Latest beam test results from RICH prototypes using hybrid photo detectors and multi anode PMTs
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Nuclear Instruments and Methods in Physics Research A 433 (1999) 159}163
Latest beam test results from RICH prototypes usinghybrid photo detectors and multi anode PMTs
E. Albrecht!, M. Alemi!, G. Barber", J.H. Bibby#, N.H. Brook$, A. Duane",S. Easo$,*, L. Eklund!, V. Gibson%, T. Gys!, A.W. Halley$, N.Harnew#, M. John",
D. Piedigrossi!, B. Simmons", N. Smale#, P. Teixeira-Dias$, D. Websdale",S.A. Wotton%, K. Wyllie!
!CERN, EP Division, 1211 Geneva 23, Switzerland"Imperial College of Science Technology & Medicine, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK
#University of Oxford, Department of Nuclear Physics, Keble Road, Oxford OX1 3RH, UK$University of Glasgow, Department of Physics, Glasgow G12 8QQ, UK
%University of Cambridge, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK
Beam tests were performed in 1998 to investigate the performance of a prototype of the downstream RICH of theLHCb using hybrid photo-diodes and multi anode PMTs. The angular resolutions obtained from these photodetectorsunder various experimental con"gurations are compared with the expectations from simulation. ( 1999 ElsevierScience B.V. All rights reserved.
Particle identi"cation in LHCb  will use twoRICH detectors. These are described in anothercontribution to this conference .
As part of the research and development forthese RICH detectors , beam tests were conduc-ted with the aim of testing the performance ofthe radiators, the optical layout and the photo-detector prototypes. The results reported brie#yin this paper are from tests performed during1998 which used an optical layout corresponding tothat of the LHCb RICH2 with CF
4and air as
f 61-pixel Hybrid Photo-Diode (HPD): This de-vice, manufactured by DEP , has an S20photocathode deposited on a quartz window.The photoelectrons are accelerated througha 12 kV potential over 12 mm onto a 61-pixelsilicon detector, resulting in a gain of about 3000.The pixels are hexagonally close packed andtheir size is 2 mm between their parallel edges.The signal is read out by a Viking VA2 analogue readout ASIC.
f 64-channel Multi-Anode PMT (MAPMT): Thisdevice, manufactured by Hamamatsu, has a bi-alkali photocathode deposited on a borosilicate-glass window and there are 64 square pixels ofside 2.3 mm. The photoelectrons are multiplied
0168-9002/99/$ - see front matter ( 1999 Elsevier Science B.V. All rights reserved.PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 3 0 5 - 8 SECTION III
Fig. 1. Schematic diagram of the RICH2 testbeam setup. One of the photodetector con"gurations (setup 1) is shown.
using a 12-stage dynode chain resulting in anoverall gain of about 106, when operated at900 V.
f 2048-pixel HPD: This device was built in collab-oration with DEP . It has electrostaticcross-focussing by which the image on thephotocathode is demagni"ed by a factor of fourat the silicon detector anode. The operating volt-age of this HPD is 20 kV. The typical gain of thesilicon detector is 5000 and it has an array of2048 silicon pixels, bump bonded to an LHC1 binary readout ASIC. The HPD has anactive input window diameter of 40 mm andthe silicon pixels are rectangles of size0.05 mm]0.5 mm. It represents a half-scaleprototype of a "nal tube which will have an80 mm diameter input window and 1024 squarepixels with 0.5 mm side.
2. Experimental setup
A schematic diagram is shown in Fig. 1. Theincident beam particle direction is measured bythree planes of silicon pixel detectors which havesquare pixels of side 1.3 mm. The "rst and last ofthese planes are separated by 8 m. The Cherenkovphotons emitted are re#ected by a spherical mirrorof focal length 4 m, which is tilted by an angle of 183
with respect to the beam axis. The detectorsare placed in the focal plane of the mirror, mountedon a plate customised to particular detectorcon"gurations.
3. Data analysis
For each hit registered by the photodetector, theCherenkov angle was calculated using parametersof the optical system and the reconstructed beamdirection. The photon emission point was assumedto be the midpoint of the particle trajectorythrough the radiator. Since the mirror is tilted, thisresults in an emission point uncertainty in theCherenkov angle determination which dependsupon the location of the detector on the detectorplate. Other factors contributing to the angularresolution are chromatic aberration and granular-ity of the pixels in the photodetector and in thebeam telescope.
A Monte Carlo program was developed to simu-late the geometry of the apparatus, the refractiveindex of the radiators, transmission characteristicsof the optical elements, quantum e$ciency of thephotodetectors, mirror re#ectivity, pixel granular-ity, etc. This was used to study the various contri-butions to the uncertainty in the Cherenkov anglereconstruction.
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Fig. 2. Reconstructed Cherenkov angle distributions for (a) the two types of HPD with pyrex "lter, using air radiator. (b) the MAPMTusing CF
4.1. Single photon resolution
In one of the detector con"gurations, a 2048-pixel HPD was placed near the top of the detectorplate with the long pixel edge tangential to theCherenkov ring and three 61-pixel HPDs wereplaced around the periphery of the plate. A beam of100 GeV pions was passed through an air radiator.Pyrex "lters were placed in front of the photodetec-tors to restrict the wavelength range of the photonsdetected and thus to reduce the chromatic aberra-tion. The distributions of the Cherenkov angle,reconstructed for each hit, are plotted in Fig. 2a forthe 2048-pixel HPD and for a 61-pixel HPD whichwere situated diametrically opposite to each otheron the Cherenkov ring. The 2048-pixel HPD showsa better resolution than the 61-pixel HPD since thepixel granularity was smaller for the former. Thevarious contributions to the Cherenkov angle res-olution in the 2048 pixel HPD are determined fromthe simulations and tabulated in Table 1 in thecolumn labelled setup 1. The overall resolution
Table 1Contributions to the single photon resolution in mrad. Thecolumn labelled setup 1 is for the 2048-HPD and the columnlabelled setup 2 is for the MAPMT
Resolution contribution Setup 1 Setup 2
Chromatic error 0.147 0.140Emission point 0.049 0.076Pixel size 0.017 0.170Telescope pixel size 0.060 0.060Alignment 0.060 0.100Total MC 0.166 0.26Total data 0.185 0.27
from this simulation is in agreement with that ob-tained from data.
In a second con"guration, CF4
was used as radi-ator and an MAPMT was mounted on the detectorplate. The reconstructed single photon Cherenkovangle distribution obtained from a beam of120 GeV pions is plotted in Fig. 2b. As can be seenfrom Table 1 in the column labelled setup 2,the resolution obtained agrees with that from thesimulation.
E. Albrecht et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 159}163 161
Fig. 3. Resolutions for each of the 61-pixel HPDs without"lter (top) and with pyrex "lter (bottom). The left-hand barrepresents the measured value and the right-hand bar is fromsimulation.
In this same con"guration, seven 61-pixel HPDswere placed around the detector plate. The resolu-tions obtained with and without the pyrex "lters infront of these HPDs can be seen in Fig. 3 in theform of a bar-chart. Good agreement is obtainedbetween data and simulation for each HPD. EachHPD shows a di!erent resolution since the contri-bution to the emission point error is dependent onits location on the detector plate. The LHCb tech-nical proposal assumes 0.35 mrad single photonresolution and this is achieved for some of the HPDcon"gurations in this "gure.
4.2. Particle identixcation
The Cherenkov angle distribution shown inFig. 4 is obtained in a measurement wherea 10.5 GeV beam composed of pions and electronswas passed through air radiator. A 2048-pixelHPD was mounted on the detector plate to recordthe hits. The "ne granularity available withthis HPD allows good separation of pions andelectrons.
Fig. 4. Single photon Cherenkov angle distribution for the2048-pixel HPD using a 10.5 GeV beam composed of pions andelectrons and an air radiator.
4.3. Multi-photon resolution
A set of seven HPDs and one MAPMT weremounted on the detector plate. No "lters were used.A beam of 120 GeV pions traversed a CF
and the mean Cherenkov angle was calculated us-ing all hits per event. The width of this distributionas a function of the number of photoelectrons de-tected per event is shown in Fig. 5. This plot isexpected to be inversely proportional to the squareroot of the number of photoelectrons (indicated bythe solid curve). The disagreement between expec-tation and observation is consistent with the resid-ual misalignment in the system which is of order0.1 mrad.
Fig. 5. Cherenkov angular resolution measured using 61-pixelHPDs versus number of photoelectrons detected in single par-ticle trigger.
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5. Summary and future prospects
The Cherenkov angle resolution obtained for thethree types of photodetectors tested agrees wellwith the expectations from simulations. Using 61-pixel HPDs, a single photon resolution of0.35 mrad, as assumed in the performance studiesof the LHCb technical proposal , is alreadyattained. Particle identi"cation capability has beendemonstrated.
The photodetectors will undergo more tests in1999 using con"gurations which are closer to the
LHCb requirements. The results of these measure-ments will be used in making the choice of photo-detector technology for the LHCb RICH detectors.
 LHCb Technical Proposal CERN/LHCC 98-4. A. Go et al., Nucl. Instr. and Meth. A 433 (1999) 153. E. Albrecht et al., Nucl. Instr. and Meth. A 411 (1998) 249. Delft Electronische Producten (DEP), Netherlands. O. Toker et al., Nucl. Instr. and Meth. A 340 (1994) 572. E. Heijne et al., Nucl. Instr. and Meth. 383 (1996) 55.
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