The 64-channel Multianode Photo Multiplier has been evaluated as a possible
choice for the photo detectors of the LHCb Ring Imaging Cherenkov detector.Comment: 3 pages, 3 figures, contributed to the Proceedings of ICHEP200

Muheim, Franz

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Franz Muheim

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Multianode Photo Multipliers for Ring Imaging Cherenkov Detectors

The 64-channel Multianode Photo Multiplier has been evaluated as a possible choice for the photo detectors of the LHCb Ring Imaging Cherenkov detector.The 64-channel Multianode Photo Multiplier has been evaluated as a possible choice for the photo detectors of the LHCb Ring Imaging Cherenkov detector

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Edinburgh 2000-26Dec 2000Multianode Photo Multipliers for Ring ImagingCherenkov DetectorsFranz MuheimThe University of Edinburgh, Department of Physics and AstronomyMayfield Road, Edinburgh EH9 3JZ, Scotland/UKE-mail: F.Muheim@ed.ac.ukAbstractThe 64-channel Multianode Photo Multiplier has been evaluated as a possible choice for the photodetectors of the LHCb Ring Imaging Cherenkov detector.Contributed to the Proceedings of the 30th International Conference on High Energy Physics,7/27/2000—8/2/2000, Osaka, Japan1 IntroductionThe LHCb experiment will exploit the large ratesof B hadrons that will be produced at the LargeHadron Collider and make precision measurementsof CP violation. Excellent particle identication isneeded for LHCb, e.g. three kaons in a large mo-mentum range are produced by the decay B0s !Ds K, D+s ! +,  ! K+K− which is sensi-tive to the CP violating phase γ. Charged parti-cles will be identied by means of two Ring ImagingCherenkov (RICH) detectors. The RICH photo de-tectors must be sensitive to single photons with aquantum eciency∫QEdE  O(1eV ) and providespatial resolution with a granularity of about 2.5 x2.5 mm2 over a large area of  3m2. The photo de-tectors must work in the magnetic fringe elds dueto the LHCb dipole magnet and must cope withtraversing charged particles.2 Multianode Photo Multipli-ersThe multianode photo multiplier tube (MaPMT)consists of an array of 64 square anodes each withits own metal dynode chain incorporated into asingle vacuum tube. The pixels have an area of2:0  2:0 mm2 and are separated by 0.3 mm gaps.The MaPMT, manufactured by Hamamatsu, hasa 0.8 mm thick UV-glass window which transmitslight down to a wavelength of 200 nm. The pho-tons are converted in a Bialkali photo cathode witha quantum eciency of maximum 22% at 380 nm.The mean gain of the 12-stage dynode chain isabout 3  105 when operated at a voltage of 800V.The ratio of the sensitive photo cathode area tothe total MaPMT area including the outer casingis only  48%. This geometrical coverage can beincreased by placing a single lens with one refract-ing and one flat surface in front of each close-packedtube (Fig. 1). If the distance d of the refracting sur-face with radius-of-curvature R to the photo cath-ode is chosen to be equal to R the demagnicationfactor is  2=3. Over the full aperture of the lens,light at normal incidence with respect to the pho-todetector plane is focused onto the photo cathode,thus restoring full geometrical acceptance.dfMultianode PMActive areaLensLightFigure 1: Schematic view of lens/MaPMT system.The focusing of normally incident light is illus-trated.3 R&D ResultsThe pulse height spectrum for the MaPMT isshown in Fig. 2, measured with a LED light source.The pedestal peak and the broad signal containingmostly one photo-electron are clearly visible. Thesignal to pedestal width ratio is 40:1.Pulse Height  [ADC Counts]Entries / [4 ADC Counts ]MaPMTFigure 2: Pulse height spectrum of a pixel.An array of 3x3 MaPMTs mounted onto thefull-scale RICH1 prototype[1] has been tested ina beam at the CERN SPS facility. The cathodevoltage was set at −1000 V. Quartz lenses weremounted onto the front face of each MaPMT. Theradiator was gaseous CF4 at a pressure of 700 mbar.1NO FILE: 3x3black_and_white_nolensNO FILE: 3x3black_and_white_lensFigure 3: Cherenkov ring measured with theMaPMT array with (right plot) and without (leftplot) quartz lenses mounted in front of the tubes.The data were recorded with a pipelined electronicread-out system based on the APVm chip[2] andrunning at LHC speed (40 MHz).The data analysis included a common-modebaseline subtraction on a event-by-event basis.With the pipelined read-out electronics cross-talkwas observed. This has been investigated usingLED runs and several sources -all in the electronics- were identied. The cross-talk is removed by re-jecting signals in a pixel if there is a larger signal inone of its cross-talk partner pixels. Genuine doublehits are lost by this procedure and the photon yieldis corrected for it.The integrated signals of two runs of 6000 eventseach are shown in Fig. 3, one with and one withoutthe lenses in front of the MaPMTs. The Cherenkovring is clearly visible and the eect of the lenses isnicely demonstrated. The gain in in photo electronsby employing the lenses is 45%. The backgroundis small. We measure 6:51  0:34 photo electronswhich is is in good agreement with a full MonteCarlo simulation.We have also exposed the MaPMT/lens tocharged particles. The measured response is usedto model the background in LHCb. The sensitivityof the MaPMT to magnetic elds has been studied.The MaPMT is aected by longitudinal magneticelds but can be eectively shielded from the ex-pected eld strengths with a -metal structure.4 ConclusionsWe have successfully tested a 3x3 array ofMaPMTs. Cherenkov light can be detected overthe full area of closely packed tubes by means ofquartz lenses focusing the light onto the sensitivearea of the device. We have demonstrated that theMaPMT meets the performance requirements forcharged particle identication in the LHCb experi-ment. The MaPMT has been chosen as the backupphoto detector for LHCb.AcknowledgmentsI thank all my MaPMT collaborators for their ex-cellent work presented here.References[1] E. Albrecht et al., Nucl. Instr. and Meth. A411 (1998) 249.[2] L.L. Jones et al., \Electronics for LHC Ex-periments", Rome 1998, CERN/LHCC/98{36(1998) 185.2

Muheim, F

MultianodePhoto Multipliers for Ring Imaging Cherenkov Detectors

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Multianode Photo Multipliers for Ring Imaging Cherenkov Detectors

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