Pla-Dalmau, A.

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Fermilab FERMILAB-Conf-00/343  January 2001Frascati Physics Series Vol. nnn (2001), pp. 000-000IX Int. Conf. on Calorimetry in Part. Phys. - Annecy, Oct. 9-14, 2000EXTRUDED PLASTIC SCINTILLATOR FOR THE MINOSCALORIMETERSAnna Pla-DalmauFermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL 60510ABSTRACTMINOS is a long-baseline, neutrino-oscillation experiment. Two iron- andscintillator-calorimeters will be built, requiring almost 300 tons of nished plas-tic scintillator. In order to lower the scintillator costs, MINOS will use anFor the MINOS Scintillator Group: I. Ambats1, D.F. Anderson2, B.Baller2, P.M. Border3, B.C. Choudhary4, J.J. Grudzinski1, R. Heinz5, N.Hill1, Y. Huang4, M. Ignatenko2, T. Joe-Minor1, H.Y. Kim4, K. Lang6, V.Makeev2, K. Mellott2, D.G. Michael4, L. Miller7, W.P. Oliver8, A. Para2, K.Ruddick3, R.L. Talaga1, J. Trevor4, J.C. Yun2.| 1Argonne National Labora-tory, 9700 S. Cass Ave., Argonne, IL 60439; 2Fermi National Accelerator Lab-oratory, P.O. Box 500, Batavia, IL 60510; 3University of Minnesota, Schoolof Physics and Astronomy, Minneapolis, MN 55455; 4California Institute ofTechnology, Pasadena, CA 91125; 5Indiana University, Bloomington, IN 47405;6The University of Texas at Austin, Center for Particle Physics, Austin, TX78712; 7James Madison University, Harrisonburg, VA 22807; 8Tufts University,Physics Department, Medford, MA 02155.extruded rectangular scintillator strip with a groove for a wavelength-shiftingber readout. This scintillator has a co-extruded titanium dioxide coating as areector. This paper presents the scintillator characteristics and describes theextrusion and quality control procedures.1 IntroductionMINOS (Main Injector Neutrino Oscillation Search) is a long-baseline, neutrinooscillation experiment. It will consist of two large iron and plastic scintillatorcalorimeters. The near detector is located on the Fermilab site. The far de-tector is situated 735 km away, 710 m below the surface, in the Soudan minein northern Minnesota. These two detectors are large iron and plastic scintil-lator calorimeters. Table 1 lists some of the experimental parameters of thesedetectors 1).Table 1: Detector parameters of the MINOS experiment.Parameter ValueNear detector mass 980 (metric) tonsFar detector mass 5400 (metric) tonsSteel plates (far detector) 8-m wide, 2.54-cm thick octagonsMagnetic eld (far detector) Toroidal, 1.5 T at 2 m radiusActive detector planes Extruded polystyrene scintillatorExtruded scintillator strips 4.1-cm wide, 1-cm thick,  8-m longScintillator planes (far detector) 484 planesScintillator planes (near detector) 160 planesScintillator strips 105,216 strips in totalThe MINOS detectors need a total of 28,000 m2 (almost 300 tons) ofnished plastic scintillator. Commercially-available, cast scintillator is too ex-pensive for such a large experiment. An experimental program at Fermilabcarried over the last few years developed extruded plastic scintillator usingindustrial-grade polystyrene pellets 2; 3). The MINOS experiment focussedin developing an extruded scintillator with a co-extruded reective coating, andin optimizing the manufacturing process for higher light yield and lower costs.2 MINOS ScintillatorExtruded plastic scintillator was considered in order to lower the costs of thedetectors. The disadvantage over cast plastic scintillator was that extrudedscintillator had poorer optical quality since standard commercial polystyrenepellets are used and the presence of additives and particulate matter in thepellets cannot be controlled. The green wavelength-shifting (WLS) ber usedto readout the scintillator minimizes the eects of the lower attenuation lengthof the scintillator. The WLS ber converts the light locally and transports itwith a much longer attenuation length.The prole of the MINOS scintillator strip is shown in g.1. It has a rec-tangular cross section of 41 mm wide and 10 mm thick. There is a 2 mm deepgroove in the top surface for the 1.2-mm WLS ber. With the exception of thegroove, the scintillator is covered with a co-extruded, reective cap. This coat-ing is 0.25 mm thick and contains 15% (by weight) TiO2 in polystyrene. TheWLS bers are glued into the groove of the scintillator strip and then coupledto clear bers that go to Hamamatsu multipixel photomultiplier tubes (PMT).In most cases, both ends of the bers are read out with 8-fold multiplexing.Figure 1: Prole of scintillator strip.The MINOS scintillator uses Dow STYRON 663 polystyrene pellets. Theprimary dopant is PPO (2,5-diphenyloxazole) which is used in a concentrationof 1% by weight. The secondary dopant is POPOP (1,4-bis(5-phenyloxazol-2-yl)benzene) and is used in a concentration of 0.03% by weight. These materialswere chosen for their spectroscopic properties as well as their relatively aord-able prices. The nal scintillator costs approximately $9/kg as compared toabout $40/kg for cast material available on the market. The extruded scin-tillator also has further advantages in labor savings over cast material. Theextruded strips can be directly epoxied into aluminum panels because of thetough outer TiO2 coating. This simplies handling and fabrication of scintilla-tor planes. These aluminum panels are built into boxes (called modules) thatprovide light tightness and strength.2.1 CharacteristicsThe MINOS scintillator strips show strong absorption at wavelengths shorterthan 400 nm. The maximum emission is at 420 nm with a tail extending tolonger wavelengths. Most of the light produced in the scintillator is collectedby a wavelength-shifting ber within 2-3 cm. The distance is relatively shortbecause of reection losses and bulk absorption. The average number of photo-electrons from a scintillator module is shown in g. 2 as a function of positionalong the strips. This module consists of 20 scintillator strips, 8 m long.Figure 2: Number of photoelectrons as a function of position along the module.On the average, green wavelength-shifting (WLS) bers extend an addi-tional 1 m from each strip-end along manifolds to an optical connector. There,the WLS bers are coupled to 2-m long clear bers. The attenuation lengths ofthe 1.2 mm diameter WLS and clear bers used for readout are 5-6 m and 11-12m, respectively. For this module, the average number of photoelectrons (pe)for a cosmic ray in the middle of the strip is 11 (both ends summed). Thesemeasurement typically ranges between 9 and 14 pe. For single-sided readout,a minimum ionizing particle through the near end of the module yields about11 pe while yielding about 3 pe when measured from the far end.3 Extrusion of the ScintillatorA schematic view of the extrusion process is shown in g. 3. The extrusion ofthe MINOS scintillator entails rst the preparation of the materials for the coreand the capstock of the strips. The raw materials are then fed to the extruderand co-extruder and the strips produced. The last step is a series of qualitycontrol (QC) tests.TiO2 mixturePS pellets and dopantmixtureEXTRUDER(64-mm single screw)CO-EXTRUDER(32-mm single screw) DIE????-Scintillator strip withreector coatingFigure 3: Extrusion process.3.1 Preparation of materialThe key to producing high quality scintillator is to remove the water and theoxygen from the raw materials (polystyrene pellets and dopants). Oxygen inthe hot environment of the extruder causes degradation of the polymer. Thisoxidation is observed as a discoloration of the plastic matrix due to an increasedabsorption. As a result, the light output of the scintillator is greatly reduced.The polystyrene pellets are dried for at least 4 hours at 77 C. After the dryer,the pellets are placed in plastic-lined steel drums for 4 hours under a nitrogenpurge. Then the dopants are added to warm polystyrene pellets. The mixtureis prepared in 45-kg batches by tumbling pre-measured dopants with the pelletsfor 15 minutes under a nitrogen purge. This mixture is returned to plastic-linedsteel drums and purged with nitrogen for at least an additional 4 hours. Thedopant/polystyrene mixture is manually fed to the extruder hopper which isalso kept under a nitrogen purge.The hopper of the co-extruder is lled with a mixture of plain polystyrenepellets and TiO2-doped polystyrene pellets. This mixture yields a reectivecoating with 15% TiO2 by weight. The hopper of the co-extruder is also keptunder a nitrogen purge. However, this mixture is not dried and purged be-fore it is loaded into the hopper as the scintillator mixture is. The TiO2-doped polystyrene is prepared in a compounding factory (color company) aspolystyrene pellets containing a high TiO2 concentration.3.2 ExtrusionTwo extruders are used to produce these scintillator strips. The main extruderhas a 64-mm diameter single screw. Its function is to melt, mix and deliverthe scintillator core material to the prole die. The co-extruder has a 32-mmdiameter single screw and is attached at 90 degrees to the prole die. Thisco-extruder processes and injects the TiO2/polystyrene mixture as a reectivecoating. The scintillator exits the die face close to the correct shape at approx-imately 200 C. The material immediately enters the vacuum-sizing toolingmounted in a 2.4-m long chilled water tank. In this vacuum-sizing tank, dif-ferential pressure draws the semi-molten material to the nal dimensions. Thematerial is further cooled in a second 2.4-m chilled water section. Upon exitingthe cooling tank, the scintillator strip enters a belt puller which maintains theline speed at about 2 m/min. For the MINOS prole, this line speed corre-sponds to a material throughput of approximately 50 kg/h. After the puller,the material is fed into a traveling saw which is programmed to cut the scintil-lator to the desired length. The nished product rst undergoes quality controltests and is then moved to storage.3.3 Quality controlDuring production the dimensions of the strips are periodically checked with acaliper. The groove dimensions are tested in-line with a gauge that monitors forcollapsing grooves. The optical quality of the scintillator is also determined atthe factory using a test that was developed by MINOS. The design of this QCtechnique was challenging because of these three main requirements: speed,simplicity and safety (no radioactive source). The nal solution uses, for agiven dopant concentration, a strong correlation between the quality of thestrips and the scintillator transmission as well as the capstock reectivity. Thiswas veried after performing transmission measurements on scintillator stripsamples with known light yields. Light yields were determined using a radioac-tive source and cosmic rays. The results of the UV measurements agreed withthe other measurements. The UV technique was implemented as the pass/failtest at the extrusion factory. This technique is shown schematically in g. 4.Figure 4: Setup for quality control at the factory.A uorescence spectrophotometer (Hitachi F-2500) is used. A specialsample holder was designed to adapt the instrument to the desired test. Lightat 450 nm is introduced through the groove by optical bers at about 8 cmfrom one end of a 14 cm long sample. The intensity of the transmitted lightis then measured from that end. The ends are prepared by simply removingthe saw cuts with a le. The response is then compared to that of a referencesample with known light yield. The scintillation light yield of the referencesample is determined beforehand using either a radioactive source or cosmicrays. The reference sample utilized at the factory was chosen to have theminimum acceptable response, so that any sample with a greater transmissionwould also have greater light output. A comparison of results from the UVtechnique at the factory and the radioactive source measurement at Fermilabis shown in g. 5. The same samples were measured by both tests. Thevertical scale of the UV measurements is shifted by 10% from that of the Cs-source measurements to correspond to a probable error in calibration at thefactory. The light yield of a sample is periodically determined by cosmic raysas a cross check and the correlation is excellent.Figure 5: Comparison of UV and radioactive source measurements.The UV test at the factory is aected by dopant concentration. Materialproduced with lower concentrations of dopants would exhibit higher transmis-sion values. A uorescence measurement was established to register possibledopant problems. This quick test is performed at Fermilab using a HitachiF-4500 uorescence spectrophotometer. The end of the sample is excited with320-nm light and the dopant emission spectra recorded. If the concentration ofthe dopants varies, the emission spectra will also change. Fig. 6 shows the u-orescence spectra for three samples with measured light outputs of 164%, 84%and 50% of the reference sample. The spectra were normalized in amplitudeto peak A which is a PPO emission band. Peak B is a POPOP emission bandThe ratio between peak B and peak A needs to be above 1.2:1 in order to havethe right dopant concentration and consequently, high light yield.Figure 6: Comparison of emission spectra of three samples with dierent lightyields.4 ConclusionsHigh-quality extruded plastic scintillator can be produced in an industrial en-vironment. The material is considerably less expensive than standard castscintillator. The strips can eectively be read out with wavelength-shiftingbers. The co-extruded reective coating that MINOS has chosen improveslight collection as well as handling of the strips at the module assembly facto-ries. The QC techniques that have been developed allows the extrusion factoryto monitor the quality of the scintillator in almost real time without the use ofa radioactive source. At the present time, MINOS has extruded approximately30 tons of scintillator, only 0.5% of which had to be rejected because of lowlight yield resulting from inadequate dopant concentration. The light yield ofthe current scintillator strips is 50% higher than the initial values used in theMINOS Technical Design Report 1).References1. The MINOS Detectors, Technical Design Report, NuMI-L-337 (1998).2. A. Pla-Dalmau, A.D. Bross and K. Mellott, Extruded Plastic ScintillationDetectors, in: DPF Proccedings, January 1999 (Fermilab-Conf-99/095).3. A. Pla-Dalmau, A.D. Bross and K. Mellott, Nuc. Instr. Meth., in print(Fermilab-Pub-00/177-E).

Extruded plastic scintillator for the MINOS calorimeters

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Extruded plastic scintillator for the MINOS calorimeters

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