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PARTICLE DETECTORS K. KLEINKNECHT Institut für Physik der Universitdt Dortmund, Dortmund, Gennany NORTH-HOLLAND PUBLISHING COMPANY AMSTERDAM

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Page 1: PARTICLE DETECTORSgruppo3.ca.infn.it/defalco/fisica/kleinknecht.pdf88 K. Kleinknecht, Particle detectors In contrast to these neutral particles, charged particles can be detected directly

PARTICLE DETECTORS

K. KLEINKNECHT

Institutfür Physikder UniversitdtDortmund,Dortmund,Gennany

NORTH-HOLLAND PUBLISHING COMPANY — AMSTERDAM

Page 2: PARTICLE DETECTORSgruppo3.ca.infn.it/defalco/fisica/kleinknecht.pdf88 K. Kleinknecht, Particle detectors In contrast to these neutral particles, charged particles can be detected directly

PHYSICSREPORTS(Review Sectionof PhysicsLetters)84, No. 2 (1982)85—16 1. North-HollandPublishingCompany

PARTICLE DETECTORS

K. KLEINKNECHTInstitut fir Physikder (JnivergitdtDortmund,Dortmund, Germany

Received 29 December1981

Contents:

1. Introduction 87 4. Identification methods 1192. Position measurement 87 4.1. Time-of-flight 119

2.1. Physical processes for detection 87 4.2. Cherenkov counters 1192.2. Proportional chambers 89 4.3. Transitionradiationdetectors 1262.3. Planar drift chambers 94 4.4. Multiple ionizationmeasurement 1302.4. Cylindrical wire chambers 96 4.5. Comparison of identification methods 134

2.5. Pictorial drift chambers 98 5. Energy measurement 1352.6. Time projection chamber 101 5.1. Electron—photon shower counters 1352.7. Bubble chambers 103 5.2. Hadron calorimeters 1422.8. Streamerchambers 104 5.3. Monitoring of calorimeters 1462.9. Flashand sparkchambers 108 6. Momentummeasurement 1482.10.Comparisonof positiondetectors 109 6.1. Magnet shapesfor fixed target experiments 148

3. Timemeasurement 110 6.2. Magnet shapesfor storagering experiments 1503.1. Photomultiplier 110 6.3. Centraltrackingdetectors 1513.2. Scintlllators 111 7. Realizationof detectorssystems 1533.3. Light collection 114 7.1. A hadronbeamdetector 1533.4. Planarsparkcounters 117 7.2. A neutrinodetector 154

7.3. A protonstoragering detector 1557.4. An electron—positron storage ring detector 158

8. Conclusion 158

References 158

Abstract:

Theprinciplesof instrumentsusedfor detectingand identifying high energyparticlesare reviewed.Recentprogressin tech-niquesand materialsis included.A few realizationsof detectorsystemsaredescribed.

Single ordersfor thisissue

PHYSICSREPORTS(Review Sectionof PhysicsLetters)84, No.2(1982)85—161.

Copiesof this issuemaybeobtainedat thepricegivenbelow.All ordersshouldbesentdirectly to thePublisher.Ordersmustbeaccompaniedby check.

SingleissuepriceDfl. 40.00,postageincluded.

0 370-1573/82/0000—-0000/$19.25© 1982North-HollandPublishingCompany

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K. Kleinknecht,Particledetectors 87

1. Introduction

Progressin experimentalparticle physicshas alwaysbeenclosely linked to improvementsinacceleratoranddetectortechnology.The searchfor smallor point-like constituentsof matterre-quired the study of scatteringandannihilationprocessesat everlarger center-of-massenergies.This was achievedeitherby largefixed-targetaccelerators,like the 400GeV Proton Synchrotronsat CERN andFermilaband the 30 GeV electronLINAC at SLAC, or by storagerings both forprotons(CERN TSR) andelectron—positronpairs(SPEARandPEPat SLAC, DORIS andPETRAat DESY, CESRat Cornell). The highestc.m. energyavailablenow is 540GeV providedby theantiproton—protoncollider at CERN. Progressin acceleratortechnologyincludesthe inventionof stochasticandelectroncooling of antiprotonbeamsandthe developmentof superconductingpulseddipolemagnetsfor the 800GeV Tevatronat Fermilab.

Experimentsare basedon the ability of the researcherto detectparticles producedby theseacceleratorsor storagerings. The detectingequipmenthasundergonethreemajordevelopmentsduring the past ten years: the size of experimentshas beenincreasing;for fixed-target experi-ments, this is a naturalconsequenceof the largermomentaof particles involved and the corre-spondinglylarger leverarmsfor magneticanalysisrequired.Also, largertargetmasseswereneededfor reactionswith small cross-section,as in neutrinophysics.For storageringdetectors,the largesize is dictatedby the necessityto covermostof the 47r solid anglearoundthe interactionpoint.The seconddevelopmentconcernsthe speedof dataacquisition:while pulseddeviceslike bubbleor sparkchamberswere limited to 1 —10 recordedeventsper acceleratorpulse,the inventionofproportionalanddrift chambershas increasedthis rate by a factor of 100 enablingexperimentswith 108 recordedevents.In parallelwith this goesthethird evolution:theincreasein complexityof the detectors.The numberof independentanaloginformationsfrom a large experimentcanreach 1 o~,andafterdigitization this yields up to 1 0~bits of informationfor oneeventfrom sucha detector.Experimentsof this type havebecomepossiblebecausethe reliability of the equip-ment has increasedconsiderablyduring this time and becausefaston-line computersenableper-manentcontrol andmonitoringof the detector.

In spiteof the complexityof largeexperiments,the basicprinciplesof detectorsaresimple. Inthis article, I go through theseprinciples andsomeof the newerdevelopments,following the listof physical quantitiesmeasuredby the detector:position, time, mass,energyandmomentum.

2. Positionmeasurement

2.1. Physicalprocessesfor detection

The physicalprocesseswhich enableus to detectparticlesaredifferent for neutralandchargedparticles.Photonscan interactby photoelectricor Comptoneffect or by pair creation,wherethelatter processdominatesat energiesabove100 MeV. The resultingelectronsandpositronscanbedetectedby their electromagneticinteraction.Neutronsof high energywill producea showerofhadronswhen colliding with detectormaterial, thus enablingthe detectionof chargedsecond-aries.Neutrinos interactby weak interactionconservinglepton number,producinghadronsandachargedor neutrallepton.

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88 K. Kleinknecht, Particle detectors

In contrastto theseneutralparticles,chargedparticlescanbe detecteddirectlyby their electro-magnetic interactionwith the atomic electronsof the detectormaterial.In thesecollisions, theenergyloss of a heavy chargedparticlewith massm > me by ionization is given by the Bethe—Bloch-formula[BE 30, BE 32, BE 33, ST 71]:

—dE/dx= (47rr~mec2NoZz2/Af32)[ln(2mec2132/((l— 132)1)) 1321

wherex is the thicknessof material traversedin g cm2, N0 is Avogrado’snumber,Z andA are

atomicandmassnumbersof thematerial,zeandv 13c the chargeandvelocity of themoving par-ticle, me the electronmass,re = 2.8 fm the classicalelectronradiusandIan effectiveatomicioni-zationpotential rangingfrom 13.5 eV in hydrogento 1 keV in lead.The dependenceof this energyloss on particlevelocity is characterizedby the 1/132 variation at low energies,by a minimum at

= p/mc 4 and finally at high energiesby the relativistic riseby a factorwhich, for gases,isaround 1.5. Fig. 1 showsthisbehaviourasmeasured[LE 78a] in anargon—methanemixture.Theminimumvalueof the energyloss aroundfry = 4 is 1.5 MeV/(gcm—

2) for iron and1.8MeV/(g cm—2)for carbon.

The energyloss by ionization is distributedstatisticallyaroundthe meanloss describedby theBethe—Bloch-formula,the distributionbeingasymmetricwith a tail [LA 44] at high lossesdueto6 ray productionanddistantcollisions.

Thecalculationof theenergylossdistributionwasfirst doneby Landau[LA 44] andSternheimer[ST 52] assumingthat the Rutherfordterm in the cross-sectionis the only sourceof the fluctua-tions and that its behaviourin the region of binding energiesis describedby a meanionizationpotential. Fig. 2 showsthat this modelreproducespoorly the measured[HA 73] energyloss dis-tribution for thin (1.5 cm) gaslayers,but thatmorerefinedcalculationsincluding the shellstruc-

1.7Argon — — —— ——

- 6cm

1 10 100 1000 10000

P//i-c

Fig. 1. Ionization energyloss in argonatatmosphericpressurerelativeto valueatj3.y = p/cm 4. Points from [LE 78a] in argon/5%CH

4 dashedline: calculation of [ST 52]; dash—dotline: [ER 77]; solid line: photo absorptionionization model [CO 75,CO 76, AL 80].

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K. Kleinknecht,Particledetectors 89

jf~\ 3 GeV/c r~ 3 6eV/c ej] \ 1,5cm 1.5 cm

I I

dN

/ pI It Ill “In - ~Ip ~.:jI lip

[~I L\\ r11 \ \J/p \ \

Il ri \~ . I -/1 I

_Jl I I - .1 I I I~ —

012345678 01 2345678L~KeV ~KeV

Fig. 2. Ionization energyloss distributionfor pions andelectronsin 1.5 cmof argon: 5% CH~at atmosphericpressure.Dataof[HA 73]. Dashedcurveof [LA 44, MA 69], dottedcurveof [LA 44, BL 50], solid curveof [AL 80].

ture of the atoms(PhotoAbsorptionIonizationModel, PAl [CO 75, CO 76, AL 80]) give a satis-factory descriptionof the data.The relativistic riseof the energyloss as measuredin fig. I is alsoreproducedwell by thesemodels,while formercalculationsgave a rise which was too large by10—15%.

The averageenergyneededfor creating anelectron—ionpair is fairly similar in differentgases,viz. 40 eV/pairin helium and26 eV/pairin argon,while it is muchsmallerin solids,e.g. 3 eV/pairfor Si, such that for solids the numberof pairs is larger andthe statisticalfluctuationsin energyloss are smaller.However, the technicalproblemswith the productionof large volumesof puri-fied semiconductorshavelimited the use of Si- andGe-countersto low-energyhigh resolutionyspectroscopy.Fordetectionof chargedparticlesin largeareadetectorswe areleft with ionizationin gases,mainly noble gases,and in liquid or solid scintillatorsconvertingthe ionization energyloss into visible light.

2.2. Proportional chambers

Proportionaltubeshadbeenusedsince a long time. A cylindrical tube(radiusra) on negativepotential and a centralwire (radius r1) on positivepotential createan electric field of the form

E(r) = V0/(r ln(ra/rj))

which reaches10~—1 0~V/cm nearthe anodewire. An electronliberatedin an ionization processgainsthe kinetic energyz~T betweentwo collisionsat radial distancesr1 andr2

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90 K. Kleinknecht, Particledetectors

~ewIres

0800.85

0.90

Fig. 3. Field configuration in a proportionalchamber;field andequipotentiallines aredrawnECH 70bj.

~T-efE(r)dr,

and if L~Texceedsthe ionization energyof thegasatoms,asecondaryionization cantakeplace.A chain of such processesleads to an avalancheof secondaryelectronsandions. The numberofsecondaryelectronsper primary electron (gasamplification a) reaches104_106in the propor-tional region,wherea is independentof the numberof primaryelectrons.

The field configurationin a proportionalchamberwith many anodewires in a planebetweentwo cathodeplanesis shownin fig. 3. Thediscoveryof Charpaket al. [CH 681 wasthat thesesep-arateanodewires act as independentdetectors.The capacitivecoupling of negativepulsesfrom

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K. Kleinknecht,Particledetectors 91

I ++ I+ /

\++ I

Fig. 4. Shapeof theionizationavalanchein a high electricfield [LO 61].

onewire to thenext is negligible comparedto the positivepulseon the neighbourwires inducedby the moving avalanche.The main contributiondoesnot comefrom the fast-movingelectronsbut from the much (1000times)slower ions in the drop-shapedavalanche(fig. 4). The time struc-ture of the negativepulse inducedon the anodewire by the avalancheshasbeenclarified subse-quently [Fl 75]. It consistsof severalpulsesinducedby differentavalanchescreatedby differentprimary ionization electronsdrifting oneafter anotherinto the high field regionnearthe anodewire. Thesepulseshavetypically a rise-time of 0.1 ns from the electronpart anda decaytime of30 ns from the ion part of the avalanche.Fig. 5 showsan oscilloscopepicturewith atime resolu-tion sufficient to resolvetheseseparatepulses,which in normal applicationsare integratedintoonepulseby sloweramplifiers.

As a practicalexamplefor operatingproportionalchambersystems,oneof the first spectrome-terswith largechambers[SC 711 useda wire distanceof 2 mm,gold-platedTungstenwiresof 20~tm,a gap betweensignal wires andcathodeplaneof 6 mm andan argon—isobutanegasmixture.The amplifiers [CU 71], basedon MECL 1035 chips, hada thresholdof 200pV on 2 k~2andaneffective resolvingtime of 30 ns.Thedetectionefficiencyfor this system,shownin fig. 6, allowsoperatingat full efficiency at 4.3 kV with a 40 nssensitivetime, i.e. the gatefor thewire signalswas openedby an externaltrigger forthistime interval.The spaceresolution for this wire distanceis a ‘-~ 0.7 mm.

One of the problemsencounteredin largechambersis the mechanicalinstability of the signalwiresdueto the electrostaticforcebetweenwires. It canbe calculated[TR 691 that the systemisstableif the wire tensionT exceedsa valuegiven by the wire geometry

T> (Vl/2 ira)24ire0

where V is the potentialdifferencebetweenanodeandcathode,1 is the lengthof thesignalwire,anda is the gap betweenwire andcathodeplane.For the exampleabove,with a wire tensionof50 p, the wires are stablefor 1� 60 cm. This meansthat for larger chambersthe wires haveto besupportedevery 60 cm by supportwires threadedacross,or by other methods.An inefficiency

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92 K. Kleinknecht, Particle detectors

b)

avalanche.

Fig. 5. (a) Oscilloscope display of proportional chamber pulse showing separate avalanches. (b) Pulse shape as simulated by com- puter [FI 751.

of 10% in a region of 5 mm width around the support wire is a consequence of some of these schemes [KL 701.

An enormous increase in spatial resolution of proportional chambers can be achieved by using the information from pulses induced on the cathode plane [RA 74, CH 78aI. For this purpose, at least one of the cathode planes is made of strips perpendicular to the direction of anode wires (see fig. 7). The pulses induced by the avalanche on the individual strips vary with the distance of the strip from the avalanche, and the center of gravity of the integrated pulse heights is a measure of the avalanche position. Fig. 8 demonstrates the precision which can be obtained with this center-of-gravity method. A soft X-ray produces ionization at three positions separated by 200

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K. Kleinknecht,Particledetectors 93

— I I I I

1.0 - ~__~—_:-—••----—•--——1--——•-—-—•—•——••---=.:~—.-—•--——•-—--.’———•—..—.—..—. 1.0

...— ...... —.—.— ..—,‘ , .—.. —..—.—

, / ,. .,.

/ ,/ / .,I, ./

I, / /

1/ ,i .. .,

7, / // ,/ / / / 40 nsec gate

/ I ? / ,~ — — — — 35 nsec gate~0.5 - / / / / . 30 nsec gate - 05f / / I —. —. — 25 nsec gate

/ / / / / — —.. — 20 nsec gate

/~.i ~//1 /1

/ ,/ ~•/ /,,•, ., .,./ S., ,

I I I

3.5 4.0 4.5 5.0high voLtage (K volt)

Fig. 6. Detection efficiency of a proportionalchamber(2 mm wire distance,2 X 6 mm gap)vs. high voltage for different gateopeningtimes [SC 71].

jim. The centerof gravity y of eachavalancheandthe integratedchargec aremeasured,and thedistribution in y showsthreepeakswith a varianceof 35 jim. Most of this resolutionerror comesfrom the range of the original photoelectron.This impressiveaccuracy,however,is achievedwitha detectorwhereboth mechanicalconstructionandelectronicpulse-heightprocessingis costly.

7/

Fig. 7. Principleof cathodereadoutfor proportionalchambers.The centerof gravityof the inducedchargeson cathodestrips(b)determinesthepositionof avalanche(a), [CH 78a].

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94 K. Kleinknecht. Particle detectors

200 400 600

y c rnlCmlS)

Fig. 8. Spatial resolution of a proportional chamber exposed to a 1.4 keV X-ray beam at 3 positions, 200 w apart; plotted is total charge on cathode vs. center of gravity [ CH 78a].

2.3. Planar drift chambers

A great reduction in cost is possible by using the experimental fact [CH 70aI that the time delay between the crossing of a charged particle through a proportional chamber and the creation of a pulse on the anode wire is related to the distance between particle trajectory and anode wire. This delay was found to be of the order of 20 nsec/mm, and if this time is measured for each anode wire with an accuracy of 4 nsec, a spatial resolution of 200 pm can be obtained.

The invention of the drift chamber [CH 70a, WA 711 exploits this possibility. A scetch of the field configuration in one cell of a drift chamber is shown in fig. 9. The electrons from the prima- ry ionization process drift in a low field (1000 V/cm) region into the high field amplification re- gion around the anode wire, where avalanche formation occurs. Typical drift velocities for differ- ent gases are shown in fig. 10 for various argon-isobutane mixtures [BR 741. For some of the mixtures, the drift velocity depends only mildly on the field strength, thus enabling a linear rela- tion between distances and drift time even without constructing a perfectly constant field in the drift region. This is important because the requirement of a constant drift field necessitates the introduction of several field shaping wires per cell. An example for such a cell [MA 771 is shown in fig. 11, where the cell dimensions are 60 X 30 mm ?. The linearity between drift time and dis- tance for this chamber is shown in fig. 12.

NEG.%i%TIAL NEG.ENTlAL

Fig. 9. Equipotential lines in a drift chamber cell.

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K. Kleinknecht,Particledetectors 95

I I I I I I j

Drift velocity

60-

4O~ Isobutone

• Ar 86.5 13.520 - o Ar 81 19

x4r75 25- +Ar70 30

£ Ar 69 ~ 31

0 l~~I62 I” I ~8 I

o 400 800 200 600 2000 2400

Drift field [V/Cm]

Fig. 10. Drift velocitiesin argon—isobutanemixtures [BR 74].

/ / z, GROUND / S.,. ‘1 .

E

+1 kV +1.5kV +2kV 4-1.5kV +1kv

p —

0SW.4O~im POT.W.~0

+3.7kV 200p.tm

15mm~5mm~5m~5mm~ 30mm —2k

S GROUND

Fig. 11. Cell structureof largeareadrift chamber[MA 77].

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96 K. Kleinknecht,Particledetectors

I I I I I

600 i~sec

400

GAP N21 ~2O4±3nsec/Cm

200

2O ~Oi~ 20_O~tance

Fig. 12. Linearrelationbetweendrift time andposition [MA 77].

2.4. Cylindrical wire chambers

For storagering detectors,cylindrical geometriesmatched to solenoidalmagnetic fields (Br= Bc1, = 0, Bz ~ 0) havebeenwidely used.Thesecentraldetectorsaim at the measurementof cur-vaturesandinitial directionsof tracksemergingfrom the interactionpoint (fig. 13).

The first detectorsof this kind usedcylindrical layers of proportional chambers(see2.2) orsparkchambers(see2.7) to determinethe (r, ~,)trajectory of the tracks (fig. 13). The wiresarestrungparallel to theB field, theE field is radial,but the displacementin I~of the avalanchesdueto the Lorentzforce is smalldueto the smalldrift spaceto the anodewire.

For the next generationof centraldetectors,cylindrical drift chamberswere used.Here thedetectorhasup to 20 cylindrical layersof drift cells with electricaldrift field in therIp-plane (fig.13). In order to savewires, the cells are openin the radial direction,but closedby at least3 p0-

tential wires in the p direction. Approximatelyhalf of the sensewiresrun exactlyparallelto theB field, the othersare inclined by a stereoangle(e.g. ±4°)relativeto this axis in order to enablereconstructionof the z positionof the tracks.

An examplefor sucha centraldetectoris the TASSOchamber[BO 80]. The drift cell (fig. 14)is radially open, the wires are 3.5 m long. The 1 5 chamberlayersextendover 85 cm radial track

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K. Kleinknee/it,Particledetectors 97OoEIII~bHV ~. _____ HV d

Fig. 13. Different types of cylindrical wire chambers;(a) proportionalchambers,(b) cylindricaldrift chambers,(c) pictorialdriftchambers, (d) time projection chamber.

~

Potential Wires: 120pm0

t~ ~.

Sense Wire: 30pm0

Fig. 14.Geometricalarrangementof wiresin theTASSOwire detector[BO 80]. Dimensionsarein mm.

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98 K. Kleinknecht,Particle detectors

S.’ —

S

S •. . . I

—I / ‘I. ~I F .• •I I •

: . . S •

• .

+ .-,

.‘ ~ : . ~ •• ~ • • /

•• -5-S...’.’

s.s c.s

Fig. 15. Hadroniceventproducedby e~ecollision at30 GeV c.m.energyin theTASSO wire detector,seenalongthebeamdirec-tion [BO 80].

length, 6 of theseare equippedwith ±4°stereowires. The resolutionachievedwas� 200 jim inthe (r, Ip) planeand 3—4 mm in the z direction.Fig. 15 showsthe (r, Ip) view of a hadroniceventat 30 GeV c.m. energy recordedby the TASSO detector.Another examplewith a closedcellstructureis the ARGUS drift chamber,shownin fig. 16 [WE 811. Herethedrift velocity is nearlyradially symmetric around the sensewire, such that the geometricalposition of all hits with afixed drift time is a cylinder aroundthe sensewire. This facilitatespatternrecognitionfor tracks.

2.5. Pictorial drift chambers

This type of chamberrecordsmany (�250) three-dimensionalpointsalongthe chargedparticletrack. Thismeasurementof true spacepointsis very instrumentalfor the reconstructionof events

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K. Kleinknee/it,Particledetectors

• 0 —

H 18.80 SignaL Wire

• Potential Wires

Fig. 16. Geometryof wires, lines of constantdrift time and drift pathsin one cell of theARGUS detectorfor a 0.9 T magneticfield parallelto the wires [WE 81].

fletd%\ a

Fig. 17. Cross-sectionthroughtwo segmentsof thejet chamberof the JADE detector.Length of drift pathd, Lorentz anglea[DR 80, WA 81b].

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100 K. Kleinknecht,Particledetectors

with a high densityof tracks.The first suchpictorial chamberwas built for the JADE detectoratPETRA [BA 79, DR 80]. Here the cylindrical volume of the trackdetectoris subdividedinto 24radial segmentsof 15°openingangle. In eachsegment(fig. 1 7), thereare64 sensewiresparallelto the magnetic field, arrangedin 4 cells of 16 wires, i.e. a total of 1 536 sensewires of 234 cmlength. The electric field is perpendicularto the sensewire plane,andthereforealsoto themag-netic field. Due to the longer drift length comparedto normal cylindrical drift chambers,theeffect of the Lorentz forceeu X B becomesnoticeablehere.

It leadsto a deviationof the directionof thedrift velocity UD from the directionof the electricfield E by an angleaL given approximatelyby the ratio of magneticandelectricforces,tan aL =

k(E) UDBIE [WA 81b], which in the JADE chamberatB = 0.45 T and4 atm. pressureis 18.5°.The factork(E) dependson chambergasandelectricfield [SC 80]. Due to the Lorentz force, thelines of equaldrift time in the neighbourhoodof the sensewire planein the JADE chamberarerathercomplicated(fig. 18). Fig. 19 showsthe (r, Ip) projectionof a two-jet eventat 35 GeV c.m.energy.Up to 48 samplingspertrackarerecordedandcanbe usedfor a measurementof the ioni-zationenergyloss. The z coordinatesalong the wires areobtain’edby chargedivision on thetwoendsof the wireswith a precisionof 1 .6 cm.

The principle of the pictorial chamberwas alsousedin the AFS [CO 81] andthe UA 1 [BA 80]centraldetectors(see5.3).

~ci~

‘(V I

_a ~SOOns

t~Q~ \300os

- — ~OOns~ \2oo>_~~ /

/ ~ \\ \

/

\ —~ ---“-~~—-~~

DISTANCE FROM WIRE PLANE5 10 15 20 [mm]

I I I I

Fig. 18. Trajectories of drifting electrons(full lines)andlines of equaldrift times(dashed)for theJADE field configuration[DR80,WA 81b].

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K. Kleinknecht,Particledetectors 101

g93 F22ILS.TPPH23O JADE A~FIStCTION ~ 15.000 GEV P~.Fl~10-‘1.532 kG OPTE 3O/O3~B0344 2154 .34 1.6 TRiGGER 0201 TIlt 12.28.18

12055‘SIUITS 0 ..—. 13L~L ~20S5~

\\\

:7//1

a

x*___vJ 3.3 3.1 ~.9

Fig. 19. Hadroniceventproducedby e~ecollisionat 30GeVc.m. energyin thejet chamberof theJADEdetector.Hits assignedto a trackareshownascrosses,remaininghits axedashes[DR 80].

2.6. Timeprojectionchamber

An ingeniousway of usingthe proportionalanddrift chamberprinciplesfor the centraldetec-tor of a storagering detectorwas proposedby Nygren [NY 74, NY 81]. Fig. 20 showsa large(1 m radius,2 m length) cylindrical volume filled with argon—methaneat 10 atmospheres.Thetwo endcapsare equippedwith onelayerof multiwire proportionalchamberssubdividedinto sixsectors.Eachof the sectorshas186proportionalsignalwires for multiple ionization measurementand 1 5 wires with segmentedcathodereadout(“pads”) for the spatialmeasurementof radiusrandperipheralangle~pin the cylindercoordinates(fig. 21).

Most importantlyhere,the electricdrift field (150kV/m) is parallelto themagneticfield (1.5 T)of the solenoidusedfor magneticanalysisof tracksoriginating from the collisions in the centerof the cylinder. E X B type forcesvanish,and it is possibleto havethe ionization electronsdriftover largedistancesto the endcaps.Furthermore,the strongmagneticfield reducesconsiderably(factor 10) the diffusion broadeningof thetrackimageon the endcapsof the cylinderby causinghelical movementsof the electronsaroundthe magneticfield lines.

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102 K. Kleinknecht,Particledetectors

Endcap wires

Endcap wires

Negative high-voltage electrode

•-5—~

192 dEldx wires per sectorBeampipe 12 spatial wires per sector

Fig. 20. Scetch of time projection chamber [MA 78].

~DRIFT PATH~—_,.

AVALANCHE,,SIGNAL WIRE .—

±T~y4mm~pADROW~~PROJECTED ~TO

PAD PLANE

Fig. 21. Principle of cathodereadoutby padsusedin TPC [FA 79].

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K. Kleinknee/it,Particledetectors 103

~~~ALUMINUM

PADS4 ~ANODE /

ANODE SIGNAL

DISCHARGE~CATHODE SIGNALS

PAD READOUT

Fig. 22. Section of TRIUMFTPCshowingpadsandanodewires [HA 81a].

The spatial reconstructionof the original tracksis obtainedby measuringthe two-dimensionalimageson the endcapsusingthe cathodeplane readoutandthe center-of-gravitymethodandbymeasuringthe drift time of eachtrack segmentparallelto thecylinder axis. In addition,thepro-portional chambersalsomeasurethe ionizationenergyloss of thetrack,thus enablingthesepara-tion of e, iT, K andp by usingdE/dxandmomentummeasurement.

Measurementson asmall testchambergavean (r, p) resolutionof less than200jim andaz res-olution along the drift pathof 0.2mm [FA 79,NY 81]. Initial testson thebig chamberhavenotyet reachedthis precision.A similar andlessambitiousTPC hasbeenbuilt and testedatTRIUMF[HA 81a]. Hexagonalendcapswith 12 sensewiresper sectormeasurethespaceposition of tracks.At atmosphericpressurefor 80% Ar 20%CH

4 andwith an electricfield of 150 V/cm the ratio ofelectric field over pressureis 0.2 V/(cm Torr), thesameas in theBerkeleyTPC.The arrangementof the cathodepadreadoutshownin fig. 22 allows a precisionof 120 jim alongthe anodewire.Preliminaryinitial testsgavea spatialresolutionof 600jim.

2. 7. Bubblechambers

The bubblechamber[GL 52, GL 58, FR 551 consistsof a pressurevesselfilled with liquifiedgasaround the boiling temperature.By an expansionmechanism,thepressureon the liquid is re-ducedfor a short time (—ms).The liquid thenbecomesunstableagainstbubbleformation, andifduring the expansiontime achargedparticle formsa trackof ionization in the liquid, bubblefor-matjon startsat this track. After a time left for bubblegrowth (jis—ms), the trackis illuminatedandphotographedthroughwindowsin the vessel.With the chamberembeddedin a magneticfield

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104 K. Kleinknecht,Particledetectors

Table 1Physicalpropertiesof bubblechamberliquid

Liquid Temp. Pressure Density Expansion Rad. length Absorption(K) (Bar) (g/cm

3) ratio iSV/V X0 (cm) length X (cm)

(%)4He 3.2 0.4 0.14 0.75 1027

26 4.0 0.06 0.7 1000 88730 4.5 0.14 0.6 900 403

20Ne 36 7.7 1.02 0.5 27 89131Xe 252 26 2.3 2.5 3.9C

3H8 333 21 0.43 3 110 176CF3Br 303 18 1.50 3 11 73Ar 35 25 1.0 1.0 20 116

of up to 30 kG, the trackcurvatureis usedfor momentummeasurement,andthe bubbledensityb canbeusedfor a measurementof thevelocity v !3c andthusthe massof the particle [GL 581.

Chamberliquids rangefrom hydrogenas a pureprotontarget,deuteriumfor studyof interac-tion on neutrinosup to xenon for experimentswhich needa high conversionprobability for ‘yrays. Table 1 gives somepropertiesof chamberliquids [BE 77, HA 81b, WE 81b].

The bubble chamberis still uniquein its capability of analyzingcomplicatedeventswith manytracks and identifying thoseparticles. A beautiful example for such a super-eventis showninfig. 23. However, the useof bubblechambersasisolateddetectorshasdiminishedbecause1) theycannotbe employedatstoragerings; ii) at high energy,showersarenot containedin thechambervolume anymoreexceptif thechamberis ahybrid of calorimeterandbubblechambertechniques,whichseemsto be possibleusingliquid argon [HA 82] ; iii) theleverarm for momentummeasure-ment is not sufficient at high momenta.Futureusewill include small chamberswith extremelyhigh resolution,e.g. the 8 jim resolutionobtained[DY 81, MO 80] by holographicreadoutin theBIBC chamberatCERN (fig. 24).Such chamberswill beusedin conjunctionwith largespectrome-ters for momentummeasurementand identification of reaction products(“hybrid systems”).

2.8. Streamerchambers

In a similar developmentstreamerchambersare used as tracksensitivetargets.In this typeofchamberelectric fields above 50 kV/cm perpendicularto the trackdirectioncreatean avalanchewith gas amplification around 108 and light emission (“streamer”). The geometryof such achamberis given in fig. 25. Very shorthighvoltagepulses(few nsec)arerequiredin order to keepthe streamersshort [SC 79, RI 74].

The excellentquality of streamerchamberspresentlyin usecan be seenin fig. 26, a picturefrom the NA5 experimentat CERN [NA 51. The developmentofavery high resolutionstreamerchamberhasbeenpioneeredat Yale [DI 781 in order to measurethe lifetimes of charmedpar-ticles, around l0’~ sec,correspondingto a flight pathof 300 jim for a time dilatation factory = 10 which is typical for a particleof 30 GeV/cwith a massof 3 GeV. Thischamberoperatesat24 atmospheres,usespulsesof 0.5 nsecdurationproducinga field of 330 ky/cm, anda spatialresolution of 32 jim has beenachieved [SA 80]. With this techniquea good measurementofcharmedparticle lifetime seemsfeasible, competitive with similar experimentsusing nuclearemulsions.

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K. Kleinknee/it,Particledetectors 105

1,,I~

I~

/ ‘N.0

<~-~ A

I I> ~ 0. Q

w ~ l~g:

~ L ~ __ ~ /4~ / ~~ ‘~— \_~ - --. - - ~ I. 11 Y ~—. I a

_ ____ ___

___________ ~ ~ ~__ - -~- S ‘~•-;~~ ~L~t~—_.1..~, ,.~-... ..- - ___ _____

M , .~. - -. . .. /.~~. / ~_______ - It J f._

~ ~

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106 K. Kleinknee/it,Particledetectors

Fig. 24. Holographicphotographof a 15 GeV 1r interactionin a smallfreonbubblechamberBIBC. Bubble sizeis 8 ~im[DY 81].

-~IDENc~j---1

HIGH J 500KV10 nsec

VOLTAGE A

I II’ ~ •I II~ ~

VIEW NORMAL TO E~FIELD

1~~VIEW PARALLEL TO E-FIELD

Fig. 25. Schematicview of streamerchamber[SC79].

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K. Kleinknecht,Particledetectors 107

~~ ———

Fig. 26. Interaction of a 300 GeV ir in a liquid hydrogen target as seen in a streamerchamberof dimensions200 X 120 X 72cm3

[EC 80].

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108 K. Kleinknecht,Particledetectors

2.9. Flash andsparkchambers

Another gasdischargechamberis theflash chamberdevelopedby Conversietal. [CO55, CO 78 1andbuilt in a similar way for the FMNN neutrinoexperimentat Fermilab[TA 78]. The chamberconsistsof an arrayof rectangulartubesmadeof polypropyleneby extrusion.This arrayis placedbetweentwo metal electrodesand filled with a neon (90%)—helium(10%) mixture (fig. 27). AtriggeredHV pulse is appliedon the electrodesgeneratinga glow dischargein thosecells whereionization has beeninducedby passingparticles.This dischargecanbe recordedby photograph-ing or by electronicreadout.The flash chamberreachesan efficiencyof 80%.Due to theextreme-ly low cost,largevolumecalorimeterswith fine grainsamplingcanbe built.

The sparkchamberhasbeenused widely as a triggeredtrack detector.A set of electrodesormassiveplates is insertedin a noble gasvolume(typically He/Ne) at atmosphericpressure.Theplatesarealternatinglyconnectedto apulsedHV supply or ground.After the passageof an ioniz-ing particle, a HV pulse is triggeredvia a sparkgap,causinga sparkbreak-throughat theplace ofthe initial ionization but parallel to the electric field (fig. 28). For chambergasesat atmosphericpressure,the magnitudeof the pulsedelectricfield hasto be 10—20 kV/cm in order to generatesparks.

The sparkposition is recordedoptically or by magnetostriction.Here the electrodesof thechamberconsistof wire planes,andamagnetostrictivewire (madeof aCo—Ni—Fe-alloy) runningacrossthesewires picksup a signalinducedby the sparkcurrentpulseon the HV or groundwires.The magnetostrictivewave travels alongthe Co—Ni—Fe-wire with a speedof 5000 rn/s suchthatits arrival time at the endof themagnetostrictivewire measuresthe sparkposition. A precisionof

FLASH CHAMBER GAP

Al ELECTRODES

~5mm ‘ kPOLYPROPYLENE

GAS MANIFOLD

Fig. 27. Partof flashchambermadeof extrudedpolypropylene[TA 78].

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K. Kleinknecht,Particledetectors 109

ionis. particle

~LOkV •1 ~

Fig. 28. Principle of sparkchamber;PM photomultiplier,F pulseshaper,C coincidenceunit,V amplifier, SG sparkgap.

200 jim is obtained.The largeamountof chargein the sparkplasmarequiresalong time(—~2ms)for clearingbeforethe chambercanbe triggeredagain[RI 74, AL 69].

2.10. Comparisonofpositiondetectors

The parametersto be comparedare spaceand time resolutionand rate of dataacquisition.Table2 gives typical values,where“deadtime” for pulseddetectorsmeansthetime neededbeforeanew trigger can be allowed to pulsethe detector,and“sensitivetime” is the time duringwhichincoming particles are registeredwhetherthey are correlatedor not with the eventcausingthetrigger. Time overlayof different eventscan only be avoidedif the meantime intervalbetweeneventsis largecomparedto this sensitivetime.

Proportionalanddrift chambersare bestsuited for preciserecordingat high datarates,whilethe pulsed bubbleandstreamerchambersstill havethe potential for optimum spaceresolutionandmultitrackanalysis.The flash chamber,becauseof its low priceandsimpleconstruction,maywell find applicationin very largedetectorsusingfine-grain calorimetry,e.g. for low-rateexperi-mentslike protondecayandneutrinoexperiments.

Table 2Propertiesof positiondetectors

Typeof Spacereso- Deadtime Sens.time Readout Advantageschamber lutions (Mm) (ns) (ns) time (ns)

normal specialProp. chb. 700 100 — 50 i0

3—i04 time resolutionDrift chb. 200 50 — 500 i03—i04 spaceresolutionBubble chb. 100 8 108 106 — complex eventsStreamerchb. 300 30 108 iO~ multiple tracksFlashchb. 4000 2000 iO~ i03 106 priceSpark chb. 200 i0~ i03 106 simplicity

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110 K. Kleinknecht,Particledetectors

3. Time measurement

3.1. Photomultiplier

The main instrumentfor obtainingtime informationon aparticle is the photomultipliertube.Visible light from a scintillator liberates,by photoelectriceffect,electronsfrom a photocathodemadeof alkali metals.Forbialkali cathodesof Cs—K—Sb,thequantumefficiencyreaches[VA 701amaximum of 25% around400nm (fig. 29). In tubesof thelinear-focussingtype, thephotoelec-trons are then focussedon the first dynode consistingof materialslike BeO or Mg—O—Cs.Withsecondaryemissionyields of 3—5 perincident electron,amplificationof 108 for 14 dynodestagescanbe achieved.The risetimeof the anodepulseis around2 ns,thetransittime is typically 40 ns.

The spreadin the transit time (“time jitter”) through the photomultiplieris mainly given bythe variation of transittime of thephotoelectronfrom the cathodeto the first dynode.Therearetwo effects, the broadvelocity distributionof thephotoelectronsandtheir differentpathlengthsfrom differentparts of the cathodeto the first dynode.The photoelectronkinetic energyspec-trum for bialkali cathodesilluminated by light of 400—430nm wavelengthextends[NA 701 from0 to 1.8 eV peakingat 1.2 eV. For an electric field of E = 150V/cm, the difference~ in transittime betweena photoelectroninitially at rest andanotheroneof kinetic energyTk 1 .2 eV is

~Nkr Etitti - ~ J~tr~ttmim~i

H Tu ~ ~‘.-1Th1TTTTTflmTnTm(mA/W)flLLjI ~ . - —-

-lU(S73)

T(S20) . .

AfSlI) . - . .

* :::i.:.L. i:...C(S1) I

161 L~IJJ L~1 . .~ .200 400 600 800 1000 X(nm)

Fig. 29. Spectral sensitivity NKr (mA/W) andquantumefficiency nq(%)for photocathodes;TU andU typeshave quartzwindow,others glass window [VA 70].

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K. Kleinknee/it,Particledetectors 111

/%Oo~o~Qo9~~Channels

Oó°oOo~0o0o0O /000 O~~)OQ

0 0~

Nichrome contact

Secondary// electron Glass tube

~ .IechnsPrimaryradiation

Fig. 30. Microchannel plateand principle of multiplication [DH 77].

= (2mTk)”2/(eE) 0.2 ns. The other effect contributingto the transittimejitter, thegeomet-rical pathlengthvariationfrom cathodeto first dynode,dependsmainlyon the cathodediameter.For adiameterof 44 mm, it is 62 = 0.25 nsand0.7 ns for the tubesXP2020andXP2232B, re-spectively [PH 78]. This seemsto be the ultimate limitation in time resolutionfor conventionalphotomultipliers.

In microchannelplate multipliers, the pathsof photoelectronsare straight channels(fig. 30).This reducesthe transit time jitter by a factorof two comparedto conventionalmultipliers; ajitter of 0.1 nshasbeenmeasured[LE 78b1.

3.2. Scm tillators

A scintillation counterhastwo functions: the conversionof the excitationcausedby the ioni-zing particleinto visible light andthe transportof this light to the photocathode.

The mechanismof scintillation FBI 641 is completelydifferent for anorganiccrystal scintilla-torsandfor organiccrystal, liquid or polymericscintillators.

For anorganic crystals dopedwith activatorcenters,the energylevel diagram looksqualitative-ly as shown in fig. 3 1. Ionizing particlesproducefree electrons,free holesandexcitons.These

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112 K. Kleinknecht,Particledetectors

conduction band •~ activator — —

6—8 eV center — — excitons

valence band 0

Fig. 31. Energy band structure in anorganic crystals.

move inside the crystal until they reachan activatorcenterA, which theytransforminto an ex-cited stateA~decayingto A with emissionof light. The decay time of scintillationlight is thengiven by the lifetime of the unstablestateA* anddependson temperaturelike exp(—E1/kT),whereE1 is the excitationenergy.Typical datafor suchcrystalsar~given in table3.

Organic scintillators, on the other hand, havevery short decay times of order nanoseconds.The scintillationmechanismhereis not a latticeeffect,but proceedsthroughexcitationof molec-ular levels which emit bandsof UV light. The absorptionlengthof this UV light in mosttranspar-ent organic materialsis short, of order mm, and the use of thesescintillators is possibleonlythrough fluorescenceexcitation in a secondmolecule,calledwavelengthshifter.The emissionofthis shifter material is usually chosento be in the blue wavelengthregion detectableby photo-cathodes.Thesetwo activecomponentsin a scintillator canbe solvedin liquids or in a monomericsubstancebeing polymerized subsequently.Two parametersdeterminethe figure of merit forsucha scintillator: the light yield andthe absorptionlength in the scintillator.

Table 4 gives structure,wavelengthof maximumemissionanddecaytime for a few primaryscintillators,as well as for two wavelengthshifters [BE 71]. Forpolymerizingplastic scintillator,eitheraromaticcompounds(styrol,vinyltoluene)or alifatic ones(acrylic glasses,“plexiglass”) areused.The aromaticonesyield about twice as much light, but the alifatic onesarelessexpensiveandmuch easierto handlemechanically.

In order to achievea goodenergyresolutionin largecalorimeters,it is veryimportantto obtainuniform responseof a long but thin scintillator over its entire length evenwhen the scintillatorlight is viewedby aphotomultiplier from oneendonly. The observedattenuationof light fromthe far end is mainly due to the absorptionof theshort-wavelengthpart of the POPOPemission

Table3Propertiesof scintillatinganorganiccrystals

NaJ (Tl) LU (Eu) CsJ (Tl) BL~Ge3012

Density(g/cm3) 3.67 4.06 4.51 7.13

Meltingpoint (°C) 650 450 620Decaytime (jJsec) 0.2 1.3 1 0.35Pulse height for electrons 1.0 0.35. 0.28 0.08Xmax (emission)(nm) 410 470 550 480Radiation length (cm) 2.5 1.12Physicalproperties hydroscopic hygroscopic nonhygroscopic

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K. Kleinknecht,Particledetectors 113

Table 4Organicscintifiatorsandwavelengthshifters

Primary Structure Xmax Decay Yield/scintillator emis. time yield

(nm) (ns) (NaJ)

Naphthalene 348 96 0.12

Anthracene ~JJJ 440 30 0.5

p-Terphenyl ~ 440 5 0.25

PBD ~ 360 1.2

Wavelengthshifter

420 1.6

bis-MSB ~CHcH~cHcH~ 420 1.2

spectrum,as shown [KL 81b] in fig. 32. In order to obtain a moreuniform responseit is there-fore possibleto filter out the shortwavelengthpart.Theeffect of a filter at 430nm canbe seenin fig. 32: the light yield at the endof the scintillator nearto thephotomultiplieris diminisheddrastically,while the onefrom the far endis influencedmuchless.

Still, by usingthe filter, light is lost, andit is interestingthereforeto searchfor an acrylic scm-tillator with long attenuationlength andhigherlight yield thancommerciallyavailable.Onenewmixture found [KL 81b] recentlycontains3% naphthalene,1% PBD and0.01% bis-MSB. The at-tenuationcurves for a scintillator of this material with size 1800 X 150 X 5 mm3 are showninfig. 33. The attenuationlengthwith blackendandfilter is A = 210cm, and the light yield at 160cm from the photomultiplierside is 20% higher thanthe one for the commercialmixture plexi-glas 1921 (1% naphthalene,1% PBD, 0.01% POPOP).This new scintillator is usedthereforefor anew neutrinocalorimeterof the CDHS collaboration(fig. 77).

Importantdevelopmentsof low costscintillatorshavebeendoneat Saclay[BO 811 for experi-mentsat the proton—antiprotoncollider at CERN. Two new groupsof scintillatormaterialhavebeendeveloped:i) the KSTI line basedon polystyrenematerial which canbe extrudedbetweentwo polishedrolls; it has80—100%of the light outputof the PVT-scintillatorNE1 10, an attenua-tion length of 80 cm for sheetsof 3 mm X 200mm cross-sectionanda decaytime of 3 ns,how-evercareful handlingis requiredas for other aromaticscintillators; ii) the Altustipe seriesbasedon polymethylmetacrylate(PMMA) with similar propertiesas Plexipop,i.e. 20—50%of the lightoutput of NEllO, andattenuationlengthsof 1.0—1.5m.

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114 K. Kleinknecht, Particledetectors

6 ~hOUtfd~AR : : I bis -MSB I

400 460 480 520 [nm] .~ •A AA

430mm wave Length £ 2 . . • •

6without filter FAR “ U •

170cm from light guide 2 U • -

400 440 480 520 [n~) 0 40 80 L 120 160 cm430nm wave Length

Fig. 33. Attenuation curvesin a scintifiatorof dimensionsFig. 32. Wavelengthspectrumoflight producedat nearend 1800 X 150 X 5 mm

3 without filter, reflecting end (fulior far end of scintillator (type plexiglas 1922, 180 cm long) dots), black end (open circles);with yellow filter, reflectingwith andwithout yellowfilter at 430 nm [KL 81b]. end (triangles), black end (squares), [KL 81b].

3.3. Light collection

The traditionalway of collectinglight from ascintillator is the adiabaticlight guide.The (blue)scintillation light travels down the scintillator plate by multiple internal reflection. The usuallyrectangularradiatingsurfacewith cross-sectionF is imagedonto the photocathodesurfacef bymeansof bent transparentplastic rodsor strips such that the radiusof curvatureof the rods islargecomparedto their thickness.In this way it canbe avoidedthat light hits an internalsurfaceunder an anglelargerthanthe oneof total reflection. The amountof light reachingthe photo-cathodeis lessthanf/F dueto Liouville’s theorem.

The time resolutionof direct-coupledscintillationcounterscomesfrom two sources:the transittime jitter of the photomultiplier and the time differencebetweendifferent light pathsin thescintillator and light guide. The lattercontributiondependsmainly on the scintillatordimensionsanddominatesfor large(�2m) counters,as shownby the data in fig. 34. For largecountersys-tems,a resolutionbelowa~ 200pshasnot beenachieved.

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K. Kleinknecht, Particle detectors 115

(1 0 LONGSCINTILLATORS

0200 0 •RISETIME-’300p

. ~ -~ ~ “

100 L SMALL• o J SCINTILLATORS

-I~1’T7~~~CULATEDRESOLUTIONRISETIME

100—400psDECAYTIME 1500ps

0 500 1000

PM Transit time spread (psI

Fig. 34. Comparison of r.m.s. time resolutionsof scintillation counters vs. the r.m.s. transit time spreadin thephotomultipliersused.Small scintillators with dimensionsbelow 1 cm arecomparedto long scintillators (length —2 m, thickness2—5 cm,width20—40cm),[CA81b].

An alternativemethod,dueoriginally to Garwin [GA 60, SH 51], hasbeenrevivedrecentlyforapplicationsin large-scalecalorimetry[BA 78, SE79]. The principle is shownin fig. 35: bluelightfrom the wavelengthshifter (e.g. POPOP)leavesthe scintillator andenters,throughan air gap, asecondshifter bar.This rod is madeof acrylic materialdopedwith a molecule(BBQ, e.g.)absorb-ing the blue light andemittingisotropically greenlight (around480nm, seefig. 36). A part (10—1 5%) of the greenlight is catchedby the shifter barby internalreflectionandreachesthephoto-

PM — — — —~ green shifter bp~~jAir gap

emi~%:ed blue

7~~NIGj~J

Fig. 35. Principle of wavelengthshifterbar technique.

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116 K. Kleinknecht, Particle detectors

I I

—. — BBQ Emission

10 - ...-. .—~. — — — Absorption\ —PBD Emission

I \POPOP Emission

200 300 400 500 600 700

X (nm)

Fig. 36. Absorption and emission spectraof BBQ.

multipliers looking at the endof the bar. Themain problemsin developingthis techniquewerei)to find the appropriateshiftingmaterialmatchedto the POPOPemissionspectrumandthe photo-cathodespectralsensitivity; ii) to find a way of optimizing the self-absorptionin the greenbar.

Theseproblemswere solved [BA 781 by takinga 90 mg/Q concentrationof BBQ in plexiglas218. The productnow is commerciallyavailableandhasfound wide applicationin largeexperi-ments.

The thicknessof thegreenshifter barneededfor absorptionof thePOPOPlight canbeobtainedfrom fig. 37, wherethe intensity of BBQ emissionhasbeenmeasured[KL 8lal as a functionofthicknessof the greenbar. An absorptionlength of A = (5.2±0.2) mm is obtainedfor the BBQconcentrationmentioned.

The shifter bartechniquecanbe usedto collectthe light from very largescintillatorswith a fewphotomultipliers.Oneexampleis the CFRneutrinocalorimeter[BA 78] with countersof 3 X 3 m

~1~2b364~ ID/mm

Fig. 37. Measurementof absorptionlengthof POPOPlight in BBQ [KL 81a].

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K. Kleinkncc/it, Particledetectors 117

500 ~ 500

400 - 400

300 - - 300 -‘I,

C

>

200 - 200

100• 100-

111111 huh 111111 111111

-24 -12 0 12 24 24 12 0 12 24t~X(crn) AY(cm)

Fig. 38. Deviation of reconstructedposition of a showerof 100 equiv.particlesfromrealposition;r.m.s.deviationo~ (7.3±0.1)cm, o, = (7.6±0.1) cm [KL 81a].

viewedby 4 phototubesat thecorners.These4 pulseheightscanbe usedto calculatethecenterofgravity of a showerof particles.Fig. 38 showsresultsof ameasurementdonewith a150X 300X 1.5cm3 acrylic scintillatorviewedin this way. The positionof a showerwith 100 equivalentparticlescanbe reconstructedwith an accuracyof a ‘— 8 cm [KL 81a]. Thismethod,therefore,hasthead-vantageof allowing to savephotomultipliers,savemechanicalwork for light guidesandpermittinga measurementof thepositionof a showerof particles.It doesnot,however,permitto disentangleseveralshowers.

One disadvantageof the wavelengthshifterBBQ is its two-componentdecaytimewith lifetimes[KL 81 a] of 18 ns and 620 ns, which causestiming difficulties whenmeasuringpulseheights.

3.4. Planarsparkcounters

Thesecountersconsist of two planarelectrodesgeneratingan electric field abovethe staticbreakdown,i.e. at a ratio of field strengthE to gaspressurep of E/p —‘ 30—60 V/(cm Torr). Theprimaryionization of a passingchargedparticledevelopsinto a spark,and the largecurrentdrawnby this sparkcan be recordedas a fastrising pulse.In counterswith metallic electrodes[KE 48]the sparkdischargesthe total capacityof the plates,leadingto hightemperaturesandburningofthe electrodes.The damagedsurfacegives spontaneousbreakdownsat lower fields. A possibilityof avoidingthis deficiencyconsistsin usingmaterialwith high resistivity(a = 10~—10’°~2cm)forone of the electrodes[BA 56] The spark then only dischargesa small areaof the condensoraround the primary ionization, leadingto a lower energydensityin the spark.Fig. 39 showsaschematicof sucha counterwith copperstrip readouton thesemiconductinganode.The imped-anceof this strip line can be matchedto the readoutcable.Countersareoperatedwith argonat

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118 K. Kleinkncc/it, Particledetectors

COPPER STRIPS .::..:::::u:::::::::::,::::::::::::,::::~~:::::::::: SPACER

- - GLASS

-HIGH VOLTAGE,/~~ ‘T.

CATHODE Cr.Cu COATED GLASS

Fig. 39. Schematicview of a typicalplanarsparkcounter [BR 81].

5—10 atmospheres,addinghydrocarbons(isobutane,ethane,1 .3-butadien)in order to absorbUVphotonsfrom the sparkthusavoidingsecondarysparking[BR 811.

The time jitter of the signal, 6, dependson the electricfield E andthe numberof primaryionsN, like 6-~-l/(E~~/7~).Measuredvalues [BR 811 are 6 30—80 ps for countersof 10 X 10 cm2areahaving detectionefficienciesof > 95%. The distributionof thetime differencebetweentwoparallelsparkcountersis not quite gaussian,showingbroadtails (fig. 40).

Despitethe excellenttiming characteristicsof thesecounters,wide applicationis not yet fore-seeablebecauseof the extremedifficulties in manufacturingandmaintaining the high-qualitysurfaces.

300

EVENTS G = 62 ps 72 kV

200

:i .{~\ .

1000 1500 2000At(ps]

Fig. 40. Distribution of timedifferenceoftwo planarsparkcounters.A gaussianis fitted to the histogram in theregionwhere thecurveis drawnassolid line [BR 81].

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K. Kleinkncc/it, Particledetectors 119

4. Identificationmethods

4.1. Time-of-flight

The identificationof chargedparticlesthroughtheir flight time betweentwo scintillationcoun-ters requires,for momentaabove I GeV/c, very good time resolutionandquite longflight path.The time differencebetweentwo particleswith massesm1 andm2 is for a flight pathL

= L/(131c) — L/(132c)= (L/c)(’Jl + m~c2/p2— \/l + m~c2/p2),

which for p2 ~ m2c2 becomesi~t~— (m~— m~)Lc/(2p2).Fig. 41 showsflight time differencesbetweenpairs of chargedparticles.Using conventionalscintillation counters(section3.3) with atime resolutionat = 300 ps, 7r/K separationat the level of 4a~would requirea flight pathof 3mat I GeV/c and 12 m at 2 GeV/c. If parallelplatesparkcounterswould comeinto operation,therequiredflight pathwould be reducedto 0.5 m or 2 m, respectively.

For this method of identification, therefore,at presenta very long flight path is needed.

4.2. Cherenkovcounters

Cherenkovradiation [CE 64] is electromagneticradiation emitted by chargedparticlesof ve-locity u traversingmatterwith refractiveindex n if v> c/n. The classicaltheoryof the effect attri-butes this radiation to the asymmetricpolarization of the medium in front of andbehindthechargedparticle resulting in a net electric dipole momentvarying with time. The radiation isemitted at an angle 0, wherecos0 = (ct/n)/(j3ct)= l/(13n). The thresholdfor Cherenkov-effect,~3> 1/n, correspondsto a thresholdin the ‘y factorof the particle,

y> l/sjl — I/n2

Typical refractiveindicesandthresholdvaluesaregiven in table5. Unfortunatelythereis agapin

~ ~0~p (GeV/c)

Fig. 41. Differencesof time of flight t

1 — t2 of particle pairseli, irK and Kp for a flight path of 1 m.

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120 K. Kleinknec/it,Particledetectors

Table 5Cherenkovradiators

Material n — 1 .y (threshold)

Glass 0.46—0.75 1.22—1.37Scintillator (toluene) 0.58 1.29Plexiglass(acrylic) 0.48 1.36Water 0.33 1.52Aerogel 0.025—0.075 4.5—2.7Pentane(STP) 1.7 X it1

3 17.2CO

2 (STP) 4.3 X 10~ 34.1He (STP) 3.3 X i0~ 123

the refractive indices betweenthe gaseswith highestindex (pentane)andpractical transparentliquids with lowest index. The developmentof silica—aerogel[CA 741 consistingof n(Si02) +

2n(H20) hasclosedthis gap andpermits avelocity measurementin the rangeof ‘y -~3—5, wherethe specificionization is nearlyconstant.

Large scaleproductionof silica—aerogelwith n = 1.03 andn = 1.05 in blocks of 18 X 18 X 3cm

3 is now possible [HE 81]. The Cherenkovlight has been collectedby cylindrical mirrors[CA 81a] or by adiffusorbox [AR 81] behindthe aerogelblock (fig. 42) andrecordedby photo-multipliers. 6—12 photoelectronshave been obtainedfrom a 15—18 cm long radiator of thismaterial(n = 1.03).

PM SIDE VIEW

— ~ _\_j

18 TOP VIEWXAxis

~L~J-~L-?~~14°2O

AEROGEL

(a) Distances in cm (b)

Fig. 42. Silica—aerogelcountersusedin EMC experiment.Particlesincidentfrom left onaerogel,diffusingbox coveredinternallywith mililpore filter [AR 81].

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K. Kleinknec/it,Particledetectors 121

From the relation cos0 = l/(13n) follows that themaximumCherenkovanglebecomessmallerif n approachesunity. The energyradiatedperpathlengthin the radiatoris

dE 2irah 1’ / 1 \—,~--——~-- jU.) c ~n>l f3n

with a beingthe fine structureconstant,a = 1/137.This leadsto the numberof photonsN emit-ted overa pathlengthL in the wavelengthinterval A1 to A2

X2

N = 2iraL f dA sin20/A2

XI

For a detectorsensitivein the visible region A1 = 400nm, A2 = 700nm, this correspondsto N/L

= 490 sin20 photons/cm.Evidently, the detectionof UV-light can increasethis yield by a factor

of 2—3.The length of Cherenkovthresholddetectorsneededfor separationof particlesof momentum

p increasesas p2: supposetwo particleswith massesm1 andm2> m1 haveto be distinguished.

Then the refractive index of the radiatorcan be chosensuch that theheavierparticlewith massm2 doesnot yet radiate,or is just below threshold,f3~ I/n

2, and n2 = ‘~/(‘y~— 1). Then theamountof Cherenkovlight from the particlewith massm

1 is proportionalto

sin2O = 1 — I/(j3~n2)

which for ~y~ I becomes

sin20~n~c2(m~—m~)/p2

In a radiator of lengthL, detectingphotonswith a quantumefficiency of 20%, thenumberofphotoelectronsis

P 100Lc2 (m~— m~)/(p2L0),

whereL0 = 1 cm. In order to obtainP = 10 photoelectrons,alength

L/L0 p2/((m~— m~)c2 10)

is required in the optimistic caseassumingthat a radiatorwith exactly the refractive index re-quired abovecanbe found.

For practicalpurposes,oneusesa combinationof thresholdcounterswith different refractiveindices,as indicatedin table 6. By usingtwo ormoreof thesecounters,pions,kaonsandprotonscanbe identified inthe momentumrangegiven in fig. 43.

Apart from this utilization of the Cherenkovthreshold,the angleof Cherenkovemission canalso be measuredin order to identify particles.Theconicalemissionpatternaroundthe radiating

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122 K. Kleinknec/it,Particledetectors

Table 6Possiblechoicesof Cherenkovthresholdcounters[LE 81c]

Counter Refractive Radiative Radiator Counter Lightindex medium length length yield

A 1.022 Aerogel 20 (cm) 50—100(cm) 5—6 (e)B 1.006 ?(Aerogel) 50—100C 1.00177 Neopentane 30 50D 1.00049 (N

20—C02) orFr14 100 ~120 ~10E 1.000135 (Ar—Ne) orH2 185 ~200 5

0.66 2.34 4.5 8.5 15.8 30 57 (GeV/c)I.

I I ICOUNTERS IAB _________________ ________ ________ABC _______ _______ _______ IABCDOABDOACD I it

ABCDE F 4 1 H

AB F -jABC I IABCO IABCDE 4 4 KABD I IACD I- I

AB I IABC I 4ABCO I 4ABCDE 4 I pABD I I I I— IACD -j I 1

Tof,t/K

Tot K/p (————-4

lot It/p (-————-1

TOF dotted line indicate best performanceachievable with a classical system

I i i I ittil I situ I i i Ii ti~

0.2 0.5 1 2 5 10 20 50 100

Fig. 43.Domainofparticle identification for threshold Cherenkov counters asgiven in table 6 [LE 81c].

particlecan be focussedinto a ring-shapedimage.An adjustablediaphragmat the focustransmitsCherenkovlight emitted in a small angularrangeinto a phototube.Changingthe radiusof thediaphragmallows a scanthroughregionsof velocity. Differential gasCherenkovcounters[Li 73 1correctingfor chromaticdispersionin the radiator(DISC) have achievedvelocity resolutionsof‘~13/13~l0~.

Since the length of thesecountersis limited to a few meters,thereis a maximummomentumat which two kindsof particlescanbe separated(fig. 44). Separationof ir andK mesonsatseveral100GeV/c is possiblewith thesedevices.A velocityspectrumof chargedhyperonsin a shortbeamfrom an externalprotontargetis shownin fig. 45, demonstratingseparationof thesehyperonsat15 GeV/cmomentum.

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K. Kleinknecht,Particledetectors 123

BEAM MOMENTUM FOR LIMITING irk SEPARATION (GeV/c)1 1) 100 1000 10000

I I T1ru I h rrr I I I TTTu I I rTr~

~ 10O~ —~k~—----- -~

-~ I~FFRE1 AL

1 RES D1D~’\ 2 ~N/A:0.4O

N \~1IHEP(1 }oDI~ ____

I 3~RNr~’PEDN)

4 CERN-IHEF ______

5 SPS- •~SC

BEAM MOMU~4TUMAT LIMITING RESOLUTION FOR THE SEF~IRATIONOFPAIRS OF PARTICLES (GeWc)

Fig.44. Highestbeammomentum for i/K separation vs. maximumCherenkovanglefor threshold,differentialandDISC Cherenkovcounters [LI 73].

An alternative to changingthe radiusof the diaphragmconsistsin changingthe gas pressureandleavingthe opticalsystemin place.

While the DISC counterscan only be used for particles parallel to the optical axis of the de-tector,a velocitymeasurementfor divergingparticlesfrom aninteractionregionrequiresadiffer-ent approach.Seguinot andYpsilantis [SE 77] haveproposedthe ideaof a Cherenkovring imag-ing detector(fig. 46). A sphericalmirror of radiusRM centeredat the interactionpoint focussesthe Cherenkovcone producedin the radiatorbetweenthe sphereof radiusRD and the mirrorinto a ring-shapedimageon thedetectorsphereof radiusRD. UsuallyRD = RM /2.

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124 K. Kleinknec/it,Particledetectors

I I I-

it-

-

1.000 0.998 0.996RELATIVE VELOCITY ~ (:v/c)

Fig. 45. Velocity distribution in a shorthyperon beamselecting15 GeV/cparticles[LI 73].

particleI

D

target 6D2

particle 2Cherenkovradialing medium

Defectorradius RD

Mirror radius

Fig. 46.Principle of ring imagingCherenkovcounter[SE77].

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K. Kleinknecht,Particledetectors 125

CaF2 Crystal

__________________________________ 5 mm thick

5k7 Stainless steel mesh~2 4~8 — — — — —— — 5OO~spacing

V3 ______‘~—‘ —— 81% transparency

V4 3.23.5 . ~

2O~~W wires‘~—‘ 2 mm spacing

V4 -—

_________________________ Armodur

Fig.47. Photon detector for ring imagingChexenkovdetectorwith CaF2window, threeparallel-plane gaps: C for conversion,PAfor amplification, T for transfer, andMWPC. Dimensionsin mm [EK 81].

Sincethe focal lengthof themirror is RM /2, theCherenkovconesof openingangle°c= arccos(1/(/3n)) emittedalongthe particle’spathin the radiatorarefocussedontoa ringwith radiusr on thedetectorsphere.ForRD = RM/

2, the openingangle°D of this ring equalsOc, in first approxima-tion. The radiusr of the ring imagegivesthe Cherenkovanglevia tan °c = 2r/R, andfrom this weobtainthevelocityj3= I/(n cosOc).The relativeerror on 13 is ~f3/f3= (tan20~(~O~)2+ (~n/n)2)’/2.Neglectingtheerror from theuncertaintyin n, oneobtains1~.y/’y= ‘y2f33n sin 0~~ andthemo-mentum of the particlep = mj3y with error i~p/p= ~y/(y132) [YP 811. The critical point of thedetector is the developmentof photoionization detectors.At present[EK 811, proportionalchamberswith an admixture of photosensitivetriethylamine (TEA) are under study. Such aphoton detectoris shown in fig. 47. Behind the CaF

2 thereare threegapsand a proportional

Fig. 48. Evidencefor Cherenkovring image usingmultistepspark chamber with TMAE photosensitivegas. Ten eventsareover-lapped in thepicture; centralspot is dueto beam [SA 81].

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126 K. Kleinknec/it,Particledetectors

chamber(PC). In gap C photonsare convertedby TEA to photoelectrons,gapPA servesfor pre-amplification, gapT for transferand PC for avalanchemultiplication. Three photoelectronsperincident 10 GeV/c pion traversing1 m of argonCherenkovradiatorat 1.2atm.pressurehavebeenobserved.Developmentof suchdetectorsis continuing.Amongstthe possibleimprovementsisthestudy of a new photosensitivegas [NA 72], Tetrakis-dimethylaminoethylene(TMAE), having aphoto-ionizationpotential of 5.4 eV, lower thanthe oneof TEA, 7.5 eV. Using thisvapour,evi-dencefor Cherenkovring imaging has beenobtainedas shownin fig. 48, whereten eventshavebeenoverlappedin the picture [SA 811.

4.3. Transitionradiation detectors

If a chargedparticletraversesa mediumwith varyingdielectricconstante.g. a periodicseriesoffoils andair gaps,radiationis emittedfrom the interfacesbetweenthe two materials.This “tran-sition radiation” (TR) was showntheoreticallyby GinzburgandFrank [GI 461 to dependon the~ factor of the moving particle, thus permitting an identification of particlesin the very highenergyregion(y> 1000)whereothermethodsfail.

The intensity of this radiation is expectedtheoretically to havea sharp forwardpeak at anangle0 ‘—j 1/y and to be proportionalto y. If a periodicsandwichof many foils is used,interfer-enceeffects [AR 75, FA 75] will producea thresholdeffect in y, such that thedetectorcanbeusedfor discriminatingbetweenparticlesof differentmass.

600- .~ -

1.04cmXe~‘ ,rWITH Li 1.4GeV/c

f ~ eWITH DUMMY

~4OO~[ ~

o - ~ eWITH LiU-

oci I2 ‘1 -

200- -

~

C ,~-,—~—.

0 10 20 30 40 50 60 70 80 keV

PULSE HEIGHT

Fig. 49. Pulseheight spectrumof transition radiation in Li foils detected by a xenonproportional chamber [FA 80].

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K. Kleinknec/it,Particledetectors 127

Practicalapplicationshavefollowed the demonstrationby Garibian[GA 73] that TR is emittedalso in the X-ray region. Actual TR countersconsist of a radiatorfollowed by a proportionalchamberfor the detectionof the X-rays emitted forward. Since the absorptionof X-rays in theradiatormaterial behavesas Z3~5,the atomic numberof the foils hasto be as low as possible.Inthe pioneeringwork of Willis, Fabjanandco-workers[CO77] thetechnologyof thin lithium (Z 3)foils has beenmastered.As a countinggas for the X-ray detector,xenon(Z 54) hasbeenused.

The pulse heightspectrumin a xenonchamberbehind1000 Li foils of 51 ~zthicknessis shownin fig. 49 togetherwith a spectrumfrom a dummy radiatornot producingTR. The pulseheightfrom TR canbe clearlyseparatedfrom the onefrom ionization loss only.

The increaseof total radiatedTR energywith y is mainly dueto an increasein the averageX-ray energy,as shownby the measurementsin different Li/Xe-detectors(fig. 50) usingelectrons

40 —

BNL1~X 50 jim ‘.1000/ 50Oj~mgap

ai3O—

- BNL2.~‘ 50 jim-500

/ ,‘ 500jim gap,4 I.,

~ 20— ~1 ‘~ CERN/ BigLil

4 / /7 /‘ BNL3

- / 50 im’.SOO// 200 jim gap

a/

//10— /

///I,

//

I’

0 1 2 3

p~ [0ev/c]

Fig. 50.Transitionradiationmeasuredin axenon/C02-filled(80/20)proportionalchamber(1.04thick)behindradiatorsof differ-

ent geometriestraversedby electronsof momentum Pe [CO 77].

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128 K. Kleinknecht,Particledetectors

with ~ —~2000—6000.From theseexperimentswe can concludethat i) TR detectorsat themo-ment canbe used for y> 1000, i.e. for electronsabove0.5 GeV/c andpions above 140GeV/c.ii) The extensionof this methodbelowy = 1000 requiresthe detectionof 1—5 keV X-rays.

Recently,Ludlam et al. [LU 81] haveshownthat an improvementin the separationof particlescan be obtainedby not only measuringthe total energydepositedby TR quantabut countingionization clustersalong the track. The numberof suchclustersfrom an ionization particletrackobeysa Poissondistribution, while the upperendof an energyloss curve hasavery longtail. Ifthereforea chargedparticlebelow transitionthresholdhasto be separatedfrom a particlewithTR, the region of overlapbecomesmuch smallerfor the clustercountingmethod.Fig. 51 showsthe principle of method [LU 81], andfig. 52 the distributionsin clusternumberN andin depos-ited chargeQ for 1 5 GeV pions andelectrons[FA 81]. Also shownis thepion rejectionvs. elec-tron efficiency for a particularcluster thresholdenergyof 4 keY, obtainedwith a detectorof 12setsof 35 ji lithium foils, eachonefollowed by axenonproportionalchamber.The detectorhasa total length of 66 cm anda thicknessof 0.04 radiationlengths.For a 90% electronefficiency,apion rejection of 8 X 1O~is obtained.The correspondingfigure for a radiatormadeof purecarbonfibres of 7 jim thicknessis 2 X iO~.With a similar detectorof 132 cm length, a kaonrejectionof 102 hasbeenachievedfor a 90% pion detectionefficiency,as shownin the pointslabelled Exp.A in fig. 53. Also shownaremeasurementsof Commichauet al. [CO 80] usingonlychargemeasurements(Exp.B). This detectornearlyreachestherejectionobtainedwith the clustermethodin Exp.A.

-DV, +H,V,

:F. TR s-ELECTRON

o THRESHOLD

BEAM 1~ ~m1~j~TIME

_____________ ...—~NODESRADIATOR : •

FIELD.- ~~wIRES

E~

LONG. DRIFTCHAMBER

Fig. 51. Principle of detection of transitionradiation by counting ionizationclustersalong the track. TR: transition radiation,D.V.: drift voltage [LU 81].

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K. Kleinknecht,Particledetectors 129

N DISC

0

_ 1~ :: e ~40Oin ~ 600z

~0 200

0 _____________ _______________III 0 ______________0 5 10 5 20 2 6 10 4

N clusters Q, keV,/setI I I

•AUIOGeVO~0 15 GeVRADIATOR Li II

cm 12 sets

z0

10~z

z00

~ADC,THR’.4 key)z0 I0~-~

N(DISC,THR~4 key)

1.00 0.90 0.80 0.70

ELECTRON EFFICIENCY

Fig. 52. Upper part: distributionin cluster number N and in deposited chargeQ for pionsand electrons.Lower part: ir/e separa-tion by thresholdin Qor in N [FA 81].

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130 K. Kleinknecht,Particledetectors

10~’

-

I I I

1.00 0.90 0.~0 070

EFFICIENCY FOR KAON

Fig. 53. ir/K separationfor a givenefficiency of K mesondetectionat 140 GeV/cbeammomentum. Exp.A [FA 81] has24 radia-torsof carbonfibres followedby Xe chambers,with total length 132 cm, Exp.B [CO 80] has 20 setsof 5 ~tmmylar foil stacksand chambers, total length 147 cm.Q: charge discrimination, N: clustercounting.

4.4. Multiple ionization measurement

Betweenthe region(7> 1000),wheretransitionradiationcanbe utilized, andthe mediumandlow energydomain,y < 100,whereCherenkovcountersandtime-of-flight measurementareprac-tical, thereis a region of y between100 and 1000 whereneitherof thesemethodsis applicable.Here a new kind of detectoris provided by the exploitation of the relativistic riseof the ioniza-tion energyloss in this domain(seefig. 1). In gases,thisenergyloss risesby afactorof 1.5, andvery precisemeasurementis required.Becauseof the Landautail from knock-on electrons,theaccuracyin the determinationof the meanenergyloss (or alternativelythe mostprobableenergyloss) does not improve considerablyby increasingthe thicknessof the detector.However, theresolutionincreasesif the energyloss is measuredin many consecutivethin detectorsandif thelargepulse-heightsfrom knock-onelectronsoccuringin someof the detectorsareremoved.Thisis done by taking the meanof the lowest 40—60% of ionization values.This samplingmethodwith truncationreducesfluctuationsin the meanandpermits a measurementof energylosspre-cise enoughin order to distinguishparticlesif their momentumis known. As can be seenfromfig. 54, the ratio of most probableenergylossesof pions andkaonsat 100 GeY/c is 1.05, suchthat 7r/K separationat this energyrequiresa r.m.s.resolutionof about2%. Sucha resolutioncanbe achievedby usingseveralhundreddetectorswith a total thicknessof a few metersof gas.For128 chambers,by measuringthe averageof the 40% smallestpulse-heights,ar.m.s.resolutionofa 2.5% hasbeenobtainedfor 50 GeY pions andprotons[LE 78a].

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K. Kleinknecht,Particledetectors 131

e

1.6

1.4

K

LU~0

1.2

p

in. I

01 1.0 10.0p(GeV/c)

Fig. 54. Most probable energy loss in onecm of argon (80%)—methane(20%) mixtureat STP, for electrons,muons,ir and Kmesons and protons [MA 78].

36 , . . .....

20

10 - ‘>( ..,,>.:~><‘,,

9%

/ 11%

2 12’>~~..>~/,~/ ..‘

b 6’5’4 ‘~ “2

1 . .. ..I10 20 50 100 200 500 1000N

Fig. 55. Resolution expectedin energy loss measurement as a function of numberof samplingsN anddetectorlengthL(m). T=

L/N is the thicknessof one samplingdetector [AD 74].

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132 K. Kleinknecht, Particle detectors

The dependenceof this resolutionon pressure,detectorlength andnumberof samplingshasbeenstudied[AD 74]. The simpleststatisticalscalinglaw for the relativeerror on the energylossmeasurementfor 3 cm argonsamplingis UE 5.6 (L p)’12 % with L beingthe detectorlei~gthinmetersandp the gas pressurein atmospheres.If oneincludesthe samplingthickness,a graphicalform of the relationis obtained(fig. 55). However,detailedmeasurementsshowthat theincreasein gaspressuredoesnot improve the resolutionas expected[LE 8la], (fig. 56). In addition,thedensityeffect for the ionization leadsto a considerablereductionof the relativistic rise(fig. 57).

14 • Ar 5%CH4

MF 64*4cm

71 PEAK

2 \

10- \~ -EPI\TEST\

EPIt8 ~ Pressure

~9..\Number~..~

6 ~—.. Expected(NTP)

Number of simultaneous particles4 $ 4 I I

I 2 3 4 5 6 7

NTP equivalent sample thickness

2~ :“ 62024 .~

Pressure ( atm ) —Fig. 56. dE/dx resolution obtained [LE Sla] by varying pressure (“Pressure”), by varying number of simultaneous particles(“Number”) comparedto expectationfrom EPI test [AD 74] at NTPwithout drift. Truncated mean of 64 samples 4 cm thick.

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K. Kleinknecht,Particledetectors 133

I ARGON

1/10 —— 0.25 atm,

+ 5 / CH4 LE78o

5’./, CH4 /

1.8 4 20’/’. CH4 ~LE81Q /~/

4 20% CH4 FA79 /

* 00/, CH4 WA79

I/I 0.5

.6 1/ /‘“•‘• 2

•i ______—--f;07 ~t5

.4 ,ç,,~

,‘/I ~5

A~I./ _.—

,!P/7 ,It,,, ,~ ~

1.2

‘I/fl,,‘~,

,/4~Jj(

IC I I II 10 02 lO~ lO~

Fig.57. Relativisticriseof ionizationin argon—CH4mixtures.Numbersindicatepressurein atmospheres.

Thesetwo effects conspireto nearly deletethe advantagesof high pressure.In fig. 58 the ratio ofdistanceD betweenthe truncatedmeanenergylossesof two particlesandthe resolutionCE forir/p ande/ir pairs is shownas a functionof pressure.Thereis only a marginalgain in going from 1to 2 atmospheres.

This result is atvariancewith measurementsdonein aJADE typetestsetup[WA 82], where 1cm samplesof 4 atm Ar—CH4 givethe resolutionexpectedfrom fig. 55 for 4 cm samplesat 1 atm.

However, on the basisof their results,Lehrauset al. [LE 81b] proposeas a rule of thumbthatan experimentaldetectorwill havea resolutionequalto theoneshownin fig. 55 but readingthegraphfor halfof the actualsamplingsize.

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134 K. Kleinknecht, Particle detectors

A EPI‘~TEST

6__— 5cH

4

~ ~ S.7 SiC4 H~

°~~—a....~20CH4~ ~ ‘~Z~~’°~— 20 C2H6

5 ~ ~ 20 Co2

X H 0~’~..20C2H4e 2 6 7r/p

4~I 5 GeV/c ME 64x4cm

b4EPI

TEST

~~ 5 cH~

// 2OCO”~’~’—.. ~~~20C2H6~ I5~C~H1~ 2,0 20C2H4 20CH4

2 I 10C02.10C2H6

e/7r

0 2 4 6 8

Pressure (atm.) —

Fig. 58. ResolvingpowerD/a (seetext) for separationof lr/p or e/lr vs. gaspressurefor 15 GeV/cparticlesusingdE/dxmeasure-ment in 64 x 4 cm samples [LE 81a].

4.5. Comparisonof identificationmethods

The identificationmethodsdiscussedaboveareusablein certainmomentumdomains:thetime-of-flight measurementat low momenta,then thresholdCherenkovcounters,DISC-Cherenkovs,multiple ionization measurementand,at ultrahigh momenta,transitionradiation.The lengthre-quired for ir/K separationin thesedetectorsis shownin fig. 59. Using atypical detectorlengthofa fixed targetexperimentof 30 metersanda lengthof 3 metersfor storagering experiments,typ-ical momentumrangesfor inK separationare calculated,as shownin table7. It appearsthat themultiple ionization measurementis necessaryfor bridging the gapbetweenthresholdCherenkovandtransitionradiationcounters.

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K. Kleinknecht,Particledetectors 135

15’ I I I

LENGTH FOR It /K SEPARATION

lm) IT.O.F. /THRESHOLD /DISC

10 - ) CERENKOV

I R~LATIVISTIC RISE

~S.

1 10 102 ~ 1O~

MOMENTUM (0eV/c)

Fig. 59. Length of detectorsneededfor inK separation by different identification methodsvs. momentum.

Table 7Identification methods

Method Domain for in/K separation Requirements

Fixed target Storage ring

geometry geometryL30m L3m

Time-of-flight p < 4 GeV/c p < 1 GeV/c = 300 psThresholdCherenkov p < 80 GeV/c p < 25 GeV/c 10photoelectronsDISC-Cherenkov p < 2000GeV/c — achromatic gas counterRingimagingCherenkov p < 65 GeV/cMultiple ionization 1.2 <p < 100 GeV/c 1.5 <p < 45 GeV/c u~= 2.5%Transitionradiation 7> 1000 7> 1000 detectionof >10 keV

X-rays

5. Energymeasurement

5.1. Electron-photonshowercounters

At energieswell above1 MeY, the ionization loss of fast(~3 1) electronsis given by

— (4d~i) = 4inN0 ~ r~mc2[ln(2mu2’y2/I) — 1]

ion

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136 K. Kleinknecht,Particledetectors

withr~= (e2/mc2)2 (2.8 fm)2, while the competingloss by bremsstrahlungtakesthe form

fdE~ N0 22 183 E

—t—---- i 4a— Zr Eln— :—,\dx/ A e z113 X

0brem

wherethe “radiationlength”X0 is definedthis way, andm is the electronmass.While ionization dominatesatlow energies,bremsstrahlungtakesoverat high energies,andthe

ratio R of bremsstrahlungloss and ionization loss comesout to be R ZE/550,whereE is mea-suredin MeV. The energyat which this ratio becomesunity, the “critical energy”Ec thereforehastheapproximatevalueE~ 550/ZMeV which for lead is E~= 6.7 MeY.

The interactionof photonsat high energyis also governedby the radiationlength: the cross-sectionfor pair creation

Upair 4aZ2r~[~ ln(183/Z1/3)—

gives a probabilityP for pair creationin one radiationlength

,Np~Xo_7

Cpair A —;;- ~

Table 8 givesradiationlength andcritical energyfor somematerials[PA 78].The interaction of a high-energyphotonor electron thereforeleads to a cascadeof electrons

andphotons;startingwith aphotonof energyE0,after 1X0 we have2 particlesof averageenergy

E0 /2, after nX0 thereare2~iparticleswith meanenergyE0 /T~.The cascadestopsapproximatelywhentheparticlesapproachthecritical energy,i.e. if E0/2’s = E~.

The numberof generationsup to the maximum thereforeis n = ln(E0/Ec)/ln 2, andthenum-ber of particlesat the maximumN~ 2~ E0 /E~.The total integralpathlengthS of all electrons

Table8Radiationlengthandcriticalenergy

Material X0 [g/cm2I E~[MeV]

H2 63 340

Al 24 47Ar 20 35Fe 13.8 24Pb 6.3 6.9LeadglassSF5 9.6 —11.8Plexiglass 40.5 80H20 36 93NaJ(Tl) 9.5 12.5BI4Ge3O12 8.0 7

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K. Kleinknecht,Particle detectors 137

or positronsin the showeris approximately

n

e—2 V IV.i a?tT —,-~v ~2 \L’ IL’

Li — ~“o ~‘ 5o3~p — ~3”~o 55O’~’~’o1~c~v1

wheres0 is thepathlengthof electronsbelowthe critical energy.

The pathlengthS is proportionalto the total energyE0 if electronsandpositronscanbe de-tecteduntil they cometo rest. In practical detectorsthereis aminimumkinetic energyrequiredfor detection(cut-off energyEk). This effect hasthe consequence[RO 52] that thevisible pathlength becomes[AM 81]

S=F(z)X0E0/E~

with F(z) ez (1 + z ln(z/1.526))andz = 4.58ZEk/(AEc).Including the effect of the cut-off energyinto Monte Carlo [CR 62, NA 65, LO 75] calcula-

tions gives the followingpropertiesof electron—photonshowers:i) thenumberof particlesat maximumN~is proportionalto theprimaryenergyE0,ii) the total tracklengthof electronsandpositronsS is proportionalto E0,iii) thedepthatwhich themaximumoccursXmax increaseslogarithmically:Xmax/X0 = ln(Eo/Ec)

— t, wheret = 1.1 for electronsandt = 0.3 for photons.The longitudinal energydepositionin an electromagneticshowercanbe seenin fig. 60 as mea-

sured [BA 70] for 6 GeV electrons.A useful parametrisationfor this distribution is given by[LO 75]

(dE/dt)E0At° e_Pt

wheret~X/X0is the longitudinaldepthX in unitsof X0, andtheparameterst~ 0.5, t ~3tmax

andA =1/~(c~+ 1) vary logarithmically with energy.For proton energiesaroundI GeY, the

distributioncanbe approximatedby (dE/dt)= E0 0.06 t

2 e”2 for aleadconverter.The transversedimensionof a showeris determinedby the multiple scatteringof low energy

electrons.It turns out thatausefulunit for transverseshowerdistributionsis the Moliere unit RM= 21 MeV X

0/E~.As shownby the measurements[BA 70] in fig. 61, the distributionof showerenergyin transverse(radial)bins scaledin RM is independentof the materialused,and99% of theenergyareinsidethe radiusof 3 RM.

The energyresolutionof an idealizedhomogeneousdetectorof infinite dimensionsis limitedonly by statisticalfluctuations.Fora cut-off energyof 0.5MeV anda critical_energyof 11.8 MeVa total track length of 176 cm/GeV anda resolutioncr(E)/E = 0.7%/v’E(G~V)have beencom-puted[LO 75].

If the showeris not containedin the detector,the fluctuationof the energyleakingout gives acontributionto the resolution.As shownin [DI 80], longitudinal lossesinducealargerdegrada-tion of the resolution than lateralones.An estimatefor thisfluctuationdueto longitudinal leak-ageis a(E) = (dE/dt)tr O(tmax), wheretr is the length of the detectorand cJ(tmax) the fluctua-tion of the position of the showermaximum. For photonsof I GeY energy,O(tmax)— I and(C(E)IE)Ieak= 0.06 t~exp(—tr/2). If the numberof photoelectronsN~detectedper incident

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138 K. Kleinknecht,Particledetectors

D(’I.)

t 0(1.)11O~u

0001 01 001

- 0.001

I~i t~I !IIlI1I11l11 III!

0 5 10 15 20 25 30 35 40 45Depth in material [xo]

Fig. 60. Longitudinal distribution of energydepositionin a 6GeVelectronshower;measurements(line) andMonteCarlo calcula-tion (histogram)[BA 70].

energyE0 is limited, the fluctuation of this numbergives an additionalcontributionto the reso-lution: a(E)/E—(N~E0Y°~

5.To thesetwo sourcesof fluctuations,valid for homogeneouscalorimeters,we haveto addthe

samplingfluctuationsif the showercalorimeterconsistsof a seriesof inactiveabsorberlayersofthicknessd interspersedwith activedetectorlayers(“samplingcalorimeter”).If thedetectorscountonly thenumberof particletraversals,N, the statisticalfluctuationin N determinesthecontribu-tion to theenergyresolution.SinceN dependson thetotaltracklength,N=S/d=E

0X0F(z)/(E~d),we obtain [AM 81]

(G(E)/E)sampi = Ik/~= 3.2%~(550/ZF(z))VEO(GeV)~

In high Z materials,the lateraldimensionof the showersis muchlargerthanin thosewith low Z,sincethe Moliere unit in units of X0, RM/X0 = 21 MeY/E~,is larger for heavymaterials.Conse-quently also the angleU of electronsandpositronsrelativeto the showeraxis is larger[AM 81].Thoseshowerparticlessee a largersamplingthicknessd/cos0, andthereforeasmallernumberoftraversalsoccurs, reducingfurther the energyresolutionby a factor (cos~> 1f2~ A Monte Carlocalculation [Fl 781 showsthat the average(cos0) = cos(21MeV/(E~ in)) 0.57 for lead. From

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K. Kleinknecht,Particledetectors 139

I I I I I10O~i

— Monte Carlo (Cu)•CuOPbA Al

10

D1.

0.1- 00000

0

.I I I I I I

0 2 4 6 8 10 12

Fig. 61. Transversedistribution of energydepositionin a 6 GeV electronshower; data: points; Monte Carlo: histrogram;RM = 21MeV Xo/E~is the Moliere unit [BA 701.

this calculation,the samplingfluctuation gives a(E)/E = 4.6%/~/E( V) for 1 mm leadsamplingthicknessandEk= 0.

Another largesourceof fluctuationsentersif the sensitivelayersof the calorimeterconsistofa gasor a very thin layerof liquid argon(~2 mm), usedas proportionalcounters.Thenlow ener-gy electronsmovingat large anglesrelative to the showeraxis induce largepulseheightfluctua-tions (“path length fluctuations”), and the Landautail of the energyloss distributionalsoleadsto a reductionof resolution.Thecomputedeffect of thesetwo contributionson the energyreso-lution of alead—argoncalorimetercanbe seenin fig. 62. The overall resolutionis 1 8%/\/E(GeV),morethantwice the samplingfluctuationof 7%/~E(GeV).

HomogeneousshowercountersThe bestresolutionsare obtainedwith anorganicscintillating crystals.NaJ(Tl) detectorswith

a diameterof 3RM = 13 cm and l5X0 = 40 cm length have yielded [PA 80] a resolution of

a(E)/E = 2.8%(E(GeV))°~25in a large scale application. For one 24X

0 long countera(E)/E= 0.9%(E(GeV))°~

25has been achieved [HU 721. The new type of crystal,_BGO(B4Ge3O12)

gives 8% of the light output of NaJ, and a resolution of cr(E)/E = 2.5%/v’E(GeV) [KO 81].

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140 K. Kleinknecht,Particledetectors

I I I I I I

+

10 -‘S

\\ •-~....~~LANDAU

PATHLENGTH

5-

SAMPLI’JG FLuCTuATIONSI I I I I

0 2 4 6 8E (GeV)

Fig. 62. Contributions of sampling, path length and Landau fluctuations to the energy resolution of a lead—gasquantameter[Fi 78].

Leadglass countersdetecttheCherenkovlight of showerelectrons,the resolutionis limited byphotoelectronstatistics.A computation[PR 80], basedon 1000 photoelectronsper GeY, givesa(E)/E = 0.006+ 0.03(~E)°~5,~ being the ratio of photocathodeareaandcounterexit area.Ac-tual measurements[BI 81] with 208blocks of 36 X 36 X 420mm3 give a resolution of a(E)/E= 0.012 + 0.053/\/E(GeV)for ~ = 0.35, in agreementwith the calculation.

SamplingshowerdetectorsThe resolutionof alead-scintillatorsandwichwith 1 mm lead and5 mm scintillator thickness

for a total lengthof 12.5 radiationlengthis shownin fig. 63 versusincidentenergy[HO 791. Thevaluesfor A = a(E)/-~J~vary from 7% GeY”2 at 100MeY to 9%GeV’12 at 5 GeY, in agreementwith a calculated5% GeY”2 from samplingfluctuations,3—4% GeV”2 from photoelectronstatistics,and2—5% GeV”2 from leakage.

In lead—liquid argon calorimeters, the ionization is sampledin a proportionalmodeby theargon chambersdefined by two lead platesas electrodes.Resolutionsfor 2 mm Pb platesand3 mm liquid argonarea(E)/E = 1 2%/..JE(GeV)[KA 811..

A summaryon the energyresolution obtainedwith electron—photonshowercountersis givenin table9.

PositionresolutionThe impactpoint of an electronor photonon an arrayof showercounterscanbe obtainedby

measuringthe lateral distribution of energyin the shower.The precisionof the positioninforma-tion increaseswith the numberof cells hit by showerparticles,anddecreaseswith the cell size.

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K. Kleinknecht,Particledetectors 141

12

10

6-

0 I 1111.11 I 1111111 I I lilt

0.03 0.1 0.3 1.0 3.0 10.0E (6eV)

Fig. 63. The quantity u/.~JE(GeV)vs. E for a lead scintillator sandwich with 1 mm lead plates. Full line is contributionof leakage[HO 79].

In particular, the accuracyis bestif the showerenergyis sharedequally betweentwo adjacentcells. Binon et al. [BI 81] usingcells of 36 X 36 X 420mm3,haveobtainedapositionresolutionof a~= 1.3 mm for 25 GeV electrons.Foralateralcell sized> 30 mm,an experimentalincreaseof ax is calculated(fig. 64) by theseauthors.On the otherhand, a variation of a, ~ IWEhasbeenfound [AK 77], confirming the assumptionthat the spatialresolution dependsmainly onthenumberof showerparticles.

With lead-scintillator sandwichesof 10 X 10 cm2 lateral dimensions[HO 79], the measuredspatialresolutionwas a, = 11 mm/’,/E(GeV).

Table9Electromagneticshower counters

Type Sampling Total a(E)/~J~ Spatial Angul Lateral Group Ref.thickness thickness %(GeV’12) resol.~.x resol. cell size(Xe) (Xo) (mm) (mm)

Na J — 24 0.9 E114 [HU 72]Na J — 16 2.8 E”4 Crystal [PA 80, K! 79]

ball [CH 78b]Pb glassF8 — 17 5.3+ 1.2~/Zr 1.3 36 X 36 IHEP [B181]Pb glassSF5 — 12.5 s162+ 2.52E 6 10mrad 80 X 104 JADE [DR 80, BA 79]

Pb glassSF5 — 20 ~J62+ 0.52E 2 NA 1 [NA 1]Pb/scint 0.18 12.5 7—9 11WE(GeV) 100 X 100 ARGUS [HO79]Pb/scint. 0.21 13 9 25/,JE(GeV) 200 X 250 LAPP—LAL [SC81]

70 X 70Pb/LAR 0.36 13.5 10—12 5 5 mrad + strips TASSO [KA 81]

20 mm

Pb/LAR 0.26 21 10 4 4mrad 23X23 CELLO [BE 81]Pb/LAR 14 11.5 Mark!! [DA 79]Pb/PWC 0.5 12 16 Mark III [H!811Pb/prop.tube 1 28 13—18 pitch7.7 NA 24 [BA 81]

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142 K. Kleinknecht,Particledetectors

I I I I

o~(y) - -

(mm) 8 E~25GeV ,‘

0 50 100d~ CELL SIZE (mm)

Fig. 64. Positionx.m.s. resolutionas a function of transverseblock size d for anarray of leadglassblocks.Full line: averageoverphotonimpact points acrosstheblock; dottedline: photonimpinging on centreof block.Point: measuredresolutionford 36mm [B! 81].

5.2. Hadron calorimeters

The scalefor the spatialdevelopmentof ahadronicshower,the inelasticproductionof second-ary hadrons,which againinteractinelastically producingtertiary hadrons,andsoon, is given bythe nuclearabsorptionlengthX. Fromthe inelasticcross-sectiona,A = A/(aN0p)canbe obtained.The experimentalvaluesof A for materialsusablefor calorimetryare77 g/cm

2(C),135 g/cm2(Fe),210g/cm2(Pb)and227g/cm2(U).Comparedto the small valuesfor the radiationlength of high Zmaterialsenablingthe constructionof correspondinglysmallshowercounters,the sizeof hadronicshowersis large; typical valuesfor Fe calorimetersare 2 metersdepthand0.5 m transversesize.The needfor suchsizesis demonstratedby the measurements[HO 78b] on thelongitudinalshowerdevelopmentshownin fig. 65,wherethe centerof gravity,the lengthfor 95% energycontainmentandthe length,wherethe averageparticlenumbergoesbelowone(“showerlength”) aredisplayedas a function of incidentpion energyfor a 5 cm Fe samplingcalorimeter.A parametrizationof L(95%) can be given: L(95%) = [9.4 ln E(GeV)+ 39] cm Fe. In a similar way, fig. 66 gives lateralshowersizesfor 95% energycontainment;

Apart from the hadronshowerparticlesleakingout longitudinallyor laterally, the energyseenin asamplingcalorimeterfor hadronsis incompletefor severalreasons: -

i) thereare particlesescapingthe calorimetercarryingawayenergy,like muonsandneutrinosfrom pion decay(1% at 40 GeY),

ii) thereis nuclearexcitationandbreakupresultingin low energy‘y rays or heavy fragments,which do not reachthe sensitivepart of the sandwich(20—30% of total energyat 10 GeV).

This loss of visible energy,typically 30%, can be seenby comparingthe light collectedfromelectron-andhadron-inducedshowersin iron (fig. 67). Sincein ahadronicshowertheelectromag-netic componentcanoccasionallybe dominantthroughenergeticin0 production,this loss inducesa fluctuationin responsewhich contributessignificantly to the resolution.

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K. Kleinknecht, Particle detectors 143

25-

20-

15-

£ Shower length ..~ • + 140 GV

• 95/. contairwnent 10 -

150 - o Centre of gravity

I I~ ~I + I

50 GeV

010 ~ 2~ I I I I

E(GV) 0 20 40 60 80 100Showerdepth (cm)

Fig. 65. Showercenterof gravity in iron, length for 95% en-ergy containmentandlengthwhereaverageparticle number Fig. 66. Lateraldimensionfor 95% energy containment asagoesbelowoneasfunctionof pion energy[HO78b]. functionof depthin iron [HO78b].

On top of this fluctuation thereis the samplingfluctuationwhich alonegives riseto a resolu-tion abouttwice as largeas in electromagneticshowers(see4.1). However, the effectsof the fluc-tuation in energyleakageandin the electromagneticcomponentof thehadronicshoweraremuchlargerhereandleadto energyresolutionsof about

o(E)/E ‘~‘ (0.9 — 0.5)WE(GeY),

if the thicknessof materialbetweenthe samplingdevices(“sampling thickness”)is below5 cm ofiron.

Two waysof improving thisresolutionhavebeeninventedandtried out successfully:i) The loss of visible energy through the nuclearexcitationandbreakupmechanismcan be

nearly completelycompensatedby the energyreleasein nuclearfission of 238U. Energeticphotonsfrom the fission contributeto the observedsignal such that the pulseheightfor hadronshowersbecomesnearly equal to the one for electromagneticshowers,as shown [FA 77] in fig. 67. Thecorrespondingfluctuationsdisappear,and the energyresolutiondecreasesby abouta factorof

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144 K. Kleinknecht,Particledetectors

3 - : : .“ U238Fe! LAp

. ~tU 238/LA V

‘p ‘-9 —

~ 2-

7/ 7V ‘—

0~1-

—70-~

I I I I I I5 10

Available Energy [0eV]

Fig.67. Detectedenergy deposition in sampling calorimetersof Fe andU for electrons, pions and protons vs. particleenergy[FA 77].

two. Experimentalresultsfor U calorimetersareshownin fig. 68, theycorrespondto

u(E)/E = 0.3/~JE(GeV),

which is only 50% higherthanthe lower limit given by samplingfluctuations.

30 x Fe 11.5mm)

U (1.7mm)

50 E— 100 150GeV

Fig. 68. r.m.s. energy resolutions obtained with hadronic sampling calorimeters; Fe (1.5 mm) and U (1.7 mm): [FA 77]; Fe(25 mm): [HO78b] and [AB 81].

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K. Kleinknecht,Particledetectors 145

ii) Anothermethod[DI 79, AB 81] reducesthe fluctuationdueto the electromagneticcompo-nent by weighting the responsein individual counters.Electromagneticparts of the showerarelocalized, thereforeproducingvery largedepositionsin individual counters.If the measuredre-sponsein onecounterEk is correcteddownwardsfor largeresponse,E~= Ek(1 — CEk), thentheresultingresolutionin the sum~ E~is markedlyimprovedoverthe onein ~ Ek,asshown[AB 811in fig. 69 for a 2.5 cm Fe samplingcalorimeterexposedto 140 GeV/cpions.The resolutiondis-playedin fig. 68 canbe approximatelydescribedby

a(E)/E= 0.58/sJE(GeV)

between10 and140 GeV/c.If the samplingthicknessis larger,the samplingfluctuationsincreaseandthe resolutionalE in-

creaseswith d; fig. 70 givessomemeasurements.Thesedata (obtainedwithout the weightingprocedure)canbe parametrizedusingan empirical

formula [AM 81], a(E)2 /E 0.25+ (R’ )2 (4t/3), wheret is the samplingthicknessin unitsof X0,t = d/X~,and the parameterR’ comesout to be 30—40%.It appearsthatareductionof d below2 cm of iron doesnot considerably_improvethe resolutionanymore,andthat the limiting resolu-tion for d -+ 0 is around0.5/V’E(GeV).

1600 - Electrons — Hadrons Hadronscorrected uncorrected

1400 -

15 0eV/c 75 0eV/c 140 0eV/c

1200-

r’~~1000-5’ I

L~800- ~ L.0E

I IZ I

600- I,

400 -

200-

L

—— . -~ , ~ . I

ido 500 lOOC)

E (nep)

Fig. 69. Pulseheightspectra(in n.e.p.)for electronsandhadronsin a2.5 cmsamplingcalorimeter[AB 81]

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146 K. Kleinknecht,Particledetectors

I I

£

>~

I .• 0•

-.

• J.PDISHAW (4000eV)(thesis)

• W Kienzle (10GeV)-~ £ CDHS1

CDHS2

0 5 10 15

Thickness, cm of Iron

Fig. 70. r.m.s. hadronicenergyresolutionfor iron calorimeterswith different sampling thickness;J.P.Dishaw [DI 79]; CDHS 1[H078b];CDHS2 [AB81].

The samplingof ionization in hadroncalorimeterscanbe doneby scintillatorsliquid argonion-ization chambers,proportionalchambers,or flash tubes.The choicebetweenthesedetectorsde-pendson thedesiredresolution,granularityandcost.For moderate-sizedgeometries,liquid argonandscintillatorsare usedfor bestresolution.For very largefine graincalorimeters(ye scattering,proton decay), the proportionaltubes or flash tubesgive granularitiesdown to 5 mm X 5 mm ata pricewhich still allows theconstructionof multi-hundredton calorimeters.

5.3. Monitoring ofcalorimeters

In a typical large-scalecalorimetertherewill be severalthousandchannelsof analogpulseheightinformation which is convertedto digits andregistered.A severeproblemwith sucha numberofchannelsis their calibrationandmonitoring.

The calibrationcan bedoneby usingsuitablehadronbeamsandcalibratingthe responseof thecalorimeter,where for eachsamplingdetectorthe pulseheightis measuredin termsof minimumionization depositedby highenergymuons.

If there arenot as manymuonsin eachsamplingdetectoras areneededfor day-to-daymonitor-ing, anothersourceof calibratedpulseheightsis needed.For liquid argon calorimeters,such asourceis obtainedby depositinga known amountof chargeinto the ion chamber.The samecanbe donefor proportionalchambers.

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K. Kleinknecht,Particledetectors 147

For scintillation counters,a novel kind of monitoringsystemshasbeenconstructedrecently[GR 80, El 80]. Thelight sourcehereis a pulsednitrogenlaseremittingat 337nm. Therearedif-ferentschemesof distributingthe light ontoa few thousandcounters.

In oneof the systems[GR 80], built for the UA1 experiment,the laserlight is injectedinto arectangularbox coveredinsidewith ahighly reflectingmaterial(millipore). After manyreflectionsinside the light is diffuse andleavesthe box throughquartzfibers of 200j.tm diameter.This fiberhasattenuationsof —400 db/km in the UY. Eachof the 8000 fibers is connectedto a plexiglasprism glued to the centerof the scintillator. The UV light pulsethenproducesscintillationlightwhich reachesthephotomultiplierandproducesa digital pulseheight.

In anothersystem[El 80], designedfor the improved CDHS detector,the laserbeampassesthrough a filter, is widenedup optically, and then illuminatesa scintillator piece glued onto aplexiglas rod (fig. 71). The blue POPOPlight emitted isotropically from the scintillator travelsdown the rod by internalreflection andis partially acceptedby the 2304 fibers groupedinto 144bundles of 16, each bundlein oneconnector.The homogeneityof illumination of the fibers iswithin 1%. A mechanicalmaskmoving acrossin front of the connectorspermits onegroup of192 fibers at a time to be illuminated.This is requiredby the numberOf ADC channelsavailable.The transmissionof the fibers of 200~tmdiameter(QSF200A) is 180 db/km for the bluescintil-lator light, such that over a length of 25 m the attenuationis a factor of 2.8. Eachfiber is con-nectedto a light guidethrougha small (4 mm dia.) cylindrical rod. With this system,by exchang-ing filters of different densityon a “filter wheel”, the linearity of all tubescanbe measuredin adynamicrange from 1 to 2000 times minimum ionization. The absolutecalibration is donebycomparingoneof the fiber outputsto the standardlight from an asourceembeddedin a scintil-lator.

D)STRIBUTOR FILTERSYSTEM LASER

~~:;: II I

Connectors Scinti(Lotor Lenses Mirror +JoulemeterFibers Wheels

50cm

Fig. 71. Dortmundlasercalibrationsystem[El 801.

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148 K. Kleinknecht,Particle detectors

6. Momentum measurement

6.1.Magnetshapesfor fixed target experiments

In a fixed target interaction,the reactionproductsareusually concentratedin a conearoundthe incident beam direction (z), becauseof their limited transversemomentaand the Lorentzboostfor longitudinalmomenta.If sucha particlewith momentum(Fr, P~~ traversesahomo-geneousmagneticfield (0,B~,0), it receivesa transversemomentumkick

1~P~= —e fB~dz,

which gives for a field integralof 10 kG Ifl atransversemomentumchangeof 0.3GeV/c.The cor-respondingdeflectionof the particle is inversely proportionalto its momentum,andameasure-mentof theprojectedanglesin the(x,z)planeyields,in thesimplestapproximation,themomentum

P = e fB~dz/(sinO~— sin 0out)~

If themagnetizedvolumeis evacuatedandthe multiple scatteringin the positiondetectorsis neg-

lected,theerrorin momentum,.SP,comesfrom measurementerror ~x in the chambersalonet5P/P— 2(P/~P~)(~x/L),

if the leverarm for the anglemeasurementbefore andafter the magnetis L. For a field integralof 50kGm, 6x = 0.3 mm andL = 3 m this gives ~P/P— 1.3%at 100 GeV/c.

£~AIR~I) _©Cd

Fig. 72. Magnetshapesfor fixed targetexperiments,(a) H-magnet,(b) C-magnet,(c) toroidal iron core magnet,(d) H-typeironcoremagnet.

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K. Kleinknecht,Particledetectors 149

These“air core” magnetscomein different forms (fig. 72): H-magnetshavesymmetricalfluxreturn yokes, C-magnetsasymmetricalones(and a lessuniform field). The amountof iron in theflux return dependson the desiredfield strengthin the air gap. For a cubic magnetizedregion,the volumeof iron needed,VFe, relativeto the magnetizedair gapvolume VMag for different fieldstrengthB in the gap is shownin fig. 73. If B hasto reachthe saturationfield strengthB~,thenVFe/VMag ‘—‘ 3, which is very uneconomical.More usualmagnetshaveB/B5 ~ to ~-

If the particlesto be analyzedare high-energymuons,a moreeconomicalform of magnetsare“iron core” magnets(1CM). Herethe field linesstaycompletelywithin iron, eitherin the form ofa toroid, wherethe field lines are circulararounda centralhole for the coils, or in akind of H-magnet,wherethe centralregion is alsofilled with iron (fig. 72). The momentumresolutionhereis limited by multiple scatteringof the muonsin iron and the measurementerror of the muontrack. Multiple scatteringresultsin a meantransversemomentumchangeof

~pMS 21 (MeV/c)~/Z7~,

whereL is the lengthof iron traversed.The momentumresolutionis given by the ratio ~pTMS/pT andis thereforeindependentof the

momentum.The resolution improveswith the lengthof the 1CM as~/Z, andis 12% at L ‘~ 5 m ifthe positionmeasurementerror is smallerthanthe errorby multiplescattering.Forhighmomenta(P> 100 GeV/c) this is not the case,sincethe momentumresolutionfrom measurementerror in-creaseslinearly with momentum.For a positionerrorof 1 mm in drift chambersafter every75 cm

of iron ~~1~/1~)meas 12% at 100 GeY/c, equal to the error by multiple scattering.The error(AP/P)measdecreaseswith a 5/2 power of the magnetlength L, becausenot only the bendingpower and the leverarm increasewith L, but also the numberof measurementsalongthe track.Fig. 74 gives thesetwo contributionsto the momentumresolution for differenth lengthof ironcoremagnets.

VE, /VMQ9

0 ‘ 05 ‘ ‘ 1

Fig. 73. Volume of iron VFeneededpermagnetizedvolumeusedin experimentfor iron coremagnets(1CM) andaircore magnets(ACM) vs. magneticflux densityB in units of saturation densityB~.

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150 K. Kleinknecht,Particledetectors

0.6 i I I / I I I I I 1 I I I

/ — — — TotaL uncertainty05 - / . — Measurement

,i7 — -— Multiple scattering

/ L=5m

0.2- /

~H-T--~LrlSm L:20m

0.3 . -

0.2

0.1

100 300 500 700 100 300 500 700

p~, GeV

Fig. 74. Momentumresolution for iron core magnetsfor different iron length,L. Contributionsfrom measurementandmultiplescatteringaregivenseparately.

6.2. Magnetshapesfor storagering experiments

Interactionratesin colliding beamexperimentsare notoriously low, and4ir solid anglecover-ageis very desirable.Variousmagnetgeometriesareimaginable(fig. 75).

i) Dipole magnetwith two compensatorsto keepparticlesin orbit: uniform field, goodana-lyzing power in the forward/backwarddirection,badanalyzingpowerfor particlesemittedparal-lel to the field lines;synchrotonradiationof the beamis prohibitive for e~erings.

ii) Split-field dipole magnet: good resolution in the forward direction, very inhomogenousfield at 90°,complicatedtrack fitting procedure;synchrotonradiation in eF e rings; beingusedatthe ISR.

iii) Toroid: the innercurrent sheetor copperbars have to be crossedby particlesbeforemo-mentumanalysis,momentumresolution is affected for low energyparticles,advantage:no fieldin beamregion.

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K. Kleinknecht,Particledetectors 151

DIPOLE TOROID

SPLIT FIELD SOLENOID

Fig. 75. Magnetshapesfor storagering experiments;lines arecurrents.

iv) Solenoid: no force on beamparticles,good analyzingpowerfor particlesemitted at 90°;accessto innerdetectoronly throughendcaps.For proton (and antiproton)storagerings,split-field anddipolemagnetsarebeingusedas well assolenoids,while for electron—positronrings the solenoidhasbeenmostwidelychosen.In general,storagering detectorsresembleeach othermuch more closely thanthe detectorsat fixed targetmachines.

6.3. Central trackingdetectors

For the solenoidalfields usedat electron—positronstoragerings, momentummeasurementisusually donein a centraltracking detectoraroundthe interactionpoint. Thisdetectorhascylin-drical shapewith cylinder coordinates;radiusr, azimuthalangleIp, andz alongthe magneticfieldwhich is parallel to the cylinder axis. If the measurementerror in the r, p planeperpendiculartothe field is a~52,the momentumcomponentin that planePT, is measuredwith an error [GL 63]

(~~.!)= (aT~,pT/(O.3BL2))~/720/(N+4),

PT

whereB is the flux densityin Tesla,L the radial tracklengthin metersandN the numberof mea-suredpointsalongthe trackat uniform spacing.

In addition to this measurementerror, thereis theerrordueto multiple scattering

(—_!.) = (0.05/BL)~Jl.43L/X0

PT

For reconstructingthe total momentumof the track,p PT/sin0, alsothepolaranglein a planecontainingthe cylinder axishasto be measured.

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152 K. Kleinknecht, Particle detectors

The errorcoming from the measurementerror in thez-coordinate,u~,is

(ao)m =-Til~N(~J~

andthe onefrom multiple scattering

(~O)ms=0.015 \1E7~.

It appearsfrom theserelationsthat the momentumresolutionimproveswith L2 andB, but thatan increasein the numberN of measuredpointsgives only an improvementwith V’N.

The three cylindrical drift chambertypesdescribedin sections2.4, 2.5 and2.6 havebeenusedas centraldetectors.Someof theirpropertiesarelisted in table 10.

In two of thesedetectorsenergyloss is measuredin conjunctionwith the tracking information.The momentumresolutionis in therange.Ap/p2 (l.5—5.0)%.

Table10

Centraltrackingdetectorproperties

Name Ref. Meas.tracklength Flux No. Gas Sense Spat.resol. Method Mom.density sampL press. wires ofz meas. resol.

radical axial (T) (bar) a(r, ~) a~ op/p2%L (cm) z (cm) (jim) (mm)

calc. meas.

TASSO [BO80] 85 330 0.5 15 1 2340 200 3—4 4°stereo 1.7

CELLO [BE 811 53 220 1.3 12 1 6432 170 0.44 cathodesCLEO [ST 81] 75 190 0.5(1.5) 17 1 250 5(0.25) 5

MARK II [DA 79] 104 0.4 16 1 200 4 1.9

JADE [DR 80] 57 234 0.45 48 4 1536 180 16 charge 2.2div.

AFS [CO 81] 60 128 0.5 42 1 3400 200 17 chargediv.

UA1 [BA8O] 112 250 0.7 —100 1 6100 drift:250~mch. div. 8—25 mm charge

div.

TPC [NY 81] 75 100 1.5 186 10 2232 <200 0.2 drift 1.0+13824

TRIUMF [HA 81a] 54 69 0.9 12 1 144 (600) (0.6) drift+630

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K. Kleinknecht,Particledetectors 153

7. Realizationof detectorsystems

Detectorsof thekind describedin the foregoingsectionsareassembledfor specificexperimentswhich nowadaysreachbig dimensions(-‘-50 m length), large mass(up to 2000 tons) andhugecomplexity (up to 10~channelsof analoganddigital information). It is evident that there is agreatvariety andversatility of experimentsat fixed targetprotonmachines,becausetherearesomanyvariationsin experimentalconditionspossible:different incidentparticles,different targets,different energies,different interactions(weak, electromagnetic,strong). On theotherhand,thehigher c.m. energiesat colliding beammachinesandparticularlythe cleanerexperimentalcondi-tions at electron—positronstoragerings haveenabledimportantdiscoverieswith thesemachines(SPEARandPEPat Stanford,DORIS andPETRAat Hamburg,CESRat Cornell). This evolutionmay continuewith the proton—antiprotoncollider at CERN in 1981 and at FNAL in 1984,Isabelle at Brookhaven, the Large Electron PositronMachine (LEP) at CERN and the ElectronProton Projectsat DESY (HERA) andlit KEK (Tristan).

Out of the largenumberof detectorsystemsin useor beingbuilt presently,I havechosenfourexamples.

7.1. A hadronbeamdetector

This experiment(NA 5 atCERN [NA 5]) usesa hybrid detector(fig. 76). A hadronbeamfromthe SPSimpinges on a hydrogentarget,embeddedin a largedipole magnet.Particlesfrom the

EXPERIMENTAL LAYOUT HORIZONTAL

~ ~ __~ _r~__

Vertexdipole magnet

Target Streamer Multiplicity Magnetostr. 1 mchamber MWPC chambers

Fig. 76. Experimental layout of Na 5 hadron experiment [NA 5].

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154 K. Kleinknecht,Particle detectors

interactionvertex are bentby the field andanalyzedin a streamerchamber.Largemultiplicitiesof chargedparticlesdo not hinder the performanceof the chamber(seefig. 26). The momentummeasurementis further improved by the eight large(6 m wide) magnetostrictivesparkchambers.A trigger on jets and their analysisis madepossibleby the hadron calorimeter [EC 77], whoseangularrangecan be changedby moving it backand forth on rails. The experiment-searchesforjet structurein hadron—hadronreactionsat high transversemomentum.

7.2. A neutrinodetector

Since theneutrino nucleontotal cross-sectionis only 1036 cm2 at 100 GeV neutrinoenergy,neutrino detectors have to be massive. This detector (fig. 77) of the CERN—Dortmund—Heidelberg—Saclay-Collaboration[HO 78a] usesthe target weightof 1500 tonsof iron, arrangedin circularplatesof 3.75m diameter,for threeotherfunctions:

i) 75 cm of iron thicknessarecombinedto form a toroidal magnet,ii) betweenthe iron slabs of 5 cm (for 7 magnets)or 1 5 cm (for 8 magnets)8 scintillators

viewedby 16 photomultiplierssampletheionization energydepositedby hadronicshowers,iii) the range in iron enablesidentification of muons. Betweentwo magnets,driftchambers

[MA 77] with 3 planesof wires strungin 120°sequencemeasurethe muonmomentumtoabout 1 0%. The detectorhasrecordedso far about3 X 106neutrinointeractions,with zero,one,two, threeandfour muons.

For future experiments,a part of the detectoris being rebuilt (fig. 78) with 2.5 cm iron platesanda finer scintillatormesh(15 cm wide stripsin bothdirections).

CDHS NEUTRINO DETECTOR

BEAM - - ~15 MAGNETIZED IRON-SCINTLLATOR CALORIMETERS 19 DRIFT CHAMBERS

lOm -I

Fig. 77. Experimental layout of CDHS neutrino experiment [HO 78a].

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K. Kleinknecht,Particledetectors 155

4500

~ ~ .—.—-—- .y~

4~I~I___ - -

~ _______________~ ~ S \~Y ~\~\\\~~‘ ~\\\\\\\\\\\\\\\\\\\~

Fig. 78. New toroidalmagneticcalorimeterof CDHSWcollaboration.

7.3. A proton storagering detector

For the proton—antiprotoncollider at CERN, one of the detectorsis the UA 1 experiment[UA 1]. Fig. 79 showsa sketchof this enormousapparatus:a dipole field configurationwith twocompensatormagnetswas chosen.The centraldetectorusesimagereadoutsimilar to the TPC,but in a different geometry,becausethe magneticfield linesareperpendicularto thebeamdirec-tion. The E andB fields arenot parallelhere,suchthat the improvementon diffusion broadeningduring drifting doesnot apply,anda drift spaceof 20 cmis used.Fromabout10000wirespulse-heightsand time information is readout to get a spatialmis resolutionof 250pm andadE/dxmeasurement.The centraldetector(radius 1.2 m, length 5.7m) is surroundedby anelectromag-netic calorimeterinside the magneticfield andby thehadroniccalorimeterembeddedin the polepiecesof the dipole magnet.The wavelengthshifting techniqueis used for the light collectionfrom the 8000 scintillators.The detectoris coveredon threesides by muon detectors(fig. 80).

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156 K. Kleinknecht, Particle detectors

EXP�RIMENTALAPIEA FOR V-~ IN LONGSTRAIGHT SECTION 5 cc TH~SPS_Veiical section in beam directIon.

1098 5463245 8910

N ~ \ L ~m I ~1

__ * _ //_ -

1 —~ ~ I ~ ~ SPStwi~t

Fig. 79. Sideview of UA 1 detectorat antiproton-protoncoilider [UA 1]: 1. centraldetectorwith imagereadout,2. largeanglecalorimeterandmagnetyoke,3. largeangleshowercounter,4. endcap showercounter,5. endcapcalorimeter,6.muondetector,7. aluminium coil, 8. forward chambers,9. forward shower countersandcalorimeters,10. compensatormagnet.

~ ~ ~n~TTh~

— ~ ‘ I

— - —. —- - t.

- . ~-- ~ :-

--..-- ..- ~ ----

Fig. 80. Perspectiveviewof UA 1 detector.

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K. Kleinknecht,Particledetectors i-si

ii ___////4 ~ø~/////////í~’hL

~ ~‘~ø’~ ~~///////////~r~

IIIY? ~ I~___Ill ::-:III -‘~\ \ - ‘ :~: 0

ItttIlTltW4— ‘ N ::::

“ \ 1MHH~k~kT ‘ ‘

7

N

I

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158 K. Kleinknechr, Particledetectors

According to the preliminary measurements[AL 821 the particle multiplicity in hadron—hadroncollisions seemsto rise logarithmicallyup to 540 GeV center-of-massenergy;the typicalnumberof chargedsecondariesis around25. Eachof the eventsrecordedby this detectorwillcontain more than l0~bits of information.On-line data reductionwill becomevery importantfor this enormousmassof information.

7.4. An electron—positronstoragering detector

One of the detectorsat the PETRAstoragering is the JADEdetector[BA 79, DR 80] (fig. 81).Aroundthe interaction point, tracksare recordedin a centraldetector(section6.3) of pictorialdrift chambers(section2.5) embeddedin a 0.45 Teslasolenoidalfield. Track coordinatesin the(r, p) plane aremeasuredwith an accuracyof 180pm, andthe momentumresolutionis ~p/p2 =

2.2%(GeV/c)~.The centraldetectoralsomeasuresdE/dx.The field is producedby a 7 cmthickaluminium coil, 3.5 m long, 2 m in diameter.Outside the coil, electronandphotonenergiesaremeasuredin an array of 2520 wedge-shapedlead glass counters,groupedin 30 rings of 84 ele-ments. Togetherwith the lead glass counterson the two endcaps,they cover 90% of the totalsolid angle. The magnetic flux of the solenoidreturns through the iron covering the cylindricaldetectorfrom all sides as a rectangularbox. The iron forms part of the muonabsorber,of totalthickness785 g/cm2or six nuclearabsorptionlengths.Penetratingtracksareregisteredby 4 layersof planardrift chambers.

8. Conclusion

During the last ten years,a lot of progressin detectortechnologyhas beenachieved,andthediscoveriesduring this exciting time of particlephysics wouldnot havebeenpossiblewithout it.Developmentswill go on: the precision of position measurementin large detectorsmay be im-proved by a calibration with nitrogen laserbeams,with planarsparkcountersthe precisionoftime-of-flight measurementscould improve, the Cherenkovring imaging techniquemaybecomeusable,electromagneticshowercounterswouldshrink in size if BGO could be producedatreason-ablecost,anddataprocessingwill becomemoreefficient byusingmicroprocessorsandspecialized32-bitemulators.

It will be necessaryandpossibleto constructfor thenewgenerationof acceleratorsandstoragerings generalpurposedetectorsystemsof the enormoussizeandcomplexityof, e.g., theUA 1 ex-periment.Theseexperimentsrequirelargeexperimentalteamsfor construction,maintenance,run-ning anddataanalysis. It is an openquestionwhetherthis is an unavoidableconsequenceof thephysicsquestionsat theseacceleratorsandstoragerings,or if thereis still a reasonablechanceforsmaller specializedexperimentsto contribute significantly to the progressin elementaryparticlephysics.

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