Lining material tests for the AUGER PROJECT surface detector

PROCEEDINGS SUPPLEMENTS ELSEVIER

Nuclear Physics B (Proc. Suppl.) 75A (1999) 386-388

Lining material tests for the AUGER P R O J E C T surface detector C.O./~scobar% A.C.Fauth ~, M.M.Guzzo a, E.H.Shibuya a qnstit.uto de Fisica Gleb Wataghin-UNICAMP, 13083-970 Campinas, Sgo Paulo, Brasil We are trying to obtain a suitable material to compose the lining of a water Cerenkov tmlk for the surface derecto1, part of a hybrid detector of the Auger Project. Resulgs of tests were compared with DuPont 1073Tyvek T M and obtained a reasonable performance for (PVC+BaSO4) material.

1. I n t r o d u c t i o n A hybrid detector proposed to the Pierre Auger Project is an array with water Cerenkov tanks and mirrors to observe fluorescence light in the atmosphere, both produced by particles of Cosmic [qadiation entering the atmosphere and multiplicating through. The main aim of the Pierre Auger Project is to detect Extensive Air Shower with energy E > 10 ~'J eV with the particular interest to increase the statistics for E ,-~ 102° eV, where GZK cut-off could exist. Our contribution to the engineering phase of the Pierre Auger Project is restricted to a search for a convenient reflective material. For that purpose we measured the reflection of some commercia.lly available materials and a specially made P \ " C + B a S 0 4 material, under request and recipes to SANSITY S/A - IND. DE PLASTICOS, a Brazilian plastic industry. First., we did measurements of relative diffuse reflection on dry materials, using a P E R K I N ELMER,, L a m b d a 9 - series 1645 - U V / V I S / N I R Spectrometer, part of I F G W / U N I C A M P facilities. In a second step, a flexible tank full of tap water, lined by ( P V C + B a S O 4 ) or DuPont t073Tyvek was used. Measurements of transmittance were also clone for both materials and we tbund that no transmitted signal was observed for the PVC. bag; this one is then sufficiently shielded against external light, effect on the Cerenkov light I)rodueed inside the tank.

2.

Measurements

Figure 1 shows the beforehand mentioned measurements of dry materials. As the Cerenkov light produced inside the tank by Cosmic-Ray particles is in a ultraviolet region, we disregard several other tested materials, for instance TiO2. Some other tested material, mainly therrnolurninescent materials (Teflon4-BaSO4:Eu or Teflon+Mg2SiO4) showed a relative diffuse reflection greater than 100%. This could be caused by Eu, that is the essential luminescent chemical element, taking into account that the reference material of the spectrophotometer for the diffuse relative reflection measurements is an A1 plate painted with BaSO4, as it is written in the spectrometer's manual. From these results and from observations we decided to concentrate the measurements to the (PVC+BaSO4) material, testing m a n y combinations of chemical element concentrations and thickness of the sheets. Afterwards we measured the signal of cosmicray particles (mainly muons) in a water tank, first with ( P V C + B a S O 4 ) and after with DuPont 1073Tyvek bags, working as lining material. The tank has d i a m e t e r = l . 2 m , h e i g h t = l . 3 m , filled with ~ 1,500 liters of tap water and provided with a photomultiplier Phillips XP2040 optically coupled to a plexiglass base. A pair of plastic scintillators, with different sizes was located over the tank. Following preliminary observations, calibrations and measurements, we moved one of the plastic scintillators under the water tank, in a vertical line linking the upper scintillator, the tank and the lower (smaller) scintillator, expect-

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C.O. Escobar et al.INuclear Physics B (Proc. SuppL) 75.4 (1999) 386-388

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Defining the ratio between the 3-fold (2 plastic scintillators + water tank) and the 2-fold (2 plastic scintillators) coincidences countings as a quantity proportional to the counting efficiency of the water C,erenkov tank, we got the set of Figure 2 (the signal threshold was 20mV/50f~). 3. C o n c l u s i o n s a n d D i s c u s s i o n s

Analysis of (PVC+BaSO4) combination shows a little less efficiency than DuPont Tyvek1073, as shown in Figures 1 and 2. But in this material the decrease of signal is more pronounced than in (PVC+BaSO.~). This effect in Tyvek was already reported by Bai Xinhua, using an American Chinese tank. His explanation of this effect is related

Figure 2. Efficiency estimation

to lower Tyvek reflectivity caused by water filling of Tyvek fibers or by the activity of remnant bacteria in the water. The Cerenkov light detected by the photomultiplier is a superposition of the totally reflected Right by the lining material. If so, when the reflective index of the material changes with time the Cerenkov signal should also change. The (PVC+BaS04) material is compact, whereas the Tyvek have air gaps, because it is made with fibers encrusted in a polyethylene sheet. So, the total reflection of Cerenkov light in Tyvek could suffer decrease as time of soaking goes, due to the progressively occupation of air gaps by more reflective material(water a n d / o r fibers composing the material). To look for a possible effect of soaking that may

388

C.O. Escobar et al./Nuclear Physics B (Proc. SuppL) 75.,1 (1999) 386--388

explain the observed decrease of signal, we tested small samples of DuPont 1073Tyvek used inside the tank, a (PVC+BaSO4) sheet also used inside t.he tank and a (Tyvek+black polyethylene sheet welded together) made and used at Fermilab. Resuits from these nleasurements are not conclusive about the effect, in spite of a small decrease of signal along the time of soaking, effect that is not observed in the (PVC+BaSO4) material probably because this ma.teriM is more compact (Figure 3).

series of tests shown in Figures 3 are different, because these tests was done with the sample materials embedded with deionized water in a small cups of 1 ml capacity. Then all observations could be consistent with a possible effect of soaking in Tyvek and so the Sansuy made (PVC+BaSO4) combination could be a reasonable alternative for the lining of the tank because it doesn't have pronounced air gaps that could be filled with water and bacteria. 4. A k n o w l e d g m e n t s

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Apparently, the wetting effect is more pronounced in tile (Tyvek+black polyehylene) than in our previously wetted DuPont Tyvek1073. In any way the effect of soaking is not conclusive, but it is necessary to remark that the wetting conditions of the Tyvek inside the tank and the

The authors would like to thanks many people for support and adviees, in particular the Brazilian branch of DuPont (Mr. O.Porto) and SANSUY S/A (T.Honda, Y.Hirasaki, S.Morita, R.O.Brostel, H.Imagawa, A.C.Iguchi, G.Hanashiro) for the financial support a n d / o r free of charge materials and technical services. Also we are indebted to Mr.A.C.da Costa for the help and measurements of reflectivity and transmittance on dried materials. We had also a financial support from C P G - I F G W / U N I C A M P for materials. One of us (EHS) acknowledges FAEP/UNICAMP, the Organizing Committee of X th ISVHECRI (Prof.O.Saavedra) for the support to present this report at the Symposium and Prof.F.Galembeek for many advices and informations about refleetivity of materiMs. Finally we are grateful to Prof. R.C.Shellard, who first called our attention to the possibility of a flexible plastic tank use.