Scintillation light from liquid argon and its use in a new hybrid detector

Nuclear Instruments and Methods 203 North-Holland Publishing Company

133

(1982) 133-140

SCINTILLATION LIGHT FROM LIQUID ARGON AND ITS USE IN A NEW HYBRID DETECTOR J.C. BERSET, M. BURNS, K. GEISSLER, G. HARIGEL, J. LINDSAY, G. LINSER and F. SCHENK CERN, Geneng Switzerland

Received 22 March

1982

Scintillation light, produced by 200 MeV/c pions in a liquid argon bubble chamber/calorimeter, was measured. Results from the test device show that this signal could be used in future large bubble chambers to trigger the flash on the occurrence of an interesting event . Furthermore, this signal gives rough information about total track lengths. hence energy contained in massive electromagnetic and hadronic showers, and could complement measurements obtained by charge collection. 1. Introduction The noble gases. in particular argon, krypton and xenon, have proved useful as scintillation detectors because of their short decay time, proportionality of light output to the particle energy dissipated over a wide range of specific ionization densities, a relatively large light output when used in conjunction with an appropriate wavelength shifter, and suitability in either the gaseous or theliquid state 11,21 . Several of thenoblegases have been extensively used also as bubble chamber liquids in high-energy experi. ments, in particular neon and its mixtures with hydrogen. as well as pure helium and xenon, the latter being expensive and not available in large quantities . Recently, we have shown in a test device that argon also can be sensitized as a bubble chamber fluid [3], that charge collection during the bubble chamber expansion is possible [4], and that narrow bubble tracks can be produced with a laser beam [5]. From a technical and financial point of view argon has the additional ad. vantage of being non-inflammable, that it can be cooled by liquid nitrogen, and that both liquids are abundant and inexpensive, In thepresent paperwe describe a newfeature of our hybrid device : the technique of measuringefficiently in argon of high purity the scintillation light produced by beam particles, and that operating the flash of the bubble chamber camera does not impair the measure. ment of the scintillation light with the photomultiplier (the charge collection device was not installed during these tests). We discuss briefly two potential applications of this scintillation signal : its use in triggering the flashtube when a (rare) event occurs, and in roughly determining 0167-5087/82/0000-0000/$02.75

9'

1982 North-Ho1land

the energy of massive electromagnetic or iadronic showers. Our bubble chamber can also be operated with liquid argon/nitrogen mixtures. However, thescintillation intensity from argon is known to drop very rapidly with increasing nitrogen concentration [6] and furthermore. the detector loses immediately its ability to collect efficiently free charges (calorimetry). Adetailed discussion of the effects of impurities on the hybrid perfof our detector is beyond the scopeof this paper2. Experimental teeMique The liquid argon scintillation consists of a riarrowband ultraviolet continuum (130=5) nm 171. dw to transitions from the excited molecule Ar` to therepulsive ground state. This excited molecule is created either by self-trapping of a free exciton (33r%) or by tctitinnbin:?tion of a self-trapped hole anda free clectron (67%), 'Flit: scintillation consists of a fast component a,=(6.3 zt 0.2)ns and aslow componentr_ = (1020 :t: 60)tä. having an amplitude ratio of At /A_=13.5=1.5 18-10] . The scintillation yield in liquid argon (and xeno,a) is comparable to the yield in Nal cristaK its dE,.'dx dependence of the scintillation yield is smatter 110]. Also, scintillations in the liquids have a faster ti response than in Nal detectors. Howri- a wms'tdaable fraction of the photons emitted in theargoon scintillations is absorbed by surrounding wall tauter ails, bl:cause there are no good reflectors far ultraviolet ph " tons. Theuse of the scintillation light in the far UV p,,for our bubble chamber/cakwinxter two ecchaàal problems, which must be solved: firstly, the light -t

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pass the chamber wind"', will it large transmission losses . and secondly . the recording photoinultiplier has to lie protected against the flash light of the camera . which is fired shortly (milliseconds) after the passage of the particles . The first problem can he solved by the use of a tcnvclength shifter (WLS). either deposited on the liquid side of the chamber window . or dissolved in the liquid itself. Since our detector is designed to operate also as charge collecting calorimeter, :my poisoning of liquid argon with electronegative atoms,'molecules trust be avoided . Therefore. we gave preference to a coating of the window. We selected out of a multitude of WLSs, used for high energy experiments (11-13], para-terphenyl (pTP)' and vacuum evaporated it onto the window. We have chosen a 10(10 tun thick layer (approximately 100 ji g,cnr). which yields a maximum gain [12) . The lifetime of the excited state of pTP is 5.5 its, which is slightly shorter even than the main component of the scintillation in argon. For our applications . there is no need for a faster decaying WLS. Para-terphenyl has a slightly opaque appearance in visible light. Therefore, we coated only two D-shaped parts (135 cm= total) of the window, leaving a centre stripe of 4cut width pependicular to the beam direction uncoveredto allow track photography (fig . I) . This enables us to relate quantitatively the number of particles passing through the liquid and registered as tracks on film, to the amplitude of the scintillation signal (or the number of scintillation counts). As yet no attempt has been made to improve the light output by covering theWLS with an antireflection coating . Magnesium fluoride (MgF.) could be used, which is transparent for light down to 115tun(12]. Such a coating could enhance the mechanical resistance of p3 P. However, the eleclronegativity of MgF. as coinpared to pTP is not yet known at cryogenic temperatures. There is a high absorption of the scintillation light by the stainless steel walls and by the scotchlite . However, at present it has not been necessary to improve the reflectivity, sino: the recorded scintillation signal is high enough for the envisaged applications. Only readily available materials were used for our feasibility test. We mounted " t Philips XP 2020 photomultiplier (PM) above the camera outside the vacuum tank of our detector, -90 cm away from the liquid (fig. 2) . ThePM hasa44 mm diameter sensitive cathode and covers a solid angle of 1.9 x 10-3 sr. A blue filter (H 320', from MTO Paris) is attached to the front of the PM as protection against the light from thef,ashtube. To improve the situation further, a high-pass yellow filter (DT gab 104/234, from Balzers) is mooted in manufacturer Merck.

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Fig. I. Photo of 200MeV/cpion tracks in argon. The chamber window is partially covered, upstream and downstream with the wavelength shifter and is therefore not transparent in these regions.

front of the annular flashtube and the camera Icns . In addition, should it become necessary, the high-voltage of the PM could be gated open for thebeam spill of the accelerator (--80 ps basewidth) and turned off befor! the flash is fired -- 1 ms later, thus avoiding any saturation effects of the photocathode and the current integrator by unwanted photons. The scintillation light, amplified by the PM, is transmitted to a charge sensitive ADC. The ADC values, together with the sealer signal from coincidence counters in the beam line in front of the detector, areread out via CAMAC to an HewlettPackardcomputer. For bright-field photography of the tracks we used

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133

Fig. 2 . General layout of the detector: (1) vacuum : ink, (21 chamber vecseh 13) expintsion piston. covered with cotchlüe, (31 chas: :^.. window, ($) wavelength shifter. (6) mirror. (7) vacuum lank window, (K) camera, (9) :mnuh;r flash. 1101 I'll- fit:cr. tltl photomultiplier, (12) blue filter . customary high-contrast, high-resolution. low-sensitivity film (Kodak microfite 2786) . Fig . 3 shows the spectral characteristics of the various optical components . Fig . 3a gives in arbitrary unit, the scintillation light in liquid argon and the light shifted by polycristalline pTi' [111. The light distribution of the xenon flashtube is sotoothed [161 for an average electrical power (20 J, ptaduced by the di,charge of 3 pF at 3700 V), tine l .'e-width of the Nigh! pulse being -- 2 ins . Fig. 3b shows the optical transmission of the tyro filters and of the chamber and vacuum tank wind(I3K7, front Schott). thickness 35 and 2" min, re. -lively Fig . 3c gives . i n arbitrary lo~garitlunic units, the s) ::teal respom,e of ottr FM and filin . 3. Experimental results r sce-s

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Fig. 3. Spectral characteristics of variou, optical cumponenis (see text).

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.Srurulhnuu, hghr /lOrrl hquu! ru;errn ++rutd rc>ult in incrca"d rhcclroo mlacluncnt . if 1,1 P is 'ha b"', of. rlrrtroncgati+e, and C0n1li TOO _. .99 .999y . . . . . . . . .. ,n c:dorimrtrr . lh. argon punt) +yas !sprritiic;tion of the manufacturer : contamin ;uion with () .<0 .1 . H, " =() .11!11, C'1-1 a - 000 . Ii1 'Ih . cell- filter in front of the annular (lash tube and the pi graphic Ien> ..daces slightly the light r~utput . and hen .. its r...pl,on in till camera . 'l'he rr~loaion in optical &nsil~' on fill,, arises from the rata++ :n- ..Pon Im,ycen 4111 and 50(1 lint, sine. our fila, is lot s .nsitia bcl- 410 lint nor above 6311 nun . ."1s ry ;+rrt.d front fig. 3 . the flash voltage had to he inc--d he - 116 +c hen tile filter +c as used . 'Ihe light intensi, was chosen to give in bright-field illumination, a harkaround density of
tile I'M nm to 4511 nn,. -1 it, ina,innnn sensitivity of being al (400 - 30) n.n. there is a comfortable overlap operational region of d,c p'l'l' enhancing the +citli tile >ensitivil~" of the detector. With :t constant inchlent h:trliclc flux and tile flash not fired. tile ratio of signal heights . nuasur.d +cilh ;md +cilltoul the hluc filter. wars

-1 :1 .5. ,as fired with 20 I. When the bubble chi her (lash and the I'M ,a, operated at 2.4 kV, tile small overlap of tltc tr;utsntission cures of the t+vo filters caused an undesirable l'M signal of - 120/(A,_k , This Value is in reasonable agreement wills tic light l,emission a ltd its reflection from tile scotch lite into 11,e M. Major uncertainties in this estimate arise front the sharp drop of reflection front scolchlite with increasing angle of incidence and the variation of this angle across our detec. Under tile above conditions, the recovery [title of tor the PM was -50 nts. lio-er. the base-line of the integrated signal used to feed the ADC's was ,till sonie-

,cop, nirtures of tl  scintillation signa : (a) recorded directly from FNI at 2 .7 kV, single bean, pulse. 450 tracks: 1(100 s drc : (h) "corded alt,, int,grninn PM at 7.4 kV, single bean, pulse, 45 tracks: 100 mV/div . 100 ps/div; (c) rucrrded . 50 gs/diva (d) recorded after integration ai 2.4 kV, 10 bean, di:rc :iy from pal at 2.4 L>V, 10 brain pidses.45 tracks each : 500 mV/div pulse,. 45 t...ks each: 100 mV," di,. 50 p,/dive

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what raised. which would snake it necessary to gate ti,_high-voltage of flu, I'M or the current intclirator dunn :, Ilu discharge of the flash tube . when even fepetili, n rates higher than I liz arc attempted . Typically. flasluubes in bubble chambers are fired some 0.5 to 10 n,s after the passage of particles. the delay depending essentially on the desired bubble diameter (size of the chamber and optical resolution) . wherthe individual scintillation signals are collected within nanoseconds. the precise time depending mainly on the duration of the beam spill. We did not find any difference in the amplitude of the scintillation signal when the bubble chamber was -pandcd or not . This implies that in liquid argon at our relatively sloe' expansions sono-lurniniscence effects do not occur. which had been preciously observed at very rapid expansions (produced by ultrasonic Iechniques) in Freon 1I (CCIF,) [181. No influence of n,ectr

anlcai vibrations on our results have been found either . In our set-up . we had to photograph the tracks and to detect the scintillation light through the same windo,c. In .ray larger detector. one would probably decouplc drew functions be using separate optic ports.

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1.= . S,mrillulion rtrrensttc C - -_, Il,e 20() M,V . - r pi .,n h-rn fthe CH"', , a spill duration of -40 Its fwhn, and a base -.ah  : - 80 Its . "F he intrn,ity distnbution i-id, this p :'._c . . gaussian-like and has a 60 ns nucro-u-ire. the t- , ; features of the bean, pal,, a,, reflected in th1 scx: . !,lion signal (fig. 4a). Ilie repetition rate of the tor is 183 's . hhc hcan, intcn,itc ,ta, n,ontiorad ;, a .~~- . .., .~.. counters in front of the detector . 1 Ian suet, that there was a one-to-oar rorre,pondrnc, -.I-en couni, and ei,ibl, 'rack, in the i,ubhlc l :_-~Fhe long-teen stability of tine hcan, ii-r it, ., :, " h t_r than about - 1', . htny-r it caricd train pal- t :, pub, approximately , :0`i r.nts. . assten on phot- . i ït-_ rnaxirnuni number of particle, during fl-, test, burs. through the chamber wits 250 Most of the results "veer obtained at a rri,,ui :, : ~ . . .~ .; of I s . For technical reason,, our n,-,ure :nl.u . hate not been take!, 11In ultaneou,ly "It'! lh . buh " i~ chamber picture, . '~ ", Since the direct signal Iron, the P\1 dec .> ::1 nano-end,, and the bran, puts ha, a -h-art :: .-r , " 611 n, . which mean, up to - I000 ,u}, puhrs_ t. .~ _ ,hsnal, -rc not eon,palibl, with the use f t: . : ,Mï4 . l Ixrr(ore a feedback rurrrnt integrator -th ," rr,pon,e occr IOU u, was built . whirl, inte,_ra,rd th ; Pi : current pul,r, brforr tl- tart,d to deca, . :\ to the ADC was delayed to be ruar t ¬ w max,n,tu ".: of 1 integrated current . For one accelerator PLO- the :,r :_ : . u;d and the integrarod P\I signal, ara ,I"own u, G_, and 4b, -p--k . and in fig, . 4, and 4d 1110 :,u ,fwnding signals . when ten acce!erat," r pu¬ - c

,upcrin,po"d on 111, -,ill I". For hl lhs, . 41,, c . tI -45 track, ,crr1 ,i,tN, ott , .. .t . . ,harsher photo. Sy,lrnuttic mrasuren,rnt, serer mad, f. "r At?, : : ..,. nP as function of particle 1-n, linear behaeiour seas found . ,cllich I1ad s ,ut fifty ADC channel, ffr",. ? and (l . d".1 _: . . . graph, are r.n,killi ;tfwn , fl", .I in these value, obtained earl, Inns it I'-t 40 t hroe:r i,_ . ..dnl, due to: I I-, error, arc n, .. a) vmiation, of 1111 numb,' of partirll, m r . . . .  a crrif!rd be counung ,traig ¬ tt tract, ott z0~i r,,n.s. dlci ;ttio  s :rrr,nm, for ;ä1'v1 t:.. . .

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.61 1oo isu zoo isu PP, - sô Fig . 5 . Dependence of ADC' count, upnn hram inlrn,m, I'\t lit 2 .25 kV .

error bars), b) it, position of J,r track, m ti,, c . ..., .. . . . . .. . ., . c) the varying d:,u~.bu :ion of 111, ,pill . The lalror effect c ;tusis a alter r! tin. m :,aarc :;r intrgr:ucd "n"n, (frvs. 4,1), Leluticr to t01 utntlc .g . .: . :\DC sure, which accounts for anotfter "j"-prerea"d,' of the-craw. ,1 , 111- error, arc cxplctlat h" ". senile of maenitudr smaller in the raw,agt,

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Fig. 6. Dependency at ADC counts upon PM voltage. With hive filter. particle flux as parameter. Solid line: gain of PM. normalized at experimental point of highest flux and 2.4 kV (big circle); dashed and dotted lines : gain of PM, scaled for lower fluxes.

tions : there, the scintillation pulses from the various tracks of an interaction are created at the same time, and the time spread of the pulse is only determined by the different distances of the tracks from the recording PM, as well as the decay time of the scintillation pulse and theWLS. Fig. 6shows thedependency of ADC readings on the PM voltage. with particle intensities as parameter. The lines are the gain curves of the PM, normalized at a PM voltage of 2.4 kV and the highest beam intensity . The experimental values fit these curves well. Theerror bars at the high PM voltage and high intensity points are somewhat smaller than elsewhere. because the current integrator reached saturation during some of the particle pulses . For the highest beam intensity the ADC values are slightly lower, when thebubble chamber flash was fired shortly after the beam pulse: this left the base line of the curre .u integrator at ahigher level for thefollowing beam spill . When the blue filter was removed, but particle flux and PM voltage were kept constant, the scintillation signal increased by approximately 35%. This result can be explained best by using theWLS spectrum for polycrystalline pTP of ref. (III (rather than refs. 12 or 14). and the cut-off by the blue filter towards large wavelengths. A rough estimate of the number of photoelectrons N(PE), to be detected by our PM, can be based on the assumptions used in Table I . Then we obtain for a single track prior to amplification by the PM N(PE)/track = 1,

Table 1 (dE/dx)..,=2 MeV g- ' cm2 , d=1.0 g/cms. (dE/dx ) 50.04(d E/dx) E=9.5 eV. N- /.N.

ale =25% .

L~ =16 em, L, = L.,/2. .t =1 .9 . 10-3 sr, Qm=25% (manu futumr) or =12% 1121, G=7x10°.

total energy loss of minimum ionizing particles density of liquid argon at 135 K energy loss of a-particles from Am-decay going into scintillation [171: no better caimates for liquid argon available energy of photons from scintillation ratio of incident to emitted photons of WLS. estimated from : (a) energy of emitted photons E =3.2 eV [111, or E.=2.9 eV 1121 (b) isotropic reemission of photons (c) transmittance of WLS to its own fluorescence light [141, but not for incident photons reflection losses on 6 glass surfaces maximum track length in bubble chamber effective track length in chamber for scintillation, dueto reduced area of WLS on wi .mdow (S=135 ent ) solid angle from each point of a track to the PM surface quantm^. _`7mciency of PM at 390 nm gain ni PM zt 2.4 kV

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and get after amplification 1 .7 x 10' electrons. If we assume further, that a scintillation pulse produced by one particle, has an overall duration of less than 60 ns (microstructure of the beam spill -g~ 60 ns, decay time(s) of scintillation Z6 ns. lifetime of excited state of WLS 5.5 ns), then this results in a "M anode current of ^" 180,.A. On the other hand, we measure on the oscilloscope (capacitance of the sable ^ 10 pF, connector -20 pF, impedance at anode of PM 4.7 U). an average pulse height of 500 to 1000 mV, with a pulse width of 300 ns (one time constant) . This corresponds to 2200 tLA, in order of magnitude agreement with our theoretical estimate. 4. Conclusions and outlook Our test detector was filled with liquid argon of high purity. In spite of the very restricted solid angle of the PM. we observed scintillation pulses produced by minimum ionizing particles. At present, the level of sensitivity corresponds to -25 cm track length, a performance which can be considerably improved in any new detector by a better geometric layout. We demonstrated that during bubble chamber operation the detection of the scintillation signal is compatible with the light pulse of the flashtube. Thecompatibility of pTP with argon purity requirements for charge collection will be investigated in our next tests when the decrease of scintillation in the presence of high electrical fields [19,12] will be studied . Because of its relatively small size and geometrical layout, ourdetector is not suited for systematic investigations of the reabsorption of the far UV light in liquid argon. The scintillation light pulse can be reliably used as a fast trigger signal for the bubble chamber camera,when an event with more than a preselected number of particles or track lengths occurs. In any future large detector of this type, several optic ports could be equipped with PMs: then the current pulse, produced by scintillation light. could be fed into a computer. Its amplitude would give information on energy contained in heavy electromagnetic and hadronic shower.:, thus complementing the results from a charge collection device. It can also trigger a gate between a charge sensitive amplifier andadouble differentiating :".mplifier of suitable shaping time constant to collee: total charge . This would allow a drastic reductio-, in microphonic noise andeffects from the change of capacitance arising front bubble chamber expan*ions [4], particularly when long beam spill times are involved. There is a comfortably long time gap between the arrival of the scintillation light at the PM and the arrival of the first free electrons. The light needs only 33 ps/cm plus the scintillation

139

time constants of argon with 6.3 ns and of the WLS with 5.5 ns, whereas the electrons, moving in an electrical field of -2 kV/cm towards the charge collection electrode, need --5 ps/cm. Furthermore. the scintillation signal could be used to trigger a laser beam 151 almost simultaneously with the event . which in turn couldproduce a string of bubbles, which have thesanx size as the bubbles of the particle tracks from an event, thus markingan interaction of interest, and providing at the same time a fiducial line, which is subject to thx same liquid and bubble movement as the tracks from the event. The authors are greatly indebted to B. Allerdyce, K. Gase, F. Schüfel and the operators of theCERN synchrocyclotron for their invaluable assistance during tax runs, to many people in the BEBC Group, in particular to W. Bichler, A. Carrere, P. Dupont . P. Rada and D. Voillat for their dedicated help in thepreparation of the chamber, to Ch . Nichols for the careful deposition of the wavelength shifter on the chamber window. and to A. Minten and H. Wenninger for their continuous support . One of us (G .H.) likes to thank T. Ypsilantis for useful discussions on scintillation in liquid argon. References [I1 C. Eggler andC.M. Huddleston, Nudcvnics 1411956)34. 121 J.A . Northrop and 1.C. Gursky, Nud. Insu. and%lath 3 (1958)207. [31 G. Harigel. G. Linser and F. Schenk. Nucl . In- and Meth. 187 (1981) 363. [41 J.C . Berset, M. Burns, G. Harigel, J. Lindsay, t1 Lit. and F. Schenk. to be submitted to Nud. Instr, and Mech [5) G. Harigel, H.J . Holke, G. Liner and F. Schenk, %mt. Instr . and Meth. 188 (1981) 517. 161 S. Hirai, T. Takahashi, J. Rua. and S KuMMa, Mk),, University. Tokyo, Japan, preprint RUP-81-IS (fAtcud>cc 1981) unpublished . 171 O. Cheshnovsky, B. R;¢ andJ. Jortncr, J. Clam- Ph- 57 (1972) 4628. `1 S. Kubota . M. Ifishida and A. Notera, -L Imer, and Me1h. 150 (1978) 561. 191. S. Kubota. M. Hishida, andJ. Raun, J, Phy~ C1 I (19788 2645 . 1101 T. Duke, Portugal, Phys, 12 (1981) 9. 1111 E.U. G.-i.. Y, Tomkiewicz and D. Tri- Next. Iuxtrand Meth. 107 (1973) 365. 1121 P. Bailkon, Y. Ikclaiti M. Ferns-Luzri, B, F~h, P, h , J.M . Perreau, J . Sfuimu andT, Ypsikutti& Next talc. and Meth. 126 (1975) 13, 1131 E.P. Kridvr, V.L. Jacobson, A.E. Pifer, P.A. Pl .os -d R,1. Kurz, Nud. hutr. and Meth, 1,14 (1976) V& 1141 1.8. Berlman, Handbook of (ùu--: sP.,,. otâ .-ticmolaules (Academic Press NewYork, 19714 [151 J.B . Birks, Photophysics of an-atic -,k-dc, (tkiky, New York, 1970).

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J .C. Berset et al. / Scintillation light from liquid argon

EG&G Ekxiro-Otties. Data Sheet F1002C-2, Linear Xenon Flashtubes Quartz Envelope 1/79 (1979). (171 J. SBguinot and T. Ypsilantis, private communication from an experiment made in 1976. ltal B . Hahn and P.N. Peacock. Nuovo Cimento XXVIII, No . 2 (1963) 334. 1161

(191 S. Kubota. A. Nakamoto. T. Takahashi. T. Hamada. E Shibamura, M. Miyajima, K. Masuda and T. Doke, Phys . Rev . B17 (1978) 2762.