricfield towards a system of segmented electrodes. Ap-proaching them, the electrons induce electric signals on

Ž .the nearest readout elements e.g., wires, strips, pads . Asthe electrons from different parts of the track arrive tothe readout system at different times, the image of thetrack is being ‘‘projected’’ in time onto the readout plane.Thus, a set of 2D images of the electron distribution inthe chamber volume is obtained allowing a full 3D pictureof the event to be reconstructed.

Liquid TPC constitutes a kind of a new generation ofbubble chamber, with the advantages of being electroni-cally read-out, operated over a very large continuously

Žsensitive volume, self-triggering if primary scintillation is.also detected , able to provide three-dimensional images

of ionising events with simultaneous identification of theparticles from their linear energy loss, dErdx, and rangemeasurements. Furthermore, this type of detector acts alsoas a homogenous calorimeter with very fine granularity.An inherent characteristic of TPCs is the low counting

Žrate capability due to the long distances sometimes �1.m that the electrons have to drift before reaching the

readout system.

Liquid argon TPC was proposed by Rubia for studyingw xsolar neutrinos and the proton decay 50 . Some details of

this device developed in the framework of the ICARUSproject will be given in Section 5.2 and further informa-

w xtion can be found in the literature 50�53 . Figure 1 showsan example of an event recorded by ICARUS liquid argon

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Figure 2. Liquid xenon TPC for �-ray imaging as a Compton tele-w xscope 54 . An incident �-ray is inelastically scattered in liquid xenon,

Ž .giving rise to an electron referred as Compton electron and a scat-Ž � .tered gamma-ray � , followed by the absorption of the scattered

photon with the emission of a photoelectron. By measuring the en-Ž .ergy deposit and the coordinates x,y,z of the two gamma-ray inter-

actions, the energy of the incident �-ray and the angle � can beŽdetermined. See Section 3.2 for more details on the principle of

.operation of this detector.

TPC demonstrating the excellent capability of this type ofdevice for recording the tracks of high energy particlesover long distances.

Liquid xenon TPC is especially suitable for demandingexperiments requiring high detection efficiency forgamma-rays in the energy range from hundreds of keV totens of MeV combined with fast self-trigger, calorimetricand imaging capabilities.

LXe TPC has been developed and successfully used forgamma-ray imaging as a Compton telescope in the energy

w xrange from about 0.2 to 20 MeV 54 . This device is re-Žferred in the literature as LXeGRIT Liquid Xenon

.Gamma-Ray Imaging Telescope and its principle of oper-ation is schematically depicted in Figure 2. The energy ofan incident gamma-ray and its direction of incidence, lim-ited to a conical surface of opening 2� , are obtained bydetermining the energy deposits and the three coordi-nates of the individual �-ray interactions in the detector

Ž .sensitive volume Figure 2 . The scintillation light pro-duced in the liquid is detected by four photomultipliersthus providing a prompt trigger. Under the applied elec-tric field E, the electrons due to ionization along theCompton electron and the photoelectron tracks drift to-wards the wire planes X and Y being collected at the col-lector electrode. The charge signals induced in the wireplanes X and Y are used for determining the x and y

coordinates of the two gamma-ray interactions, while thez coordinate is obtained by the drift time of the electronsusing the light as start signal. A detector with an activearea of 400 cm2 and a drift gap of 7 cm has already beentested both on ground and in balloon flights. With a posi-

Ž .tion resolution of f1mm r.m.s. along all three direc-Ž . Žtions and an energy resolution FWHM f8.8%r�E E in

.MeV , it provides Compton imaging of 1.8 MeV �-raysŽ . w xwith an angular resolution of about 3 degrees r.m.s. 55 .

A program for the upgrading of LXeGRIT to enhance itsenergy sensitivity and efficiency of triggering is envisagedw x56 .

Liquid Xenon TPCs have also been developed and suc-cessfully applied in particle physics experiments. An ex-ample is the 60-litre LXe TPC that has been used forstudying the decay of a pion into a muon, a neutrino and

Ž . w xa gamma-ray i.e., � ™�� 10,57 . In this experiment,low threshold for gamma-rays and fast trigger are the mainrequirements for the detector. A minimum measurableenergy of 230 keV was achieved.

A LXe segmented ionization chamber triggered by scin-tillation, which was proposed for Positron Emission To-

Ž .mography PET , can also be considered, to some extent,a high counting rate TPC. We shall describe it in Section5.

4 LRGD FOR THE SEARCH OF RAREPROCESSES

The new generation of underground experiments with-out accelerator for studying nuclear and high energy rareprocesses, i.e., processes whose expected probability ofoccurrence is very low, pose new and specific challenges,for which novel technologies and detecting methods arerequired. In these experiments, the remotely probable sig-nal that constitutes the experimental signature of the phe-nomenon under study has to be disentangled from thebackground. Consequently, there are two key issues to be

.considered: i lowering the background as much as possi-ble by going deep underground, shielding and using ultra-

.low background materials; ii using detection methods ca-pable of revealing the signal as much unambiguously as

Žpossible, discriminating the residual background active.discrimination .

Liquid rare gas detectors are being increasingly usedfor rare event experiments, such as those for direct dark

Žmatter search, study of neutrinoless double beta decay 2.� 0 , proton decay, and some aspects of neutrino physics.

In fact, LXe and LAr present very attractive characteris-.tics for these experiments: i facility in obtaining very large

continuously sensitive fiducial volumes of very pure mate-.rial with very low intrinsic background; ii high density al-

.lowing to obtain a large mass of the detector medium; iiiflexibility for implementing active background discrimina-

.tion methods; iv in most cases, the liquid medium can

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combine detector and source functions thus increasing thesensitivity of the experiment.

The use of LRGD for the direct detection of dark mat-ter will be considered in some detail, but, due to the limi-tation of space, the applications of LRGD to other rareevents experiments are mentioned only briefly.

4.1 DARK MATTER DIRECT DETECTIONThe nature of the dark matter, which makes f90% of

the mass of the Universe, is one of the current issues inŽastrophysics. WIMPs Weakly Interacting Massive Parti-

.cles are possible dark matter candidates. The most directmethod of detecting such a particle is to look to the nu-clear recoil resulting from the elastic scattering of theWIMP with a targetrdetector nucleus. However, the prob-ability of the process is very small and the recoil signalsare very weak. Therefore, the mass of the detector oughtto be large, the energy threshold should be as low as pos-sible and very good rejection of the background events isindispensable. Furthermore, it is of crucial importance toachieve an extremely high radiopurity of all the compo-nents and the sensitive medium of the detector. This setof requirements with an emphasis on an energy thresholdas low as possible and high background rejection effi-ciency, called for new detection techniques and innovativedetector designs.

.Liquid xenon has appropriate properties: i it has 48%isotopic content with non-zero nuclear spin, thus beingsensitive to both spin-dependent and spin-independent in-

. Ž .teractions; ii it has a large atomic mass As131 suitablefor searching WIMPs with masses between 50 and 500

2 Ž . .GeVrc c is the speed of light in vacuum ; iii it is free oflong living radioactive isotopes, except for possible con-

85 w x .tamination with Kr but that can be removed 58 ; iv ithas high light and charge yields, therefore allowing a low

.energy threshold; v large mass detector is feasible at a

.reasonable price; vi its properties allow the implementa-tion of a variety of techniques for discrimination betweenthe signals due to recoil nucleus and those due to back-ground electrons and gamma rays.

Liquid xenon offers several signalrbackground discrimi-.nation methods based on two facts: 1 the electron-ion

recombination along the particle track depends signifi-.cantly on the dErdx of the particle; 2 a large fraction of

the scintillation photons comes from de-excitation of ex-cimers formed in the sequence of the recombination pro-cess. One method is based on pulse shape discrimination:in the absence of electric field, the nuclear recoil eventsproduce light signals, which are shorter than those from

w xthe electron background events 59 . Another discriminat-ing method takes advantage of the fact that, under an ap-plied electric field, the fraction of electrons escaping re-combination is much larger for an electron track than fora densely ionized nuclear recoil trace. For the energies of

w xFigure 3. A two-phase detector for WIMP detection 63 . The Xenuclear recoil resulting from the elastic scattering of a WIMP pro-

Ž .duces scintillation referred to as primary and ionization in the liq-uid. The electrons drift under the electric field E , are extracted into1the gas phase by the field E and are transported into a region where2there is a field E high enough for producing secondary scintillation.3

Ž .interest �1 to �100 keV , the ionization signal is verysmall and charge multiplication, although occurring in liq-

w xuid xenon 60,61 , requires very high electric fields. Oneway of overcoming these difficulties is to use a two-phase

w x Ž .detector 62,63 Figure 3 : under an applied electric fieldthe electrons drift to the phase boundary and are ex-tracted into the gas phase where they can easily produce

Ž .secondary scintillation electroluminescence or be multi-plied. In the case of using secondary scintillation, thereare two light signals for each event: the primary and thesecondary scintillation signals separated in time by thedrift time of electrons. Measurements showed that the ra-tio of amplitudes of the secondary to the primary signals

w xis much larger for electrons than for recoils 33,64,65 . ThisŽallows to discriminate nuclear recoils from �-rays Figure

.4 . Other discrimination schemes potentially available inw xliquid xenon detectors can be found in references 66�68 .

In fact, very low background liquid xenon detectors havebeen already taking data underground, searching forWIMPs, and others are being developed for future instal-lation. In the framework of the DAMA Collaboration, aliquid xenon scintillation detector has been providing datain the Gran Sasso National Laboratory for already several

w x Ž .years 69 . It has f6.5 kg i.e., f2.2 litres of LXe, Kr-freeŽto lower the intrinsic radioactivity due to the residual

85 . 129contamination with Kr , enriched at 99.5% in Xe inorder to increase the sensitivity to possible spin-depen-dent interacting WIMPs. The background rejection ismade by pulse shape discrimination. For more details, the

w xreader is referred to the literature 58,69,70 . Another ex-ample of a liquid xenon detector entirely based on scintil-lation readout is ZEPLIN-I, which is taking data since

w x2001 at Boulby Mine 35 . It is a single phase detector andthe fiducial volume with 4 kg of liquid is seen by 3 photo-

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Lopes and Chepel: Liquid Rare Gas Detectors: Recent De©elopments and Applications1000

ŽFigure 4. Secondary scintillation here referred to as proportional.scintillation in the gas vs. primary scintillation in the liquid in a

two-phase xenon detector with a mixed gamma-ray and neutronw xsource 33 . Arbitrary units are used in both axis.

Figure 5. Schematic drawing of the liquid xenon detector ZEPLIN-Iw x71 . 1, liquid xenon chamber; 2, photomultipliers; 3, liquid organicscintillator for Compton veto shielding.

Ž .multipliers see Figure 5 . No electric field is applied, andpulse shape analysis is used for background discrimina-tion.

Among the two-phase xenon detectors proposed forWIMPs search, two are being manufactured at the time of

Ž .writing ZEPLIN II and ZEPLIN III and will also be in-stalled in Boulby Mine. ZEPLIN-II has 35 kg liquid xenontarget seen by 7 photomultipliers placed in the gas phase.Electrons extracted from the liquid into the vapour pro-duce secondary scintillation when approaching a plane ofwires where a high field region exists. The ratio between

primary and secondary scintillation signal amplitudes isexpected to provide a good discrimination of the nuclear

w xrecoils against the electron background 65,72�74 .ZEPLIN-III is a kind of ‘‘upside-down’’ version ofZEPLIN-II with smaller gap and lower mass but with adesign that improves the light collection and allows opera-tion at higher electric field in order to lower the detectorenergy threshold which is expected to be of the order of 1

ŽkeV: it has a thin slab geometry 3.5 cm drift layer and 0.5.cm electroluminescence gap with 31 photomultipliers im-

mersed in the liquid. It uses a much higher electric fieldŽ .up to 20 kVrcm , which is expected to enable the detec-tion of the small ionization signal from the nuclear recoils,thus improving the background discrimination power. Thepossibility of obtaining position information for further

w xreduction of the background is also investigated 66,75 .

The array of ZEPLIN detectors intends to be a test bedfor scale-up to a 1-ton liquid xenon detector, which is apart of the WIMP dark matter search programme of the

w xUKDMC at Boulby Mine 66,76 .

Recently, two other large double phase xenon detectorsfor direct WIMPs search have been proposed and are be-

w xing developed 33,68,77 .

4.2 NEUTRINO EXPERIMENTSw x w xLiquid argon 50 and more recently liquid xenon 78

have been proposed as detectors to study the problem ofneutrino oscillations, namely, to determine oscillation pa-rameters and neutrino masses.

The ICARUS liquid argon TPC, described in Section3.2, can be used in multi-purpose experiments and is par-ticularly suited to perform neutrino studies with longbaseline beam, atmospheric, supernovae and high energy

w xsolar neutrinos 79 . After small scale tests started in 1989to verify the feasibility of such detector, a 3 ton prototypewas constructed and submitted to a Research and Devel-

w xopment program which was completed in 1993 51,52 . Thefollowing years have been spent in the development oftechniques suitable for the industrial production of largescale detectors. A first 600 ton module, an intermediatestep towards the multi-kiloton detector, is already con-structed and has successfully undergone above-ground

w xtests 53 .

In view of the success of the ICARUS programme, a 70Žkton liquid argon TPC in a second phase expected to

. Ž100�200 k ton , named LANNDD Liquid Argon Neu-.trino and Nucleon Decay Detector , has been proposed

for studying neutrino oscillations and search for the pro-ton decay, aiming at overcoming the sensitivity of other

w xsimilar experiments 80 .w xThe XMASS collaboration 78 proposed a 10 tons

Ž .fiducial mass liquid xenon detector to measure the lowenergy p-p and the 7Be solar neutrino spectra. The neu-trino detection would rely on the occurrence of e elastic

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scattering in liquid xenon, the recoil electron resultingfrom the neutrino scattering being the signature of theevent. The detector is basically a 4� spectrometer basedon liquid xenon scintillation. Pulse shape analysis is in-tended to be implemented for background discrimination.

4.3 DOUBLE BETA DECAYVarious types of detectors and different emitters are

being used to investigate the neutrinoless double beta de-w xcay in direct counting experiments 81 . Common goals are

to produce the lowest possible background level of theexperimental devices, to get the best energy resolution andto obtain redundant information.

An experiment has been running for several years usinga multisectional liquid argon ionization chamber where100Mo is the source of the rare neutrinoless double beta

w xevents 37 . The liquid xenon setup of DAMA experimenthas also been proved to be useful in searching for neutri-

134 136 w xnoless double beta decay in Xe and Xe 82 .

5 LRGD FOR MEDICAL IMAGINGDue to its excellent properties as gamma-ray detector

medium, liquid xenon is very attractive for nuclearmedicine imaging detectors where gamma-rays of energy

Žranging from 140 keV typical energy used in scintigraphy.and Single Photon Emission Tomography, SPECT to 511

Ž .keV in the case of Positron Emission Tomography, PEThave to be detected with the highest possible efficiency inorder to reduce the dose given to the patient.

As for gamma-cameras for scintigraphy and SPECT,Ž .besides high efficiency, good position resolution �5 mm

along two directions, as well as good energy resolutionŽ .better than 15% at 140 keV are currently required. Thedetectors for PET are even more demanding: in additionto the above mentioned requirements, they must have

Ž .good time resolution better than a few ns and highŽ 2counting rate capability A� F4 �s.cm , A and be-d d

ing the irradiated area and the dead time of a detector. w xmodule, respectively 83 . Furthermore, it is desirable to

have 3D position information instead of the 2D tradition-ally provided. This additional feature is fundamental tocorrect for the parallax error, thus allowing to reduce thesize of the ring, and consequently, its geometrical accep-tance and price, and to attain the physical limit of theposition resolution in the image set by the positron anni-hilation process itself. It is also necessary for the so-called3D PET, in which all the pairs of �-rays are intended tobe used and not only those emitted in a thin slice perpen-dicular to the ring axis.

First proposals for using liquid xenon for a gamma cam-era were made some 30 years ago and a liquid xenon

w xmulti-wire proportional chamber was tested 84 . How-ever, at the high voltages required to attain charge multi-plication in the liquid, the wires could not stand the ten-

sion and broke. To overcome this problem, a two-phasew xcamera was proposed 85 , with �-ray absorption in the

liquid followed by the drift of electrons towards the liq-uid-gas interface and the emission of the electrons intothe gas where they were multiplied around wires. In thiscase, the wires could be much thicker because the electricfield required to obtain charge amplification in the gas ismuch lower than in the liquid. The wires were read indi-vidually, which required tens of charge sensitive preampli-fiers, very expensive at that time. To overcome this draw-back, a similar two-phase liquid xenon camera was devel-oped but using the secondary scintillation produced by the

w xelectrons in the gas for obtaining position information 86 .The light was detected by an array of photomultipliers andthe localisation was attained by computing the centre of

Žgravity of the distribution of light among the PMTs i.e.,.by the so-called Anger method . More details about these

attempts to develop a liquid xenon gamma camera can bew xfound in 87 .

Recently, a new approach taking advantage of the goodposition resolution potentially available on the basis of thecharge signal in the liquid but avoiding the thin wires re-quired for charge amplification has been used. Hence, the

w xdesign of the chamber is similar to that of 84 but theplane of wires is replaced by a ‘‘mini-strip’’ plate made of0.5 mm thick glass with metal strips deposited on both

w xsides 88 . The chamber is operated without charge multi-plication. The strips are read individually using very low-noise multichannel integrated electronics, nowadays com-mercially available at a very low price per channel. Posi-tion resolution better than 2 mm was obtained with 122

w xkeV gamma-rays 89 . The full characterisation and devel-opment of this device is presently under way.

Another recent application of liquid xenon to medicalimaging detectors is a liquid xenon multiwire drift cham-ber with scintillation trigger, developed as a detector

Ž . w xmodule for a PET scanner Figure 6 90,17 . The detectorprofits from the fast scintillation of xenon to make coinci-dences and to obtain a trigger for the data acquisition,while ionization charge collection is used for accurate 3Dlocalisation in the way similar to the time projection

Žchamber technique, i.e. with one of the coordinates x in.Figure 6 being determined by measuring the electron drift

time. In order to minimise the dead time, the drift dis-tance has to be as short as possible. Therefore, the detec-tor is made of an array of ionization cells with drift gap of5 mm. Most results were obtained with a multiwire read-out of ionization signals but better performance is ex-pected with the mini-strip plate. For 511 keV �-rays, coin-

Ž .cidence time resolution of 1.3 ns FWHM , energy resolu-Ž .tion of 17% FWHM , total detection efficiency of about

60% have been obtained with a prototype of a detectorw x Ž .module 91,92 . Position resolution of 0.8 mm FWHM

Žalong the drift direction and the depth-of-interaction z in.Figure 6 with precision of 5 mm were measured. It was

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Lopes and Chepel: Liquid Rare Gas Detectors: Recent De©elopments and Applications1002

Figure 6. A segment of the liquid xenon ionization chamber forPET. 511 keV �-rays interact with liquid xenon producing scintilla-tion light and ionization in the liquid. The scintillation is detected by

Ž .the photomultiplier tubes PMT while the charge is collected to aŽ .segmented anode wires, in this version , each segment being read

Ž .individually numbered by 0 to 9 .

shown that with the mini-strip plate readout, position res-Ž .olution of F1 mm in 2D x and y in Figure 6 and 2 mm

Ž .for depth-of-interaction z can be obtained.Recently, alternative designs of liquid xenon detector

for PET based only on scintillation of the liquid were pro-w xposed 31,93 . The scintillation is detected with position

sensitive photomultipliers allowing reconstruction of thew xinteraction position. In 31 , the light collection is signifi-

cantly improved by surrounding the sensitive volume withphotomultipliers, which form a parallelepiped open at onesurface, thus minimising the loss of photons on the re-flecting surfaces.

6 CONCLUSIONŽ .HE field of liquid rare gas detectors LRGD is farTfrom being fully exploited, as demonstrated by the in-

novative concepts and new applications that are emerging.

Electromagnetic calorimetry based on LRG continuesto make progress that will lead to the construction of de-

vices meeting the requirements of the new generation ofhigh-luminosity high energy particle experiments.

The feasibility of large LAr and LXe time projectionchambers has been demonstrated and these extremelypowerful devices have found increasing application in dif-ferent research fields.

Some underground experiments for studying rare pro-cesses, such as WIMP direct detection, neutrinoless dou-

Ž .ble beta decay 2� 0 , and some aspects of neutrinophysics, is presently one of the most challenging and ac-tive fields for the application of LRGD.

Finally, liquid xenon detectors for medical imaging arebeing developed with very promising results.

ACKNOWLEDGMENTThis work was done in the framework of the FCT pro-

ject CERNrFISr43729r2001.

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This paper is based on a presentation gi®en at the IEEE International Confer-ence on Dielectric Liquids, Graz, 7 � 12 July 2002.

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