Mouëllic, B

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 EUROPEAN ORGANISATION FOR NUCLEAR RESEARCHCERN-PPE/94-1944.10.94A Comparative Study of the Reflectivityof Several Materials Used for the Wrapping of Scintillatorsin Particle Physics ExperimentsBernard MouëllicCERN, PPE DivisionAbstractWe have carried out a comparative measurement of the reflectivity of several materials which could be used for thewrapping of scintillators for either triggering, tracking or calorimetry purposes. We used either a beam of pions at 800MeV/c or a 207Bi source to irradiate a plastic scintillator, the light signal being read out by a photomultiplier. The fourbest materials were the Tyvek L-1085B, the Millipore HAWP 00010, the Tyvek L-1073D and the millipore GVHP 00010.1 Introduction The initial aim of this work was to investigate thepossibilities of improving the collection of the yieldlight in the scintillators to allow the use of a large arrayof very thin plastic scintillators with high efficiency inan experimental fast trigger. The very good resultsobtained with the first tested reflecting materials haveled us to prospect a larger range of products.Many experimental setups with either a very lownumber of generated photons or a weak front-endamplification could be realistically considered if wecould increase sufficiently the light collection.2 Setup descriptionIn a first method we used a plastic scintillator(NE102, all surfaces polished) successively wrappedby the different materials to be tested (Fig 1) andirradiated either by a minimum ionising particle beamor by the g rays of a 207Bi source. A slit in thescintillator between the particle impact point and thephotocathode avoided any direct light.In the second method (Fig 2) a tube made in thedifferent reflecting materials was introduced into acylindrical box with a small naked scintillator hangingfrom the cover. At the bottom side the tube coveredthe whole photocathode surface area.75mm80mm60mm10mmBeam ImpactThe scintillator is wrappedin different materialsThe light guide isalways wrapped in milliporeFig 1: Test Setup 1Reflector Naked ScintillatorMask (no direct lightto the photocathode)PhotocathodeFig 2: Test Setup 23 Experimental MethodWe have used two different setups to check thatthe results were not dependent on the method. Both setup were read out by the same linear chain.The pulse from the photomultiplier was sent to anADC LRS 2249, the gate being given by a telescope ofsmall scintillator counters in the test beam line ( p  + e,800 MeV/c) or by the counter signal itself when usingthe 207Bi source.The purpose of these measurements was to obtaincomparative results. Precautions were taken tominimise the fluctuations of the phototube responseand we have made repetitive checks of some referencepoints regularly during the tests.The light collection in setup 2 is a pure diffusingprocess while it is a combination of diffusion and totalreflection in setup 1.We simply recorded the ADC peak values. In thesetup 1 we subtracted from each peak value the valueobtained with the naked scintillator to exclude by thisway the part of light collected by total reflection on thewalls of the scintillator.4 ResultsThe two setups have given the same results within2% (figure 3 and table 1) .It is clear that these results are only valid with alight source centred on 425 nanometers and convertedby a standard photocathode with a spectral responsecentred around 420 nanometers. Extrapolating thequantitative results reported here without anynormalization to other setups could be misleading.00.20.40.60.811.2Reflectivity of Different MaterialsLight Collection related to the HAWP00010 Millipore (ADC peak value)Light Source : scintillation (MIP)Tyvek L-1085BMillipore HAWP0010Tyvek L-1073DMillipore GVHP0010Tyvel L-1057DTyvek L-1058DTyvek L-1422AAluminium Neher rigidAluminium Neher flexibleBicron Paint BC-620Printer PaperMuhlebachFig 3: General ResultsThe two Tyvek 1085 and 1073 show very niceperformances. These materials are extremely strongand inexpensive. The thickness of the 1085 can be adrawback in some instrumentations.The Millipore HAWP 00010 collects 1.9 timesmore light than "standard" aluminium does. Thismaterial is very fragile and it needs to be reinforced forinstance by a thin tape on the back side. It permanentlylooses its reflecting properties in case of direct contactwith water (leakages), fortunately there is no ob servedwater absorption effect in the normal hygrometricconditions.The Millipore GVHP 00010 is a good reflector. Itsmechanical qualities are good enough to be usedwithout any reinforcement and it is hydrophobic. Itcollects 1.55 times more than aluminium does.Unfortunately the Millipore products are veryexpensive and the comparison with the Tyvek 1073could be dissuasive.This study is centred on the applications where thethickness of the scintillators is much smaller than theirlength. Here the total reflection is an importantcontribution to the light collection. This part falls offwhen a refective paint is directly applied on thescintillator. As an example the total reflection in setup1 contributes respectively for 21% and 29% to the totallight collected with the millipore HAWP and theTyvek 1057. The last tested material, the Bicron paintBC 620 directly painted on the scintillator, is not in thedirect field of interest of this paper. We recorded itsresults just as reference in our setup.Table 1Material ADC chpeakvalue% of<aluminium>ThicknessmmStrengthTyvekL-1085B634 205 0.3– 0.06 excellentMilliporeHAWP00010591 191 0.2* very bad(good with tape)TyvekL-1073D496 160 0.12– 0.03 excellentMilliporeGVHP00010481 155 0.12 goodBicronpaintBC-620437 141 3 layersTyvekL-1057D371 120 0.12– 0.03 excellentTyvekL-1058D364 117 0.12– 0.03 excellentTyvekL-1422A337.5 109 0.12– 0.03 excellentAluminiumNEHER340 110 0.02 rigidvery goodAluminiumNEHER280 90 0.02 flexiblevery goodPrinterpaperMuhlebach171 55 0.09 good* When it is reinforced by a thin tapeAfter the completion of the measurementsreported above we have modified the setup 2 to findout what happens when the walls of the box were nottotally covered by the reflecting material. We havereplaced the reflecting side of the cover by a black tape.Then the results are well in favor of aluminium over allthe other ones. The explanation is easy: aluminiumensures an important light collection by directreflection while the diffusion on the other materialsforces the light to pass about everywhere. That is tosay the missing reflecting areas act as traps for thephotons.This last check shows us that either carelesswrapping or shaping necessities leading to a lack ofcoverage can dramatically reduce the collected light.5 ConclusionAlthough the aluminium foil currently used is farfrom being the best from the point of view of thereflectivity it is very thin and easy to use. It remainsappreciated when the number of generated photons ishigh enough or when the gaps between the adjacentscintillators have to be minimized.We have chosen the Millipore HAWP 00010 towrap up 60 thin scintillator counters and 48 waterCherenkov counters used in the fast trigger of thePS202 experiment at LEAR. The excellent resultsfrom the  test have been confirmed (and also thehandling difficulties).Another field of interest is the use of siliconphotodiodes to read cheap plastic scintillators in amagnetic field or when the bulkiness of thephotomultipliers prohibits their use.The absolute light yield of a NE110 or NE102plastic scintillator is around 10000 photons perMeV[1] with a bad spectral match with the photo–diode . One of us has reported in a previous internalnote[2] the possibility to obtain a signal of up to 4000electrons per cm 2 of read out photodiode from aminimum ionising particle crossing a NE102scintillator (thickness: 1.5 cm) wrapped with MilliporeHAWP 00010.This feature has been very successfully exploitedin the L3 experiment for the instrumen–tation of theirforward tagger. They report a signal over noise ratiofrom 3 to 5 depending on the configuration forminimum ionising particles crossing 1cm of plasticscintillator (NE102, HAWP00010 Milliporewrapped)[3].AcknowledgementsThanks to Claude Detraz for his technical support,to Jean-Marie Perreau for his contribution to this studyand to Kryno Geissler for his suggestion to test themillipore HAWP.References[1] I. Holl et al., IEEE Vol. 35, No 1, Feb 1988 – AMeasurement of the Light Yield of Common InorganicScintillators.[2] B. Mouëllic  PS202-89 note –  Test of a Fast AcrylicScintillator with a Photodiode Read Out.[3] M. Chemarin et al., L3 note 1580, March 1993 – TestBeam Results for an Upgraded Forward Tagger of the L3Experiment at LEP II.Addresses and specificationsMilliporeSwitzerland: Millipore AG France: Millipore SAChriesbaumstrasse 6 BP 307CH 8604 VOLKETSWIL F 78054 St Quentin Yvelines CedexCharacteristicsHAWP: type MF, mixed cellulose ester (nitrate+acetate) pore size: 0.42 m m. Price (1991): Filter-Rolle 30x300cm2: SFr509 GVHP: polyvinylidene difluoride, pore size: 0.22m m, hydrophobicTyvekSwitzerland: Du Pont de Nemours Internatiopnal SAPO Box 50CH 1218 Le Grand SaconnexCharacteristicsHigh-density polyethylene fibres. Price (1993) 100 sheets 76x102cm2 : SFr 233PaintThe Netherlands: Bicron Corporation, European OfficeMarktstraat 27A, P.O. Box 271CharacteristicsBC-620: Diffuse reflector, reflectivity=96% above 420nm, special grade of titanium dioxide in a water soluble binder, forsolid scintillators

A comparative study of the reflectivity of several materials used for the wrapping of scintillators in particle physics experiments

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