The liquid xenon set-up of the DAMA experiment

Nuclear Instruments and Methods in Physics Research A 482 (2002) 728–743

The liquid xenon set-up of the DAMA experiment

R. Bernabeia,*, P. Bellia, A. Bussolottia, F. Cappellaa, R. Cerullia, C.J. Daib,A. Incicchittic, A. Matteic, F. Montecchiaa, D. Prosperic

aDipartimento di Fisica, Universit !a di Roma ‘‘Tor Vergata’’ and INFN, Sezione Roma2, via della Ricerca Scientifica 1,

I-00133 Roma, Italyb IHEP, Chinese Academy, P.O. Box 918/3, Beijing 100039, China

cDipartimento di Fisica, Universit !a di Roma ‘‘La Sapienza’’ and INFN, sez. Roma, I-00185 Rome, Italy

Received 19 March 2001; received in revised form 28 July 2001; accepted 7 August 2001

Abstract

The main features of the low background DAMA pure liquid xenon scintillator set-up running deep underground inthe Gran Sasso National Laboratory of the INFN are described. r 2002 Elsevier Science B.V. All rights reserved.

PACS: 29.40.Mc; 61.25.Bi

Keywords: Scintillation detectors; Liquid noble gases; Low background detectors

1. Introduction

Various detection strategies have been proposedfor deep underground searches of Dark Matterparticles. In particular we pointed out the interestin using liquid xenon (LXe) as target-detector forWIMP search deep underground since Ref. [1].Several prototypes were built and related resultspublished [2].The choice to use liquid xenon as a pure

scintillator, directly collecting the emitted UVlight, was performed because in this case no othermaterial apart from xenon is present in thesensitive volume. In fact, this assures an improvedradiopurity, a higher light response and thepossibility of a pulse shape discrimination for

electromagnetic background rejection [3]. A highradiopurity is also assured by the use of Kr-freexenon gas, since the 85Kr is a b� emitter with ahalf-life of 10.7 years.Preliminary results were achieved both on

elastic and inelastic WIMP-129Xe scattering[4,5] by using xenon gas enriched at 99.5%in 129Xe: The response of a similar pure LXescintillator to recoil nuclei as well as its pulseshape discrimination capability have also beenmeasured [3]. Several upgradings of the set-up have been performed with time deep under-ground to improve its sensitivity, obtainingnew results on the WIMP search [3,6]. Severalother rare processes have also been searched forby means of this set-up; results on charge non-conserving processes [7–9] as well as on thenucleon and di-nucleon stability [10] have beenpublished.

*Corresponding author. Tel./fax: +39-06-725-945-42.

E-mail address: [email protected] (R. Bernabei).

0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 1 9 1 8 - 0

At present the set-up is running filled with Kr-free xenon gas enriched at C68:8% in 136Xe; butthe set-up configuration is such that the sensitivevolume can be filled alternatively with xenon gasenriched in 136Xe or with xenon gas enriched in129Xe:In this paper the main features of the low

background DAMA pure liquid xenon scintillatorset-up are reviewed.

2. Liquid xenon as scintillator

A detailed description of the main character-istics of liquid xenon as a detector mediumconsidering scintillation, ionization and propor-tional scintillation properties was given in Refs.[11,12]. In the following we recall only the mainphysical properties useful for its application asregards direct scintillation.The scintillation is produced through the

processes summarized in the following:

Rn þR-Rn2-2Rþ hn

or

Rþ þR-Rþ2 ;

Rþ2 þ e�-R* * þR;

R* *-Rnþheat;

RnþR-Rn2-2Rþhn

where Rn and Rþ are excited atoms and ionsproduced by ionizing radiation, respectively. Herehn is an ultraviolet photon having l ¼ 17871 nm[13] with a bandwidth of Dl ¼ 1472 nm asmeasured at 160 K in the presence of a contam-ination level in N2; H2 and rare gaseso0:01%: Anextensive study of the optical properties ofcondensed xenon was performed in the period1960–1970 [14].Since the scintillation emission is in the far-UV

region, an efficient light transmission and collec-tion is a fundamental point for an LXe scintillator.Impurities that absorb UV light (such as nitrogen,water, hydrocarbons, etc.) could act as an ineffi-cient wave shifter or as quenchers degrading thelight response. Therefore, particular care has to bedevoted to the purification of the used xenon andto the choice of materials that enter in direct

contact with it. These requirements, in case ofexperiments searching for possible rare processes,have to be matched with the necessity to use lowradioactivity materials in order to reduce thebackground as much as possible.As regards scintillation, first indications that

liquid xenon could approach NaI(Tl) in lightoutput date back to 1958 [15]. In particular thehigh atomic number, the high density and thescintillation efficiency similar to those of NaI(Tl)have increased the interest in this scintillator fordifferent fields of application, due also to its fastresponse and to the estimated high tolerance toradiation damage. Some numbers of comparisonwith NaI(Tl) are given in Table 1.The scintillation from liquid xenon has two

decay components (see Table 1) corresponding,respectively, to the life time of singlet and tripletstate of the excited molecule. An additional decaycomponent (with decay time tr) can be consideredin order to take into account the recombination[17,18] between electrons and ions. In case ofelectron excitation the time tr is longer than t1 andnear to t2:

1 In case of a particle excitation, therecombination process is instead fast due to thehigh density of ionization. The recombination

Table 1

Few numbers of comparison between LXe and NaI(Tl)

scintillators

Liquid Xe NaI(Tl)

Density ðg=cm3Þ 3.06 at boiling point 3.67

Fast decay time t1 (ns) 2.2a 230

Slow decay time t2 (ns) 27a FA1=A2 ratio 0:6a

Ratio a=e 1:170:2 0.5–0.6

(Birks’ constantÞ�1 ðMeV=cmÞ 30 600 3670

ðdE=dxÞmin (MeV/cm) 3.9 4.85

aLXe at 179 K in case of electron excitation and electric field

of 4 kV=cm applied [16].

1 In this case tr ¼ 45 ns and A2=Ar ¼ 0:6: In terms of

intensity, considering iðtÞ ¼ A1e�t=t1 þ A2e

�t=t2 þAre�t=tr withR

N

0 iðtÞ dt ¼ 1; it follows I1 ¼ 0:01; I2 ¼ 0:25 and Ir ¼ 0:74where Ik ¼

RN

0 Ake�t=tk dt with k ¼ 1; 2; r; as quoted in

Ref. [19].

R. Bernabei et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 728–743 729

effect does not appear when a strong electric fieldis applied. We recall in particular that ana=e light ratio C1:1–1.2 has been measured byvarious authors in pure LXe scintillators (see e.g.Refs. [2,20–22] for details) showing the im-portant role that recombination can play in liquidxenon.A relevant quantity, when scintillation is

considered, is the mean energy needed for produ-cing a scintillation photon, Ws: Various measure-

ments and estimates are available in literaturefor different types of initial particles as shownin Table 2. Detailed comments can be found inRef. [21].For completeness, we recall that an important

point, as regards the search for dark matterparticles, is the light response versus the releasedenergy in case of a WIMP-nucleus scattering; seeSection 4.2 for some details.As regards measurements of the scintillation

light attenuation length in liquid xenon we refer toRef. [2] and the references quoted therein.The diagram of state of xenon is given in Fig. 1.

A deep stability and control of the thermodyna-mical working conditions and in particular of theuniformity with which these conditions arereached is fundamental in order to have a goodand stable light collection.Finally, some estimates of the refractive index

were given in Ref. [27], while more recently it hasbeen quoted to be 1.7 at 170 nm decreasing downto 1.4 at 360 nm (where liquid Xe is considered at173:2 K and 1 atm) [19]. Moreover, a recentmeasurement has been given in Ref. [28], where

Table 2

Theoretical estimates and experimental measurements of Ws

(eV) for LXe scintillatorsa

Liquid xenon Relativistic a particles Relativistic

electrons heavy particles

Theoretical [21] 23:7þ5:8�6:3 19:672:1 14:7þ1:5�3:3Experimental o35 [23] 16:370:3 [20]

67722 [24] 39 [25]

29:671:8 [2]

14.2 [26]

aFor comparison we recall that WsC23 eV in NaI(Tl).

Fig. 1. Diagram of state of xenon.

R. Bernabei et al. / Nuclear Instruments and Methods in Physics Research A 482 (2002) 728–743730

it is quoted to be 1:565570:002470:0078 at180 nm for the xenon triple point.

3. The experimental set-up

The DAMA LXe experimental set-up is runn-ing deep underground at the Gran Sasso NationalLaboratory of the INFN since several yearsand various upgradings to improve its perfor-mance and sensitivity have been carried outwith time; the latest one has been performed inAugust 2000.The main parts of this set-up (see Figs. 2 and 3)

are: (i) the inner vessel where the xenon under-goes the transition between the gaseous andthe liquid phase; (ii) the external vessel where theinsulation vacuum is made and where threephotomultipliers are located which collect the

UV light through the optical windows of theinner vessel; (iii) the shield; (iv) the cryogenicsystem constituted by a He compressor whichdrives a one-stadium cold head and a cryo-pump; moreover, a heater and a turbo-molecular pump are also operative; and (v) avacuum/filling/purification/recovery system for thexenon.The whole installation is under air condit-

ioning and this assures the utmost stabilityconditions on environmental temperature inorder to avoid any possible influence on theworking conditions. The electronics andthe acquisition system (DAQ) are placed on theupper floor of the building, which is also airconditioned.

Fig. 2. A view of the vacuum/filling/purification/recovery

system; the shield of the set-up is visible at the bottom.

Fig. 3. A view of the inner vessel.


3.1. The inner vessel

The sensitive volume of the inner vessel isof C2 l; corresponding to C6:5 kg of liquidxenon. The inner vessel is made by highlyradiopure OFHC copper and its external shape isa parallelepiped with squared base. In orderto optimize the light collection, the inner sensitivevolume has been excavated smoothly in aquasi-spherical shape cut forward, backward,upward, downward and laterally. A simplifiedschema of the LXe sensitive volume is shown inFig. 4.Three CONFLAT flanges assure the connection

with the optical windows 10 cm in diameter madeof cultured crystal quartz. They are bakeable up to2001C: The optical windows offer a transparencyof about 80% (including reflection losses) to theUV emission ðlC178 nmÞ of the LXe purescintillator. The mechanical features of thesewindows assure the possibility to safely work withxenon gas at about 2 bar pressure during theliquefaction. A UHV valve (electropolished, allmetal, bakeable) directly connected to this vesselallows an independent and direct way for itsevacuation and the possibility to insert a particularhome-made getter, that can work directly in liquidxenon. An upper flange allows the connection witha stainless steel tombak to insert the radioactivesource for calibration.

The temperature monitoring of the inner vesselis performed by a Lakeshore system having fourminiaturized semiconductor sensors placed insideholes along the Cu vessel and an externalcontroller.The leakage of this vessel was tested to be

o10�9 mbar l s�1:Further details on specific solutions chosen for

the UHV connections and the thermic link aregiven in Section 3.6.

3.2. The external vessel and the shield

The external insulation vessel is a 316 stainlesssteel cylinder (about 100 l volume). It has variousvacuum feedthroughs and flanges in order to workon photomultipliers, cold head, sensors, etc.The shield is made of 5–10 cm of low radio-

active copper inside the vacuum insulation vessel(to shield, at the most, mainly possible back-ground contribution from PMTs), 2 cm of steel(wall of the insulation vacuum vessel), 5–10 cm oflow radioactive copper, 5 cm of Polish lead and10 cm of Boliden lead, C1 mm cadmium andC10 cm of polyethylene plus paraffin with athickness fixed by the available space. Thecadmium and the polyethylene/paraffin are usedto shield the set-up from environmental neutrons;measurements on neutron fluxes deep under-ground in the Gran Sasso National Laborator-yFin the various energy regionsFcan be foundfor example in Ref. [29]. Finally, an externalplexiglass box, sealed and fluxed with HP nitrogencloses the external shield.

3.3. The vacuum and cryogenic system

The vacuum/filling/purification/recovery systemhas electropolished 316L stainless steel tubeswith VCR fittings and valves (electropolished, allmetal, bakeable). A schematic view is given inFig. 5.An ultra-high vacuum, UHV, (typically not

worse than 10�6 mbar) is required for the vacuum/filling/purification/recovery line and for the innervessel, while less stringent requirements are neededfor the insulation chamber (typically not worsethan 10�3 mbarÞ: In this chamber the vacuum

Fig. 4. Simplified schema of the LXe sensitive volume in the

inner vessel (not in scale). On the left a schematic view from the

top is shown, on the right a central vertical section is depicted

(the dashed circle corresponds to the lateral optical window).

The upper part of the vessel is closed by a suitable flange (see

Fig. 3) not shown here.


is produced by a turbomolecular pump and readout by a Pirani gauge through a LEYBOLDcontroller mod. IG3 that gives a logarithmicanalogic output (0–10 V full scale) and an outputas NC contact (normally closed) for alarm andmonitor. This control allows to pilot an electro-pneumatic valve that can close the insulationvacuum line, in order to trap the xenon gas incase of accidental break of the cultured crystalquartz windows.The UHV for the inner vessel and the vacuum/

filling/purification/recovery line is firstly producedby a LEYBOLD turbomolecular pump and,secondly, by a LEYBOLD cryo-pump mod.RPK 900 (at 20 K). Part of the vacuum procedureis executed at 1601C for a long time. The UHV isread out by two Bayard-Alpert gauges throughtwo LEYBOLD mod. IG3. On the line also abakeable Pirani gauge is also connected. All theline has a leakage o10�9 mbar l=s:A helium compressor LEYBOLD mod. RW

5000 (located outside the experimental site in aconcrete container specially equipped in theinternal part to avoid any vibration) pilots thecold head (LEYBOLD mod. RGS120 singlestadium 80 K) and the cryo-pump. The stableworking temperature to keep the xenon as a liquidis obtained with a heater directly connected to thecold head. The temperature on the cold head ismonitored with a PT100 sensor. During normalrun conditions a voltage (relative stability of2� 10�4) is supplied to the heater to maintain a

stable operating temperature. During the recoveryprocedure of xenon (for which higher voltage andhigher current are needed) a LEYBOLD mod.HR1 controller, properly modified for our use,allows to gradually heat the inner xenon vessel. Asuitable thermic link made of low radioactivityOFHC copper connects the xenon vessel to thecold head/heater system. The cold head is sup-ported by a suitable basement that protects theapparatus from any vibration noise.The compressor as well as the turbomolecular

pump, is water cooled and its correct operationmust respect fixed parameters of capacity andtemperature of the water. In particular, a tank ofwater outside the experimental site is operative forthe cases of lack of the current water. The watercirculates through two twin circuits (in order to atleast guarantee the operation in case of break-down) and is normally replaced from currentwater at the actual capacity of 7 l=min: Thetemperature ðT1Þ of the outgoing water at the exitof the compressor unit is controlled through asensor dipped in the water itself; it is an integratedsensor of National Semiconductor LM35 that canwork between �40 and 1001C with a linear outputof 10 mV=1C:The control on the correct working conditions

of the compressor unit is performed by aLEYBOLD sensor inside the system with a remotecontrol through a 24 V d.c. output transformed inan NC contact. In an analogous way thecompressor unit supply is monitored.

Fig. 5. Schematic view of the vacuum/filling/purification/recovery line.


This cooling/heater system was preferred toother systems less expensive (as possible with coldbaths) because it can guarantee a high level ofthermodynamical stability. The temperature of theliquid xenon typically has a stability of 0:11C over1 month of work.As regards the purification system firstly a

cold nitrogen trap is used whose working tem-perature is set and monitored by a temperaturecontroller (typically 190 K). It purifies Xe frompossible Rn, H2O and other impurities thatcondense at this temperature. It is used duringthe full procedure of gas filling and liquefaction.Moreover, a Monotorr getter by SAES and home-made getters allow, each one, outlet impuritieso1 ppb for any component: O2; N2; CO, etc. [30].The getters are firstly activated at 4001C and thenused at different and/or environmental tempera-ture during the liquefaction procedure and runconditions. One getter in liquid phase can also beused.The used xenon gas is Kr-free enriched xenon

(two options are possible at present: enrichment at99.5% in 129Xe or enrichment at 68.8% in 136Xe).Two lines of filling/recovery allow the separate useof the two gases. The liquefaction and recoveryprocedures are performed slowly in order to avoidthermic stress on optical windows and photomul-tipliers. During the filling procedure for a giventemperature we maintain higher pressure withrespect to the equilibrium line (see Fig. 1) betweenliquid and vapour in the diagram of state. Thepressure in any case for safety reasons is keptt2 bar:When the liquefaction procedure is completed

and the stability in temperature is attained, givinga suitable constant voltage supply to the heater,the xenon vessel is insulated and kept in directcontact only with getters. Therefore, temperatureand pressure move along the equilibrium curve. Inthis way we can take advantage of a large plateauof work, typically in the range between 168 and178 K: Lower temperatures are not preferredagain to avoid stress on photomultipliers.The stainless steel bottles of xenon are inserted

into stainless steel dewars. When the recoveryprocedure of the used xenon gas starts, thecorresponding dewar is filled with liquid nitrogen

and the xenon is gradually stored back in its bottlewhile the xenon vessel is heated.

3.4. The photomultipliers

Three 3:5 in: diameter flying leads photomulti-pliers with magnesium fluoride windows collect thescintillation light through the cultured crystalquartz optical windows. The photomultipliers usedin this experiment were specially realized with thepeculiar requirements of UV light collection andmaximization of quantum efficiency at the UVwavelength of interest. The photomultipliers areEMI D631/FL, that is without sockets. Theirflying leads are directly connected to suitablevoltage dividers with miniaturized SMD resistorsand capacitors (whose low radiopurity has alsobeen measured by Ge detector), mounted on thinTeflon sockets. The solders have been performedby using low radioactive Boliden lead and lowradioactive resin. The voltage dividers have alsobeen optimized to reach the best signal/noise ratio;a sketch is shown in Fig. 6. The PMTs work nearthe maximum high voltageFwithin the linearityplateauFin order to assure a suitable energythreshold. The PMTs have 10 dynodes with linearfocus, their measured quantum efficiencyFfornormal incidenceFranges between 18% and32% at the LXe scintillation wavelength with aflat behaviour around the measured value; more-over, they have good pulse height resolution forsingle photoelectron pulses, low dark noise rateand a gain of C106–107: The three PMTs work incoincidence at a single photoelectron threshold.

3.5. The electronic chain

The three photomultipliers have groundedcathode supplied by positive high voltage. Thevoltage suppliers have a stability of 0.1%. Eachphotomultiplier is connected to a low noisepreamplifier with a 0–250 MHz bandwidth range,with a voltage amplification of 10 and an integrallinearity of 70:2%: Particular attention has beenpaid to the low voltage power system in order tolimit at the most possible electronic noise.A schematic drawing of the electronic chain is

shown in Fig. 7.


Fig. 7. Schematic view of the electronic chain.

Fig. 6. The voltage divider.


The amplified signals of the three PMTsFbymeans of Linear Fan-in Fan-outFprovide: (i) theinputs of the sum of the three pulses; (ii) a signal tothe charge ADC in order to record the pulseenergy collected by each side only (for possiblesingle/sum correlation analysis); and (iii) theinputs of three timing filter amplifiers with 50 nsintegration time. The subsequent discriminatorshave triggers at photoelectron level.The summed signal of the three PMTs is split

by a Linear Fan-in Fan-out and is addressed to: (i)a Transient Digitizer LeCroy 8828D with adigitizing sampling rate of 200 MS=s recordingthe pulse shape in a 1000 ns time window; (ii) acharge ADC with a scale suitable for low energy;(iii) a charge ADCFafter 22 db attenuationFsui-table for events in the intermediate energy range;and (iv) a timing filter amplifier and a discrimi-nator with threshold at the single photoelectronlevel.The detector trigger requires the coincidence

of the four logical signals: from the threePMTs and from the sum. A Gate Generatorrejects events occurring within a time windowof 500 ms after event collection; this requirementintroduces a negligible relative systematic error.A subsequent coincidence takes into accountthe busy signal from data acquisition in orderto give the correct trigger to the TransientDigitizer and the gate of 100 ns to the ADCswith a suitable circuit to avoid triggers duringthe acquisition time of an event. The correspond-ing dead time is evaluated by comparing thehardware triggers and the acquired triggers; itis typically of a few per cent. The data acquisitionis performed by a CAMAC system. For each eventthe following information is recorded: amplitudesof each single PMT pulse; amplitudes and shapeof the sum pulse (recorded by the TransientDigitizer).Finally, we note that an upgrading of

the electronics is in preparation in order tohave in the near future a Transient Digitizeron each photomultiplier and in order to introducea passive power splitterFbefore the signalenters in the preamplifierFto allow in the samerun the collection of higher energy signalsby ADC.

3.6. The radiopurity

The inner vessel, the surrounding insulat-ion vessel, the thermic link, the vacuum/filling/purification/recovery line, etc. were realizedwith materials selected for low radioactivitymainly by sample measurements with Ge detectordeep underground in the Gran Sasso NationalLaboratory (LNGS). Most of the detectorsand the shield materials as well as the used xenongases have been kept deep underground sincemany years.The internal vessel is made of OFHC low

radioactive copper. This detector has the uniquefeature that no O-rings were used for the flangesconnected to it (windows, vacuum/filling tube,etc.), the stainless steel material was minimizedand the Cu itself acts as O-ring in all theconnections steel/copper. This was obtained bymatching a suitable inclination of the knife on thestainless steel flange with the tightness of thescrews. Where a copper/copper connection wasneeded, as for the upper flange of the internalvessel (see Fig. 3) this was obtained by using asandwich structure copper/steel/copper with a thinstainless steel ring. Also, the thermic link wasperformed all with low radioactive OFHC Cuwithout any additional material (such as indium,etc. which is used normally).Results on residual radioactivity in materials

close the LXe sensitive volume are given inTable 3.In our set-up Kr-free xenon gas enriched in

129Xe at 99.5% by ISOTEC company has beenused. The enrichment was verified at the arrival ofthe xenon bottle by mass spectrography [31,32].

Table 3

Residual radioactivity in materials close the LXe sensitive

volume measured with various techniques; the limits are at 95%

CL

Materials 238U 232Th natK

Cu vessel o16 ppt o16 ppt o10 ppb

Cultured crystal F o6 ppb o11 ppm

quartz

Stainless steel o3 ppb o10 ppb o33 ppm


Since August 2000, the vessel can also alternativelybe filled with Kr-free xenon gas enriched in 136Xeat 68.8% by Oak Ridge [33,34].2 Both the Kr-freeenriched gases are deep underground since manyyears.Due to the enrichment procedure, giving

the separation from Kr, the radiopurity ofthe gas is greatly increased with respect to naturalXe. Moreover, only xenon radioactive isotopeswith very short half-life exist; the longest half-

life isotope is 127Xe with 36.41 days. The otherXe isotopes have very fast decay time andtheir possible cosmogenic presence can beremoved after a few months of storage deepunderground.In Tables 4 and 5, respectively, the isotopic

composition of the Kr-free enriched xenon gasesused in our set-up are given. For the first case alsothe original (before any purification cycle) contentof trace contaminants is given, while for thesecond one it is not available here. For comparisonwe recall that e.g. natural Xe of 99.998% puritywhich is commercially available typically contains> 400 times higher Kr concentration.The radiopurity of the purification system was

also deeply investigated. As regards the coldnitrogen trap the only part directly in contact withxenon is the spiral electropolished stainless steeltube where it goes through. Moreover, due to thestrong contamination of Rn in Oxisorb (seeTable 6), after some tests, we chose to avoid itsuse preferring the use of getters. The alloy St707(see Table 6), used in our getters, is a suitablematerial for radiopurity considering also that wecan use getters in cascade at different tempera-tures. In addition, the getters’ material is as-sembled in electropolished housing by SAES or byus and can be equipped with a PALL microfilter(removal rating > 0:003 mmÞ with electropolished316L stainless steel housing, no O-ring and allfluoropolymer Super-Cheminert element [35]. Thematerial of the home-made getters is originallyavailable in the form of pills sealed in nitrogenatmosphere.As regards the residual contamination in the

used PMTs whose characteristics were largely fixed

Table 4

Isotopic composition of the Xe enriched in 129Xe and original

(before the first purification cycle) content of trace contami-

nants [31,32]

129Xe 99.5%128Xe ð0:2570:05Þ%130Xe ð0:2570:05Þ%O2 o3 ppm

N2 o5 ppm

H2 o2 ppm

H2O o3 ppm

Hydrocarbon o1 ppm

Kr o0:02 ppmCF4 o1 ppm

CO2 o1 ppm

Table 5

Main isotopic composition of Xe enriched in 136Xe [33,34]; the

original (before the first purification cycle) content of trace

contaminants is not available here

136Xe 68.8%131Xe 2.9%132Xe 8.5%134Xe 17.1%

Table 6

Residual radioactivity in oxisorb (not used in the DAMA low

background LXe scintillator) and in getters as measured at the

LNGS with the low background Ge detector; the limits are at

95% CL

Materials 238U (ppb) 232Th (ppb) nat K (ppm)

Oxisorb 298733 43179 2475

St707 o25 o10 o7

2The choice to consider enrichments in these two particular

xenon isotopes depends on the scientific purpose of the

experiment. In fact, in case spin dependent coupled WIMPs

are searched for, the enrichment in 129Xe increases by about a

factor 3 the target mass with respect to natural xenon andFin

additionFthe 129Xe offers a higher spin factor with respect to

other targets with the exception of 19F: The present availabilityof the Kr-free xenon gas enriched at 68.8% in 136Xe also allows

to perform data taking in the same set-up with a target once

sensitive and once not to spin-dependent coupled WIMPs as

well as to investigate the bb decay of 136Xe:


by the previously mentioned requirements, themeasured values for main parts are given inTable 7.As described before, to retain the thermic

insulation the external vessel is under vacuum.Moreover, copper bricks are used to fulfilthis chamber as much as possible to act as ashield mainly from possible background contribu-tion from PMTs. This chamber is surroundedby the external shield and by a plexiglass boxthat is continuously fluxed with HP nitrogengas and kept in small overpressure with respectto the environment. This assures the insulat-ion from the environmental air, which containsradon in traces. The 222Rn ðT1=2 ¼ 3:82 daysÞand 220Rn ðT1=2 ¼ 55 sÞ isotopes from 238U and232Th chains, respectively, are in gaseous formand their daughters attach themselves to surfacesby various processes. In underground laboratories,222Rn and 220Rn are due mainly to decays inthe rocks and accumulate themselves in closedspaces. Therefore, in rare event searchesany contact between the detectors and theenvironmental air has to be avoided in order toreduce both the background counting rate andthe permanent pollution of the detectorsurfaces. These requirements are important notonly in running conditions but also in allthe procedures followed during the assembling ofthe full set-up.The radiopurity characteristics of the shield are

given in Table 8. Before the installation, the leadbricks have chemically been etched by HNO3

aqueous solution and the Cu bricksFas all the Cuparts of the set-upFby HCl aqueous solution; theused water was extremely radiopure. Before theinstallation of the external plexiglass box, a

Supronyl (permeability: 2� 10�11 cm2=s [36]) en-velopFfluxed with HP N2Fwrapped the shield.

4. Calibrations

4.1. Calibration with g source

The energy calibrations in keV electron equiva-lent unit are typically performed with a 109Cd(peaks at 22 and 88 keVÞ isotope by Amersham. Itis sealed in a small stainless steel capsule with athin window (C190 mmÞ suitable to work inthe operative conditions of our set-up (ultra-highvacuum, low temperature and high pressureof Xe). The source is supported by a tombakð15 cm linear motion) which can vary its verticalposition and is normally placed in a centralposition inside the inner vessel when it is evac-uated; so the source is already inside during the Xeliquefaction phase.The typical response of the low radioactivity

LXe scintillator to bare 109Cd isotope is shown inFig. 8. The energy dependence of the detectorresults: s=E ¼ 0:056þ 1:19=

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiE½keV�

p:

The uniformity of the light collection wasinitially verified by moving the source in thesensitive volume and by Monte Carlo calculations;both have shown no significant variation of theenergy spectra given by the sum pulse of the threephotomultipliers above the considered 13 keVsoftware energy threshold. An example is shownin Fig. 9.

Table 8

Measurements of the residual radioactivity in some components

used in the passive shield as performed at the LNGS low

background facility with the low background Ge detector; the

limits are at 95% CL

Materials 238U (ppb) 232Th (ppb) natK (ppm)

Cu bricks o0:5 o1 o0:6Boliden Pb o8 o0:03 o0:06Boliden2 Pb o3:6 o0:027 o0:06Polish Pb o7:4 o0:042 o0:03Polyethylene o0:3 o0:7 o2

Plexiglass o0:64 o27:2 o3:3

Table 7

Residual radioactivity in PMT components as measured at the

LNGS low background facility with the low background Ge

detector; the limits are at 95% CL

Materials 238U (ppb) 232Th (ppb) natK (ppm)

MgF2 window F o10 o20

Dynodes o50 o50 o200


A 13 keV software energy threshold (thehardware threshold is at a single photoelectronlevel) has been considered so far to reject atthe most any possible contribution from the tail of

PMT noise pulses. Procedures to discriminate, atvery low energy, noise pulses from LXe scintilla-tion pulses cannot effectively be pursued becauseof the slight difference in the timing structure ofthe two kinds of pulses. In fact, the scintillationdecay time of LXe is relatively fast and theavailable number of photoelectrons/keV is rela-tively modest.

4.2. Calibrations with neutrons

When the low energy data are analysed interms of WIMP-nucleus scattering, it is necessaryto evaluate the energy of the recoil nucleus inkeV from its value known in keV electronequivalent (as given by g source calibrations).For this purpose it is necessary to determinethe ratio (r) of the measured amount of light froma recoil nucleus to the amount of light from anelectron of the same kinetic energy.In 1992 our group considered the Lindhard

theory [37] to evaluate a preliminary estimateof r [38], but it is unsuitable for this purposesince it does not account for all the processeswhich occur in pure LXe scintillation, such as forexample the recombination effect (see Section 2).

Fig. 8. Response of the low radioactivity LXe scintillator to

bare 109Cd isotope.

Fig. 9. Example of the uniformity of the light collection in the detector considering the sum pulse of the three photomultipliers. There

L is the relative light output at various distances, d; from the vessel centre for the 88 keV 109Cd line.


In particular, for comparison, we recall thatthe a/electron light ratio is very high in pureLXe scintillators: C1:2 [39,40], and that estimatesby considering xenon data [17,39,41] supportsa recoil/electron light ratio not worse thanC0:5 [22].Afterwards, we have realized dedicated neutron

calibrations [3]. The calibrations have beenperformed by a dedicated 40 cc detector, sincethe use of a neutron source in a low backgroundinstallation is forbidden by the consequent activa-tion of the irradiated materials. The measure-ments of r have been carried out both by themethod of Refs. [42,43] with an americium–boron

(Am–B) neutron source (obtaining a value aver-aged between 10 and 50 keV: r/10�50 keVS ¼0:6570:10Þ and by detecting scattered neutronsat fixed angles using the ENEA-Frascati neutrongenerator giving about 14 MeV neutrons [44](obtaining a value averaged between 150 and230 keV: r/150�230 keVS ¼ 0:4570:12Þ: Although asimilar behaviour with energy has already beenobserved for some light ratios [45–48], the overallmean value and the half dispersion have beencalculated: 0:5570:11; since the two determina-tions agree within the errors. Afterwards the lowervalue of 0.44 has cautiously been considered in theevaluation of WIMP results. For details see

Fig. 10. The alarm system.


Ref. [3].3 New data have been collected at the endof 2000 at the ENEA neutron generator withneutrons of 2:5 MeV: Measurements have beenperformed by detecting scattered neutrons at fourfixed angles overlapping in part the energy region

previously investigated by the Am–B source. Fordetails see Ref. [50], here we only summarize thatresults compatible within the errors have beenobtained; in particular, an overall (all dataincluded: old and new, for the cumulative energywindow) averaged value has been derived /rS ¼0:4670:10:The neutron measurements have also allowed a

quantitative investigation of the possibility of apulse shape discrimination between electromag-netic background and xenon recoils after pre-liminary studies on the different decay timesbetween a and electron pulses [2,51]. The resultshave been described in Ref. [3].

5. Monitoring of sensors and alarm system

The main control parameters are the pressureand the temperature of the xenon vessel. Tomonitor them a high precision temperature sensor

Fig. 11. Display of the behaviour of sensors values in running conditions. Top left: the environmental temperature (1C); top right: the

temperature (1C) of the cooling water when outgoing from the compressor unit; bottom left: the temperature of the electronics (1C);

bottom right: the LXe vapour pressure with respect to external pressure.

3For the sake of completeness, we mention that recoil/

electron light ratiosC0:2 are quoted in Ref. [49]. Similar resultswere preliminarily released by H. Wang in 1996 at IDM96 and

this motivated an additional care in our measurements in 1998

summarized in the text. It can be noted that: (i) significant

differences are often present in different experimental determi-

nations of recoil/electron response for the same nuclei in

detectors even more similar than those mentioned here; (ii) in

operating conditions, various specific experimental features

(such as e.g. the initial purity of the used xenon gas, the inner

surface treatment, the reached vacuum before filling, the used

purification line components and the degassing/release char-

acteristics of all the materials ofFand insideFthe inner vessel,

etc.) can give rise to the presence of different trace contaminants

in different LXe set-ups which can differently affect the

measured recoil/electron light ratio, similar to different dopant

concentrations in doped scintillators.


with an external display (as mentioned in Section3.1) and a pressure sensor DIGIBAR mod. PE 300ð�1; 10 barÞ are used.As already mentioned, in the inner vessel

four temperature sensors are working, whiletwo pressure sensors are active on the xenonline next to the inner vessel; however, in Fig. 10only the two (one for temperature and onefor pressure) used for the remote control arepointed out.The sensor values are continuously monitored

with time by a PC (see Fig. 10) with an internalboard National Instruments mod. PC-LPM16 PnPhaving 16 analogic inputs for an ADC with 12 bitresolution and range from þ5 to �5 V: ByLabView software, a program that managesmonitoring data and presents a front panel withtheir values during each run of data taking has

been developed. In Fig. 11 the front panel during arun is shown.To allow a remote control of the experi-

mental conditions a web site has been createdto monitor all the needed parameters. In particu-lar, a WebCam has been installed and inter-faced with the web server in order to show inreal time (2 min resolution) the display of a sensor(see Fig. 12) which cannot be accessed by electro-nic devices and whose substitution would require alarge dismounting of the set-up.The sketch of the circuit realized for the alarm

system is shown in Fig. 10. An interface isoperative with the general security network insidethe Gran Sasso underground laboratories.The alarm conditions, visual and acoustic in theinstallation, are therefore also sent to the generalalarm system in order to allow a prompt interven-ing action. Seven alarms are connected: ð1Þ lackof supply to the compressor unit; ð2Þ failureof the compressor unit (internal sensors); ð3Þ lackof insulation vacuum; ð4Þ high pressure insidethe xenon vessel (P1 sensor); ð5Þ high temperatureof cooling water outgoing from the compressorunit (T1 sensor); ð6Þ high temperature of thecryogenic set-up room or in the electronic set-uproom (T2,T3 sensors); ð7Þ failure of cooling waterplant.

6. Conclusions

In this work we have described the main featuresof the low background DAMA pure liquid xenonscintillator running deep underground in the GranSasso National Laboratory of INFN.It has already achieved several physical results

on the searches for various rare processes. Furtherdata taking is in progress.

Acknowledgements

We thank the LNGS laboratory of the INFNfor support. We thank Mr. F. Bronzini,S. Parmeggiano and G. Ranelli for their qualif-ied technical help, the INFN Sezione di Romaand INFN Sezione di Roma2 mechanical and

Fig. 12. WebCam image of: topFthe external vacuum IG3

display as measured by Pirani gauge (end scale is shown);

bottomFtemperature sensor no. 1 value, that is the reference

for the alarms (not calibrated value).


electronical staffs for support and the ACF stafffor hardware works and assistance.We wish to thank Dr. M. Angelone, Dr. P.

Batistoni and Dr. M. Pillon for their effectivecollaboration in the neutron measurements atENEA-Frascati and Dr. C. Arpesella and Ing.M. Balata for their contribution to measurementson samples and gases. It is also a pleasure to thankProf. E. Fiorini and the Mibeta group for makingavailable to us the xenon enriched in 136Xe:Finally, we are indebted to Prof. S. d’Angelo formany stimulating discussions.

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The liquid xenon set-up of the DAMA experiment