*Corresponding author.E-mail address: samo.korpar@ijs.si (S. Korpar)

Nuclear Instruments and Methods in Physics Research A 433 (1999) 128}135

The HERA-B RICH

S. Korpar!,*, I. Arinyo", M. Atiya#, D. Broemmelsiek$, J. Carvalho%, D. DujmicH &,R. Eckmann&, L. Garrido", A. Goris\ ek!, I. Ivaniouchenkov%, M. Ispirian',S. Karabekian', P. Kriz\ an!, K. Lau), P. Maas$, J. McGill&, R. Miquel",

R. Pestotnik!, J. Pyrlik), D. Ramachadran), K. Reeves&, J. Rosen$, R. Schwitters&,M. Staric\ !, A. Stanovnik!, D. S[ krk!

!J. Stefan Institute and University of Ljubljana, Ljubljana, Slovenia"University of Barcelona, Barcelona, Spain

#BNL, Upton, USA$Northwestern University, Evanston, USA

%LIP Coimbra, Portugal&University of Texas, Austin, USA

'DESY, Hamburg, Germany)University of Houston, Houston, USA

Abstract

The essential components of the ring imaging Cherenkov detector for the HERA-B experiment are brie#y described.Results of the "rst test measurements with the HERA proton beam are presented and the capability of the RICH toidentify kaons is estimated. ( 1999 Elsevier Science B.V. All rights reserved.

1. Introduction

The HERA-B experiment [1] aims at measuring:CP violation in B0PJ/tK0

Sand B0Pp`p~

decays, B0sBM 0s

mixing and BBM decays with two lep-tons in the "nal state. The B mesons will be produc-ed in collisions of 820 GeV/c protons with a "xedtarget. The target will consist of 8 ribbons in thehalo of the proton beam in order not to disturbexperiments measuring ep collisions.

One of the essential components of the spectro-meter is the ring imaging counter (RICH), with the

prime function of separating kaons from pions[2,3]. The identi"cation of kaons serves to tag the#avour of one of the B mesons in the CP violatingmeasurement.

In what follows we shall brie#y describe theessential components of the RICH counter and willthen discuss recent results obtained with the820 GeV/c proton beam interacting in the HERA-Btargets.

2. Radiator

The 100 m3 vessel for the C4F10

gas radiator hasbeen constructed out of stainless steel, except forthe particle entry and exit windows (1 mm Al) and

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 2 4 - 1

Fig. 1. A photograph of the mirror system.

Fig. 2. The optical system for light collection and demagni"ca-tion.

the photon exit windows (UVT perspex). Also, twobeam shrouds close the gas volume around the twobeam pipes for protons and electrons.

The puri"cation and circulation system for theC

4F10

gas has been constructed at CERN and isbeing commissioned at DESY. It is worth notingthat the initial "lling of the volume will amount toabout 1100 kg of C

4F10

.

3. Mirrors

Two mirror systems, a spherical and a planarone, are made of hexagonal and rectangular units,respectively. The hexagonal mirror segments with11.4 m radius of curvature have been produced byGlass Mountain Optics from 7 mm thick grindedglass. The mirror quality has been remeasuredupon delivery [4]. For each segment the radius ofcurvature was measured, as well as the fraction ofscattered light. In addition, a Ronchi image hasbeen recorded by a camera to check the homogen-eity of the mirror surface. The re#ectivity is

S. Korpar et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 128}135 129

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Fig. 3. Base board with PMT sockets and front end electronics.

required to exceed 85% in the wavelength interval250}600 nm. This was veri"ed by measuring there#ectivity of the two witness samples.

By making use of the data gathered on all themirror segments, it was possible to group them inthe tiling scheme according to their optical qualityand resolution requirements [5].

Each spherical mirror segment is supported atthree points, two of which can be moved, and aremotor driven via a transmission mechanism witha feed through to the exterior of the vessel.

The planar mirrors are made of #oat glass, thusbeing signi"cantly cheaper at the required opticalquality.

A photograph of the mirrors as mounted in theradiator vessel is seen in Fig. 1.

4. Photon detector

The initially foreseen detectors based on wirechambers had to be abandoned; the TMAE de-tector showed a prohibitive decrease of avalanchegain due to aging e!ects, while the CsI photo-cathode could not be routinely produced and main-tained with su$ciently high quantum e$ciency inaddition to problems with rates in excess of a fewkHz per pixel. The "nal photon detector thus con-sists of Hamamatsu multianode R5900 M16 andM4 photomultiplier tubes. Due to the smallerphotocathode surface compared to the photomul-tiplier cross section, a two lens demagni"cationsystem (2 : 1) was designed in order to adjust therequired pixel size to the PMT pad size (Fig. 2). The

130 S. Korpar et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 128}135

Fig. 4. Cherenkov rings of 3 GeV/c electrons as the argon gas radiator is progressively replaced by the C4F10

gas.

granularity of the photon detector has been chosenon the basis of expected occupancy and the re-quired position resolution to be either 9]9 or18]18 mm2. Recent tests have shown satisfactoryperformance under an elevated counting rate of3 MHz per channel during a period of 30 days [6].

The required demagni"cation of a factor of 2 isachieved with a two lens system consisting of a "eldlens and a condensor lens as is shown in Fig. 2 [7].The lenses are made by WAHL Kunststo!optikGmbH of UVT perspex with high transparencyover most of the wavelength region wherethe photocathode is sensitive [4]. The angular

acceptance of the optical system is also satisfactoryand is uniform for incident angles below about110 mrad [4].

The base board with sockets for the multianodephotomultipliers and front end electronics is shownin Fig. 3. The base board accepts four PMTs andprovides positioning. In addition it houses the volt-age divider, signal lines and the front-end elec-tronics consisting of a 16-channel board based onthe ASD8 chip.

In order to reduce the contribution of sphericalaberration to the overall resolution of the Cheren-kov angle, an optimal surface of the Cherenkov

S. Korpar et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 128}135 131

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Fig. 5. The count rate on a partly equipped photon detectorversus interaction rate in the HERA-B targets.

Fig. 6. Count rate versus high voltage for an M4 (top diagram)and M16 (lower diagram) photomultiplier.

photon detector has been calculated [8]. Eachhalf-detector (upper and lower) consists of 5 #atsupermodules placed in order to approximate theoptimal surface, which is close to the shape ofa #attened (ellipsoidal) cylinder. Such an arrange-ment also ensures better acceptance for the Cheren-kov photons, which should be incident onto the #atsupermodules at angles below 110 mrad.

5. Test measurements

An array consisting of 36 ("6]6) M16 photo-multipliers has been tested with 3 GeV electrons in5 m of argon gas radiator at the T24 test beam inDESY. Fig. 4 shows a sector of the measuredCherenkov ring with the ring radius growing whilethe argon gas is being substituted with the C

4F10

radiator. The number of detected photons scaled tothe full ring with complete photocathode coverage,increased from 21 for pure argon to 72 after 28 h of#ushing with C

4F10

. These numbers correspondto a "gure of merit N

0"70 cm~1 for the test

apparatus.The full set of 1500 M16 and 750 M4

Hamamatsu R5900 photomultipliers has been tes-ted on the bench. The individual channel count ratedue to Cherenkov radiation caused by a 90Srsource in a quartz radiator has been measured asa function of high voltage and threshold. On thebasis of these tests, the photomultipliers have beengrouped according to similar HV characteristics, sothat all PMTs within a group could be set to thesame high voltage value. Monte Carlo calculationsof occupancy and resolution indicated that theouter region of the RICH photon detector could beoccupied by M4 PMTs (9]9 mm2 pad), while M16PMTs (4.5]4.5 mm2 pad) should be used for thecentral part.

With the photon detectors in their proper posi-tion in the HERA-B spectrometer, the photomul-tipliers cabled to the readout system and with air asCherenkov radiator, some data have been taken.Fig. 5 shows the linear dependence of the Cheren-kov photon count rate on the proton interactionrate in the HERA-B targets. Fig. 6 gives the countrate versus high voltage for one M16 and one M4photomultiplier. It is seen that the curves measured

132 S. Korpar et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 128}135

Fig. 7. The occupancy of the upper and lower photon detectors shows the region occupied by M16 (inner region) and M4 PMTs (outerregion).

in-situ with the HERA proton beam agree nicelywith the 90Sr source measurements. The occupancyis shown in Fig. 7, where the region occupied byM16 PMs is clearly distinguished from the regionoccupied by M4 PMs.

Some events obtained by a random readout trig-ger are shown in Fig. 8. In most events Cherenkovrings can be seen by the naked eye. Especiallyinteresting is the event in the lowest image ofFig. 8, where the ring consists of some 15 hits,which is about two times the expected number.The interpretation is that this ring is due to an

electron}positron pair with nearly coincidenttracks.

6. Conclusions

On the basis of tests described above, we expectto detect 32$2 photons per ring of a b"1 par-ticle, when the radiator vessel is "lled withC

4F10

. This corresponds to a "gure of meritN

0"42 cm~1 and should allow identi"cation of

kaons up to momenta of at least 50 GeV/c.

S. Korpar et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 128}135 133

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Fig. 8. Display of three events as registered by the upper and lower photon detectors.

134 S. Korpar et al. / Nuclear Instruments and Methods in Physics Research A 433 (1999) 128}135

Fig. 8. Continued.

References

[1] T. Lohse et al., Proposal for HERA-B, DESY PRC-94/02,May 1994.

[2] P. Kriz\ an et al., Nucl. Instr. and Meth. A 371 (1996) 295.[3] J.L. Rosen, Nucl. Instr. and Meth. A 408 (1998) 191.[4] P. Kriz\ an et al., 1997 IEEE Nuclear Science Symp. and

Medical Imaging Conf., November 1997, Albuquerque,New Mexico, Conference Records, pp. 353}356.

[5] D. DujmicH , K. Reeves, HERA-B RICH Note 97-182, Ham-burg 1997.

[6] P. Kriz\ an et al., Nucl. Instr. and Meth. A 394 (1997)27.

[7] D. Broemmelsiek, Nucl. Instr. and Meth. A 433 (1999)136.

[8] P. Kriz\ an, M. Staric\ , Nucl. Instr. and Meth. A 379 (1996)124.

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