Nuclear Instruments and Methods in Physics Research A 433 (1999) 98}103

The HERMES RICH detector

on behalf of HERMES Collaboration

D. Ryckbosch

Department of Subatomic Physics, University of Gent, B-9000 Gent, Belgium

Abstract

The new HERMES RICH detector is presented. This is the "rst fully operational RICH using an aerogel radiator, aswell as the more standard C

4F10

gas. The design of the detector is discussed and "rst results are shown. ( 1999 ElsevierScience B.V. All rights reserved.

1. Introduction

In late 1996 the HERMES collaboration hadsuccesfully operated its spectrometer for almosttwo years. When discussing possible upgrades tothe spectrometer it was obvious that particle identi-"cation should have high priority. Excellent lep-ton/hadron separation was provided by the use ofan electromagnetic calorimeter, a transition radi-ation detector (TRD) and a pair of scintillatorhodoscopes. However, the only hadron identi"ca-tion was by means of a threshold Cherenkov de-tector "lled with a mixture of N

2and C

4F10

. Thisgave pion identi"cation only between pion andkaon threshold. Several alternative schemes to im-prove on this situation were discussed, but ulti-mately it was seen that only the implementation ofa RICH with a radiator with refractive indexaround 1.03 would "t our requirements. The factthat a recent test at CERN had shown the feasibil-ity of a RICH based on an aerogel radiator [1] wasan important argument to decide this discussion.

Tel.: #32-9/264-6542; fax: #32-9/264-6699.E-mail address: dirk@inwfsunl.rug.ac.be (D. Ryckbosch)

The timeline for the design and construction ofany new detector in HERMES was extremely tight.At that time it was clear that the RICH would haveto be installed in the spring of 1998 if it were tocontribute signi"cantly to the completion of theinitial HERMES physics programme. The nextlong shutdown of HERA where installation couldtake place was scheduled for 2000. A team consist-ing of the HERMES groups from Argonne, Bari(who joined speci"cally for this project), Caltech,Frascati, Gent, Rome, Tokyo and DESY-Zeuthenthen started the design of the RICH. In May 1998,after only 18 months, the detector was installed inthe HERMES spectrometer. Since the start of datataking in 1998, during August, the RICH has beenfully operational.

In this paper the design and construction of theHERMES RICH are presented. Some typical re-sults as obtained in the "rst weeks of data takingare shown.

2. The HERMES experiment

The HERMES experiment was installed by thespring of 1995 at the 27.5 GeV HERA electron

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 5 6 - 3

storage ring of DESY. Its aim is to study the spinstructure of nucleons through deep inelastic scat-tering of polarized leptons o! polarized nucleons.After storage in the ring the electrons become po-larized transversely to the beam direction, due tothe Sokolov}Ternov e!ect. Two spin rotatorsplaced symmetrically around the HERMES inter-action point rotate the beampolarization to a longi-tudinal direction at the target. This target is a thingas jet with polarized 3He (acting as an e!ectiveneutron target), H or D. Another feature that setsHERMES apart from the other polarized DIS ex-periments is its ability to study semi-inclusive reac-tions. The polar angular range of the spectrometerdownstream of the target extends from 40 to220 mrad. The spectrometer consists of two identi-cal halves situated above and below the beamline.Scattered leptons are identi"ed using the informa-tion from the electromagnetic calorimeter, theTRD and a pair of scintillator hodoscopes (onepreceded by a Pb sheet to induce showering),together with the measurement of particlemomentum in the spectrometer magnet. A fulldescription of the HERMES experimental arrange-ment can be found in Ref. [2].

From high-precision inclusive polarized DIS ex-periments it is well known that the spin of thequarks generates only about 30% of the nucleonspin. To study this quark contribution in moredetail and to determine the contribution from thevarious quark #avours separately, one must gobeyond inclusive reactions and also perform semi-inclusive experiments where at least part of theproduced hadrons are detected and identi"ed. Thusa measurement of e.g. kaon asymmetries gives moreprecise information on the polarization of thequark sea (the K~ being an all-sea object), and thuson the contribution of the s-quarks to the nucleonspin. To obtain a complete picture of the variousquark contributions one must combine, of course,the results for pion-asymmetries, kaon-asymmet-ries, etc., which obviously requires completehadron identi"cation over a large and preferablyuninterrupted momentum range.

A second topic which generated a lot of interest isthen, if the quarks do not combine to give thenucleon its full spin, where does the rest come from?A large contribution from the gluons seems to be

the most reasonable mechanism. To determine thisexperimentally is at least challenging. One of thepossible ways to obtain information on this gluonicspin contribution is through the study of charmproduction in polarized lepton}nucleon scattering.It is argued that most of the charm productionshould proceed through the photon}gluon fusionmechanism, thus opening a window on the gluonicsector. For HERMES the most promising routeto study this is in the production of open charm.Reconstruction of the decay products of thecharmed hadrons is challenging due to the ratherlarge combinatorial background of uncorrelatedpions. This can be largely suppressed with goodpion/kaon separation.

3. Requirements for a RICH in HERMES

The "rst requirement for the HERMES RICHfollowed from the physics programme thatHERMES pursues. This implies the need to havegood pion/kaon separation over the entire mo-mentum range of HERMES: between 1 and20 GeV/c. The high-momentum end can easily becovered with a gas radiator, e.g. pure C

4F

10has the

correct index of refraction for RICH operation inthe momentum range between 10 and 17 GeV/c.At the very low momenta, liquid radiators canserve, but without extremely good photon angularresolution they give no pion}kaon separation formomenta above about 4 GeV/c. This would leavea gap in the particle identi"cation precisely in themost interesting part of the HERMES momentumspectrum. The only material which has a suitablerefractive index of about 1.03 is silica aerogel. Al-though aerogel has been used for many years inthreshold Cherenkov detectors, its use as a radiatorin a RICH was only recently demonstrated in a testsetup [1]. This renewed interest is prompted by therecent availability of aerogel that is both hydropho-bic and much more transparent than the earliermaterial. The former property is of course veryimportant when used in a detector that is inaccess-ible over long data taking periods.

The second HERMES requirement was that theRICH detector had to "t inside the HERMES spec-trometer with as little impact on the spectrometer

D. Ryckbosch / Nuclear Instruments and Methods in Physics Research A 433 (1999) 98}103 99

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Fig. 1. Schematic view of (one half of) the HERMES RICH. 1Manufactured by Matsushita Ltd, Japan.

as a whole as possible. This at once meant thatthe space available for the detector was "xed toless than 1.2 m, the size of the former thresholdCherenkov. At the same time the amount of mater-ial introduced in the path of particles traversingthe RICH had to be minimized. This put ratherstrong restrictions on the construction of themirror.

Finally, the whole RICH project had to producea working detector in less than two years. The onlytime slot available for installation of the RICHdetector was the spring of 1998. The implication ofthis schedule was that there was very little time, ifany, for extensive R&D. Most of the componentsfor the RICH would have to be of more or lessstandard design.

4. Design

The requirements given above quite naturallylead to the following general design: the RICHwould be a dual radiator type. As a "rst radiator,aerogel with a refractive index around 1.03 is used.The second radiator is pure C

4F10

with a refractiveindex of 1.0014. Photons produced in both mater-ials are re#ected from a single spherical mirror ontoa photon detector located outside the acceptance ofthe HERMES spectrometer. A schematic drawingof the RICH is given in Fig. 1.

The aerogel radiator is located immediately be-hind the entrance window of the RICH detector. Itcontains stacks of aerogel tiles: 5 rows, 15 columnsof each 5 tiles thick, to cover the entire entrancewindow with a thickness of nominally 5 cm aerogel.The individual stacks of aerogel are lined withblack tedlar tape to avoid internal re#ection o! thesides of the tiles (for more detail about the aerogelmaterial used in this RICH, see the presentation byP. Carter in this proceedings [3]).

The transparency of the aerogel is usually dis-cussed in terms of the clarity coe$cient C appear-ing in the Hunt formula:

¹"A expA!C¸

j4 B (1)

where ¹ is the transmission through a thickness¸ of aerogel. Typical values for the aerogel1 used inthe HERMES RICH are A"0.964, and C"

0.0094 lm4cm~1. With this kind of clarity coe$c-ient the cuto! wavelength for an aerogel thicknessof 5 cm is well above 300 nm. Consequently, thephoton detector must work with visible light.

Thus the aerogel is clear enough to have a radi-ator thickness of 5 cm. This thickness is a goodcompromise between Cherenkov photon yield andbackground from Rayleigh scattering. This back-ground must be taken into account in the designhowever: right behind the aerogel box a sheet of3.2 mm thick UV transmitting lucite is installed.(Incidentally this also serves to separate the aerogelfrom the C

4F

10gas volume). This exit window

absorbs very few of the Cherenkov photons, butcuts away about half of the Rayleigh scatteringbackground which is situated mainly at lowwavelengths.

The space behind the aerogel detector is "lledwith pure C

4F10

gas. The gashandling system isessentially identical to the one already in use for theold threshold Cherenkov detector [2]. Photonsoriginating in either the aerogel or the gas radiatorare re#ected o! a spherical mirror at the far sideof the gas volume. This mirror with a (nominal)radius of 2.2 m consists of eight individually aligned

100 D. Ryckbosch / Nuclear Instruments and Methods in Physics Research A 433 (1999) 98}103

Table 1(Expected) resolution (in mrad) for the HERMES RICH

Aerogel C4F10

Pixel size 5.3 5.3Optical dispersion 1.4 0.6Tile-to-tile variations in n 2.7Scattering 2.7Emission point uncertainty 0.3 0.6Photon resolution 6.7 5.4d PMTs "ring per ring 8 12Track resolution 2.4 1.6

segments, and has a total area of about250]80 cm2. It consists of a carbon "bre backingof 3 mm, with a special coating to give the requiredsurface quality. The re#ective layer is 70}100 nmaluminium. The total weight of the mirror is about3 kg/m2, consistent with the need to minimize theamount of material introduced in the path oftraversing particles.

With this kind of optics the total area to becovered by the photon detector is of the order oftwice 0.72 m2 (for the two halves of the HERMESspectrometer). At the present state of developmentof hybrid photon detectors there were essentiallyonly two possible choices for the detector: multi-anode PMTs or a complete array of small standardPMTs. The major impediment to the use of multi-anode PMTs was the large dead area associatedwith them. A solution with focussing lenses is notpossible for the HERMES detector where a largespread in incoming photon angles has to be accep-ted. Thus it was decided to cover the focal planesurface with a hexagonal-close-packing of standardphotomultipliers, as done already in e.g. theSELEX RICH detector [3]. Tubes from di!erentmanufacturers were tested and eventually thechoice was made to buy Philips XP1911-UV/A 3/4Aphototubes. The guaranteed active photocathodediameter of these PMTs is 15 mm. The tubes aremounted in a soft steel matrix at a distance of23.3 mm from each other. It was necessary to usesuch a housing to reduce the stray "eld from theHERMES spectrometer magnet, which at the posi-tion of the phototubes was almost 100 G. With thisdesign both RICH-detectors were "tted with 1934PMTs each. Every individual PMT has a lightcollecting funnel in front of the photocathode toreduce the dead area. These funnels are made out ofaluminized mylar foil with a re#ectivity of about90%, yielding a total geometric e$ciency of 80%(9% of the light is always lost in the hexagonal closepacking).

The almost 4000 phototubes were gain matchedat a nominal gain of 3]106. At this gain the noiserate is always well below 5 kHz (at a threshold of0.1 photoelectron), which gives a contribution ofless than 1 random PMT "ring per "ve events.Readout of the tubes is done with the LeCroyPCOS4 system. The major advantage of this ar-

rangement is the large reduction in the number ofcables that must run from the experiment to thecounting electronics. A more detailed account ofthe photon detector is given in the contribution byAschenauer to this proceedings.

5. (Expected) performance

In Table 1 the di!erent expected contributions tothe resolution of the RICH are listed for both theaerogel and the gas radiators. As expected from thedesign, the largest contribution by far comes fromthe pixel size. But also the quality of the aerogel isan important source of loss of resolution. The aero-gel tiles delivered to HERMES were extensivelytested for their mechanical (size, density,2) andoptical properties (index of refraction, transmis-sion, scattering pro"le,2). The chromatic errorcontribution is estimated from the measured indexof refraction at 633 and 544 nm. The e!ect of vari-ations in refractive index from tile to tile is alsobased on the measured indices. A large contribu-tion "nally comes from the scattering of photonsinside the aerogel radiator: this is mainly caused bysurface imperfections and local density variations.It is precisely this quantity which is a good indica-tion for the quality of the aerogel tiles.

The resolution measured so far with theHERMES RICH is somewhat worse than expectedfrom the design. Up to now the alignment of theoptical elements and the alignment of the RICHwith respect to the tracking detectors in the spec-trometer has not been adequately introduced inthe reconstruction code. However, preliminary

D. Ryckbosch / Nuclear Instruments and Methods in Physics Research A 433 (1999) 98}103 101

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Fig. 2. Ring distributions as observed in the HERMES RICH. The rings from individual tracks were all transposed such that the trackcenter lies at the origin. The left "gure shows events with a single lepton in the spectrometer, on the right are events with a single lowmomentum ((2.8 GeV/c) hadron. The geometry is preliminary. Dimensions are in cm.

numbers are close to expectation: a photon resolu-tion of about 7 mrad for the gas rings was observed,and with full knowledge of the alignment furtherimprovement is to be expected.

The number of photons per ring is as usual animportant parameter. Monte Carlo simulations ofthe RICH indicated a average number of 8 PMTs"ring per aerogel ring and about 12 per gas ring.(For the aerogel ring this is also the number ofphotons per ring, while for the smaller gas ring mostPMTs see more than 1 photon). These numbers arecon"rmed in the actual data taken since August1998. The overall resolution per track is then pre-dicted to be about 2.4 mrad for the aerogel radiatorand 1.6 mrad for the gas, neglecting contributionsfrom imperfect knowledge of the alignment.

6. First rings

Since August 1998 the HERMES RICH has beenfully operational. Some early problems with thereadout were rapidly solved and since then thedetector has worked stably. The uncorrelatedbackground seen in the event sample is as low asexpected: very little random "ring of PMTs isobserved. Most of the background hits seen areassociated with Cherenkov light being produced inthe aerogel radiator and are presumably due to

Rayleigh scattering. Some background from theHERA-proton beam which passes o! centerthrough the HERMES spectrometer is also ob-served. The number of PMTs hit by Cherenkovphotons on the aerogel and gas rings is also whatwas expected from the design.

In Fig. 2. ring distributions for some events areshown. For this plot all (re#ected) track centerswere shifted to the origin, so that the individualrings belonging to di!erent tracks form a ring dis-tribution with high statistics. The left-hand side ofthe "gure shows events that were selected to haveonly one track belonging to a scattered e~ in thespectrometer. The rings from both the aerogel andthe gas radiator are clearly seen. (Note also theo!set of the center of the rings due to the imperfectimplementation of the detector geometry in thispreliminary analysis). The right-hand side of the"gure shows the residuals for hadrons (as identi"edby the other PID detectors) with a momentumbelow 2.8 GeV/c, i.e. below the Cherenkov thre-shold in C

4F

10. Obviously, only the aerogel ring is

present in this case.

7. Conclusion

The HERMES collaboration has in the spaceof only 18 months designed, constructed and

102 D. Ryckbosch / Nuclear Instruments and Methods in Physics Research A 433 (1999) 98}103

commissioned a dual radiator RICH. This is the"rst full-scale RICH to use aerogel as a radiatormedium. The "rst analysis of the data show clearlywell-developed aerogel rings. The "rst priority nowis to "nalize the geometry information in the analy-sis programs, after which the full implementation ofthe hadron identi"cation algorithms can start.With its RICH detector running stably, HERMEShas opened a new window on the semi-inclusiveDeep Inelastic Scattering physics.

References

[1] R. De Leo et al., Nucl. Instr. and Meth. A 401 (1997) 187.[2] K. Ackersta! et al., Nucl. Instr. and Meth. A, hep-ex/

9806008, in press.[3] P. Carter, Nucl. Instr. and Meth. A 433 (1999) 392.[4] J. Engelfried et al., Nucl. Instr. and Meth. A, hep-ex/

9811001, submitted for publication.

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