The HERMES Spectrometer

K. Ackersta�5, A. Airapetian30, N. Akopov30, M. Amarian30, V. Andreev24, E.C. Aschenauer11;21,R. Avakian30, H. Avakian9, A. Avetissian30, B. Bains13, S. Barrow23, W. Beckhusen5, M. Beckmann10,

St. Belostotski24, E. Belz4, Th. Benisch8, S. Bernreuther8, N. Bianchi9, J. Blouw21, H. B�ottcher6,A. Borissov6;12, J. Brack4, B. Braun8;19, B. Bray3, S. Brons6;27, W. Br�uckner12, A. Br�ull12, H.J. Bulten15,G.P. Capitani9, P. Carter3, P. Chumney20, E. Cisbani25, S. Clark4, S. Colilli25, H. Coombes1, G.R. Court14,P. Delheij27, E. Devitsin18, C.W. de Jager21, E. De Sanctis9, D. De Schepper2;17, P.K.A. de Witt Huberts21,

P. Di Nezza9, M. Doets21, M. D�uren8, A. Dvoredsky3, G. Elbakian30, J. Emerson26;27, A. Fantoni9,A. Fechtchenko7, M. Ferstl8, D. Fick16, K. Fiedler8, B.W. Filippone3, H. Fischer10, H.T. Fortune23,J. Franz10, S. Frullani25, M.-A. Funk5, N.D. Gagunashvili7, P. Galumian1, H. Gao13, Y. G�arber6,

F. Garibaldi25, G. Gavrilov24, P. Geiger12, V. Gharibyan30, V. Giordjian9, F. Giuliani25,A. Golendoukhin16, B. Grabowski5, G. Graw19, O. Grebeniouk24, P. Green1;27, G. Greeniaus1;27,M. Gricia25, C. Grosshauser8, A. Gute8, J.P. Haas20, K. Hakelberg5, W. Haeberli15, J.-O. Hansen2,

D. Hasch6, O. Haussery 26;27, R. Henderson27, Th. Henkes21, R. Hertenberger19, Y. Holler5, R.J. Holt13,H. Ihssen5, A. Izotov24, M. Iodice25, H.E. Jackson2, A. Jgoun24, C. Jones2, R. Kaiser26;27, J. Kelsey17,E. Kinney4, M. Kirsch8, A. Kisselev24, P. Kitching1, H. Kobayashi28, E. Kok21, K. K�onigsmann10,M. Kolstein21, H. Kolster19, W. Korsch3, S. Kozlov24, V. Kozlov18, R. Kowalczyk2, L. Kramer17,B. Krause6, A. Krivchitch24, V.G. Krivokhijine7, M. Kueckes27, P. Kutt23, G. Kyle20, W. Lachnit8,

R. Langsta�27, W. Lorenzon23, M. Lucentini25, A. Lung3, N. Makins2, V. Maleev24, S.I. Manaenkov24,K. Martens1, A. Mateos17, K. McIlhany3, R.D. McKeown3, F. Mei�ner6, F. Menden27, D. Mercer4,A. Metz19, N. Meyners5, O. Mikloukho24, C.A. Miller1;27, M.A. Miller13, R. Milner17, V. Mitsyn7,G. Modrak6, J. Morton14, A. Most13, R. Mozzetti9, V. Muccifora9, A. Nagaitsev7, Y. Naryshkin24,A.M. Nathan13, F. Neunreither8, M. Niczyporuk17, W.-D. Nowak6, M. Nupieri9, P. Oelwein12,

H. Ogami28, T.G. O'Neill2, R. Openshaw27, V. Papavassiliou20, S.F. Pate17;20, S. Patrichev24, M. Pitt3,H.J. Plett8, H.R. Poolman21, S. Potashov18, D. Potterveld2, B. Povh12, V. Prahl5, G. Rakness4,V. Razmyslovich24, R. Redwine17, A.R. Reolon9, R. Ristinen4, K. Rith8, H.O. Rolo�6, G. R�oper5,P. Rossi9, S. Rudnitsky23, H. Russo8, D. Ryckbosch11, Y. Sakemi28, F. Santavenere25, I. Savin7,F. Schmidt8, H. Schmitt10, G. Schnell20, K.P. Sch�uler5, A. Schwind6, T.-A. Shibata28, T. Shin17,B. Siebels1, A. Simon10;20, K. Sinram5, W.R. Smythe4, J. Sowinski12, M. Spengos23, K. Sperber5,

E. Ste�ens8, J. Stenger8, J. Stewart14, F. Stock8;12, U. St�o�lein6, M. Sutter17, H. Tallini14, S. Taroian30,A. Terkulov18, D. Thiessen26;27, B. Tipton17, V. Tro�mov24, A. Trudel27, M. Tytgat11, G.M. Urciuoli25,

R. Van de Vyver11, J.F.J. van den Brand21;29, G. van der Steenhoven21, J.J. van Hunen21,D. van Westrum4, A. Vassiliev24, M.C. Vetterli26;27, M.G. Vincter27, E. Volk12, W. Wander8,

T.P. Welch22, S.E. Williamson13, T. Wise15, G. W�obke5, K. Woller5, S. Yoneyama28, K. Zapfe-D�uren5,T. Zeuli2, H. Zohrabian30,

1Department of Physics, University of Alberta, Edmonton, Alberta T6G 2N2, Canada2Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA

3W.K.Kellogg Radiation Lab, California Institute of Technology, Pasadena, CA, 91125, USA4Nuclear Physics Laboratory, University of Colorado, Boulder CO 80309-0446, USA

5DESY, Deutsches Elektronen Synchrotron, 22603 Hamburg, Germany6DESY, 15738 Zeuthen, Germany

7Joint Institute for Nuclear Research, 141980 Dubna, Russia8 Physikalisches Institut, Universit�at Erlangen-N�urnberg, 91058 Erlangen, Germany

9Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Frascati, I-00044 Frascati, Italy10 Fakult�at f�ur Physik, Universit�at Freiburg, 79104 Freiburg, Germany

11Department of Subatomic and Radiation Physics, University of Gent, 9000 Gent, Belgium12Max-Planck-Institut f�ur Kernphysik, 69029 Heidelberg, Germany

13Department of Physics, University of Illinois, Urbana, IL 61801, USA

1

14Physics Department, University of Liverpool, Liverpool L69 7ZE, United Kingdom15Department of Physics, University of Wisconsin-Madison, Madison, WI 53706, USA16Physikalisches Institut, Philipps-Universit�at Marburg, 35037 Marburg, Germany

17Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA18Lebedev Physical Institute, 117924 Moscow, Russia

19 Sektion Physik, Universit�at M�unchen, 85748 Garching, Germany20Department of Physics, New Mexico State University, Las Cruces, NM 88003, USA

21Nationaal Instituut voor Kernfysica en Hoge-Energiefysica (NIKHEF), 1009 DB Amsterdam, Netherlands22Department of Physics, University of Oregon, Eugene, OR 97403 USA

23Department of Physics, University of Pennsylvania, Philadelphia PA 19104-6396, USA24Petersburg Nuclear Physics Institute, St.Petersburg, 188350 Russia

25Istituto Nazionale di Fisica Nucleare, Sezione Sanit�a, 00161 Roma, Italy26Department of Physics, Simon Fraser University, Burnaby, British Columbia V5A 1S6 Canada

27TRIUMF, Vancouver, British Columbia V6T 2A3, Canada28Department of Physics, Tokyo Institute of Technology, Tokyo 152, Japan

29 Department of Physics and Astronomy, Vrije Universiteit, 1081 HV Amsterdam, The Netherlands.30Yerevan Physics Institute, 375036, Yerevan, Armenia

Abstract

The HERMES experiment is collecting data on inclusive and semi-inclusive deep inelastic scattering

of polarised positrons from polarised targets of H, D, and 3He. These data give information on the spin

structure of the nucleon. This paper describes the forward angle spectrometer built for this purpose.

The spectrometer includes numerous tracking chambers (micro-strip gas chambers, drift and proportional

chambers) in front of and behind a 1.3 T.m magnetic �eld, as well as an extensive set of detectors for

particle identi�cation (a lead-glass calorimeter, a pre-shower detector, a transition radiation detector, and

a threshold �Cerenkov detector). Two of the main features of the spectrometer are its good acceptance and

identi�cation of both positrons and hadrons, in particular pions. These characteristics, together with the

purity of the targets, are allowing HERMES to make unique contributions to the understanding of how the

spins of the quarks contribute to the spin of the nucleon.

(Submitted to Nuclear Instruments and Methods)

2

1 Introduction.

The HERMES experiment (HERA MEasurement ofSpin) is a second generation polarised deep inelasticscattering (DIS) experiment to study the spin struc-ture of the nucleon. It is being run at the HERAstorage ring at DESY1.

Several experiments over the last decade have pro-vided accurate data on the polarisation asymmetry ofthe cross-section for inclusive scattering where onlythe scattered lepton is detected. These experimentshave been interpreted as showing that at most 30% ofthe nucleon spin comes from the spins of the quarks.Further knowledge of the origin of the nucleon's spincan be gained by studying semi-inclusive processesinvolving the detection of hadrons in coincidence withthe scattered lepton. These data o�er a means of` avour-tagging' the struck quark to help isolate thecontributions to the nucleon spin of the individualquark avours, including the sea quarks. The inter-pretation of semi-inclusive data is made clearer if thetype of hadron is identi�ed. This is a central theme ofthe HERMES experiment, which can identify pionsand will add kaon identi�cation for 1998.

The physics program for HERMES is very broad.The experiment contributes inclusive data with qual-itatively di�erent systematic uncertainties to improvethe world data set for the x dependence and the inte-gral of the spin structure function g1(x) for both theproton and the neutron (see below for the de�nitionof x). Most importantly, HERMES is providing newprecise data on semi-inclusive processes by virtue ofthe good acceptance of the spectrometer combinedwith hadron identi�cation and the purity of the tar-gets.

The HERA storage ring can be �lled with ei-ther electrons or positrons, which are accelerated to27.5 GeV. Positrons have been used since 1995 be-cause longer beam lifetimes are then possible. Sincewith few exceptions such as the luminosity measure-ment, the physics processes are the same for positronsand electrons, the term \positron" will be used forboth in this paper.

In addition to studying polarised DIS using~H; ~D; and ~3He targets, data are collected with unpo-larised gases (H2;D2;

3He;N2). This provides in a rel-atively short period of time (2-3 weeks) high statisticsdata sets that are used to study important propertiesof the nucleon not related to spin, such as the avour

1HERA is an e-p collider but only the e beam is used by

HERMES

asymmetry of the sea, as well as hadronisation in nu-clei.

2 General Description.

The HERMES experiment is located in the East hallof the HERA storage ring complex at DESY. Thespectrometer is a forward angle instrument of con-ventional design. It is symmetric about a central,horizontal shielding plate in the magnet. Due tothis symmetry, the description of the detectors con-tained in this paper will apply typically to only onehalf of the spectrometer, in particular with respectto the number of detectors quoted and their dimen-sions. A diagram of the spectrometer is shown inFig. 1. The coordinate system used by HERMEShas the z axis along the beam momentum, the y axisvertical upwards, and the x axis horizontal, pointingtowards the outside of the ring. The polar (�) andazimuthal (�) scattering angles as well as the initialtrajectory for the determination of the particle's mo-mentum are measured by the front tracking system,which consists of microstrip gas chambers (MSGC,referred to as the vertex chambers (VC)) and driftchambers (DVC, FC1/2). The momentum measure-ment is completed by two sets of drift chambers be-hind the magnet (BC1/2 and BC3/4). In addition,there are three proportional chambers in the magnet(MC1/3) to help match front and back tracks as wellas to track low momentum particles that do not reachthe rear section of the spectrometer.

Particle identi�cation (PID) is provided by a lead-glass calorimeter, a pre-shower detector (H2) consist-ing of two radiation lengths of lead followed by a plas-tic scintillator hodoscope, a transition radiation de-tector (TRD) consisting of six identical modules, anda threshold �Cerenkov detector (�C). The particle iden-ti�cation system was designed to provide a hadron re-jection factor of 104 to yield a very clean DIS positronsample, and exceeds this in practice. Pion identi-�cation is provided by a threshold �Cerenkov detec-tor. An upgrade to a ring imaging �Cerenkov detectorthat will identify both pions and kaons is being pre-pared for operation in 1998. The calorimeter andpre-shower detector are included in the trigger alongwith a second hodoscope (H1) placed in front of theTRD. An additional trigger hodoscope (H0) was in-cluded in 1996 in front of FC1 to reduce the triggerrate from background caused by the proton beam.

The acceptance is limited at small angles by an iron

3

Figure 1: Schematic side view of the HERMES spectrometer. See the text for the meaning of the labels.

plate in the beam plane, which shields the positronand proton beams from the magnetic �eld of the spec-trometer magnet. Both beams go through the spec-trometer, separated by 72 cm. Particles with scat-tering angles within �170 mrad in the horizontal di-rection and between +({)40 mrad and +({)140 mradin the vertical direction are accepted. Therefore, therange of scattering angles is 40 mrad to 220 mrad.Spin structure functions depend on x (x= Q2/2M�where Q is the four-momentum transfer in the DISreaction, � is the energy transfer, and M is the massof the nucleon). The variable x can be interpreted asthe fraction of the nucleon's momentum carried bythe struck quark. The x range covered by the HER-MES experiment is 0.02-1.0, although there is littlecount rate for x�0.8 when W > 2 GeV (W is thephoton-nucleon invariant mass).

The experiment is mounted on a large platformthat can move on rails together with an attachedtrailer (ET) containing the electronics and the gassystems. A �xed but removable concrete wall be-tween the platform and the ET shields the mainpart of the hall from radiation and hence allows ac-cess to the electronics while the accelerators are run-

ning. Cables and gas pipes between the detectorsand the ET are routed beneath the platform to alarge cable tray passing under the shielding wall.The experiment was assembled in the East hall out-side the shielding during the 1994 HERA run. Af-ter the shielding wall had been dismantled, the plat-form/trailer was moved into place in the ring. Theshielding wall was re-erected between the platformand the electronics trailer in such a way that theexperiment could be moved far enough within theshielding to allow the HERA maintenance tram ac-cess to the tunnel on 24 hours notice.

3 The Target Region.

The HERMES target consists of gas from a polarisedsource fed into a storage cell internal to the HERApositron ring. In 1995, an optically pumped polarised~3He cell was used to supply gas to the storage cell[1], while an atomic beam source (ABS) was used in

1996-97 to produce a ~H target [2]. The ABS will be

modi�ed for operation with ~D in 1998-99. It is alsopossible to inject unpolarised gases into the storage

4

cell. The following were used in the �rst three yearsof operation: H2;D2;

3 He;N2.

The target region is shown schematically in Fig. 2.The gas enters an open-ended (ie. windowless) T-shaped tube that con�nes the gas atoms in a regionaround the positron beam. The storage cell increasesthe areal target density by about two orders of mag-nitude compared to a free atomic beam. The gasatoms leak out the open ends of the target cell andare pumped away by a high speed di�erential pump-ing system. In this way, the number of atoms seen atthe target location by the beam is maximized whileminimizing the e�ect on the stored beam. The stor-age cell is an elliptical tube, 9.8 mm high by 29.0 mmwide and 400 mm in length. It was made of 125 �mthick ultra-pure aluminium in 1995 and 75 �m alu-minium in 1996-97. The transition from the cell tothe beam pipe was made smooth to avoid the gen-eration of wake �elds that could cause heating andincrease the emittance of the beam. This was accom-plished using thin perforated tubes called \wake �eldsuppressors".

The amount of synchrotron radiation created up-stream of the HERMES area is considerable, eventhough it is reduced by the addition of two weakdipoles downstream of the last bending magnet in thearc of the accelerator. If it were allowed to hit thetarget cell, it would damage the cell coating, and onthe order of 1013 - 1014 soft synchrotron photons persecond would Compton scatter into the spectrometeracceptance. This is prevented by a system of two col-limators installed near the target. C1 is placed 2 mupstream of the target and protects the next colli-mator from direct radiation. It actually consists oftwo moveable collimators, C1H in the horizontal di-rection and C1V in the vertical, separated by 0.5 m.These collimators are opened during beam injectionand operated at a narrow but safe setting (15� +0.5 mm) during HERMES operation. During the�rst three years of operation, typical beam param-eters were �x= 0.31 mm and �y= 0.07 mm. C2 hasa �xed aperture that is slightly larger than C1, andis located right in front of the storage cell to pro-tect it from photons scattered in C1. It also protectsthe cell from direct radiation during beam injection.Note that only C2 is shown in Fig. 2.

Particles scattered into the spectrometer accep-tance exit the target chamber through a thin(0.3 mm) stainless steel foil. Background from show-ers (initiated mostly by 's from �0 decay) is reducedby also making the beam pipe just downstream of the

gas injection

storage cell

fixed collimator

e-beam

spectrometer

exit window

pumps

suppressors

wake field

target

vacuum chamber

thin walled

beam pipe

Figure 2: Schematic of the target region. See the textfor more explanation.

target chamber of thin stainless steel.

4 The Magnet.

The HERMES spectrometer magnet is of the H-typewith �eld clamps in front as well as behind in orderto reduce the fringe �elds at the position of the driftchambers FC2 and BC1. A massive iron plate inthe symmetry plane shields the positron and protonbeams as they pass through the magnet.

The most important features of the magnet are:

� It is capable of providing a de ecting power ofRBdl = 1.5 T�m, although it is operated at

1.3 T�m to reduce power consumption. Thesmaller �eld integral does not signi�cantly im-pair the performance of the spectrometer. Thevariation of the de ecting power within the ac-ceptance is less than 10 %.

� The gap between the pole faces encloses the geo-metrical acceptance of �(40..140) mrad in thevertical direction. In the horizontal direction�170 mrad plus another �100 mrad startinghalfway through the magnet is provided. Thepole faces are tilted parallel to the limits of thevertical angular acceptance. Due to the limitedspace of 8.5 m for the HERMES spectrometerbetween the center of the target and the �rstquadrupole magnet of the electron machine, only2.2 m between the drift chambers FC2 on thefront and BC1 on the rear side are utilized forthe magnet.

� The fringe �eld at the positions of the adjacentdrift chambers does not exceed 0.1 T.

5

� An e�ective magnetic shielding substantially re-duces the e�ect of the magnet on the proton andpositron beams. In particular, the sextupole mo-ment of the magnetic �eld in the beam tubes isminimized.

� A correction coil with a de ecting power of0.08 T�m is accommodated inside the shieldingof the positron beam pipe. This coil is usedto correct for fringe �elds and the imperfectionsof the magnetic shielding in this section of theiron plate. It is also intended to compensate thetransverse holding �eld of the target when oper-ating with transverse polarisation. It serves asan element of a closed orbit bump with no netglobal e�ect on the positron beam.

Magnetic model calculations were done with the 3D-codes MAFIA [3] and TOSCA [4]. The calcula-tions agree with subsequent magnetic measurementswithin a few percent. The �eld measurements weredone with a 3D-Hall probe on an automated 3D-mapping machine. The results exhibit an overall re-producibility of about 10�3, and a �eld pattern asexpected. No homogeneous regions are found due tothe high gap to length ratio of the magnet, and apronounced step in the �eld is observed that re ectsthe structure of the iron shielding plate. The detailed�eld map was integrated into the track reconstructionalgorithm, as described later.

5 The Tracking System

The tracking system serves several functions, whichvary somewhat in their performance requirements:

� Determine the event vertex in the target cell toensure that reconstructed events come only fromthe sub-mm beam envelope in the target gas,with the expected z-distribution. Also, the anal-ysis of events with multiple tracks requires eithera common vertex, or possibly an additional dis-placed vertex for a reconstructed particle decay.

� Measure the scattering angles for kinematic re-construction.

� Measure the particle momentum from the trackde ection in the spectrometer dipole magnet.

� Identify the hits in the PID detectors associatedwith each track.

When all tracking detectors are fully operational,the momentum resolution for positrons is limited byBremsstrahlung in the material of the target cellwalls, the 0.3 mm thick stainless steel vacuum win-dow, as well as the �rst tracking detectors. Eventhe resolution in scattering angle is normally limitedby multiple scattering in this material. An excep-tion was 1995, the �rst year of operation, when thelow e�ciency of most planes of the Vertex Cham-bers (improved for 1996 for the top detector) made itmore attractive to reconstruct the tracks using onlythe drift chambers (DC), as will be explained later inmore detail. This mode of operation resulted in theresolution of reconstructed kinematic quantities be-ing much more sensitive to the DC spatial resolution.

The locations of the tracking detectors are shownin Fig. 1 and listed in Table 1, which also summarizestheir properties.

The microstrip gas counters VC1/2, the drift ver-tex chambers DVC's, and the front drift chambersFC1/2 provide both vertex reconstruction to the tar-get, and de�nition of the scattering angle. In conjunc-tion with the front tracking, the back drift chambersBC1/2 and BC3/4 measure the magnetic de ectionand hence the momentum. The BC's also identifythe cells in the PID detectors associated with eachtrack. The proportional chambers inside the mag-net (MC1/3) were originally intended to ensure thatmulti-track ambiguities could be resolved. As it hap-pens, chamber occupancies are low enough that thiscan be accomplished using the drift chambers alone.However, the MC's are found to be very useful formomentum analysis of low energy decay productsthat are de ected too much to reach the downstreamtracking detectors.

The horizontal length of the BC's precluded the useof long horizontal wires for y measurements. Henceall planes are one of three types with wires orientedeither in the vertical ( for X measurement), or tilted30� right or left (U and V planes). In order to allowthe use of fast reconstruction algorithms, all trackingdetectors are restricted to this geometry. Like the restof the spectrometer, the tracking system is symmetricabout the beam plane.

5.1 Vertex Chambers

The purpose of the vertex chambers is to providehigh-precision measurements of the scattering angleand the vertex position over the full acceptance ofthe experiment, in the presence of a signi�cant back-

6

Table 1: Tracking Chamber properties.

CHAMBER

Vertex

DriftVertex

Front

Magnet

Back

Detectorname

VC1

VC2

DVC

FC1

FC2

MC1

MC2

MC3

BC1/2

BC3/4

mmfromtarget

731

965

1100

1530

1650

2725

3047

3369

4055

5800

ActiveArea:

Horizontal(mm)

323

393

474

660

660

996

1210

1424

1880

2890

Vertical(mm)

137

137

290

180

180

263

306

347

520

710

Celldesign

microstripgas

horizontal-drift

horizontal-drift

MWPC

horizontal-drift

Cellwidth(mm)

0.193

6

7

2

15

A-Cplanegap(mm)

3

3

4

4

8

Anode(A)material

200�mglass(Al)

W(Au)

W(Au)

W(Au)

W(Au)

Anodewirediameter

7�m

30�m

20�m

25�m

25�m

Potentialwiremat0l

Alstrip

Be-Cu(Au)

Al(Au)

Be-Cu(Au)

Potentialwiredia.

85�m

50�m

50�m

127�m

Cathode(C)material

Alonglass

AlonMylar

AlonMylar

Be-Cuwires

ConKapton

Cathodethickness

200�m

34�m

6.4�m

90�m@0.5mmpitch

25.4�m

Gascomposition:

DME/Ne

Ar/CO2/CF4

Ar/CO2/CF4

Ar/CO2/CF4

Ar/CO2/CF4

(%)

50/50

90/5/5

90/5/5

65/30/5

90/5/5

U,Vstereoangle

+5�,�90�

�30�

�30�

�30�

�30�

Resolution/plane(�)

65�m

220�m

225�m

700�m

275�m

300�m

WiresinXplane

1674

2046

80

96

96

496

608

720

128

192

WiresinU,Vplane

2170

2170

96

96

96

512

608

720

128

192

Modulecon�g.

VUX

XVU

XX0UU0VV0

UU0XX0VV0

UXV

UU0XX0VV0

Rad.length/module

0.8%

0.25%

0.075%

0.29%

0.26%

Numberofmodules

1

1

1

1

1

1

1

1

2

2

(upperorlower)

Channels/module

6014

6386

544

576

576

1520

1824

2160

768

1152

Totalchannels

24800

1088

2304

11008

7680

7

Figure 3: Schematic view of a drift cell in a Mi-croStrip Gas Counter, showing the �eld lines. This�gure has been taken from ref.[6].

ground ux. The proximity of the VC to the targetimplies that at the minimum scattering angle, thetracks from the downstream end of the target cellpass the VC active area 20 mm from the beam axis,or only 5 mm from the beam tube. In combinationwith the need to minimize high-Z materials near thebeam tube that could convert the numerous photonsfrom �0 decay into high multiplicity showers, this ge-ometry presented severe problems in mechanical de-sign that would have been very di�cult to solve inthe context of wire chamber frames. All these re-quirements are met through the use of a relativelynew technology, a microstrip gas chamber (MSGC).In this technique [5], metal strips are etched on athin, semi-rigid substrate of high resistivity, allow-ing for higher drift �elds and much smaller cell pitchthan in a conventional drift chamber. Each of theupper and lower VC assemblies contains six MSGCplanes, grouped into two \modules" (VUX and XUVfor VC1 and VC2 respectively), which are su�cientlyseparated to provide a precise determination of thefront track by the VC alone. The total radiationlength of one module is 0.8% X0 and the thicknessof the target chamber vacuum window is 1.7% X0,so that for momenta below about 7 GeV the angularaccuracy becomes limited by multiple scattering. Aninherent cost associated with the MSGC choice is thelarge channel count { 24,800 channels in total. Thisrequires a highly integrated readout system, includ-ing analog electronic signal storage on the planes andmultiplexed digitization at the detector.

The basic features of the MSGC drift cell are shownin Fig. 3. Ionization electrons created by a travers-ing particle in a 3 mm thick gas volume move in adrift �eld of about 5 kV/cm directly away from aplanar cathode and towards a 200 �m thick glasssubstrate that carries a pattern of �ne etched alu-minum strips. The �eld is produced by a 1800 Vpotential on the cathode planes. A voltage di�erenceof 580 V is applied between the 7 �m wide anodestrips and the 85 �m wide cathode strips forming thepattern on the substrates. As a result, gas multipli-cation occurs in the vicinity of the anode, a processcomparable to that in a wire chamber. High gas am-pli�cation at 580V is obtained by using a DiMethyl-Ether(DME)/Ne mixture (50/50%). The gas gain isset at a level such that the signals are about threetimes that of silicon detectors of the same materialthickness, leading to a better signal to noise ratio.Even with only single bit per cell digital read-out, aspatial resolution much better than the cell pitch isachieved, due to charge sharing between cells.

The metallic strip pattern that creates the electri-cal �eld con�guration is carried on substrates, thematerial of which is a critical choice. Low Z is pre-ferred to minimize the e�ects of multiple scatteringand Bremsstrahlung. Surface properties are impor-tant as electrons, ions, and gas pollutants may stickand cause �eld modi�cations, discharges, ageing, andradiation damage. Since part of the avalanche fallson the substrate instead of on the conductive strip,the substrate should have a certain degree of conduc-tivity in order to remove the collected surface charge.D-263 glass was chosen as the substrate material [7].It consists of SiOx (64.1 %), Al2O3 (4.5 %), K2O(6.6 %), TiO (4.2 %), ZnO (6.1 %), Na (6.1 %), withthe remainder (8.4 %) unspeci�ed. It has a bulk resis-tivity of 1015 cm, equivalent to a surface resistivityof 1017 =2, caused by the migration of electrons andNa+ ions. The thickness of the substrates is 0.2 mm,the minimum required for safe handling and to havesu�cient rigidity to resist electrostatic distortions.

The active area of the U or V planes is as largeas 484�137 mm2. The largest substrate area thatcould be processed is 150 � 200 mm2. Therefore, 5substrates are combined to form one 30� plane. Thesubstrates are cut along the cathode strip, and gluedon a carrier frame, leading to a gap of on average461 �m between the anode wires of consecutive sub-strates. No loss in e�ciency at these boundaries hasbeen observed.

The PC carrier frame to which the substrates are

8

glued has su�cient mechanical sti�ness to be self-supporting. The carrier frames are mounted in a�xture attached to the inside of the lid of the boxthat acts as the gas enclosure. The �xture providesa means of precisely aligning the planes with respectto �ducials on the outside of the lid. The wall of thebox, which is shaped to curve around the beam tube,is only 0.5mm thick and made of low-Z material. Thethin windows for the detected particles have two lay-ers enclosing non- ammable ush gas to ensure noleaks of ammable gas.

In order to minimize external data ow, digitiza-tion of the signals is done in an electronic assemblymounted on the detector. The high degree of digi-tizer multiplexing required for economy is made pos-sible by continuously storing the charge buckets col-lected by each anode strip in a switched-capacitoranalog pipeline. There are 64 preampli�ers andpipelines, each 32 steps deep, integrated on each Ana-log Pipeline Chip (APC). The pipelines are clockedwith the HERA bunch frequency (10.4 MHz). If anevent trigger occurs any time within 3.1 �s (32 �96 ns) after an event, the pipelines are stalled beforethe data are shifted out and lost. The pipelines areadvanced until the buckets of interest in the vicinityof the bunch causing the trigger are about to appearat the pipeline output stage. Then the pipelines areshifted much more slowly as the buckets of interestare presented to the \tracking discriminators", whichwere designed to compensate the rising baseline ofthe APC chip. A sample and hold circuit is used tokeep the threshold at a �xed value above the base-line o�set, given by the readout value of the previ-ous channel. If a hit is detected, the baseline restoreoperation is suspended, until the following channelno longer exceeds the discriminator level. For thatreason, the system is limited to handle a maximumcluster size of up to four neighboring anode strips.The �rst two channels of each chip are not connectedto an anode strip, and serve as a reference for thebaseline restoration. The data from each discrimina-tor (62 channels) are stored in its shift register. Assoon as all digital data have been stored, the inputsof the APC chips are enabled and a new trigger canbe accepted. The dead time during readout of theAPC chips and digitization into the shift register is56 �s. Simultaneously, serial readout of the shift reg-isters is started. Addresses of signals above thresholdare loaded into memory stacks. If during this opera-tion a new trigger arrives, the new event waits in theAPC pipeline until the data of the previous event are

transferred into the memory stacks. Subsequently,the memory stacks are read out serially via a datadriver into a Struck ECL300 FASTBUS module.

The e�ciency of the vertex detector is measured intwo ways. In the internal tracking approach, tracksare identi�ed using VC data alone, requiring hits in�ve of six planes. The track is reconstructed fromfour planes, and a hit is accepted in the �fth plane if itis within 200 �m of the track. Only tracks are consid-ered that point to the target and are reconstructableby the rest of the tracking system. The e�ciencyof the �fth plane is then de�ned as the fraction ofevents with a hit close to the track. In the externaltracking approach, the e�ciency is determined withtracks reconstructed using the drift chamber system,extrapolated to the vertex chamber. In both cases,e�ciencies in the range 60-90% were found for the1995 data, where a prototype version of the APCchip was used that exhibited a strongly non-linearbaseline. This caused high noise. Due to the fragilityof some of the anode resistors that were etched onthe substrates, the high voltage could not be raisedsu�ciently to get a high enough signal-to-noise ratio.For 1996 running, the substrates and APC chips ofthe upper chamber were replaced, resulting in muchimproved e�ciencies of around 95%. The lower halfwas similarly upgraded for 1997 running. The resolu-tion has been determined by comparing the distancebetween the track, formed with 5 planes, and the hitin the sixth plane, and also by comparing the hori-zontal angle of the track calculated from the X planeswith that from the U and V planes. Both methodsyield a resolution of about 65 �m (�) per plane.

The vertex reconstruction capability of the VC isillustrated in Fig. 4, which is a histogram of the trans-verse coordinates of the point of closest approach oftwo tracks in an event. A well de�ned beam enve-lope is seen. Furthermore, the �gure shows that thevertices are well separated from the target cell wallrepresented by the ellipse.

Over three years of operation, there has been noobserved degradation of performance that would sug-gest ageing e�ects.

5.2 The Drift Chambers

The drift chambers DVC, FC1/2, BC1/2 and BC3/4are of the conventional horizontal-drift type. Eachlayer of drift cells consists of a plane of alternatinganode and cathode wires between a pair of cathodefoils. The cathode wires and foils are at negative

9

x (mm)y (mm)-15 -10 -5 0 5 10 15

-8-6

-4-2

02

46

8

0

50

100

150

200

250

300

350

Figure 4: Vertex reconstruction in the transverse co-ordinates using the VC alone. This plot was madeusing two-track events only to get a well de�ned ver-tex. The outline of the target cell is shown as theellipse.

high voltage with the anode sense wires at groundpotential. The chambers are assembled as modulesconsisting of six such drift cell layers in three coordi-nate doublets (UU 0, XX 0 and V V 0). The wires arevertical for the X planes and at an angle of �30�

to the vertical for the U and V planes. The X 0,U 0 and V 0 planes are staggered with respect to theirpartners by half the cell size in order to help resolveleft-right ambiguities. Each sense plane consists ofa �berglass-epoxy frame laminated to a printed cir-cuit board (PCB) that has solder pads for the wires.Except in the case of the DVC's, the PCB's carrythe traces leading past O-ring gas seals to externalconnectors. The opposite ends of the cathode wiresconnect to internal distribution buses for the negativehigh voltage. Each module is enclosed between met-allized Mylar gas windows mounted on metal frames.Identical modules are situated above and below thebeam pipe.

The frames of all the drift chambers except theDVC's have su�cient length to allow both ends of allwires in the U and V planes to terminate on the longedges of the frames. Hence all U and V wires andtheir PCB traces have the same length and similar

capacitance. The onboard electronics are mountedalong only the one long edge opposite to the beampipes. This arrangement does not sacri�ce mechani-cal sti�ness of the frame. Since the inside aperture ofeach frame is only as large as the active area in com-mon with all the frames, each stressed frame memberis as short as possible. A shallow relief is milled inthe inactive end regions of the U and V frames toallow the wires to stand o� from the frame material.Proximity to the beam of the active areas of all cham-bers required external notches to be machined in theframes to accommodate the beam pipes. As thesenotches are short, they do not seriously compromisethe mechanical sti�ness of the frames.

The choice of gas mixture for the drift cham-bers was governed by the serious inconvenience ofcontrolling the hazards of a ammable gas in atunnel environment. As well, there is an advan-tage in limiting the drift cell occupation time andhence the number of extraneous hits that mustbe accommodated in track �nding. The mixtureAr(90%)/CO2(5%)/CF4(5%) is both fast and non- ammable [8]. Its drift velocity has been measured asa function of �eld - eg. > 7 cm/�sec at E=800 V/cm[9]. FC2 and BC1 operate in a dipole fringe �eldthat reaches 0.1 T. Mixtures containing CF4 havebeen shown to result in compromised resolution dueto electron attachment [10], and large Lorentz driftangles in magnetic �elds. However, they have the ad-vantages of short occupation time and long chamberlifetime [11].

The gas pressure and ow through all drift cham-bers is regulated by a system of valves and mass ow controllers, all under the supervision of an indus-trial programmed controller. A large fraction of the ow (80-90%) is recirculated through puri�ers thatremove oxygen and water vapour, but not nitrogen.In 1995, nitrogen from gas leaks in the drift chambersand from di�usion in the window foils accumulatedin the loop to an equilibrium concentration of 1.1%.This in uenced the drift velocity in the gas mixtureand resulted in a gain reduction as con�rmed by inde-pendent studies [12]. Work on the chamber gas leaksas well as decreasing the re-cycled fraction from 90%in 1995 to 80% in 1996 (20% exhaust), reduced thenitrogen contamination to a stable level of 0.4% withnegligible in uence on both drift velocity and gain.

The DC readout system consists of Ampli-�er/Shaper/Discriminator (ASD) cards mounted on-board the drift chambers, driving ECL signals on30 m long at cables to LeCroy 1877Multihit FastBus

10

TDC's in the external electronics trailer. The designgoal (especially in the case of the FC's and DVC's)was chamber operation at low gas gain (� 104) tomaximize chamber lifetime and reduce particle uxdependence of the gas gain due to space charge ef-fects. To combine this goal with good spatial resolu-tion requires operation at low threshold { well below105 electrons from the wire. This in turn requiresboth low loise performance of the ampli�ers as wellas great care in the radio-frequency design of the elec-tronics enclosure on the chambers, to maintain elec-tronic stability. All ground elements including theASD cards themselves should be considered as radio-frequency resonators, and the lowest frequency of anyresonant mode should be kept well above the pass-band of the ampli�ers. For example, this dictatesthat the ground plane of each 16-channel ASD cardcontinuously contact the electronics enclosure alongboth sides of the card via special high-conductivitycard guides.

During the �rst year of operation in 1995, all driftchambers performed satisfactorily and alone providedthe tracking data that were used in the physics anal-ysis. As discussed later, performance was improvedin 1996-97 by the addition of the DVC's.

5.2.1 The Front Drift Chambers.

The front drift chambers FC1 and FC2 provide goodspatial resolution immediately in front of the spec-trometer magnet. Each chamber above or below thebeam consists of one module of six sense planes; thusthe total front drift chamber package presents twelvesense planes to a charged particle. Design parametersare listed in Table 1.

A drift cell size of only 7 mm (3.5 mm maximumdrift length) was chosen to provide �ne spatial gran-ularity to maximize tolerance to both correlated anduncorrelated background. It emerged that uncorre-lated background is mostly from collimator showers,at the average level of only 0.1 tracks per event. For aminor fraction of the events, the FC occupation fromcorrelated tracks can be substantial, but is alwaysacceptable.

The ASD design is similar to that of the LeCroy2735DC card, except with a low-noise front end andpulse shaping added. With the input transistor(MMBR941LT1) used in common emitter mode withfeedback, channel to channel gain variation is about�10%. In order to optimize pulse pair resolution andhence rate capability, pulse shaping between stages

100

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0 0.5 1 1.5 2 2.5 3Drift Distance (mm)

Res

olut

ion

(µm

)

Figure 5: FC resolution as function of drift distance.

is also added. A long tail due to positive ions fromthe avalanche drifting to the cathodes is cancelled byshaping with two stages of pole-zero-cancelled di�er-entiation. To facilitate testing of the chamber system,a linear monitor point is provided for each channel.

The threshold can be varied from approximately0.07 to 1.25�A at the input. At the lowest value, itwill trigger e�ciently on an impulse charge of approx-imately 12,000 electrons, resulting in 8 ns wide pulsesat both the linear monitor and ECL logic outputs.An impulse charge of 1600 electrons produces a lin-ear pulse of about the same magnitude as the RMSnoise. In the 1995 running period, the FC thresholdwas dictated by ambient noise radiated by the tar-get r.f. system. In 1996, the target r.f. noise wasreduced and the electronics enclosure was improved,allowing a threshold of 140 nA from the wire, corre-sponding to a charge impulse of about 20,000 elec-trons. The power consumption per board is 0.3A at+5V and 0.55A at �5:2V, or 270mW per channel.Hence moderate forced air cooling is provided.

Values of the resolution and e�ciency of the cham-bers in 1996 are plotted as a function of track positionacross the drift cell in Figs. 5 and 6. The values shownfor the resolution are the result of a �tting techniquethat removes the contribution of the tracking reso-lution to the track residuals. These values thereforebetter represent the resolution of the chambers.

11

Drift Distance (mm)

Effi

cien

cy

0.8

0.9

1

0 0.5 1 1.5 2 2.5 3 3.5

Figure 6: FC single-plane e�ciency as function ofdrift distance.

5.2.2 The Back Chambers.

The back drift chambers BC1/2 and BC3/4 are inthe size regime where special attention is required inthe mechanical design to maintain the rigidity andstability necessary for uniform wire tension and pre-cise alignment. These detectors are described in de-tail in [13]. The active areas of the modules werechosen according to their z-positions and the accep-tance of the spectrometer. The geometrical parame-ters are listed in Table 1. Each module contains 14

�berglass/epoxy (GFK) frames of 8 mm thickness,including 6 wire frames. They are clamped betweentwo 38 mm thick non-magnetic stainless steel frames.The total thickness of a module is 190 mm. Becauseof their great length, the frames were prestressed tomaintain the wire and foil tensions. The wires arelocated precisely by contact with accurate pins madeof Polyoxymethylene with 5 �m tolerance, insertedin holes drilled in the GFK frames with a precisionof better than 50 �m. The holes are located along acurved convex locus that compensates the expectedstress de ection of the frames, in order to prevent dis-placement of the U and V wires. A mean deviationof 14 �m for the position of the holes was measured.Pins precisely located in holes can be produced muchmore easily than combs and allow convenient instal-lation of the wires. The gas is injected directly intothe wire gaps and exhausted via the window gaps.

Work on the chamber gas leaks after the 1995 run-ning period reduced the leak rate of the BC's to thelevel of di�usion through the window foils. The cath-ode wires and foils are operated at the same negativehigh voltage of typically 1770 V; the anode wires areat ground potential. More details on the BC mechan-ical construction can be found in [14] [15].

The chamber BC1 is located in the fringe �eld ofthe magnet. It has been veri�ed by simulations usingthe program GARFIELD [16] that a vertical compo-nent of 0.1 T contributes to a deviation in the mea-sured coordinate of < 60 �m, which can be neglectedin the analysis.

The BC performance was studied extensively intest beam measurements illuminating essentially onlyone drift cell per plane. Under these conditions theHV/threshold setting 1750 V/50 mV was found tobe the optimum working point, yielding � = 175 �mresolution (� = 150 �m in the central region of theBC drift cell) and 96.3% e�ciency at 6% cross talk.The e�ciency was determined from the occurrenceof chamber signals in a rather narrow � 4� corridoraround the reference track, which was de�ned by asilicon micro strip detector telescope [17, 18].

The BC performance was evaluated using produc-tion data from normal running. The widths of Gaus-sian �ts to the central portions of the residual distri-bution for all tracks crossing the plane with an angle< 1� is given in Fig. 7 as function of the drift dis-tance for both BC1/2 and BC3/4. This plot wasobtained by combining the data from all cells of allplanes. Tracks close to the sense wire (i.e. smalldrift distances) are excluded because this data pointis strongly a�ected by the di�culties in resolving theleft/right ambiguity. However, a smooth behaviourof the residual can be assumed. As expected, thebest value is measured in the central region of thedrift cell. The results are approximately 210 �m forBC1/2 and 250 �m for BC3/4.

The average BC plane e�ciency for electron andpositron tracks is found to be well above 99% for 1996data. The situation is somewhat worse for all tracks,which are mainly hadrons, due the smaller energydeposited in the chambers by hadrons. The plane ef-�ciency drops to 97% when all tracks are considered.The e�ciency is calculated by using the reconstructedtrack as a reference and adopting the same corridorwidth as used in the reconstruction program to �ndthe hits belonging to a track, i.e. about �3�, where� is the plane resolution. The dependence of the ef-

12

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0 1 2 3 4 5 6 7

BC3/4

BC1/2

Drift Distance (mm)

Tra

ck R

esid

ual W

idth

(µm

)

Figure 7: Width of the BC1/2 and BC3/4 residualdistribution as function of the drift distance for trackswith an incident angle < 1�.

�ciency on the drift distance averaged over all driftcells is shown in Fig. 8.

Investigations of the long term stability of the 1995data showed the expected correlation between cham-ber performance and atmospheric pressure, which ledto varying working conditions.

A system to compensate for atmospheric pressurevariations by dynamically adjusting the high voltagewas introduced during the 1996 running period. A'nominal' high voltage setting of 1770 V was chosenfor an atmospheric pressure of 1013 mbar. The actualoperating voltage was computed using a parametri-sation of the gain versus atmospheric pressure givenin Ref. [19]. The high voltage setting is corrected insteps of 1 Volt over a range of � 20 Volts. Thisscheme signi�cantly improves the stability of thechambers as can be seen in Fig. 9. The dependence ofthe average plane e�ciency on the atmospheric pres-sure is presented for data before and after introduc-tion of the gain stabilisation scheme in the middle of1996 running. The overall improvement in e�ciencyusing this system is 1 to 2 %. Threshold settings of65 mV for the BC1/2 and 80 mV for the BC3/4 havebeen used.

The operation of the eight BC modules was re-markably stable and reliable in the �rst 2.5 years withlow dark currents.

96

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0 1 2 3 4 5 6 7Drift Distance (mm)

Effi

cien

cy (

%)

Figure 8: BC e�ciency as a function of the drift dis-tance for the upper BC modules under normal run-ning conditions.

93

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980 990 1000 1010 1020 1030 1040

96 DATA

95 DATA

Atmospheric Pressure (mbar)

Effi

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%)

Figure 9: Averaged BC plane e�ciency as functionof the atmospheric pressure for the 1995 data sample(open circles) and a 1996 data sample after introduc-ing a gain stabilisation scheme (�lled circles). Fordetails see the text.

13

5.3 The Drift Vertex Chambers

Notwithstanding the adequate performance of thetracking system during the 1995 commissioning year,redundancy in the front region was insu�cient. Lossof a single FC plane could have had serious conse-quences. There was also foreseen the need for fronttracking elements with larger acceptance for a futureupgrade to provide muon detection beyond the stan-dard acceptance. Hence an additional Drift VertexChamber (DVC) pair was constructed, and installedimmediately following the VC's for use starting inthe 1997 running period. The DVC drift cell de-sign is similar to but slightly smaller than that ofthe FC's. However, the DVC acceptance extendsvertically from �35 mrad to �270 mrad, and covers�200 mrad horizontally.An acute design problem arising from the DVC lo-

cation near the target chamber exit window is theclose proximity of the active area to the beam pipe.This is solved in part by separating the functions ofgas enclosure and chamber frames. The gas enclosureor box design is similar to that of the VC's, with avery thin wall less than 1 mm from the beam tube.Each of six wire planes in a box is clamped betweenits own pair of cathode foil planes. All planes canbe installed together in the box while they are matedwith the signal feed-throughs in the lid. However,the planes are thereafter precisely aligned by two pinsequipped with gas seals, externally inserted throughthe box and all the planes. The ends of these pinsprovide external alignment �ducials. A gas injectoris also externally inserted through the box wall intoeach anode plane after its installation in the box.The gas spills out of the active gaps into the box,and hence to a common exit port. Both the signalfeed-throughs and the gas injectors are designed formechanical accommodation su�cient to avoid con-straining the alignment of the planes with respect tothe box.The readout system of the DVC's is identical to

that of the FC's, resulting in similar performance in1997 except for one of the twelve DVC planes, whichsu�ered a broken wire immediately after installation.

5.4 The Magnet Chambers

The proportional wire chambers MC1 through MC3located in the gap of the magnet were originallyintended to help resolve multiple tracks in case ofhigh multiplicity events. Since low backgrounds havemade this unnecessary, their primary function is now

the momentum analysis of relatively low energy par-ticles, from the decay of �'s for example. Since theMC's operate in a strong magnetic �eld and res-olution of �1mm is su�cient, multi-wire propor-tional chambers with digital single bit-per-wire read-out were chosen. The magnet has a tapered pole con-�guration that implies a di�erent size for each pair ofchambers. The active areas of the three chambersgiven in Table 1 are dictated by the spectrometer ac-ceptance together with magnetic dispersion at a nom-inal trigger threshold positron energy of 3.5 GeV.

Each chamber consists of three submodules U;Xand V , laminated into one module with common gasvolume. Each submodule consists of an anode planeat ground potential and two cathode planes with acommon negative HV connection unique to this sub-module, typically at 2850 V. The distance betweenanode and cathode planes is 4�0.03mm and the cen-ters of neighbouring submodules are separated by21mm. The gas mixture has the same constituentsas the drift chamber gas, but with proportions (Ar-CO2-CF4 65:30:5) optimized for MWPC operation(see Table 1).

The most di�cult aspect of the MC design was thelimited space inside the magnet for the frames andonboard electronics. The electronics are mounted onthe front and back faces of the chambers in the barelyadequate space between the magnet pole face and thespectrometer acceptance. Thin exible printed cir-cuit foils connect the wire frames to the electronics.The readout is the LeCroy PCOS IV system, includ-ing an on-chamber card design con�gured speci�callyfor this application [20]. In addition to signal ampli-�cation, these cards provide discrimination and de-lay as well as latching in response to the event trig-ger. The high speed serial readout (up to 20 Mbit/s)greatly reduces the cabling in the very con�ned spaceof the magnet gap. The most severe constraint on MCoperation is the inaccessibility of the detectors andthe on-board electronics for the entire running pe-riod. It is not possible to work on the detectors dur-ing regular monthly accesses because they are buriedinside the magnet. Unfortunately, a faulty batch ofdiscriminator/latch chips together with a less-than-optimal design of the output motherboards used in1995 created a reliability problem. The failure of asingle chip that reads out only 8 wires disabled asmany as 16 cards, corresponding to 256 wires. Thisprocess in combination with a Low Voltage distribu-tion malfunction eventually resulted in the failure ofa large fraction of the MC system, with overall track-

14

ing e�ciency of only 45%. A redesigned motherboardeliminated this problem for 1996 operation. The MCsystem then performed reliably with typical e�ciencyper plane of 98-99%. Electronics failures in the 1996-97 running period resulted in the loss of only 0.5% ofthe channels (3 of 688 cards).

The very restricted air ow in combination with thepower dissipation of 250mW per channel makes watercooling a necessity. Flexible metallic �ngers contactthe tops of the SMD chips and conduct the heat towater-cooled plates between the cards. The watersystem operates at a pressure less than atmosphericto eliminate the danger of leaks.

The contribution to the spectrometer momentumresolution for 10 GeV positrons by Coulomb multiplescattering in the magnet chambers and air betweenFC2 and BC1/2 is �rms = 0.15 mrad. (If the air werereplaced by helium, it would be decreased to 0.11mrad.) Thus the ratio of �rms to the magnet de ec-tion angle is equal to 0.0024, independent of energy.This is small compared to other contributions.

5.5 Alignment

Errors in the relative alignment of the various ele-ments of the tracking system are an important con-tribution to the resolution in reconstructed kinematicquantities. The initial alignment during detector in-stallation was done using conventional optical tech-niques, but di�culties with optical access to the com-ponents limited the expected accuracy of this processto a few hundred micrometers in x and y, and of or-der 1 mm in z. It was always expected that the rela-tive detector alignment would have to be improved inthe data analysis through the study of track residualswith the spectrometer magnet turned o�. This proce-dure was re�ned over the �rst three years of operationto the point that drift chamber alignment errors areno longer a major contribution to tracking resolution[21].

5.5.1 Laser Alignment System

It was considered essential to have a means to contin-uously monitor the relative alignment of the trackingdetectors for two reasons. First, the detectors aremounted on a complex support system that is notsimply interconnected so that its response to tem-perature changes and gradients is di�cult to predict.Also, the front detectors are supported ultimately viathe spectrometer magnet �eld clamp, which might

24.5 mm

Fresnel Zone Plate

Figure 10: Fresnel zone plate made of 13 �m thickstainless steel with a diameter of 24.5 mm.

su�er distortion from magnetic forces. Such an e�ectwas detected with this monitoring system, and elim-inated by stabilizing the clamp mount. Secondly, if adetector must be replaced with a spare after the labo-rious alignment analysis has been done, it is desirableto be able to avoid having to repeat this process im-mediately with new �eld-o� data.

A solution to both of these problems is to have oneach detector module optical targets that are continu-ously accessible to a remotely operated image record-ing system [22] . The type of system that was cho-sen employs two well-collimated parallel laser beamspassing through the entire spectrometer in the beamplane, on either side of and parallel to the beam axis.The 2 cm diameter beams are created by one He-Ne laser and a semi-transparent mirror. Two opticaltarget assemblies are mounted precisely and repro-ducibly on each detector element. Each assembly in-cludes a remotely-controlled actuator that can movean optical target into the path of a laser beam, ar-riving at a precise and reproducible stop. Only onetarget is in a laser beam at a time. The type of targetused { the Fresnel zone plate { is similar to that usedin the SLAC accelerator alignment monitoring sys-tem. It is made by etching a precise pattern througha thin metal foil. This avoids any de ection of thebeam from imperfections in the planarity of trans-parent material. A Fresnel zone plate is shown inFig. 10. The large unperforated central regions wereleft for the sake of mechanical stability. They havethe disadvantage that they produce secondary fringesin the images.

The zone plates on each detector are speci�ed witha focal length equal to the optical path length from

15

Figure 11: Typical Fresnel pattern image detectedby the CCD camera. A one-dimensional projectionis shown at the top and on the right. Nine peaks arevisible at the center. The position of the central peakis �tted.

the target to the image recording component, whichis a lensless CCD camera with a pixel size of 11 �m� 11 �m. Hence any shift in the detector or targetis reproduced with unity magni�cation as a shift inthe focal pattern at the camera. The camera imageframes are digitized and recorded via a VME basedsystem.

Since there are two targets on each detector atx=�45 cm, all transverse detector coordinates canbe monitored. No absolute alignment is attemptedwith this system. In fact, long term stability of thelaser beams is not assumed, since the optical pathscontain several mirrors that are subject to instability.The beams must remain stable only for the 10 minuteperiod required to cycle through all targets in eachbeam path. Only the relative x and y positions ofthe images recorded within that cycle are interpretedas being signi�cant. Thermal wavering of the beamsis reduced by insulating the beam path through thewarm magnet shielding plate using a foam-cell tube.Several camera frames are averaged to minimize thee�ects of the wavering.

A Fresnel pattern image is shown in Fig. 11 . TheFWHM of the central peak is about 300 �m. Theprecision of the laser alignment system is 30 - 50 �m.

5.6 Track Reconstruction

The HERMES reconstruction program (HRC) [23] isvery e�cient because it makes use of two unusualmethods: the tree-search algorithm for fast track�nding and a look-up table for fast momentum de-termination of the tracks. The full reconstruction of30 Monte Carlo events on an SGI R4400 processortakes only one second.

5.6.1 The Fast Pattern Recognition Algo-

rithm

The main task of the reconstruction program is toidentify particle tracks using the hits in the trackingchambers. Each detector plane gives spatial informa-tion in one coordinate, and only by combining theinformation of many detectors is it possible to recon-struct the tracks uniquely in space. There are severaltrack-�nding algorithms in common use. HRC usesthe tree-search algorithm, which turns out to be veryfast. This algorithm is implemented in the followingway:

As a �rst step, \partial" tracks have to be foundin projections, separately in the region in front ofand behind the spectrometer magnet. The track pro-jections are approximately straight lines in those re-gions, except for small curvatures caused by the mag-netic fringe �elds, and kinks coming from secondaryinteractions and straggling.

The basic idea of pattern recognition using thetree-search algorithm is to look at the whole hit pat-tern of the detectors with variable (increasing) res-olution as illustrated in Fig. 12. At each step ofthe iteration the entire partial track is seen. How-ever only at the end is the full detector resolutionreached. The detector resolution is roughly 250 �mand the active width of a chamber is of the order ofa few meters. Therefore after about 14 steps in thebinary tree, the resolution of the detector of � 1 : 214

would be reached. For the purpose of track �nding(not �tting) a resolution of 1 : 211 is su�cient, whichreduces the maximum number of iterations in the HRCtree-search to about 11.

In each step of the iteration, the algorithm checksif the pattern (at the given resolution) contains asub-pattern that is consistent with an allowed track.Fig. 13 shows examples of allowed and forbidden pat-terns. All allowed patterns are generated and storedin a data base at the initialisation phase of the pro-gram. The comparison is very fast as only look-up

16

5

4

3

2

1

LEVEL TRACK 1TRACK 2

DET. 1DET. 2

DET. 3DET. 4

Figure 12: The tree search algorithm looks at thehits of the tracking detectors with arti�cially reducedresolution. In every tree search level the resolution isdoubled until a resolution is reached that is optimalfor track �nding.

DET. 1DET. 2

DET. 3DET. 4

ALLOWED

FORBIDDEN

Figure 13: At each tree search level, the allowed andforbidden patterns are well de�ned. The programcompares the measured pattern with the data baseand determines if the measured pattern is consistentwith an allowed track.

tables are used; no calculations have to be done dur-ing event processing.

The number of allowed track patterns at the fullresolution is of the order of 108. Without optimiza-tion, this large number would make the algorithmuseless for two reasons: memory space problems tostore all the patterns, and CPU time problems tocompare with each of them. The following methodsavoid these problems:

� If one compares the allowed patterns from oneiteration to those from the following one (calledparent and child in the following) it becomes ob-vious that only a very limited number of childrenexist for each parent. At initialisation time alllinks between the allowed patterns of each gen-eration are calculated, i.e. all children for eachparent are stored in the data base. The numberof comparisons now becomes small: if a patternis recognised at one generation, only its childrenhave to be checked in the next generation. Thenumber of children for each parent is typically 4to 8. This solves the CPU problem.

� Symmetry considerations can be used to reducethe number of patterns that have to be stored. Iftwo patterns are mirror symmetric, or if they areidentical except for a transverse shift, they arestored as one pattern. Most important is that if

17

a child is identical to a parent, then the child islinked to the parent and all grand children be-come identical to the children. This simpli�ca-tion is based on the fact that we are looking onlyfor approximately straight tracks. This reducesthe number of branches in the tree-search signif-icantly. The number of patterns in the data baseis of the order of 50,000, which is small comparedto the original number of 108.

After applying the tree search algorithm to the Uand V planes, the partial tracks in these projectionsare de�ned. They are called tree-lines. By testingall combinations of tree-lines and merging them withhits in the x coordinate, the partial tracks in spaceare found. The tree-search algorithm is not applied tothe x-coordinate directly as portions of the X-planesof the VC chambers are tilted and thus do not ful�lthe symmetry conditions. However, the x projectionsare used for track �nding in the back region.The procedure described until now is applied inde-

pendently to the front and back regions, resulting ina set of front partial tracks and another set of backpartial tracks. All combinations of front and backpartial tracks are tested to see if they match spa-tially within a speci�ed tolerance at the x-y plane inthe center of the magnet. Those combinations thatmatch are combined to form a full track, after re-�tting the track geometry to the chamber hits, sub-ject to the match condition. The matching conditionis re�ned by various small corrections depending onpartial track geometry and de ection angle from the�rst iteration.

5.6.2 The Fast Momentum Look-Up

As a second feature in HRC, a very fast method hasbeen developed to determine the momentum of atrack, which is given by the de ection in the inhomo-geneous �eld of the spectrometer magnet. Trackingthrough a magnetic �eld is very CPU intensive. Thenew method makes obsolete the tracking through themagnetic �eld on a track by track basis. Instead, alarge look-up table is generated only once during ini-tilization. It contains the momentum of a given trackas a function of the track parameters in front of andbehind the magnet. The relevant track parametersare the position and the slope of the track in frontof the magnet and the horizontal slope behind themagnet. The resolution of the table has been chosensuch that, using interpolation methods, the contribu-tion by HRC to the precision of the track momentum

determination is better than �p=p = 0:5%. The look-up table contains 520,000 numbers. This method ofmomentum determination is extremely e�cient.

The vertical slope and position of the track behindthe magnet are not kinematically relevant degrees offreedom and are used only to determine the trackquality and reduce the number of ghost tracks. Theyare not used in the determination of the kinematicparameters, as the resolution and alignment of theVC-FC system is superior to the resolution of theback chambers.

5.6.3 Tracking in 1995 without VC's and

DVC's

As mentioned earlier, the VC performance in 1995was not optimal due to di�culties in production ofthe APC readout chips. The performance of thesubstitute prototype chips resulted in marginal ef-�ciencies (60-90%) and many "hot" channels. TheDVC's were conceived later and became availableonly for 1997. Hence an alternative track reconstruc-tion method was developed using only the drift cham-ber data. The back partial tracks are generated asusual and the front partial tracks are generated usingonly the FC data. The front-back matching searchis done as usual, except with somewhat larger toler-ances. Then, for all matching pairs of partial tracks,the match point at the centre of the magnet de�nedby the back partial track is used to re�ne the frontpartial track by pivoting it about a conserved spacepoint midway between FC1 and FC2. Thus the frontpartial track is forced to agree at the magnet mid-point with the presumably higher quality informationfrom the back partial track. This process was usedsuccessfully for the 1995/96 physics analysis, with ap-proximately a factor of two loss of resolution in kine-matic quantities relative to what could be expectedif the VC's were fully operational.

5.7 Tracking System Performance

The performance of the tracking system has beenstudied using the detailed HERMES Monte Carlo(HMC) simulation of the experiment, which is basedon the GEANT [24] software package. Resolutionsfor several quantities were estimated by performing aGaussian �t to spectra of the di�erence between thegenerated and reconstructed quantities. Results forthe measured quantities p (momentum) and � (scat-tering angle) are shown in Figs. 14, 15. The mo-

18

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0 5 10 15 20 25 30

P (GeV)

∆P/P

Figure 14: Momentum resolution in the HERMESspectrometer, deduced from Monte Carlo studies.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 5 10 15 20 25 30

P (GeV)

∆Θ (

mra

d)

Figure 15: Resolution in scattering angle deducedfrom Monte Carlo studies.

mentum resolution is 0.7-1.25% over the kinematicrange of the experiment, while the uncertainty inthe scattering angle is below 0.6 mrad everywhere.Bremsstrahlung in materials in the positron pathcause the momentum resolution function to havea standard deviation considerably larger than thatof the �tted Gaussian. The resolutions for x andthe square of the momentum transfer are shown inFigs. 16, 17. The x resolution varies from 4% to 8%while the Q2 resolution is better than 2% over mostof the kinematic range.

Finally, the absolute calibration of the spectrom-eter can be checked using the decay of KS mesonsinto two pions. The invariant two-pion mass is plot-ted in Fig. 18. The reconstructed KS mass agrees toone part per mil with the value of the Particle DataGroup [25] (497.4 MeV vs 497.7 MeV). The widthof the peak (�= 0.0057, or �1.1%) agrees with theresolution determined by Monte Carlo methods.

5.8 Kinematic Variables

Numerous measured and calculated quantities havebeen compared to the Monte Carlo simulation ofthe experiment (HMC). Fig. 19 shows some of thesecomparisons for the momentum p of the scatteredparticles, the variable x de�ned earlier, the four-momentum transfer squared (Q2), and the squareof the photon-nucleon invariant mass (W2). Thenormalisations of the histograms are arbitrary. Thepoints represent data while the shaded histograms arethe Monte Carlo simulation. The agreement is goodover most of the range of the variables.

The kinematic plane for the experiment (Q2 vs �)is shown in Fig. 20. The typical DIS acceptance isde�ned by the following software cuts shown as thebold lines on the plot:

- Scattering angle: 40 mrad < � < 220 mrad;- Fractional energy transfer: y= �/E < 0.85;- W2 > 4 GeV2;- Q2 > 1 GeV2.

6 The Particle Identi�cation

Detectors

6.1 General Description

The HERMES particle identi�cation system discrim-inates between positrons, pions, and other hadrons.It provides a factor of at least ten in hadron suppres-

19

0

0.02

0.04

0.06

0.08

0.1

0.12

10-1

1x

∆x/x

Figure 16: Resolution for the Bjorken x variable, de-duced from Monte Carlo studies.

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0 1 2 3 4 5 6

Q2 (GeV)2

∆Q2 /Q

2

Figure 17: Resolution for the square of the four-momentum transfer, Q2, deduced from Monte Carlostudies.

π+π-- mass (GeV)ev

ents

mΚ=0.4974±0.0001

σ=0.0057

0

100

200

300

400

500

600

0.4 0.425 0.45 0.475 0.5 0.525 0.55 0.575 0.6

Figure 18: Invariant mass of �+ � �� pairs. Thereconstructed KS mass agrees very well with the PDGvalue.

sion at the DIS trigger level to keep data acquisitionrates reasonable. The rate of DIS positrons is muchexceeded by that of hadrons from photoproduction bya factor as high as 400:1 in certain kinematic regions.The system provides a hadron rejection factor (HRF)of at least 104 in o�ine analysis to keep the contam-ination of the positron sample by hadrons below 1%for the whole kinematic range. The HRF is de�ned asthe total number of hadrons in the spectrometer ac-ceptance divided by the number of hadrons misidenti-�ed as positrons. It depends on the selected e�ciencyof positron identi�cation (see Table 3 later for typ-

ical e�ciencies and contaminations). Since most ofthe hadrons are pions, we will often refer to a pionrejection factor (PRF). The PID system also discrim-inates pions from other hadrons for the importantsemi-inclusive measurements that will allow the con-tribution of the valence and sea quarks to the nucleonspin to be isolated.

The PID system consists of four sub-systems: alead-glass calorimeter, two plastic scintillator ho-doscopes, one of which is preceded by two radiationlengths of lead and which acts as a pre-shower de-tector, a transition radiation detector, and a thresh-old �Cerenkov detector (see Fig. 1). The calorimeterand the hodoscopes are used in the �rst level trig-ger to select DIS events. Beam tests at CERN haveshown that this combination gives a HRF of severalthousand for an electron e�ciency of 95% in o�ineanalysis. The rejection factor is estimated to be ap-

20

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0.4

0.6

0.8

1

0 5 10 15 20 25 30

P ( GeV )

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0 0.1 0.2 0.3 0.4 0.5 0.6

X

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0 1 2 3 4 5 6 7 8

Q2 ( GeV2 )

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25 30 35 40 45 50

W2 ( GeV2 )

Figure 19: Kinematic variables p, x, Q2, and W2.The points are data while the shaded histogram is aMonte Carlo simulation. The normalisations of thehistograms are arbitrary.

ν (GeV)Q

2 (G

eV2 )

0

2

4

6

8

10

12

14

16

18

0 5 10 15 20 25

Figure 20: Kinematic plane for the HERMES exper-iment. See text for details.

proximately 10 in the trigger. The TRD consists of 6modules in each half and provides an additional HRFof over 100 for 90% e+ e�ciency (several hundred ifa probability based analysis is used). The main func-tion of the threshold �Cerenkov detector is to distin-guish pions from other hadrons for the semi-inclusivemeasurements. In future, kaons will also be identi-�ed using a ring imaging �Cerenkov detector (RICH).An upgrade of the current threshold detector to aRICH is being prepared for 1998. Each sub-system isdescribed in detail in the following sections.

6.2 The Calorimeter

The function of the calorimeter is to provide a �rstlevel trigger for scattered positrons, based on energydeposition in a localized spatial region (�3.5 GeVin 1995 and the beginning of 1996 and �1.4 GeV inlate 1996 and 1997); to suppress pions by a factor of�10 at the �rst level trigger and �100 in the o�-lineanalysis; to measure the energy of positrons and alsoof photons from radiative processes or from �0 and �decays.The solution chosen to meet these requirements

consists of radiation resistant F101 lead-glass blocks[26] arranged in two walls of 420 blocks each aboveand below the beam. The properties of F101 glassare listed in Table 2.

21

Chemical composition (weight %)

Pb3O4 51.2SiO2 41.5K2O 7.0Ce 0.2

Radiation Length (cm) 2.78

Density (g/cm3) 3.86

Critical Energy (MeV) 17.97

Moli�ere radius (cm) 3.28

Refractive Index 1.65

Thermal Expansion coe�cient (C�1) 8.5 x 10�6

Table 2: Properties of F101 Glass.

Each block is viewed from the rear by a photo-multiplier tube (PMT). The blocks have an area of9�9 cm2, a length of 50 cm (about 18 radiationlengths), and are stacked in a 42�10 array. Theblocks were polished, wrapped with 0.051 mm thickaluminized mylar foil, and covered with a 0.127 mmthick tedlar foil to provide light isolation. Each blockis coupled to a 7.62 cm Philips XP3461 PMT by a sili-cone glue (SILGARD 184) with refractive index 1.41.A �-metal magnetic shield surrounds the PMT. Asurrounding aluminum tube houses the �-metal andprovides the light seal. It is �xed to a ange that isglued to the surface of the lead glass. This ange ismade of titanium, matching the thermal expansioncoe�cient of F101.

To prevent radiation damage of the lead glass, bothcalorimeter walls are moved away vertically from thebeam pipe by 50 cm for beam injection. The mon-itoring of gain and ageing is achieved using a dyelaser at 500 nm, which sends light pulses of variousintensities through glass �bres to every PMT of thecalorimeter, and additionally to a reference counterphotodiode. The various intensities are produced bya rotating wheel with several attenuation plates. Thelight is split in several stages before being fed into theglass �bres. As the gain of the photodiode is stable,the ratio of the PMT amplitude to that of the pho-todiode signal can be used to monitor relative gainchanges in the photomultipliers. Over three years ofoperation, there has been no observed degradation ofperformance that would suggest ageing e�ects.

Radiation damage to the lead-glass is also moni-tored indirectly using TF1 [27] blocks placed behindthe calorimeter. This material is much more sensitiveto radiation damage than F101. Therefore, a degra-dation of the response would be seen much sooner

in these monitor detectors if there were a large ra-diation dose incident on the back of the calorimetercaused by showers produced by beam loss in the pro-ton machine. So far no variation has been observed intheir response, indicating that the e�ect of radiationdamage is negligible.

Measurements with 1-30 GeV electron beams wereperformed at CERN and DESY with a 3�3 array ofcounters. Each lead-glass block was also calibratedwithin �1% at DESY in a 3 GeV electron beam in-cident at the center of the block.The performance of a 3�3 array of counters showed

[28]: i) an energy response to electrons linear within1%, over the energy range 1-30 GeV; ii) an energy res-olution that can be parameterized as �(E)=E [%] =(5:1� 1:1)=

pE [GeV ] + (1:5� 0:5): this is similar to

that obtained for other large lead-glass calorimeters;iii) a spatial resolution of the impact point of about0.7 cm (�); iv) a PRF of � (2:5�1:2) �103 integratedover all energies in combination with the preshowerdetector, for a 95% electron detection e�ciency.The central values of E=p distributions for

positrons, measured for tracks incident on each blockduring the 1995 data taking period are plotted inFig. 21. E and p are respectively the energy ofpositrons measured by the calorimeter and the mo-mentum determined by the spectrometer. The ra-tio has a central value of 1.00 with a width (�) of0.01, demonstrating that the response of the coun-ters is uniform to � 1%. The long term stabilityof the detector response of 1% is determined fromthe mean value of the E=p distribution, measured foreach run for the counters nearest to the beam. Thisvalue also includes the contribution of the radiationdamage produced in one year of operation, indicatingthe e�ect of the radiation damage is negligible.

The overall calibration of the calorimeter can be de-termined by the reconstruction of �0 decays. Fig. 22shows the �0 invariant mass for events with two clusters. The centroid of the peak for the 1995 datais (134.9�0.2) MeV with a �= (12.5�0.2) MeV ingood agreement with the particle data group value[25]. The calorimeter is described in more detail in aseparate paper [29].

6.3 The Hodoscopes

A scintillator hodoscope (H1) and Pb-scintillatorpreshower counter (H2) provide trigger signals and

22

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20

40

60

80

100

0.9 0.95 1 1.05 1.1E/p

Num

ber

of L

G b

lock

s

Figure 21: Central values of the E=p distribution foreach block (one entry each) measured for positronsfor data collected during the �rst year of operation.

0

200

400

600

800

1000

0.05 0.1 0.15 0.2 0.25mγγ (GeV)

coun

ts

Figure 22: �0 mass reconstructed from 2 clusterevents.

particle identi�cation information. Both counters arecomposed of vertical scintillator modules (42 each inthe upper and lower detectors), which are 1 cm thickand 9.3 cm x 91 cm in area. The material for themodules is BC-412 from Bicron Co., a fast scintil-lator with large attenuation length (300 - 400 cm).The scintillation light is detected by 5.2 cm diameterThorn EMI 9954 photomultiplier tubes, one coupledvia a light guide to the outside end of each scintillator(away from the beam plane). The modules are stag-gered to provide maximum e�ciency with 2 - 3 mmof overlap between each unit.

In addition to providing a fast signal that is com-bined with the calorimeter and H1 to form the �rstlevel trigger (H0 was implemented in 1996; see be-low), the H2 counter provides important discrimina-tion between positrons and hadrons. This is accom-plished with a passive radiator that initiates electro-magnetic showers that deposit typically much moreenergy in the scintillator than minimum ionizing par-ticles. The passive radiator consists of 11 mm (2radiations lengths) of Pb, sandwiched between two1.3 mm stainless steel sheets. The response of the H2counter to positrons and hadrons is shown in Fig. 23.While pions deposit only about 2 MeV, positrons pro-duce a broad distribution of deposited energies witha mean of 20 - 40 MeV that depends weakly on theenergy of the incident positron. A PRF of � 10 ispossible with 95% e�ciency for positron detection.

6.4 Forward Trigger Scintillators

In 1995, a large number of showers generated by theproton beam were able to satisfy the trigger require-ments for DIS positrons. There were typically be-tween 20 and 100 triggers per second of this type,compared to about 35 positron triggers and 35 pion-induced triggers per second (with the standard 3Hetarget operation and about 30 mA of positron cur-rent).

A forward trigger scintillator (H0) placed directlyupstream of the front drift chambers was introducedin 1996 to eliminate these triggers by distinguishingforward and backward going particles using the timeof ight between the front and rear scintillators.

The total time required for a light-speed particle totraverse all trigger detectors is on the order of 18 ns,and hence a backward-going particle produces a pulsein the front counter that is displaced in time by about36 ns from the normal trigger condition. Such a largetime displacement allows for easy elimination of these

23

0

0.025

0.05

0.075

0.1

0.001 0.01 0.1Deposited Energy (GeV)

Cou

nts

(Arb

. Uni

ts)

Figure 23: Response of the pre-shower detector tohadrons and positrons.

background events in the trigger.

The front trigger detectors consist of a single sheetof standard plastic scintillator, 3.2 mm thick (0.7%of a radiation length). The occupancy of the scin-tillators is such that segmentation of the detector isnot necessary. The rate in each paddle is about 106

per second. The scintillation light is collected alongthe edge farthest from the beam axis into two sets oflucite strips and then into two 5.08 cm Thorn EMI9954SB phototubes.

6.5 The �Cerenkov Detector

Pion identi�cation is provided by a pair of single-gasradiator threshold �Cerenkov counters, one each aboveand below the beam. The units are located betweenthe back drift chamber groups, BC1/2 and BC3/4.The radiator is a gas mixture at atmospheric pressureof nitrogen and per uorobutane, C4F10, the composi-tion of which can be varied to control the momentuminterval over which pions can be distinguished un-ambiguously from other hadrons. The depth of theradiator volume is 1.17 m. During the �rst year of op-eration, the radiator was pure nitrogen for which the�Cerenkov momentum thresholds for pions, kaons, andprotons are 5.6, 19.8, and 37.6 GeV, respectively. In1996-97, a mixture of 70% nitrogen and 30% per uo-robutane was used, which gives corresponding thresh-

olds of 3.8, 13.6, and 25.8 GeV.

The body of each unit is constructed of aluminum.The entrance and exit windows have areas 0.46 m �

1.88 m and 0.59 m � 2.57 m, respectively. Each win-dow consists of a composite foil of 100 �m of mylarand 30 �m of tedlar separated by a 1 cm gas gapfrom a second identical composite foil. Dry nitro-gen gas continuously owing through this gap in eachwindow provides a barrier against the di�usion of at-mospheric gas through the windows into the radiatorgas volume. An array of 20 spherical mirrors (radiusof curvature: 156 cm) mounted at the rear of thecounter on Rohacell forms focuses the �Cerenkov lightemitted along particle trajectories onto correspond-ing members of an array of phototubes. The mirrorsare coated with aluminum and magnesium uorideand have a measured re ectivity of 90% at 400 nm.The mirror array is mounted with individual adjust-ment on a plane of 3.0 cm Rohacell foam reinforcedwith �bre glass epoxy �lms. The average e�ectiveparticle path length in the gas viewed by the mirrorsis 0.95 m.

The phototubes (Burle 8854) have 12.7 cm diame-ter photocathodes, and are �tted with Hinterberger-Winston light cones to maximize the light collection.The funnels have an entrance diameter of 21.7 cmand can accept light at angles of up to 26 degreesfrom the optical axis. They are coated with alu-minium to give a re ectivity of 85% at 400 nm.Magnetic shielding for the phototubes is providedby three concentric �-metal shields and a soft ironhousing. The photocathode faces are coated with alayer of 200 �gm/cm2 of p-Terphenyl and a protec-tive coating of magnesium uoride. This wavelengthshifter increases the photoelectron yield [30]. Themean number of photoelectrons for a �=1 particlewith a pure nitrogen radiator was measured to beslightly less than 3. LEDs mounted on each Win-ston cone provide an on-line gain monitoring system.Calibration spectra are accumulated during normalrunning with the LED amplitudes set su�ciently lowso that strong single-photoelectron peaks are resolvedcleanly. These peaks provide an accurate measure ofthe gain per photoelectron for each channel.

The typical response of the �Cerenkov detector isshown in Fig.24. The single photoelectron peak isprominent and reasonably well separated from thepedestal.

24

0

1000

2000

3000

4000

5000

6000

0 1 2 3 4 5 6 7 8Numb. of photoelectrons

Cou

nts

Figure 24: Response of the �Cerenkov detector from1995 data; note that events below 0.15 photoelectronshave been suppressed.

6.6 The Transition Radiation Detec-

tor

The purpose of the transition radiation detector(TRD) is to provide a pion rejection factor of at least100 for 90% positron e�ciency at 5 GeV and above.

Only positrons produce transition radiation in theHERMES energy regime and the detection of one ormore TR X-rays in coincidence with the charged par-ticle can be used to discriminate between positronsand hadrons. The X-ray is di�cult to distinguishbecause it is emitted at an angle of 1/ to the mo-mentum of the charged particle and is therefore notseparable in the detector from the positron.

Because the space restrictions imposed on the TRDwere not severe, a relatively simple and economicaldesign was chosen. Rather than optimize each mod-ule of the TRD for maximum performance, the overallperformance is assured by the addition of extra mod-ules. The design of the TRD was optimized using aMonte Carlo program that tracks charged particlesand photons down to an energy of 1 keV. Care wastaken to model properly the production of �-rays inthe radiator and the detector gas as well as hadronicshowers in the radiator. Elements of the TRD sim-ulation were later integrated into the general spec-

trometer simulation [31].

The HERMES TRD consists of 6 modules aboveand below the beam. Each module contains a radi-ator and a Xe/CH4 �lled proportional chamber. Inaddition, there are two ush gaps on either side ofeach detector through which CO2 is owed to reducethe di�usion of oxygen and nitrogen into the cham-ber. Such contaminants would a�ect the responsesigni�cantly. CO2 is used in the ush gaps becauseit is easily removed from the chamber gas during re-circulation.

The use of polyethylene foils as a radiator is notpractical for HERMES because of the large size ofthe modules (active area: 325 x 75 cm2). It is notpossible to maintain a uniform separation of the foilsfor such a large radiator. A good approximation toa foil radiator is provided by a pseudo-random butpredominantly two-dimensional matrix of �bers [32].Several radiators were compared in a test beam atCERN: foils, �ber matrices with di�erent �ber diam-eters, and polyethylene foams. Fiber radiators wereshown to produce a response only slightly less thanthat from a foil radiator. The �nal design uses �bersof 17-20 �m diameter in a material with a density of0.059 g/cm3. This density is signi�cantly less thanwould be found to be optimal if hadronic showersand delta rays from the radiator were ignored. Theradiators are 6.35 cm thick, which corresponds to anaverage of 267 dielectric layers.

The detectors are proportional wire chambers ofconventional design, each of which consists of 256 ver-tical wires of 75 �m gold coated Be-Cu separated by1.27 cm. The wires are not soldered but rather held incrimped pins. This makes it much simpler to replacea broken wire in situ. The wires are positioned with25 �m accuracy using precisely machined holes for thecrimp pins in plastic strips (Ultem) that are glued tothe aluminium chamber frames. The chambers are2.54 cm thick. Xe/CH4 (90:10) is used as the detec-tor gas because of its e�cient X-ray absorption. Theelectric �eld in the chamber is produced by a +3100 Vpotential applied to the anode wires; the cathode foilsare at ground potential. The large voltage on the an-ode wires is needed to produce an electric �eld in thecritical region between the wires of su�cient strengthto collect the ionization electrons within the 1 �secADC gate used in the readout electronics. The wirediameter was chosen to be unusually large to allowoperation at this high voltage while limiting the gasgain to about 104. The drift cell properties were opti-mized using the computer program GARFIELD [16].

25

The active gas volume of the detector is de�nedby the cathodes which are 50 �m thick mylar foilsaluminized on both sides. The foils are stretched toan unusually high tension (1.75 kN/m) to make themmore stable against pressure di�erences between thedetector and the ush gaps. This is important be-cause a 10 �m deviation in the position of the cath-ode causes a 1% gain shift in the detector. All framessupporting the anode wires and the cathode foils werepre-stressed during construction.

The tension of each wire was measured using reso-nant vibration induced by the combination of a cur-rent owing through the wire and an external mag-netic �eld. Furthermore, each wire was scanned withan X-ray source every 2.54 cm during operationaltests after assembly. Any gain variations of more than10% caused by imperfections in the wire resulted inthe wire being removed and re-strung.

The signals from the wires are ampli�ed by elec-tronics mounted along the long edge of the detectorfurthest from the beam. Balanced di�erential signalsare transmitted over 35 m long twist/ at cables tothe electronics trailer. The use of di�erential signalsreduces common mode noise, which would severelydegrade TRD performance. Receiver modules �lter,amplify, and delay the signals by 256 ns while a �rstlevel trigger decision is made, at which time theyare digitized by LeCroy 1881M Fastbus ADC's. Testpulses are injected via an antenna at the other endof the wires to allow for a quick check of the detectorand readout electronics without beam.

Since Xe is expensive, the chamber gas must berecirculated and puri�ed. Impurities in the gas arekept at acceptable levels by molecular sieves and ac-tivated copper (N2 � 1%, CO2 � 0.25%, O2 � ppm).As a further precaution to keep impurities low, alldetectors were He leak checked during construction.This was particularly important to identify and plugleaks in the crimped pins used to �x the wires. Asophisticated pressure control system based on an in-dustrial programmed controller adjusts the ows intothe detector and the gaps in such a way that the dif-ferential pressure between these volumes is less than afew �bar [33]. A pressure stability of 1-2 �bar corre-sponds to a gain stability of a few percent. The cham-bers track atmospheric pressure in order to avoid us-ing very thick foils to contain the gas. However, ex-perience has shown that even during periods of largeatmospheric pressure variations, the di�erential pres-sure between the detector volume and the ush gapsis maintained below 1-2 �bar by the control system.

0

200

400

600

800

1000

1200

10 20 30 40 50 60 70 80Deposited Energy (keV)

Cou

nts

Figure 25: Response of a single TRD module topositrons and hadrons, integrated over all momenta.Data from 1995 were used for this plot.

The gas system is described in more detail in [33].

Both positrons and hadrons deposit energy in thedetector due to the ionisation of the chamber gas(dE=dx). The most probable energy deposited bya 5 GeV pion is about 11 keV. Positrons deposit onaverage approximately twice this amount of energydue to transition radiation and the relativistic rise indE=dx. The response to positrons and hadrons in one

module is shown in Fig. 25. Pure positron and hadronsamples are selected by placing stringent, ine�cientconditions on the other PID detector responses. Thedistributions are very broad and it is clear that in-formation from several modules must be combinedfor high quality hadron rejection. In particular thelong, high energy tail in the hadron distribution over-laps signi�cantly with the positron distribution. Asimple but e�ective method of analysis is the trun-cated mean method. In this technique, the largestsignal from the six modules is discarded and the av-erage of the remaining �ve modules is used. Thisprocedure reduces the mean of the positron distribu-tion but has a much more signi�cant e�ect on thehadron tail that is due to rare events - productionof energetic knock-on electrons. Fig. 26 shows thetruncated mean distributions for the HERMES TRDderived from 1995 data. It is clear that the separa-

26

0

250

500

750

1000

1250

1500

1750

2000

10 20 30 40 50 60Deposited Energy (keV)

Cou

nts

Figure 26: Truncated mean for six modules forpositrons and hadrons, integrated over all momenta.Data from 1995 were used for this plot.

tion of the hadrons and positrons is greatly enhanced.The pion rejection factor varies with energy becauseboth the energy loss by charged particles and the pro-duction of transition radiation depend on the energyof the incident particle. It is customary when de-scribing the performance of a TRD to place a cuton the response of the detector such that 90% of thepositrons are above the cut. The cut is set for highere�ciency for physics analysis. Using the truncatedmean method, the energy-averaged PRF is about 150for 90% positron e�ciency. If the data are analysedas a function of momentum, a PRF of 130 is deducedat 5 GeV, exceeding the design goal. The PRF issomewhat smaller for lower momenta (80 at 4 GeV)and larger for higher energies (up to 150).

The PRF can be improved using a probabilitybased analysis similar to the one described in the sec-tion on PID system performance below. A probabil-ity function can be de�ned as

�TRD= log10[Pe+=Ph],

where Pe+ and Ph are the probabilities that a particlewas a positron or a hadron respectively. The resultof such an analysis for 1996 data is shown in Fig.27.A cut for 90% positron e�ciency results in a PRFof 1460 � 130. Even at 95% positron e�ciency, thePRF remains very good (489�25).

ΓTRDC

ount

s

0

1000

2000

3000

4000

5000

-10 -8 -6 -4 -2 0 2 4 6 8 10

Figure 27: The logarithmic likelyhood �TRD, inte-grated over all momenta, for hadrons (positrons) isplotted as the solid line (dashed line). Data from1996 were used for this plot. The vertical line indi-cates the location of a 90% e�ciency positron cut.

6.7 PID System Performance

The responses of the four PID detectors are combinedto provide good hadron rejection. This has been donein several ways. The simplest technique is to considerthe responses individually, determine a cut that sep-arates positrons and hadrons in each detector, andde�ne a positron/hadron as a particle identi�ed assuch in all detectors. A very clean sample of a givenparticle type can be produced in this way but at thecost of e�ciency. Typically, this technique is usedonly for detector studies. The e�ciency of the sys-tem can be improved while retaining good particletype separation by using a probability based analy-sis. The responses of the calorimeter, the pre-showerdetector, and the �Cerenkov detector are combined toproduce the quantity PID3 de�ned asPID3= log10 [(P

eCalP

ePreP

eCer)=(P

hCalP

hPreP

hCer)],

where P ij is the probability that a particle of type i

produced a given response in detector j. The P ij 's

are determined by comparing the detector responsesfor each track to typical response functions for eachdetector called parent distributions. The latter are ei-ther derived from the data or modelled using Monte

27

-20-15

-10-5

05

05

1015

2025

3035

4045

50

0250500750

100012501500175020002250

x 10 2

PID3

TRD S

ignal

(keV

)

Figure 28: Plot of the PID3-TRD plane. Positronsappear at positive PID3 and large values of 'TRDSignal'.

Carlo techniques. The use of momentum dependentparent distributions is required to account for thevarying responses of the detectors. PID2 is de�ned ina similar way to PID3 using only the calorimeter andthe pre-shower detector. PID4 including the TRD isalso de�ned but will be discussed later.

For the 1995 data, PID3 was used in combina-tion with the TRD truncated mean to make a two-dimensional cut identifying positrons and hadrons.This is shown in Fig. 28 where it is clear that a cleanpositron sample can be isolated at positive PID3 andlarge TRD truncated mean (TRD Signal). The ad-vantage of using this technique rather than combiningall four detectors is that it allows detailed systematicstudies to be done on the probability analysis [34].For example stringent cuts (low e�ciency, but highpurity) can be applied to the TRD to de�ne cleansamples of positrons and hadrons that can be used tostudy the PID3 response and vice-versa.

One can incorporate the TRD into the probabilityanalysis to improve performance even further. Theresulting function is labelled PID4. However, onethen loses the straightforward systematic cross-checksdescribed above and Monte Carlo simulation must beused to estimate the contamination of each particlesample as well as the detection e�ciency. A graphi-

0

500

1000

1500

-30 -25 -20 -15 -10 -5 0 5 10

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pid2

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nts

pid3

Cou

nts

pid4

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nts

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250

500

750

1000

-30 -25 -20 -15 -10 -5 0 5 10

Figure 29: Comparison between the PID parame-ters PID2, PID3 and PID4 for the 1995 data set.Positrons have positive values of PID2, PID3, andPID4, while hadrons have negative values for thesequantities.

cal comparison of PID2, PID3, and PID4 is shown inFig. 29. In order to keep as many events as possiblefor the �nal analysis, a so-called PID downshiftingscheme was implemented. If all PID detectors pro-vide valid data, a cut on a 2-dimensional PID3-TRDplot is used, which is equivalent to a straight cut onPID4. However, if information from either the TRDor the �Cerenkov detector is not available a more lim-

ited scheme is used: if the TRD data are not availablePID3 is used, while if the �Cerenkov data are not use-ful a two-dimensional cut on the PID2-TRD plane isconsidered. For the analysis of the inclusive data in1995, the full PID3-TRD scheme was used for over97% of the useful events.

The hadron contamination of the positron samplewas determined in several ways. In the more limitedPID schemes, a value can be derived from clean par-ticle samples obtained using the detector not consid-ered in that particular method. However, for the fullPID scheme, the hadron contamination can only beestimated due to the fact that all detectors are usedin the analysis and there is no control detector to de-�ne clean samples. The di�erent analysis techniquesgive consistent results. The most straightforward ofthese methods is a simple �t to the total probabilitydistributions using an assumed shape for the tail for

28

each particle sample in the overlap region, for exam-ple a Gaussian or an exponential. The values of thehadron contamination for a given positron e�ciencyare listed in Table 3 for the x bins used in the anal-ysis of the 1995 data. More details on the particleidenti�cation system can be found in ref. [34].

x-Bin PID3-TRD

e+ e�. h+ cont.

0.023 - 0.04 97.77 1.180.04 - 0.055 98.38 0.810.055 - 0.075 98.78 0.550.075 - 0.1 99.21 0.370.1 - 0.14 99.44 0.220.14 - 0.2 99.64 0.160.2 - 0.3 99.71 0.160.3 - 0.4 99.72 0.160.4 - 0.6 99.72 0.11

Table 3: Positron E�ciency and Hadron Contamina-tion in % for the PID3-TRD Method (1995).

Time of ight information from the hodoscopescan be used to separate pions, kaons, protons, anddeuterons at low momentum (up to �2 GeV). Thetime of ight is referenced to the accelerator bunchsignal. The TDC values must be corrected for verticalposition in the scintillators to take into account thetransmission time of the light. The time resolution is300 ps. Fig. 30 shows the time of ight as a functionof momentum. Protons, deuterons and perhaps somekaons are visible.Another very e�ective way to do particle identi-

�cation is to reconstruct the mass of particles withdecay products in the spectrometer. This has beendone for �0, �, �, KS, �, !, J/, and �. More detailson decaying particles can be found in ref. [35].

7 The Luminosity Monitor

The luminosity measurement is based on the elasticscattering of beam positrons from target gas electronse+e� ! e+e� (Bhabha scattering) and the annihila-tion into photon pairs e+e� ! . The cross sectionsare known precisely, including radiative corrections,e.g. e+e� ! e+e� , e+e� ! [36]. In the fu-ture with an electron beam in HERA, the processe�e� ! e�e� (M�ller scattering) will be used [37].The scattered particles exit the beam pipe at z =7.2 m and are detected in coincidence by two smallcalorimeters with a horizontal acceptance of 4.6 to

10

15

20

25

30

35

40

45

50

55

60

0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3

P (GeV)T

OF

(ns

)

Figure 30: Time of ight vs momentum for hodoscopeH1.

8.9 mrad, which is limited by the size of the beamaperture in the magnet shielding plate. For a beamenergy of 27.5 GeV the symmetric scattering angleis 6.1 mrad and both scattered particles have half ofthe beam energy.

Density � (g/cm3) 7.57

Radiation length X0 (cm) 1.03

Moli�ere radius RM (cm) 2.38

Critical energy EC (MeV) 9.75

Index of refraction n 2.15

Optical transparency (nm) � 380

Radiation hardness (Gy) 7 � 105

Table 4: Properties of NaBi(WO4)2 crystals.

Due to the high radiation background in the regionvery near to the beam, the calorimeter consists of�Cerenkov crystals of NaBi(WO4)2 [38, 39, 40], whichhave a very high radiation hardness on the order of7 � 105 Gy. To further minimize radiation damage,the calorimeters are moved away from the beam pipeexit window by � 20 cm in the horizontal directionfor beam injection and dumping. The properties ofNaBi(WO4)2 are summarized in Table 4. The smallradiation length and the small Moli�ere radius allowa very compact calorimeter design. Each calorime-

29

ter consists of 12 crystals of size 22�22�200 mm3

in a 3�4 array. The crystals are wrapped in alu-minized mylar foil and coupled with optical greaseto 1.9 cm Hamamatsu R4125Q photomultipliers witha radiation hard synthetic silica window and a bial-kali photocathode. For monitoring the gain of thecounters, each crystal is fed with light pulses froman optical �bre of the HERMES gain monitoring sys-tem described in section 6.2. An energy resolution of�(E)/E ' (9.3�0.1)% /

p(E) (E in GeV) has been

determined for a 3�3 matrix from test beam mea-surements with 1-6 GeV electrons.

beam pipe

beam

60 mm66 mm

88 mm

Figure 31: Hit distribution in the calorimeters of theluminosity monitor. The size of the boxes is propor-tional to the number of hits per channel. The shadedarea shows the beam pipe acceptance.

Fig. 31 shows the hit distribution in the calorime-ters. Most of the scattered particles hit the innercrystals near the beam pipe. The deposited energyis reduced by lateral shower leakage, especially forhits near the inner crystal edge. A scatter plot ofthe energy in the left detector versus the energy inthe right detector is shown in Fig. 32. Most of thebackground events have a high energy deposition inonly one of the detectors while Bhabha events havea high energy deposition in both detectors. They areseparated from background by triggering on a coinci-dent signal with energy above 5 GeV in both the leftand right calorimeter. A Bhabha coincidence rate of132 Hz was measured with this trigger scheme for abeam current of 20 mA and a 3He areal target den-sity of 1 � 1015 nucleons/cm2. This provides a statis-tical accuracy of the luminosity measurement of 1%,within about 100 s.

-5

0

5

10

15

20

25

30

35

-5 0 5 10 15 20 25 30 35

Eright (GeV)E

left

(GeV

)

Figure 32: Scatter plot of the deposited energy in theleft luminosity detector versus the deposited energyin the right detector. The dashed line shows the trig-ger threshold of 5 GeV in both detectors for Bhabhaevents.

8 The Trigger

8.1 Trigger Requirements

The function of the trigger system is to distinguishinteresting events from background with high e�-ciency, and initiate digitization and readout of thedetector signals. HERMES requires physics triggerscorresponding to deep-inelastic positron scattering,photoproduction processes (where no positron is de-tected) and additional triggers for detector monitor-ing and calibration. The trigger hierarchy is poten-tially capable of four levels. However, for running in1995-97 only the �rst-level trigger was required.

The �rst-level trigger decision is made within about400 ns of the event using prompt signals of the scin-tillator hodoscopes, the calorimeter, and a few wirechambers. This can be divided into the time neces-sary for signal formation in the detectors and trans-portation to the electronics trailer (� 250 ns includ-ing the transit time of the particle from the target),and the time needed for a decision by the triggerelectronics (� 150 ns). The �rst-level trigger ini-tiates digitization by the readout electronics. The

30

second-level trigger could use any information avail-able within the 50 �s during which time a hardwareclear of the Fastbus readout electronics is possible.The third-level decision could be made on a timescaleof a few hundred micro-seconds by existing digital sig-nal processors in Fastbus, which would �rst read outonly that part of the detector information requiredfor the decision. In fourth level, the full data streamcould be �ltered by a farm of Intel Pentium Pro pro-cessors that can make a decision on a timescale of�1 ms. Only the �rst-level trigger is described be-low.

The DIS trigger selects positron events by requir-ing hits in the three scintillator hodoscopes, H0, H1,and H2, (only H1 and H2 in 1995) together with suf-�cient energy deposited in two adjacent columns ofthe calorimeter, in coincidence with the acceleratorbunch signal (HERA Clock). The forward H0 scin-tillator was installed in 1996 because backward-goingparticles from the HERA proton beam are not ad-equately suppressed by the H1 { H2 timing alone.With H0 included, the primary sources of backgroundare hadrons either produced from positron beam in-teractions in the upstream collimators, or photopro-duced in the target. The requirement of hits inthe H0 and H1 hodoscopes suppresses neutral par-ticle background. The primary charged backgrounddiscrimination is done using a calorimeter thresholdset above the minimum ionizing energy deposition of0.8 GeV. The calorimeter has high e�ciency for elec-tromagnetic showers, but relatively low e�ciency forhadronic showers. The hodoscope thresholds wereset below minimum ionizing. A calorimeter thresh-old setting of 3.5 GeV (used for the 1995 running andcorresponding to y � 0:85) suppresses the chargedhadronic background rate by a factor of � 10 in thetrigger. The typical trigger rate at a luminosity of5 � 1032 nucleons/cm2/sec, was about 50 Hz in 1995.The threshold was lowered to 1.4 GeV during 1996 toincrease the y range to y � 0:95 and improve the ac-ceptance for semi-inclusive particles.

The photoproduction trigger detects hadrons suchas K, �, D0, J/ and �0 that are produced at low Q2

and decay to two or more charged particles. Typicallythe scattered positron angle is too small for detection.The trigger requires charged particle tracks in boththe top and bottom detectors, as identi�ed by thethree hodoscopes and the BC1 chamber as well asthe HERA Clock. The back chamber requirementeliminates those showers originating in the upstreamcollimators, which are con�ned near the beampipe

and hit the tips of the hodoscopes but not the wirechambers which are well shielded by the magnet steel.

8.2 First-level Trigger Architecture

The signals needed for the �rst-level triggers are thehodoscopes (H0, H1, H2), a multiplicity of two ormore hits in the back wire chambers (BC1), and acluster of calorimeter blocks with total energy abovethreshold. Hits in the top AND bottom halves ofthe spectrometer are required for the photoproduc-tion trigger.

Each hodoscope photomultiplier (PMT) signal ispassively split with one output going to a LeCroy1881M analog to digital convertor (ADC) and theother going to a LeCroy 3420 Constant FractionDisciminator (CFD). The individual CFD outputsare fed to LeCroy 1875A time to digital conver-tors (TDC). The high impedance OR outputs of theCFD's for each hodoscope wall are connected to-gether in a chain to form the hodoscope logic signals.

Each calorimeter wall consists of 42 columns of 10lead-glass blocks each. PMT signals from each blockin a given 10-block column are split to provide ADCinputs, and added linearly to form a Column Sumusing custom-built NIM modules. Segment Sums of20 blocks are formed by linear addition of pairs ofadjacent Column Sums, in LeCroy 428F modules. Atleast one Segment Sum contains more than 95% ofthe energy of each electromagnetic shower. All com-binations of adjacent columns are used to fully covera calorimeter wall. The analog Segment Sum signalsare discriminated by CFD's to form TDC and logic

signals.

The back chamber signals are used only for thephotoproduction trigger. ECL signals are utilizedfrom each of the 256 wires of the top and bottomX-planes of BC1. LeCroy 4564 modules are used toproduce logical OR's of adjacent wires in groups of16. The multiplicity of 16-wire groups that �red isdetermined in a LeCroy 4532 module.

The resulting ECL logic signals are reshaped by aLeCroy 4415A di�erential discriminator and collectedinto a signal bus. Relative delay adjustments can bemade with a LeCroy 4518 programmable CAMAClogic delay module. The trigger logic is established inLeCroy 4508 programmable logic units (PLU), whichallow changes to be made easily. The signal bus pro-vides the inputs to the PLU, which is strobed withthe HERA clock. The PLU ECL outputs are con-verted to NIM, prescaled appropriately with CERN

31

prescalers, and brought together in the Master EventOR. The Master Event is retimed by the HERA Clockand gated by the data acquisition NOT BUSY signalto produce the Master Trigger that initiates digitiza-tion and readout of the event.

8.3 Trigger Performance

The purity of the deep inelastic scattering (DIS) trig-ger is good for a high calorimeter threshold (3.5 GeV)and acceptable for a low threshold (1.4 GeV). For a3.5 GeV calorimeter threshold, two thirds of the trig-gers had tracks, 95% of reconstructed tracks camefrom the target, and one third had accompanyingpositrons. For a 1.4 GeV calorimeter threshold, thephysics trigger rate increased by a factor of about 6while the fraction of DIS positrons only went up byabout 10%. These runs had signi�cant contaminationcoming from the collimator just upstream of the tar-get. About two thirds of the events had tracks, butonly 70% of the tracks came from the target, withmost of the remaining coming from the collimator.

Approximately one third of the photoproductiontriggers have reconstructable tracks. Of these, over3/4 have two or more tracks, due to the requirementof hits in the lower and upper part of the detectorsimultaneously. Only 6-7% have positrons, which isnot surprising given that the aim of the trigger isto detect events at low Q2 where the positron goesdown the beampipe. However, most of the trackscome from the target region.

During normal polarised running with the highcalorimeter threshold, 15% of the photo-productiontriggers were also DIS triggers. Roughly 2.5 photo-production triggers were collected for each DIS trig-ger. During polarised running with the low calorime-ter threshold the overlap increased to 40% due inpart to the increased rate in the high-y DIS triggers.The number of photo-production triggers remainedthe same, since the calorimeter is not in this trigger.

9 Data Acquisition and Read-

out Electronics

The backbone of the data aquisition system is con-structed in Fastbus. It consists of 10 front-end crates,the event collector crate, and the event receiver crateconnected to the online workstation cluster via 2SCSI interfaces. CERN Host Interfaces (CHI) actas Fastbus masters. To enhance their readout per-

formance they are equipped in most places withStruck Fastbus Readout Engines (FRE), featuringone or two Motorola 96002 DSPs. The event col-lector in the electronics trailer is connected to theevent receiver crate in a computer room via a Fi-bre Optical Link (STR330/FOL) plugged into therespective CHIs. Pairs of Fastbus crates, read viaonly one CHI/FRE, are formed by cluster intercon-nects (CI), where overall readout timing consider-ations allow for it. The drift chamber signals aredigitized using LeCroy multi-hit multi-event 16 bit1877 sparsifying multiblock 96 channel TDCs with aresolution of 0.5 ns/channel. Charge from the vari-ous photomultiplier tubes and the TRD is digitizedby LeCroy multi-event 64 channel 1881M multiblockADCs. The resolution is 50 fC/channel and the fullscale range is 13 bits above pedestal. During a multi-block transfer from 1881M ADCs or 1877 TDCs,the CHI/FRE combination results in a readout timeof 127 ns/word (CHI without FRE: 270 ns/word),where the slave timing is contributing 90 ns/wordand the FRE 37 ns/word. The multiblock passingtime (time for two neighbouring modules to pass thetoken) is found to be 640 ns. Reading beyond a clus-ter interconnect adds a penalty of about 50 ns/word.This results in a block read speed of more than 30MB/s in crates having a master and more than 20MB/s in crates beyond a CI. As soon as �rst dataare collected into the FRE's input FIFO, the activeDSP starts processing the incoming data, writing theoutput to its output FIFO. The LeCroy multi-event10 bit 64 channel 1875A was choosen for digitizing thetime of ight signals. The resolution is set to 25 or50 ps/channel depending on the detector. As only six1875A's are read out in total, the lack of multiblockcapability on this unit is only a minor disadvantage.The magnet chamber readout is instrumented withthe LeCroy VME based PCOS4 system, consisting ofone 2749 and twelve 2748 modules. The readout time(including the transfer to the event collector mem-ory) is 450 ns/hit wire. Vertex chamber data arrivefrom the detector as a 16 bit ECL STR330/ECL datastream, one 16 bit word containing the plane and stripnumber of one hit strip. Incoming data are passed indirect mode from the Struck ECL interface to the as-sociated DSPs, where hot channels are suppressed us-ing a linear 16 bit lookup table. The trigger pattern isavailable to all front ends, to allow for selective read-out. Event collection is done from each FRE outputFIFO via a cable segment, connecting directly to theFRE cable port. Double bu�ering is implemented in

32

all essential places to provide a low front-end dead-time. Event collection is done in parallel with thenext event readout with incoming events processedalternately between two DSPs in each of the FREs.A typical overall reduction in event size from �18 to10 KB/event is achieved at the DSP level.

In addition to the standard detector readout, thereis implemented a variety of asynchronous indepen-dent events, capable of trigger rates exceeding 5 kHz,including the luminosity monitor and various highspeed calibration and monitoring equipment. De-pending on the application, the data from theseevents are either collected by the event collector dur-ing idle time or by dedicated additional CHIs andFREs. They can be de�ned either as so-called userevents, or as a set of histograms read directly fromCHI RAM or FRE DSP BRAM where they have beenaccumulated.

One VME branch with 4 crates and three CAMACbranches with a total of 9 crates are connected to theevent builder crate to handle special data acquisitiontasks, such as obtaining the electron bunch number,and slow control. The VME interface is a CHI/VSB2module and VDB. The VME crates are connected viaa STR723 in combination with a STR725. The CA-MAC crates are controlled by standard CCA2 typecrate controllers. Scaler information is obtained fromSTR200 32 channel 32 bit Fastbus scalers.

The code for both the 68030 and the 96002 is writ-ten in assembly language. An online library of Stan-dard Fastbus, CAMAC and VME procedures for jobsthat are not time-critical can be called from FOR-TRAN or C in the workstation cluster. These proce-dures are executed in the respective front-end CHIs.All event data and the slow control information areavailable for online monitoring and analysis in the on-line data stream, and are easily available to code inhigh level languages.

Data are written to 9 GB staging disks over thecourse of a �ll of the storage rings, typically lasting8-12 hours. The data are copied between �lls to stor-age silos on the DESY main site via a FDDI link andassociated hardware. In parallel, they are stored onlocal Exabyte tape drives for redundancy. The localsystem was switched to DLT tapes late in 1996. A to-tal of 2 TB was recorded in 1995 and 4.1 TB in 1996.The deadtime during standard running is typicallywell below 10% and the downtime due to the dataaquisition system is estimated to be below 1%. Thepossible throughput of 1.5 MB/s was determined bythe CPU and I/O bandwidth of the event distribut-

ing work station (Alpha 3000X), which correspondsto a 150 Hz event rate at the average event size. Byreplacing the 175 MHz 3000X with a 266 MHz 5/266,this was doubled for 1997.

10 Beam Diagnostics and Tun-

ing

Since HERMES is a �xed target experiment, the scat-tered particles are mostly forward and the detectorsare necessarily in close proximity to the beamline.This makes the experiment sensitive to beam inducedbackground. The two main sources of backgroundassociated with the positron beam are synchrotronradiation and showers from halo particles strikingthe collimators just upstream of the target. Severalclosed orbit corrections (i.e. that a�ect the beam onlylocally) are available to help minimize background.These are of two types: symmetric which a�ect theposition of the beam at the interaction point (IP),and asymmetric which vary the angle at the IP. Itwas determined to be crucial to tune the beam inthe straight section upstream of the experiment tominimize synchrotron radiation. In particular, back-ground from this source is acceptable only when aspecial asymmetric orbit correction well upstream ofthe IP is optimized. The detectors most susceptableto backgrounds are the front tracking chambers andthe TRD, the former because they must operate infront of the magnet and hence in the presence of alarge ux of low energy particles, the latter becauseit is particularly sensitive to photons by design. TheTRD was made less vulnerable to synchrotron radia-tion by adding a thin sheet of lead (�1 mm) in frontof the �rst chamber to absorb some of the low energyincident photon ux.

To help optimize background conditions before thedetectors are turned on, dedicated scintillators wereinstalled around the beam pipe. This system is calledthe Tuning Scintillation Telescope (TST). The TSTconsists of 2 sets of scintillators (8 each) that are lo-cated upstream of the front chambers at z = +1.4 mand behind the luminosity detector at z = +8.3 m.Each set has two layers of 4 scintillators surround-ing the beampipe to view the beam halo. The �rstlayer is unshielded and therefore is sensitive to syn-chrotron radiation as well as charged particles. Thesecond layer is shielded with 3 mm of lead, so thesynchrotron radiation is mostly suppressed. Chargedparticle halo and synchrotron radiation can be identi-

33

�ed using coincidences and anticoincidences betweenthe �rst and second layers of scintillators. Beam tun-ing is then a matter of minimizing the counting ratesand making them symmetric. The TST is used pri-marily during initial beam tuning when the acceler-ators are �rst turned on at the start of a runningperiod, and at the beginning of a �ll of the storagerings.Beam position monitors (BPM) on either side of

the IP allow the beam to be adjusted to a pre-determined position and angle in a short time. TheBPMs are particularly useful at the beginning of a �llof the storage rings.

The experiment is also exposed to particles thatcome from showers originating from the proton beam.A set of scintillators was installed around the pro-ton beam pipe just behind the calorimeter to mon-itor this background. Background from this sourcevaries signi�cantly from �ll to �ll and even duringa �ll. However, essentially none of it is correlatedwith the events of interest so its main e�ect is to in-crease the trigger rate and decrease the proportionof good events going to tape. As outlined previously,the addition of the front trigger hodoscope (H0) hassigni�cantly improved the situation.

11 Summary

This paper describes the spectrometer built to mea-sure the products of deep inelastic scattering of po-larised positrons from polarised internal gas targetsat HERA. After a relatively short commissioningphase, the experiment started recording data in 1995.All essential detectors have performed well with onlya few component failures over the �rst two and a halfyears of running. The acceptance of the spectrom-eter allows the detection of hadrons in coincidencewith the scattered positron. These important semi-inclusive measurements with the identi�cation of pi-ons and kaons are unique to HERMES and will allowthe determination of the spin dependent quark dis-tribution functions.

12 Acknowledgements

The HERMES collaboration would like to thank allthose who participated in the design and constructionof the spectrometer. We are particularly indebted tothe MEA sta� at DESY, many more of whom con-tributed than are mentioned in the author list. We

also gratefully acknowledge the DESY managementfor its support.This project has been supported by the FWO-

Flanders, Belgium; the Natural Sciences and Engi-neering Council of Canada; the Dutch Foundationfor Fundamenteel Onderzoek der Materie (FOM);the German Bundesministerium f�ur Bildung, Wis-senschaft, Forschung und Technologie; the GermanAcademic Exchange Service (DAAD); the Italian Isti-tuto Nazionale di Fisica Nucleare (INFN); MonbushoInternational Scienti�c Research Program, JSPS, andToray Science Foundation of Japan; the United King-dom Particle Physics and Astronomy Research Coun-cil; the U.S. Department of Energy and National Sci-ence Foundation; and INTAS, HCM, and TMR con-tributions from the European Community.

References

[1] D. De Schepper et al., submitted to Nucl. Inst.and Methods.

[2] A. Golendoukhin et al. and B. Braun et al., con-tributions to the 12th International Symposiumon High Energy Spin Physics, Amsterdam,Sept.'96 (World Scienti�c, eds. C.W. de Jageret al) p.241 and p.331; and J. Stewart andB. Braun, contributions to the 7th Interna-tional Workshop on Polarised Gas Targets andPolarised Beams, Aug.'97, Urbana-Champaign,to be published as AIP conference proceedings.

[3] MAFIA Userguide, Version 3.2, Computer Sim-ulation Technology, Lauteschlogerst. 38, 64289Darmstadt, Germany; tel. +49 6151 717057,andM. Bartsch et al., Computer Physics Commu-nications 72 (1992) 22.

[4] TOSCA computer program. Vector Fields Inc.,24 Bankside, Kidlington, Oxford OX5 1JE,England; tel.: (0865) 370151.

[5] M. Geijsberts et al., Nucl. Instr. and Meth.A313 (1992) 377.

[6] J. van der Marel, PhD thesis, Delft University(1997).

[7] D-263 glass is produced by Desag, Germany.

[8] J.M. Butler et al., Nucl. Instr. and Meth. A290(1990) 122.

34

[9] A. Trudel, private communication; U. Becker etal., private communication.

[10] S.F. Biagi, Nucl. Instr. and Meth. A310 (1991)133.

[11] R. Openshaw, R. Henderson, W. Faszer, andM. Salomon, Nucl. Instr. and Meth. A307(1991) 298.

[12] F. Schmidt, Diploma thesis, Friedrich Alexan-der Universit�at Erlangen-N�urnberg, May 1996.HERMES internal note 96-043�.

[13] S. Bernreuther et al., DESY 98-011, hep-ex/9803005, submitted to Nucl. Inst. and Meth.

[14] S. Bernreuther, Diploma thesis, FriedrichAlexander Universit�at Erlangen-N�urnberg,1995; HERMES internal note 95-068�.

[15] F. Neunreither, PhD thesis, Friedrich Alexan-der Universit�at Erlangen-N�urnberg, 1997.

[16] R. Veenhof et al., program GARFIELD, a driftchamber simulation program; CERN ProgramLibrary, Long Writeup, W5050, 1991.

[17] S. Bernreuther et al., Nucl. Instr. and Meth.A367 (1995) 96.

[18] D. Hasch, Diploma thesis, Humboldt Univer-sit�at zu Berlin, 1995; HERMES internal note95-028�.

[19] J.C. Armitage et al., Nucl. Instr. and Meth.A271 (1988) 588.

[20] LeCroy Research Instrumentation Catalog -1996.

[21] E. G�arber, Diploma thesis, Humboldt Univer-sit�at zu Berlin, November 1996. HERMES in-ternal note 96-062�.

[22] T.-A. Shibata et al., accepted in Nucl. Instr.and Meth. A.

[23] W. Wander, PhD thesis, Friedrich AlexanderUniversit�at Erlangen{N�urnberg, April 1996.(English translation: HERMES internal note97-031�)

[24] GEANT Detector Description and SimulationTool, CERN Program Library, Long Writeup,W5013, 1994.

[25] Particle Data Group, Phys. Rev. D54 (1996) 1.

[26] The F101 glass blocks were provided by theLytcarino factory, 140061 Lytcarino, Russia.

[27] M. Kobayashi et al., KEK Internal Report 93-178 (1993).

[28] H. Avakian et al., Nucl. Instr. and Meth. A378(1996) 13;A. Fantoni, PhD thesis, Universit�a della Cal-abria, February 1996, HERMES internal note96-015�.

[29] H. Avakian et al., accepted in Nucl. Inst. andMethods.

[30] P. Baillon et al., Nucl. Instr. and Meth. 126(1975) 13.

[31] J.V. Emerson, MSc thesis, Simon Fraser Uni-versity; July 1995. HERMES internal note 96-038�.

[32] The �bers were provided by: Carl Freuden-berg Faservliessto�e, P.O. Box 100363 Wein-heim 69465. The commercial designation of thematerial is VILEDON C 1900/034.

[33] D.M. Thiessen, MSc thesis, Simon Fraser Uni-versity; March 1996. HERMES internal note96-028�.

[34] R.B. Kaiser, PhD thesis, Simon Fraser Univer-sity; April 1997. HERMES internal note 97-017�; andR.B. Kaiser (for the HERMES PID Group),HERMES internal note 97-025�.

[35] K. Ackersta�, Contribution to DIS96, Rome,Apr.'96; eds. G. D'Agostini and A. Nigro,World Scienti�c, p.526; and \Cross-Sections forVector Meson Production at HERMES", paperin preparation.

[36] F.A. Berends and A. B�ohm, in High EnergyElectron{Positron Physics, eds. A. Ali andP. S�oding, World Scienti�c Publishing, Singa-pore, 1988, p. 27.

[37] Th. Benisch, Diploma thesis, Friedrich Alexan-der Universit�at Erlangen-N�urnberg, March1994; HERMES internal note 94-030�; andPhD thesis, Friedrich Alexander Universit�atErlangen-N�urnberg, March 1998.

35

[38] G.I. Britvich et al., Nucl. Instr. Meth. A321(1992) 64.

[39] A.V. Antipov et al., Nucl. Instr. Meth. A327(1993) 346.

[40] V. Samsonov, Proc. of the 'Crystal 2000' Inter-national Workshop, Chamonix, France, 1992,eds. P. Lecoq et al. (Ed. Fronti�eres, 1993) p.383.

* The theses mentioned in these references are avail-able as HERMES internal notes, and can be accessedon the HERMES web pages:http://www-hermes.desy.de/notes/

yDeceased.

36