Nuclear Instruments and Methods in Physics Research A 345 (1994) 230-243North-Holland

Performance of a three-ton liquid argon time projection chamber

P. Cennini, S . Cittolin, J-P. Revol, C. Rubbia, W-H. TianCERN, CH-1211, Geneva 23, Switzerland

P. PicchiLab. Naz. di Frascati dell'INFN, via E. Ferma 40, Frascati, Roma, Italy

F. Civanna, G . Piano Mortari, M. VerdecchiaDipartamento da Fisaca e INFN, Uneversuà dell Aquala, via Vetoao, Coppito

D. Cline, G. Muratori, S. Otwinowski, H. Wang, M. ZhouDepartment of Physics, UCLA, Los Angeles, CA 90024, USA

A. Bettini *, F. Cisagrande, P . Cisoli, S . Centro, B. Diinese, C. De Vecchi, A. Pepito,F. Pietropaolo, P . Rossi, S . VenturaDapartkmento di Fisica e INFN, Uniuersità di Padoua, via Marzolo 8, Padova, Italy

P. Benetti, E. Calligarich, R. Dolini, A. Gigli Berzolari, F. Miuri, L. Mizzone, C. Montinari,A. Piizzoli, A. Rippoldi, G.L. Riselli, D . ScannicchioDapartemento di Fisica e INFN, Unaversità dt Pauta, via Bassi 6, Pauaa, Italy

L. Periale, S . SuzukiICGF del CNR da Torino, corso Fiume 4, Tormo, Italy

(Received 3 January 1994)

We have constructed and operated a 3 ton liquid argon time projection chamber as part of the R&D programme for theICARUS project . We report on the analysis of events from cosmic rays and from radioactive sources collected from June 1991 toJune 1993 . We have systematically investigated the performance and the physical parameters of the detector . We present here theresults obtained from the analysis of the cosmic rays data on the following items: the electron draft velocity, the electron lifetime,the free electron yield, the electron diffusion coefficient, the space resolution and the particle identification capability. The datafrom radioactive sources are used to study the energy resolution in the MeV range. The in depth understanding of the basic physicsaspects of the liquid argon TPC allows us to conclude that such a detector can be built in large sizes and reliably operated over longperiods of the time, providing a new instrument for physics experiments .

1. Introduction

The ICARUS collaboration proposed [11 in 1985 amulti-kiloton LAr TPC to be operated in the GranSasso Laboratory to search for rare underground phe-nomena ranging from proton decay to real time solarneutrino observations, as well as neutrino oscillationsand relic supernovae neutrinos.

* Corresponding author .

0168-9002/94/$07.00 C 1994 - Elsevier Science B.V. All rights reservedSSDI0168-9002(94)00075-1

(AQ), Italy

NUCLEARINSTRUMENTS& METHODSIN PHYSICSRESEARCH

Section A

As a part of our research and development pro-gramme we built a medium scale prototype, with amass of 3 tons . Beginning on June 1991 we started tocollect data to measure the relevant physical parame-ters of the detector, to understand its response toionising events in a wide energy range from the MeVto the GeV scales and to gain operating experience ona long time . The results reported here are based on thecosmic ray data collected from June 1991 to the end ofDecember 1992 .

2. The three-ton detector

P. Cennini et al. / Nucl. Instr. and Meth . in Phys. Res A 345 (1994) 230-243

We have described the construction of the 3 tondetector in a separate paper, where we have alsoshown some bubble chamber grade event images [2] .Physics results based on the analysis of the data will bediscussed in the present paper. We have measured theparameters that determine the behaviour of the detec-tor : electron lifetime, electron drift velocity, free elec-tron yield, electron diffusion coefficient, space resolu-tion and particle identification capability . We have alsotaken data from radioactive sources to study the energyresolution .

We have fully described the three-ton prototypedetector, its construction and operation in ref. [21, andsimply recall here its principal characteristics. The in-ner volume (2 .61 m3) is split into two independentsemicylindrical sections . Both sections are equippedwith identical drift chamber systems. Each wire cham-ber covers a surface equal to 2.4 x 0.9 m2 and issupported by frames . The drift volume is defined bythe chamber itself, a system of "race tracks", used toproduce a uniform electric field in the drift region . Themaximum drift distance is 42 cm . The monitor cham-bers we use for measuring and controlling the electronlifetime have been described elsewhere [3] . We onlymention here that the monitor is a doubly griddedionisation chamber; the maximum measurable lifetimeis between 2 and 3 ms . A monitor chamber is locatedat the bottom of the detector and is used to monitorcontinuously the electron lifetime . With one exceptionthat we will now describe, during the more than twoyears long operation it has always been found to bebetween 2 and 3 ms . These values have been checked,always finding consistent results, by measuring (seesection 7) the charge deposited on the wires by verticalcosmic ray muons and observing the shift of the distri-bution peak position for increasing distances of thetrack from the collecting wires.

The ultra pure liquid argon, even in the absence ofany leak, can be contaminated by outgassing of thewalls and of the various materials (electrodes, cham-bers, spacers, cables, high-voltage resistors, etc.) con-tained in the dewar. In practice the main outgassingsources are the parts of the system that are not indirect contact with the liquid and have as a conse-quence higher temperatures . The closing flange andthe signal cables in particular have large surfaces in theclean volume at almost room temperature and are themain source of outgassing . To keep the liquid argon atthe necessary purity we have provided a recirculationsystem. The naturally evaporating argon is broughtthrough a pipe to a purification system similar to themain one. The argon is then recondensed in a serpen-

231

tine . The hydraulic circuit just described is ultra-clean .The system provides a natural compensation of thethermal losses of the detector dewar. As we will show,the recirculation system, described in ref. [2], proved tobe an essential part of the apparatus necessary tomaintain the purity of the argon.

This conclusion has been strengthened by the inci-dent and the subsequent recovery that we will nowdescribe .

During the night of December 31, 1992 a power-offcaused the stop of the pumping system that keeps thevacuum in the insulation space between the internaland the external vessels of the dewar. The fault wasdetected only after a few hours; during this time excessheat flowed into the liquid . When we measured theelectron lifetime after the accident we found it to be150 ws . Searching for leaks, we found that the rupturedisk (a 100 Rm thick stainless steel membrane used forsafety reasons) had a small crack.

Tests on ultra pure liquid argon, in separate con-tainers, had shown that the contamination of the liquidby oxygen present in the gas above its free surface isextremely slow, provided there are no convective mo-tions in the liquid . It is on the other hand practicallyimmediate in the presence of motions in the liquid [3] .This behaviour is due to the slowness of the diffusionfrom gas into the liquid . It is of great importance for areliable operation of the detector . We then interpretedthe lifetime degradation as due to the insurgence ofconvective motions in the ultrapure liquid during theexcess heat input period . These motions have mixedthe liquid with the gas above it that was contaminatedwith oxygen .

The proof that our interpretation is likely to becorrect is given by the fact that, after having repairedthe rupture disk, the measured values of the lifetimestarted to increase exponentially in time at the rateexpected from the known evaporation rate . This de-mostrates clearly the power of the recirculation system .We display in Fig. 1 the electron lifetime, as measuredwith crossing muons tracks, as a function of time (thevalues measured by the monitor chamber are compati-ble) . During the exponential rise the slope correspondsto an evaporation rate of 3.2 1/h. Subsequently thelifetime reached, and even exceeded, the value it hadbefore the accident .

We can conclude that, even in the presence of anexceptional event that causes contamination of theliquid, a suitably designed recirculation system canlead to a quick recovery of the operational conditions .

3. The readout

The readout is performed with a chamber consistingof three parallel wire planes located at the end of the

Cennint et al. /Nucl. Instr. and Meth . in Phys. Res. A 345 (1994) 230-243

J

________ 40_--____ f

--- ~__ --- __-------- ------------- -------

15/7 23/10 31/1 11/5 19/8Date

Fig. 1 . Electron lifetime from 15/7/92 to 19/8/93 as mea-sured using crossing tracks .

drift length . We call z the coordinate along the electricfield, x and y the coordinates on the plane of thechamber. The absolute z coordinate is given by ameasurement of the drift time, provided that t = 0time and drift speed are known. The drifting electronsreach and cross in sequence the following wire planes :1) a plane of wires running in the y direction, function-ing as the screening grid ; 2) a plane of wires (2 mmpitch) running again in the y direction, located at adistance of 2 mm below the screening grid where itsfunction is to measure by induction the x coordinate(we call it "induction plane" in the following); 3) aplane of wires running in the x direction, with 2 mmpitch, located at a distance of 2 mm below the induc-tion plane; its function is to measure the y coordinate .Since this is the last sensitive plane the electric field is

arranged to collect the drifting electrons (collection

plane) .The ratio of the electric fields above the screening

grid (E t ) and between it and the induction plane (E,)is chosen to achieve full transparency of the grid . Fig. 2

shows the average charge produced by a minimum

ionising track and collected by the single sense wires as

a function of E, : E, As predicted by the Buneman's

formula [41, full transparency is reached at E2 : E,1 .4. We will discuss the data collected at E2 : E, = 1 .5 .

The collection plane is made of sense wires at a 2mm pitch separated by screen wires. The fields above

(E2) and below the induction plane (E3) are chosen to

assure complete transparency of the induction plane.

Fig. 3 shows the average charge collected by the singlesense wires produced by a minimum ionising track as afunction of E3 : E2. As predicted, full transparency is

reached at E3 : E2 = 3.1 . The data to be discussed here

have been collected at E3 : E2= 3.5 .The current signals from each sense wire of the

induction plane and of the collection plane are inte-

U

U

Û10

U

9.5

8.5

ô 8.00.5 1.0 1 .5 2.0 2.5 3.0

Fig. 2. Charge collected by the induction wires as a functionof field ratio across the grid .

grated by charge sensitive preamplifiers. The preampli-fiers are located close to the signal feedthroughs tominimise the input capacitance and hence the noise.

For a minimum ionising track parallel to the cham-ber the shape of the charge signal from the inductionplane is triangular with a maximum of about 6000electrons for a 2 mm sample (depending on the fieldintensity). Fig. 4 shows an example of a digitised induc-tion signal, where two tracks are present, sampled witha 200 ns period . As expected from our calculations, theinduced charge is not the full charge crossing the wireplane, but only 60% of it .

The charge signal from the collection plane is a stepfunction followed by the exponential discharge of thecapacitor that holds the charge . The height of the step

10a0Û

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Û

Û 8.0

---------------

Calculated field ratio_ _

;_ _ _ _ - ;for fùll transparancy _

_ _

E21El

- ;-1-Calculated field ratiofor full transparency

------ ------

2.0 2.5 3.0 3.5 4.0 4.5E3/E2

Fig. 3. Charge collected by the induction wires as a functionof field ratio across the induction plane.

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P. Cennine et al /Nucl. Instr. and Meth . in Phys. Res. A 345 (1994) 230-243

Drift Time (lts)Fig. 4. A digitised induction signal on one doublet, showing

the presence of two tracks . Sampling period is 200 ns

is around 10000 electrons for a 2 mm sample. Fig. 5shows an example of a digitised collection signal, sam-pled with a 200 ns period, together with the curveresulting from the filtering and fitting procedure wewill describe . Two close tracks contribute to the signal :their distance in time is 4.6 ws (from fit). The driftvelocity is 1.24 mm p,s: the distance between the twotracks along the drift direction is then 5.7 mm. It isclear from the example that the two track resolution isclose to the 2 mm length of the gaps between the wire

0 8 16 24 32 40Drift time (its)

Fig. 5. A digitised signal from one collected wire due to twotracks separated by 5.7 mm . The curve is the result of the

fitting procedure described in the text .

planes . The value of the rise time obtained from fit is2.4 p,s .We notice that the signal may become very long,

several microseconds, if the track has a large dip angle.This requires that the low frequency components mustbe accepted by the amplifier without distortion .

The capacitance at the input of the amplifiers is thesum of the wire and of the cable capacitances, thelatter being the larger . The input capacitance rangefrom 100 pF for the upper collection wires to 200 pFfor the induction wires. The corresponding designequivalent noise charge (ENC) evaluated with 1 p,sshaping time is ENC (electrons) = 500 + 2.5C, where Cis the input capacitance in pF, corresponding to valuesbetween 700 and 1000 electrons. These values havebeen confirmed by direct measurements on the detec-tor.

The charge signals are sampled with flash ADCs(one per channel) at a frequency that provides severalmeasurements for each pulse. The sampling periodranges from 50 to 400 ns ; we have used 200 ns periodin the data collection described in this work . Thecomplete readout system is described in ref. [2] . Thecharge measurement calibration is obtained for allchannels by injecting the same voltage pulse on testcapacitances (one per channel) . The test capacitanceshave been measured with 0.5% accuracy. The gainshave been periodically measured and found to be sta-ble within 2% .

Two different types of cross talk are present be-tween electronic channels . The first one is the crosstalk between two contiguous wires: its average value is1 .2%. The second one is due to the structure of ourcables . The signals from groups of eight contiguouswires are brought from the chamber to the amplifiersby specially designed flat cables . A number (32) ofthese flat cables are sandwiched together to form astack as described in ref. [2] . The cross talk between awire in one cable and the octet that it faces is onaverage 1%.

4. Extraction of the signal

233

As mentioned above the charge signals are smalland the signal-to-noise ratios are S/N ~ 10 for thecollection wires and S/N = 6 for the induction dou-blets. We are discussing here the software filteringalgorithms and procedures developed to extract therelevant parameters (pulse time, rise time, height, etc.)from the digitised data.

The data used in this paper were obtained mainlyfrom cosmic ray muons crossing almost vertically thedetector : the information comes, as a consequence,mainly from the horizontal collection wires. We willdescribe the algorithms used to extract, from the digi-

234

a)

b)

Fig. 6 . (a) shows a sketch of the signal of Fig. 5, with the filterparameters symbols. (b) shows the filtered signal .

tised collection signals, the relevant information and itsevaluation based on a sample of simulated verticaltracks .

The collection signal carries three main characteris-tics (Fig . 6a) which are relevant for the physics analysis :peak time position (pp), which gives the drift coordi-nate ; pulse height (ph), proportional to the collectedcharge ; rise time (rt), which depends both on the trackangle with the plane, and on the diffusion of theelectron cloud during the drift. These parameters areextracted in two steps : first, the valid regions (thosewhich contain some signal over the noise) inside thelarge event data must be traced and a hit table is built;then, a least-squares fit is performed locally on eachsingle region traced on the table, to recover the signalbehaviour. The search region must be wide enough toaccept inclined track signals and multiple hits fromclose tracks : we take a range of ±50 samples aroundthe peak. Searching in a long (20 ws) window corre-sponds to accept the low frequency part of the spec-trum, where 1/f and microphonic noises give impor-tant contributions (larger than the signal itself) . The hitfinding algorithm must then be robust against noise

60U 40cwv 20

0

P. Cennint et al. /Nucl. Insu. and Meth . i n Phys . Res. A 345 (1994) 230-243

rt risetimépp peak positionph pulse heigth

th thresholdoth over threshold

lenght

----------------------------

9 14 19 24 29 34Threshold (ADC counts)

and it must perform with an efficiency close to 100%on the entire range of collection signals .

4.1 . The hit fender

To find the hits we have chosen a two stage nonlin-ear filtering procedure. A nonrecursive median filter (5samples long) is first applied to reduce high frequencydisturbances contribution (electromagnetic pickup fromsurrounding devices, spikes due to FADC output swingon most significant bits) . Then the local variance iscalculated over a narrow window that spans the entiresignal . The output (Fig . 6b) is a flat baseline aroundthe noise level, with a peak quickly rising up as thecollection rising edge enters the window . Given itssmall slope, the long (many window sizes) falling tailgives a negligible contribution to the variance, and theoutput lowers back just above the noise level. Thisallows for a clear separation of overlapping tracks .When the variance output stays over a given threshold(th) long enough (oth), that region is tagged as no .i .(region of interest) and passed to the fitting algorithm.A Monte Carlo simulation has been developed to

study the filter performance concerning signal andnoise characteristics and to optimise the choice of therelevant filter parameters : window size, threshold level,over-threshold width) .

Fig. 7 shows the efficiency of the hit finding algo-rithm, for two different noise levels, as a function ofthe threshold parameter. Units are ADC counts, oneADC count corresponding to 350 electrons. A whitenoise is included in the Monte Carlo simulation . Itsrms value is 3 ADC counts in Fig. 7a, 4.5 ADC countsin Fig. 7b . As mentioned in section 2 the ENC mea-sured at 1 ws shaping time is typically 1000 electronscorresponding to 3 ADC counts . From the figures wesee that, in both cases, the efficiency is close to 100%in a wide range of the parameter. We fix the thresholdparameter at four times the noise level (i .e . at 12 ADCcounts in Fig. 7a, and at 18 in Fig. 7b).We then consider the hit finder efficiency (Fig . 8)

(the rms noise is 3 ADC counts) changing the windowwidth parameter. Again the efficiency is close to 100%

100

T806040200

b)

Fig . 7 . Efficiency of the hit finding algorithm as a function of the threshold parameter . The rms noise is different in the two figures :3 ADC counts in (a), 4 .5 ADC counts in (b) . 1 ADC count = 350 electrons .

80

60

40wW

20

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100

s 80v

60Uw 40W

1001________

20

P. Cennini et al. /Nuct. tnstr. and Meth . to Phys . Res. A 345 (1994) 230-243

--------- ------

4 9 14 19Window width (clock counts)

Fig . 8 . Efficiency of the hit finding algorithm as a function ofthe window width . The threshold is four times the noise rms,the noise rms is 3 ADC counts . 1 clock count is

200 ns .

in a wide range and, as expected, the parameter mustbe chosen close to the rise time value, typically be-tween 1.2 Rs (6 clock counts) and 1 .6 [Ls (8 clockcounts). We fix the window width parameter at 13clock counts (arrow in Fig. 8) .

Having fixed the threshold and the window width,we now study the hit finder efficiency as a function ofthe width over threshold in Fig. 9 (the rms noise is 3ADC Counts). Again the efficiency is close to 100%over a wide range . The value we chose for this parame-ter is 7 clock counts (arrow in Fig. 9) .

In conclusion the efficiency of the hit finder ap-pears to be high (> 98%) and stable around the chosenvalues of the parameters, which, as a consequence, arenot critical . We then can have confidence in our algo-rithm for the analysis of cosmic ray muons. Someadjustment of the parameters might nonetheless benecessary for tracks at vastly different angles .

0 3

5

7

9

11

13Width over threshold (clock counts)

Fig. 9. Efficiency of the hit finder vs width over threshold.Noise rms = 3 ADC counts, threshold = 4 noise rms, windowwidth =12 clock counts . 1 clock counts = 200 ns, 1 ADC

count = 350 electrons.

4.2. Fitting procedures .

The best way of extracting the physical quantitiesfrom the raw data in the r.o .i . is to perform a least-square fit to the data with a set of theoretical functionsnumerically computed by means of a simulation thatincorporates the electron trajectories in liquid Ar, thesignal they induce on the sense wires and the elec-tronic response function . In practice, to speed up thecalculation we use an analytical shape that reproduceswell a wide range of signal shapes by means of fewparameters (see Fig. 6) : pulse height (ph), position intime (pp), rise time (rt), amplifier decay time (a) andincludes a linear baseline (a + bt). The function of thetime t we use in the fit is (the decay time is fixed bydirect measurement with the test pulse)

phe(pp - t)/dS(t)

1+e(pp-t/rt)+a+bt .

5. Electronic noise

235

In the least square fitting procedure we chose an erroron the single point equal to the rms noise used in thesimulation . The minimum X` is always close to theexpected value, showing that the assumed analyticalshape reproduces the signal correctly .

The effective noise after off-line filtering and fittingis relative to a large bandwidth determined by thelength of the search window and by the responsefunction of the amplifier ; its value may then be differ-ent from that, mentioned in section 2, measured with 1ws shaping time . To measure it, we inject a series oftest pulses of a fixed amplitude and a fixed rise time(similar to those of the track signals) in all the chan-nels . The digitised outputs are then fitted with exactlythe same procedures we used for the track signals,described in section 4. The distribution of the pulseheights, obtained by the fit, is shown in Fig. 10 for atest pulse height of 12000 electrons . The curve is aGaussian fit to data : the fitted value of the rms is1300 ± 100 electrons. Notice also that any difference inthe effective t = 0 amongst the channels due to differ-ent lengths of cables or different rise times is includedin the rms value. We will see in section 10 that this is asmall contribution .

The shape and the height of the charge signals fromthe tracks depend on the energy and angle of the track,on its distance from the wire plane and on the value ofthe electric field (due to recombination) . The height ofthe charge signal depends on the energy deposited in asegment of track, whose length projected on the wireplane in the direction normal to the wires is equal tothe pitch (2 mm). It also depends on the electric field,due to the recombination process (section 8) and, fi-

236

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300

ô200

vo.

ô 100U

0

P. Cennint et al . INucl. Instr. and Meth. in Phys. Res. A 345 (1994) 230-243

5 10 15 20Pulse Height (1000 el.)

Fig . 10. Distribution of the measured charged for a test pulsesof 12000 electrons . The width measures the electronic noise.

nally, on the drift time for finite values of the electronlifetime . For a constant value of the rms noise the S/Nratio and the resolution in the drift time measurementare correlated. The resolution in the drift time mea-surement is also a function of the signal rise time : for agiven S/N ratio the resolution is of course smaller forshorter rise times. The rise time of the charge signalfrom the track elements depends mainly on two effects :a) the time taken by a point charge to cross thedistance between the screening electrode and the sensewire plane. (With our geometry and field intensitiesthis time varies between 1 and 2 ws) ; b) the time takenby the actual electron cloud to cross a given point; thisis proportional to the diameter of the cloud and is anincreasing function of the distance of the track fromthe wire plane, due to the diffusion process (section 8) .Notice that the effect of diffusion is more importantfor a liquid TPC than for a gas TPC, where, due to the

cc0

.t~aôvv

200

150

100

50

00.0 0.5 1 .0 1.5 2.0 2.5 3.0

Pulse rise time (its)

Fig . 11 . Resolution on the time measurement as measured fortest pulses of different rise times and of different amplitudes

(different signal to noise ratios).

c

ô

v

w

E~

100

75

50

°' 25v

05 10 15 20 25

Signal/noise

Fig 12. Contribution of the electronic noise to the resolutionof the drift time measurement for two different values of the

test pulse rise time . Curves are meant to guide the eye

amplification process, the resolution depends only onthe fluctuations in the diffusion.We use test pulse data to study the dependence of

the time measurement on S/N and on rise time, byvarying its amplitude and its rise time . For a series oftest pulses of the same characteristics we define theresolution as the rms of a Gaussian fit to the measuredtime distribution .

Fig. 11 shows the measured time resolution as afunction of the rise time for two different values of theS/N ratio . As it can be seen, the resolution is a linearfunction of the pulse rise time . Fig . 12 shows the timeresolution as a function of the S/N ratio for twodifferent rise times: a very fast one (0 .6 lts) and onetypical of our measurements (1 .2 ws) . In section 10 wewill discuss the drift coordinate resolution for tracksand we will come back to these points .

6. Monte Carlo simulation

To have complete understanding of the physicalprocesses that determine the behaviour of the detectorwe have developed a Monte Carlo simulation program,based on the LEANT Monte Carlo [5], that fullyreproduces the detector response to all ionising events .

The simulation is performed in four steps:a) In the LEANT framework the geometry of the

detector (dewar, wire chambers, race track) is de-scribed as well as the characteristics of the materialsused to build the detector (mainly steel and liquid At)and the definition of the active media (ix. : the liquidAr volume seen by the wire chamber) .

which takes into account the increase of recombinationprobability with ionisation density. The parameters aand /3 have been chosen to reproduce the data ob-tained with stopping muon tracks in our detector (seesection 8) . The charge is then propagated in the liquidAt taking into account the drift velocity given by theapplied electric field and the diffusion coefficient .When each amount of charge reaches the wire cham-ber its value is stored into two bidimensional arrays,whose indexes are respectively the collection/induc-tion hit wire number and the drift time (binned as inthe real data).

d) When b) and c) have been completed, the streamof data corresponding to each wire is convoluted with astep function having finite rise time (typically = 1 lts)to simulate the drift of the electrons between twosuccessive wire planes . An equivalent noise charge isadded to each time bin. The noise spectrum is assumedto be white for simplicity and its rms value is chosen toreproduce, in the output, the value (1300 electrons)measured with the test pulse as discussed in section 4.The result is then convoluted with the response func-tion of the electronic chain (a double exponential withrise time = 0.3 p,s and decay time = 42 ws) and isquantised in 256 levels following the ADC range usedin the experiment ( = 350 e/count) .

Each event simulated in the way described above isthen analysed using the same algorithms as for thedata : the physical quantities extracted are then directlycomparable with those obtained with analysis of' the 3ton detector data .

7. Drift velocity measurements

P. Cennini et al. INuel. Instr. and Meth. i n Phys . Res. A 345 (1994) 230-243

b) Each particle initially defined by the user (par-ticle type, energy, momentum and position) is propa-gated inside the detector by GEANT returning at eachstep the identity of the current particle, its energy,position and direction, the length of the last step, theamount of energy deposited and the mechanism ofenergy loss . The steps are performed until the particleexits the detector or looses its identity (i .e . : when itdecays or its energy falls below the cut).

c) When a particle reaches the active medium, theenergy deposited by ionisation is converted into charge(electrons) by means of the following formula (so-calledBirks law) :

dEdQ=a dE/1 +0 dx ,

We have measured the electrons drift velocity as afunction of the drift field using the purity monitorchamber outside the main dewar. The vapour pressurein the chamber was 1 bar, corresponding to a tempera-ture of 87 K. The charge pulse length from the inte-

2.0

ô 1.0

wC 0.5

0.00 200 400 600 800 1000

Electric field (V/cm)

Fig . 13 . Free electrons drift velocity in liquid argon as afunction of the electric field . Circles at 87 K, squares at 92 K.

Curves are to guide the eye

8. Electron diffusion

237

grating amplifier is equal to the drift time between thecathode and anode grids. For a given field the driftvelocity is obtained dividing the grid distance (50 mm),by the measured drift time . Results are reported inFig. 13 .

An independent measurement was performed inthe detector using muon tracks crossing diagonally thedrift volume. The distance from the entrance pointthrough the cathode (the first point of the track) andthe point where the track crosses the collection plane(recognised as the point where the track image re-flects) is known. The drift time is measured andrecorded by the readout electronics . The vapour pres-sure was 1.5 bar corresponding to T= 92 K. Resultsare reported in Fig. 13 . Data show a strong depen-dence of the drift velocity on temperature, implyingthat an accurate control of the vapour pressure isrequested to have a reliable operation of the detector .

At low field values the drift velocity is approxi-mately a linear function of the field, i.e . the mobility isa constant . When the drift velocity reaches values closeto the sound speed (0.85 mm/1,,s) it increases lessrapidly, approximately as F. Mobility in the linearzone is 545 cm2 V-1 s-1 at 87 K and 480 cm2 V-1 s - Iat 92 K. In the following analysis we will use themobility values measured at 92 K. Our data are consis-tent with those reported by other authors [6,7].

The trigger for cosmic ray events was made up ofthree scintillators placed two on the top and one at thebottom of the dewar. They are in coincidence with thesignals from two groups of 16 horizontal wires each,

238 P. Cennint et al. / Nuel. Instr. and Meth . in Phys. Res. A 345 (1994) 230-243

Fig. 14 . Collected charge distribution for 2 mm samples ofvertical muon tracks . (E = 500 V/cm ; drift time = 20-40 ws).The continuous curve is the result of the fit described in thetext ; the shaded area is the Gaussian contribution of the 1400

electrons rms.

one group at the top and one group at the bottom ofthe wire chamber. The trigger for muons stopping inthe detector was the same but without the lowestscintillator and with the lowest group of wires in anti-coincidence. The external trigger provides the t = 0 tothe system . Events were visually selected requiring theabsence of large multiple scattering and of delta rays toobtain a sample of minimum ionising particle (m .i .p .)tracks (barring the relativistic rise). The sample wasdivided in bins, 20 Ws wide, of the measured drift time .For each drift time bin we plotted the distribution ofthe charge collected on the (horizontal) collection sensewires, proportional to the energy deposited by a m.i .p .in 2 mm length .

Fig. 14 shows, as an example, the distribution forthe 20-40 [.Ls bin of drift time at 500 V/cm drift field.The distribution is fitted with the convolution of aLandau distribution and a Gaussian function ; the latter(shown as a shaded area in the figure) takes intoaccount of the effects of the electronic noise, themuons spectrum (not all the muons are minimum ionis-ing) and of the effect of the finite electron lifetime .The fit gives, in the example, a peak value of 11100electrons and an rms of the Gaussian of 1400 electronscompatible with the above mentioned effects . The in-trinsic width of the Landau distribution is much smaller,500 electrons ; this implies that the energy resolutionfor dE/dx is dominated by the electronic noise.

The sample of minimum ionising tracks crossingvertically the drift volume was used to measure theelectron lifetime, as mentioned in section 2, and to

extract a second parameter, the longitudinal diffusioncoefficient D. This parameter is important because itcan influence the definition of track images and theaccuracy of the drift coordinate measurements . Accu-rate measurements of the diffusion coefficient havebeen done by Shibamura et al . [8], but they are relativeto the transverse coefficient, that in principle can bedifferent from the longitudinal one, and are at fieldsabove 2 kV/cm . We do not know of published mea-surements at lower fields .We obtain the longitudinal diffusion coefficient from

the analysis of the rise time (RT ) of the signal of thecollection wires . The square of the latter is the sum ofa constant term (R 2 ) and of a term proportional tothe square of the spread (Q

2 ) of the signal due todiffusion . Also, if t is the drift time and v the driftvelocity, we have o- 2 =2Dt/v 2 . In conclusion a linearfit of R2 (or equivalently 0,2) versus drift time givesdirectly the longitudinal diffusion coefficient D. Wetook data at drift field intensities of 350, 250, 150 and100 V/em . For each sample we fitted the signal riseshape with the convolution of a step function and of agaussian . The fitted values of the variance of theGaussian are shown in Fig. 15 . Linear fits to the datagive, at the different fields, values that are equal insidethe errors . Taking their average we obtain D =4.8 ±0.2 cm 2/s .

At the relatively low field intensities of our data,the electrons are expected to be thermal; in this casethe Einstein relation qD/w = kT should hold . Withthe mobility value that we have measured, A = 480cm' V- ' s-1 , we then expect D = 3.8 Cm2/s . A furthercontribution to the diffusion is known to come fromthe Coulomb repulsion amongst the electrons. Calcu-lating their contribution with the approximated methodof ref. [8] we find a value of about 2 cm2/S, that shouldbe added (linearly not quadratically) . We can concludethat our measured value agrees with the expectations .

9. Free electron yield and recombination

Using again the vertical muons sample we studiedsystematically the electron-ion recombination at lowfields, namely between 100 and 500 V/cm . The verticalmuons are, in a first approximation, minimum ionisingtracks, parallel to the coordinate planes . The (horizon-tal) collection wires sample 2 mm long segments of thetrack. The peak values of the charge distribution ateach electric field are presented in Fig. 16 : it is, asexpected, an increasing function of the field intensity.The asymptotic value at high fields is obtained fromthe known value of dE/dx through Monte Carlosimulation . We calculate the most probable energydeposition in 2 mm long track to be 326 keV, corre-sponding to 13 800 electrons (23.6 eV/pair) .

3.0

2.5

0.5

0.00 100 200 300 400 500 600

Drift time (gs)

Fig. 15 . Pulse rise time squared vs drift time for differentfields .

In Fig. 16 we show also the prediction of the On-sager model (arbitrarily) normalised at the highest field(500 V/cm). As expected it disagrees with the data .

An interesting feature of the liquid argon TPC is itsability to measure dE/dx, giving important informa-tion for particle identification and for determining thedirection of a stopping particle . To study the capabilityof the detector of identifying particles by dE/dx as afunction of the range and the recombination depen-dence on dE/dx we studied a sample of muons stop-ping within the chamber, with an identified decayelectron . Fig. 17 shows how the ionisation increaseswhen the muons slow down .

At any given point of the muon track the residualrange is measured, hence the muon energy is known.The corresponding (dE/dx) is obtained, by the MonteCarlo simulation described in section 6. The averagevalue of dQ/dx for that value of the range is extractedfrom the data sample . The correlation between

P. Cennini et al. /Nucl. Instr. and Meth. t o Phys . Res A 345 (1994) 230-24.3

Fig. 16 . Charge collected on 2 mm length as a function of fieldfor vertical muons. The curve is the Onsager model nor-

malised at 500 V/cm .

^ 251~20

15

`Ô 10

g5

dQ dE/dx

dx-A

dE '1 +k b-

dx

250

200V

150

50

00

5

10dFJdx (MeV/cm)

239

2

4-

6

8

10

12Distance from stopping point (cm)

Fig. 17 . Average collected charge as a function of the distancefrom the end point of stopping muons. Curve is the result of a

Monte Carlo calculation not including recombination .

(dQ/dx) (data) and (dE/dx) (Monte Carlo), forthree different fields, is shown in Fig. 18 . At each fieldthe point at the lowest energy loss is obtained fromminimum ionising muon tracks .

As we lack a full theoretical understanding of therecombination process, we fitted the data in Fig. 18 ateach field with a phenomenological formula, called theBirks' law:

where A and kb are constant to be obtained from thefit : the constant A is the asymptotic value of thecollected charge per unit length corresponding to aunitary specific energy loss . It is expected to be inde-

15

Fig. 18 . (dQ/dx) as a function of (dE/dx) at differentfields (stopping muons data). Curves are from fit described in

the text .

11

.w 0

E=250 /cm

E=3 OV/cm

240 P Cennsni et al . /Nucl. Instr. and Meth. i n Phys . Res. A 345 (1994) 230-243

pendent of field and approximately (given the rough-ness of the model) equal to 42 electrons/keV. TheBirks constant kb gives the nonlinear dependence : it isexpected to be a decreasing function of the field. Thecurves in Fig. 18 are the results of the fits, that de-scribe well the data . The corresponding values of theconstants at 100, 350 and 700 V/cm are respectively:A = 36 ± 1, 33 ± 1 and 36 ± 1 electrons/keV and k,, =0 .326 ± 0.019, 0.130 ± 0.008 and 0.072 ± 0.004cm/MeV .

The nonlinear dependence of the collected chargeon the deposited energy at the low field values, wherethe liquid argon TPC is operated, affects both thedE/dx measurements and the energy resolution . Dueto the recombination process, especially at high spe-cific ionisation values and at low fields, part of theenergy deposited by the ionising particle appears notas free charge but as photons, that are not detected .We are considering the possibility of doping the liquidargon to recuperate as free charge part of the energyescaping detection.

10. Space resolution

The space resolution along the drift coordinate hasbeen measured using the tracks from vertical muons atdistances from the wire planes such that the maximumdrift time is, for each field, less than 250 ws. To avoidthe contribution of multiple scattering we evaluate theresolution taking in turn the drift times measured onany three contiguous collection wires; we then take thedistance (in time) from the line through the two outerpoints to the middle one. If d is the correspondingdistance along the drift coordinate, the resolution 0 isgiven as a function of its root mean square value by:

0 = 3 (d 2 ) . As can be seen in Fig. 19, the resolutionis around 150 /-Lm and appears to be independent ofthe field in our range of intensities.

We expect, on the other hand a dependence on thedrift time . The result at E = 350 V/cm, again for drifttimes less than 250 Ws, is shown in Fig. 20 (data atdifferent fields show similar behaviour) . The curve isthe result of a Monte Carlo simulation taking intoaccount the contributions of the noise and of thediffusion.

Different sources contribute to the measured spaceresolution in the drift direction : 1) the combined effectof electronic noise and diffusion of the electron cloudduring drift (rms 0), 2) the channel to channel differ-ence in the effective origin of time due to differentcables lengths and to different slopes of the outputsignals (02), 3) out of plane position of the wires (03) .The convolution of contributions 1 and 2 (0) has beenmeasured by means of the test pulse, as shown in Figs .

Fig. 19 . Space resolution m the drift coordinate vs drift fieldfrom vertical tracks within 250ws drift time .

11 and 12 . The quantity 02 is evaluated by measuringthe average time difference for the test pulse in neigh-bouring channels . It has a small value, 02 = 18 ns .From Fig. 12 we see that, in our operating conditions(S/N = 7-10) this is a small contribution to o-,, andthat it is reached at high values of S/N. The quantity03 is evaluated, using vertical muons, measuring thedrift time difference between a channel and the aver-age of the two contiguous ones . The result is 03 = 18wm, again a small contribution . The remaining contri-bution to the space resolution comes from the com-bined effect of longitudinal diffusion and electronicnoise. The signal rise time comes from the two causes :the longitudinal charge distribution due to diffusionand the time taken by the electrons to cross the dis-tance (2 mm) between screening and sense planes .

To interpret the data in Fig. 20 note that they aretaken at 350 V/cm, where we have a signal to noise

200

1750

ucà 125Cua`n 100

Electric field (V/cm)

Drift time (its)Fig. 20 . Resolution in the drift coordinate measured vs drifttime (350 V/cm). Curve is from the Monte Carlo simulation

described in the text .

,1 200 ,, ,

0

,

0 150' -------------a. ,

,

a

1txU0 100 200 300 400 500

11 . Energy resolution at a few MeV

P. Cennrni et al. INucl. Instr. and Meth. in Phys . Res. A 345 (1994) 230-243

ratio S/N = 8. (We see from Fig. 16 that the signal is10 500 electrons and from the discussion in section 5the noise is 1300 electrons) . The pulse rise time, aver-aged on the sample, is 1.5 Vs . From test pulse datasimilar to those shown in Figs . 11 and 12 we have forcomparison o-, = 120 ns . The measured drift velocity(see Fig. 13) is v = 1.25 mm/Vs. We can finally calcu-late this contribution to space resolution as vo,, = 1 .25wm/ns x 120 ns = 150 wm. In conclusion the calcu-lated value is equal to the measured total resolution asshown in Fig. 20 : the combined effect of the rise timeand electronic noise determines the space resolution .

As we have noticed the resolution is almost inde-pendent of the field intensity. To understand this as-pect we recall that the data are relative, for all fields,to tracks at drift times less than 250 Vs . As a conse-quence the average signal rise time is the same for allthe samples. At lower fields the signal is smaller, dueto increased recombination, and o-, becomes larger (forexample going from 500 to 250 V/cm S/N decreasesby 20% and o-, increases by about 23%) . On the otherhand the drift velocity decreases (by 25% in the aboveexample) and both effects almost perfectly compensateeach other.

The curve shown in Fig. 20 is the prediction of theMonte Carlo simulation and agrees with the data .

The energy resolution for low energy electron trackscan be evaluated by studying the Compton spectrumand the pair production peak produced by amonochromatic gamma ray source placed just outsidethe dewar. At energies higher than 3 MeV only com-posite sources are available able to produce monochro-matic gamma rays (e .g . : Pu-Ca giving a 6.13 MeV y orAm-Be emitting a 4.43 MeV y) . Unfortunately thesegamma rays are always accompanied by neutrons whoseinteraction rate is equal or higher to that of the gam-mas. As the energy spectrum of the neutrons extendsfrom thermal energies to about 8-10 MeV, their inter-action products with the argon nuclei (nuclear captureor inelastic collisions with subsequent emission ofgamma and beta rays) could result in a non-negligiblebackground not easily removable. We have chosen touse an Am-Be source that emits monochromaticgamma rays of 4.43 MeV. Its n-y ratio is about one.The source is placed outside the dewar, close to itslateral wall 1 m below the top flange and it is colli-mated by means of lead blocks in such a way that theCompton and pairs electrons are produced as close aspossible to the wire chamber. As the radiation lengthin liquid Ar is 14 cm, nearly all the ys entering theactive volume of the detector (45 cm in radius) shouldproduce a visible ionising event.

The source emits 2 x 103 y/s as well as 2.5 x 103n/s over 4,rr . The presence of neutrons increases thebackground due to the natural radioactivity. In fact,due to inelastic and capture reactions both on argonand stainless steel, the neutrons produce y rays with awide energy spectrum . The total background can bemeasured by shielding completely the source with 20cm of lead which absorbs all the y emitted directly bythe source and leaves the neutron spectrum practicallyunchanged. In this test it is crucial to exploit theself-triggering capabilities of our device . Our recentmeasurements demonstrated that we can trigger onisolated events with energy down to = 1 MeV simplyusing the integrated signal coming from the collectionwires. We collected data with the threshold at 1.8MeV. With this threshold and using 16 wires the trig-ger rates are the following : without source 5.1 Hz, withshielded source 5.6 Hz, with unshielded source 8.5 Hz .

In practice, to reduce the event size hence to max-imise the acquisition rate, the analogue signals fromthe collection wires are grouped sixteen by sixteen andare used independently to trigger the acquisition andto localise the chamber wires hit by the ionising event.Only a small volume around the triggering track (16wires times 51 Vs = 4 kbyte) is stored on tape . Thecosmic ray events are rejected with good efficiencysimply requiring that only one group of sixteen wireshas a signal above threshold.

We have collected data at a drift field of 500 V/cmand analysed them using the algorithms described insection 4. Fig. 21 shows as an example the collectionview of two electrons of 7.5 and 2 MeV respectively .

The Compton scattering theory predicts a slowlyincreasing energy distribution of the scattered electronwith a sharp falling edge corresponding to the 4.20MeV (= Ey - 2me/Ey, Ey = incoming y energy); thepair production peak should appear as a narrow linecentred at 3 .41 MeV (Ey - 2me). The pair productionprobability versus Compton scattering is at this energyabout 30%. In the experimental data this value issomehow reduced because the ys having Compton-scattered once can subsequently reinteract producing

24 1

Fig. 21 . Collected view of 7.5 and of a 2 MeV electrons . Thedrift coordinate corresponds to the horizontal axis.

24 2

600G

8 500

400G16 300

â 200Û

100

0

P. Cennini et al. /Nuel. Instr. and Meth. in Phys Res. A 345 (1994) 230-243

80 100 120 140 160 180 " 103Depositedcharge (electrons)

Fig . 22 . Compton spectrum (background subtracted) . Thedotted curve shows the Monte Carlo calculation described in

the text .

other electrons which in turn can trigger the dataacquisition . The ratio between the position of theCompton edge and the pair peak is a check that we arereally dealing with the interaction products of the 4.43MeV y source ; the smearing of the edge and the peakin the experimental distribution is directly related tothe detector energy resolution.

Fig. 22 shows the background subtracted energyspectrum; it corresponds to about 45 000 triggers withthe unshielded source and 28 000 with shielded source(background). The Compton edge and the pair peakare clearly visible . To interpret the data we have gener-ated 100 000 events with the Monte Carlo programdescribed in section 6; the simulation takes into ac-count all the interactions of the photons and electronsin the detector ; its geometry is completely described inthe program. The deposited energy is transformed intodetected charge taking into account the loss due to thefinite electron lifetime, taken to be 2.5 ms (the datacannot be corrected for this effect, because we do nothave a t = 0 signal, hence we do not know the distanceof the event from the wire plane) . The free electronyield is adjusted to 66% to fit the data . The detectedcharge is then smeared with a Gaussian shape witho,Q/Q =7% also adjusted to fit the data. The MonteCarlo reproduces well the data . We conclude that,after subtracting the electron lifetime contribution, theenergy resolution for electrons around 4 MeV is Qe/E= 7%. The main contributions to the energy resolutionfor an electron of = 4 MeV are:

1) The effective electronic noise contribution, in-cluding that of the off-line extraction of the signalwhich is = 1300 electrons, as we discussed in section 5,while the average number of wire hit is 6; this gives acontribution to the energy measurement error of 3.8% .

2) Uncertainty on the charge amplifiers calibrationconstants (2%) .

3) Missing parts of the track: since the signal extrac-tion algorithm has a threshold at about 6000 electrons(in order to reject efficiently the noise), we miss part ofthe deposited charge especially at the start and end ofthe track; this translates into a rms of = 3500 electronscorresponding to 3.5% .

The best results obtained in liquid argon are thoseof Aprile et al . [10] . They measured the energy resolu-tion for the 976 keV internal conversion electrons from207Bi at different field values, finding a noise sub-tracted resolution of ±4.7% at 500 V/cm . In order tocompare the two results we must subtract from ourresolution the contribution, due not only to the elec-tronic noise, but also to points 2) and 3), absent in themeasurement of Aprile et al . Subtracting in quadraturewe are left with QE/E = 4.3% in good agreement withthose authors. As we have mentioned 66% of the ionpairs produced by electrons of a few MeV at a field of500 V/cm, do not recombine. This value can be com-pared with the measurement of Aprile et al . [10] at 1MeV and a field of 500 V/cm, that found 67% andwith the 60% obtained by Scalettar et al . [9] using 364keV electrons, i.e . at a lower energy . As discussed insection 8, we have measured the free electron yieldalso for minimum ionising particles; we find that thefraction of free ionisation charge at 500 V/cm is 83% .These differences could be interpreted as conse-quences of the nonlinear dependence of the free chargeon the deposited energy at high values of dE/dx,because at lower energy the ionisation increases rapidlynear the end point of the track.

12 . Conclusions

We have performed the analysis of the data takenwith a three-ton liquid argon TPC, that we operatedsmoothly and without interruptions for more than twoyears at CERN. The detector is providing bubblechamber grade three-dimensional pictures of eventsinduced by cosmic rays and radioactive sources. Wehave systematically investigated the performance andthe physical parameters of the detector analysing theseevents . The physical parameters (electron drift velocity,electron lifetime, free electron yield, electron diffusioncoefficient) have been found to be consistent with theexpectation, confirming the design detector perfor-mance. The space resolution in the drift coordinate,found to be of the order of 100 wm, and the superiorparticle identification capability are powerful tools forthe detailed study of rare underground events as fore-seen by the ICARUS experiment . The energy resolu-tion in the MeV region, as determined from the datafrom radioactive sources, is of a few percent, as ex-

pected and as needed for the solar neutrino programof ICARUS .

The in-depth understanding of the basic physicsaspects of the liquid argon TPC allows us to concludethat such a detector can be built in large sizes . Thebasic assumptions we made in the original proposal ofthe ICARUS experiment have been experimentallyverified . The experience gained in operating a mediumsize prototype has shown that a multi-kiloton detectorcan be reliably operated over long periods of time .

Acknowledgements

P. Cennins et al. /Nucl. Instr. and Meth. i n Phys . Res . A 345 (1994) 230-243

The mechanical drafting of the different compo-nents of the detector and of those used for the prelimi-nary tests was skillfully and accurately done by M.Benvenuto. The precise work of A. Galvani, R. Pa-vanello and P. Currarrie in the construction of themechanical parts has been essential, as that of J. Muf-fat-Jolie and N. Piffart in the wiring of the chambers .

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