Abstract   The experimental results coming from an intense R&D study about the possibility of detecting the light produced by the liquid argon scintillation certify the use of glass-window photomultiplier tubes. The devices, working in cryogenic liquid, are made sensitive to the VUV photons by means of a wavelength shifter coating. This is a useful detection method to provide an effective way for the absolute time measurement and trigger of ionizing events occurring in Time Projection Chambers

C. Montanari

C. Vignoli

G.L. Raselli

M. Rossella

P. Benetti

English

Open Access Repository

Nuclear Instruments and Methods in Physics Research A 505 (2003) 89–92Detection of the VUV liquid argon scintillation light bymeans of glass-window photomultiplier tubesP. Benetti, C. Montanari, G.L. Raselli*, M. Rossella, C. VignoliIstituto Nazionale di Fisica Nucleare (INFN), Sezione di Pavia, via A. Bassi 6, I-27100 Pavia, ItalyAbstractThe experimental results coming from an intense R&D study about the possibility of detecting the light produced by theliquid argon scintillation certify the use of glass-window photomultiplier tubes. The devices, working in cryogenic liquid, aremade sensitive to the VUV photons by means of a wavelength shifter coating. This is a useful detection method to providean effective way for the absolute time measurement and trigger of ionizing events occurring in Time Projection Chambers.r 2003 Elsevier Science B.V. All rights reserved.PACS: 85.60.Ha; 29.40.Mc; 29.40.GxKeywords: Scintillation light; Photomultipliers; Liquid argon detectors1. IntroductionThe liquid argon (LAr) is successfully used fornuclear particle detection, thanks to the possibilityto measure ionizing events with high energy andhigh spatial resolution by means of the TimeProjection Chamber (TPC) technique [1].The particles interacting with the LAr producefree electrons and scintillation radiation in theVUV spectrum. The scintillation light can be usedto define the interaction absolute time and totrigger the events in TPC detectors [2,3].The experimental results presented in this papercertify the use of glass-window photomultipliertubes (PMTs) directly immersed in LAr to detectthe scintillation light.2. Liquid argon scintillationCharged particles interacting in LAr createionized and excited molecular states which pro-duce scintillation radiation through recombinationand deexcitation processes [2,3]. This scintillationis characterized by a prompt ðo2 msÞ photonemission in the VUV spectrum with l ¼ 128 nmand by a yield of about 5  104 photons MeV1:In a TPC detector, the photon emission is afunction of the electric field intensity used to driftthe free electrons (300–500 V cm1). Moreover thepresence of impurities limits the photon transmis-sion in the liquid.3. Detection systemThe most practical and straightforward methodto detect the light coming from the LAr scintilla-tion makes use of photomultiplier tubes.*Corresponding author. Tel.: +39-0382-507-410; fax: +39-0382-423-241.E-mail address: gianluca.raselli@pv.infn.it (G.L. Raselli).0168-9002/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0168-9002(03)01027-1Conventional glass-window photomultiplierswith bialkali photocathode are not fit for thispurpose since they are not sensitive to VUVphotons and they do not work at LAr temperatureð87 KÞ: Moreover the use of magnesium-fluoridewindow photomultipliers is not an economicalsolution when used in large detectors.The suitable solution makes use of large surfaceglass window photomultipliers specifically manu-factured to work at cryogenic temperatures. Thesensitivity to VUV photons is achieved coating thewindow with a proper compound which acts asfluorescent wavelength shifter to the PMT sensi-tive spectrum.3.1. PhotomultiplierThe 9357FLA Electron Tubes photomultiplierwas chosen as detection device. The model, shownin Fig. 1, is an 8 in: diameter photomultiplier withhemispherical glass window, manufactured in asingle carrying glass structure without welded orglued parts. The bialkali photocathode is depos-ited on a platinum under layer which allows thelow-temperature working. Table 1 shows the mainphysical and electric characteristics of the device.The PMT working parameters at cryogenictemperature are good enough for the LAr scintil-lation detection, though the PMT gain at 87 K isonly E20% of the nominal one.3.2. Wavelength shifterThe VUV detection needs a 128 nm radiationsensitive compound with a high emission prob-ability in the blue region, such as the tetraphenyl-butadiene (TPB). If we prepare a solution with theproper solvent (toluene) the compound can beeasily deposited on glass surfaces by means of anebulizer. This method provides fine-grained anduniform layers on large surfaces.We tested coatings of different thickness atcryogenic temperature. We found that layerthicknesses under 200 mg cm2 do not presentmechanical instability after immersion in LAr.Moreover the glass sand-blasting makes the coat-ings more adherent and abiding.4. Quantum efficiency measurementWe measured the global quantum efficiency ofthe system (PMT + wavelength shifter) as afunction of the following parameters: the TPBcoating thickness, the photon angle of incidenceand the backing surface type (smooth or blastedglass) [4]. The measurements were performed bymeans of a monochromator and the results wereobtained comparing the PMT output current1 tothe intensity of a 128 nm light beam.The system quantum efficiency as a function ofthe coating thickness is presented in Fig. 2; somevalues obtained with a sodium-salicylate coating,Fig. 1. 9357FLA Electron Tubes photomultiplier with blastedglass window.Table 1Nominal physical and electric characteristics of the 9357FLAElectron Tubes photomultiplierLength and diameter 293 mm; 203 mmCathode size 190 mmSpectral response 300–500 nmDynodes 12LF CsSbSER p/v 2Rise time and FWHM 5 ns; 8 nsMaximum gain 5  107Quantum efficiency (blue) E20%HV supply 1200 V1We used for this purpose a flat 30-mm-diameter PMT,manufactured by the Electron Tubes with the same cathode andmain characteristics of the 9357FLA.P. Benetti et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 89–9290which is the most used as reference shifter, areplotted in the same figure. The measurementsshow a high conversion efficiency of the TPBcoatings, no angular dependence and, on thewhole, a better and uniform response of thecoatings on blasted glass. As a result, we foundout that the response of the PMT with theoptimized shifter thickness allows a VUV quantumefficiency around 10%.Our data, together with the value of the 128 nmlight attenuation length in LAr (E90 cm [5])allows the evaluation of the detection efficiencyas a function of the deposited energy and to definethe best PMT assembling geometry by means ofMonte Carlo programs.5. LAr scintillation light measurement in largedetectorsA demonstration of our detection methodcapability to define a PMT-based trigger andto determine the particle absolute interactingtime in a LAr TPC is given by the recent resultsobtained by the ICARUS Collaboration [6].Twenty 9357FLA TPB-coated photomultipliers,assembled in the first 300 t LAr cryostat of theT600 detector, were extensively used duringthe test run in Pavia in 2001 summer. Fig. 3shows the two-dimensional image (about 1:5 m1:5 m) of three muon tracks crossing a TPCdrift volume region. The event was recorded bymeans of a trigger logic based on a four-foldcoincidence of the photomultipliers (2 m spaced)placed behind the TPC wire planes. The PMToutput, facing the event, was shaped ð10 msÞand sampled at 200 MHz by means of anFADC, as shown in the plot on the right inFig. 3. The different time position of the PMTTPB on smooth glassTPB on blasted glassSodium-Salicylate0 200 400 600 8000286410Global quantum efficiency (%)Coating density (µg cm-2)Fig. 2. Quantum efficiency measurement as a function of theTPB coating thickness. The gray area refers to thickness valueswith no adhesion instabilities at cryogenic temperature.500600700800900100011000 100 200 300 400 500Drift coordinate (time)Spatial coordinate (wire)Sampling time (µs)PMT amplitude (A.U.)123123Fig. 3. Image of three muons crossing the first cryostat of the T600 ICARUS detector. The PMT output looking the drift volume isshown in the plot on the right. A detailed description of this event is given in Ref. [6].P. Benetti et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 89–92 91pulses, due to the scintillation light, shows thatthe tracks correspond to consecutive events cross-ing the detector drift volume at three differenttimes.6. ConclusionsA PMT system sensitive to the LAr scintillationis able to provide an effective method for thetrigger and for the absolute time measurement ofionizing events occurring in Time ProjectionChambers. Our results testify the good confidenceand reliability of the detection method making useof glass-window photomultipliers. A full demon-stration is given by the recent results obtained bythe ICARUS collaboration during the T600detector test run.AcknowledgementsThis work, part of the ICARUS project R&Dactivities, was carried out with the collaboration ofthe ‘‘Dipartimento di Elettronica e Informaticadell’Universit"a di Padova’’ and in particular withthe experimental contribution of Dr. G. Nalettoand Dr. L. Placentino.References[1] For a complete set of references and documents about theLAr TPC functioning and the ICARUS project see thefollowing web pages: http://www.aquila.infn.it/icarus/http://www.cern.ch/icarus/[2] P. Cennini, et al., Nucl. Instr. and Meth. A 432 (1999) 240.[3] C. Vignoli, Studio della luce di scintillazione dell’argonliquido per la determinazione del tempo assoluto deglieventi nel rivelatore ICARUS, Ph.D. Thesis, Univ. degliStudi di Pavia, AA 1995/1996 (in Italian).[4] G.L. Raselli, M. Rossella, C. Vignoli, A. Votano, LArscintillation light detection in the ICARUS T600 apparatus,INFN Report FNT/AE-2001/12, 2001.[5] ICARUS Collaboration, private communication.[6] ICARUS Collaboration, A second-generation proton decayexperiment and neutrino observatory at the Gran Sassolaboratory, LNGS-EXP 13/89 addendum 2/01, 2001.P. Benetti et al. / Nuclear Instruments and Methods in Physics Research A 505 (2003) 89–9292

Detection of the VUV liquid argon scintillation light by means of glass-window photomultiplier tubes

https://www.openaccessrepository.it/record/137865/files/fulltext.pdf

Detection of the VUV liquid argon scintillation light by means of glass-window photomultiplier tubes

Abstract

Similar works

Full text

Available Versions

Open Access Repository