Present status of silicon detectors in C ompass

Nuclear Instruments and Methods in Physics Research A 512 (2003) 229–238

Present status of silicon detectors in Compass$

H. Angerer, R. De Masi*, A. Esposito1, J. Friedrich, S. Gerassimov,B. Grube, B. Ketzer, I. Konorov, R. Kuhn, S. Paul, L. Schmitt,

R.M. Wagner2, M. Wiesmann*

Physik Department E18, Technische Universit .at M .unchen, James Franck Strasse, 85748 Garching, Germany

Abstract

In 2002 the Compass experiment at Cern has started to take first physics data. The fixed target experiment at the SPS

uses muon and hadron beams of very high intensity to investigate the structure of the nucleon. For beam definition and

small angle tracking silicon microstrip detectors are used. This article describes the requirements which are set by the

physics program of Compass for these detectors and the ways they were met, amongst which the operation at a

temperature around 130 K (Lazarus effect) is the most prominent. Measurements at low temperatures as well as first

results from the operation at room temperature in the Compass physics run 2002 are presented.

r 2003 Elsevier B.V. All rights reserved.

PACS: 29.40.Wk; 29.40.Gx

Keywords: Compass; Cern NA58; Silicon detector; Double sided silicon; Radiation hard silicon; Lazarus effect; Cryogenic

1. Introduction

The Compass experiment (NA58, COmmonMuon and Proton Apparatus for Structure andSpectroscopy) is a high intensity, high rate, fixedtarget experiment at the Cern/SPS [1].

Compass will use different types of beamparticles to investigate hadron properties. The spinprogram uses polarized muons scattering offpolarized nucleons to access spin distributions

(longitudinal and transverse) inside the nucleon.First goal is the measurement of the spin distribu-tion of gluons Dg=g via open charm production.

The hadron program uses pion, proton andkaon beams on various targets. Fields of interestare the measurement of pion and kaon polariz-abilities via Primakoff scattering, spectroscopy ofhybrid mesons with exotic quantum numbers andglueballs via diffractive production as well as thestudy of charmed and doubly charmed hadrons.

The two key features of Compass are highstatistics and high precision. The statistics will beachieved with beam rates of the order of 108

particles per SPS spill3 and high trigger rates up to

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$This work is supported by the BMBF and the Maier-

Leibnitz-Labor of the Universities of Munich.

*Corresponding authors.

E-mail addresses: [email protected] (R. De Masi),

[email protected] (M. Wiesmann).1Now at: A4Vision, Geneva 1207, Switzerland.2Now at: Max-Planck-Institut f .ur Physik (Werner-Heisen-

berg-Institut), F .ohringer Ring 6, M .unchen 80805, Germany.

3Per spill (5:1 s in 16:8 s) 2 � 108 muons with 160 GeV on a

spot size down to 6 � 7 mm2 RMS or 108 hadrons with 200–

300 GeV on a spot size down to 1:1 � 1:5 mm2 RMS.

0168-9002/03/$ - see front matter r 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0168-9002(03)01898-9

100 kHz: For the precision a double magneticspectrometer with tracking, electromagnetic andhadronic calorimetry and particle identification ineach section is used: the first stage detects lowmomentum particles with large deflectionsð7180 mradÞ: High momentum particles are ana-lyzed in the second part ð725 mradÞ using a longlever arm and a higher magnetic field than in thefirst stage. Like this an angular resolution down to10 mrad (for small scattering angles) and a lateralresolution down to 7 mm can be achieved. Aschematic view of the spectrometer is givenin Fig. 1.

The tracking system comprises a set of the so-called tracking stations, each made up of threenested tracking detectors with increasing granular-ity and rate capability towards the center of thebeam: Large Area Trackers (LAT) covering theouter region with lowest track density, Small AreaTrackers (SAT) for the inner region and Silicon andScintillating fiber detectors directly in the beam.

Upstream of the target additional silicon detec-tors and scintillating fibers are used for beamdetection.

The very high beam intensity is a big challengefor silicon detectors: the detectors have to providehigh spatial resolution (few mm) and precise timeinformation (few ns) and at the same time have tocope with severe radiation damage caused byhadron beams, which limits their lifetime to only 1year (estimated dose of 150 kGy) if no special care

is taken. The readout electronics has to stand atrigger rate of up to 100 kHz:

For the 2002 run two silicon stations (8 pro-jections) were operated at room temperatureupstream of the target. The operation at atemperature of B130 K to improve the radiationhardness (see Section 2) will be implemented in2003. As the experiment ran with a high intensitymuon beam, the radiation damage is expected to beless critical in 2002.4

After a short introduction (Section 2) to theLazarus effect, which reduces the influence ofradiation damage, this paper describes the require-ments and the design choices of the critical parts ofthe detectors in Section 3. In Section 4 measure-ments at low temperatures are presented. The lastsection shows first results of the performance inthe 2002 run.

2. Lazarus effect

New high luminosity experiments in high energyphysics are currently challenged by the problem ofradiation damage. Owing to their excellent spatialresolution, silicon strip detectors are very attrac-tive and much effort has been spent on thisproblem. Much of this concerned improvementsof the radiation hardness of silicon detectors via‘‘solid state engineering’’5 (see e.g. Ref. [3] for anoverview of the present status and an introductionto radiation damage and its origins).

A different and promising approach is to exploitthe behavior of silicon at cryogenic temperatures.Studies have shown [4] that a detector which isalready unusable at room temperatures recoversits Charge Collection Efficiency (CCE)6 up to 80%when cooled down with liquid nitrogen. The CCEand its temperature dependence is shown in Fig. 2:from low values at room temperature, the CCE

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50mpolarizedtarget

beam

SM1

RICH 1

muon filter 1

SM2

muon filter 2

HCAL 2

ECAL 2

HCAL 1

ECAL 1

Fig. 1. Schematic view of the Compass spectrometer (SM:

spectrometer magnet, ECAL: electromagnetic calorimeter,

HCAL: hadronic calorimeter).

4According to Ref. [2] the damage effect of muons is 10 times

smaller than that of pions in the energy range of several GeV.5Optimization of the treatment of the wafer material in the

production process, e.g. using varying oxygen atmospheres,

additional dope atoms, etc.6The CCE is the fraction of the charge deposited by a

traversing particle in the crystal which is actually collected at

the readout strip, normalized to the total charge.

H. Angerer et al. / Nuclear Instruments and Methods in Physics Research A 512 (2003) 229–238230

rises to a maximum at around 130 K; showing theso-called Lazarus Effect. The different curvescorrespond to different radiation damages anddifferent bias voltages. Furthermore, it was shown[5] that originally undamaged detectors can standfluences up to the equivalent of the damage of 1015

neutrons with 1 MeV energy7 per cm2 whenoperated at these temperatures from the beginning.

The main reason for the withering CCE is theloss of parts of the signal charge in additional,radiation induced energy levels deep in the bandgap of the semiconductor. Their trapping time isdominated by thermal excitation and is muchlonger than that of normal acceptor and donorlevels. Therefore the signal charge gets delayed oreven lost, so that the CCE decreases. An intuitivebut simplified explanation of CCE recovery at lowtemperatures is that once these levels are filled withcharge, the thermal energy is not enough to drainthem again: the levels ‘‘freeze out’’ and do notinfluence the charge collection anymore.

Although not being understood in all details,the Lazarus effect is already used in severalexperiments [6,7] and is planned to be used forthe Compass silicon detectors as well.

3. Design of the detector

3.1. Requirements from Compass

The high beam rate is a key feature of theCompass experiment. In the final configuration thefull beam with 108 hadrons per spill with 200–300 GeV will pass the silicon detectors on a spotsize as low as 1:1 � 1:5 mm2 RMS. Up to 250 000of these hadrons will initiate a trigger signal andneed to be tracked. With an average time of only25 ns between consecutive beam particles, pileupwill occur in readout systems where signals areshaped with a time constant longer than 25 ns:Therefore, a good time resolution is needed to findthe hits which are correlated to the trigger signaland reject noise or hits from particles out of time.

A ‘‘side effect’’ of the intense hadron beam isradiation damage, which imposes serious problemsto the design of a silicon detector: in the centralbeam area a fluence equivalent to the damage of1013 neutrons with 1 MeV energy per cm2 and yearwill be reached. To improve the radiation hardnessand allow for several years of running, the LazarusEffect (Section 2) will be exploited. Thus, thesilicon detectors have to be operated at atemperature of 130 K:

The material budget of the detector and itsmounting, however, has to be kept as low aspossible to reduce multiple scattering and second-ary interactions. The cooling mechanism thereforehas to be light. The low temperature parts have tobe thermally well insulated to avoid icing on theirsurfaces. The shielding should have a very smallmass.

The readout electronics has to be able to digestdata at high speed: high multiplicities and pileuplead to a high occupancy of the detectors.

3.2. The silicon detector

The silicon wafer was originally designed andproduced for HERA-B. It provides a readout pitchof about 50 mm with intermediate strips on thep-side to enhance charge sharing for more precisetracking. With this a space resolution of betterthan 16 mm is expected. The active area is 5 �7 cm2 and fits well with the beam size in Compass

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Temperature in K80 100 120 140 160 180 200 220 240

CC

E in

%

0

20

40

60

80

100 , 100V141*10 , 250V141*10 , 250V145*10 , 250V151*10

Fig. 2. CCE dependence on the temperature (measurements

from [4]): the different curves show the CCE for different

radiation damages and different bias voltages.

7 Important for radiation damage is the Non Ionizing Energy

Loss (NIEL). To compare the damaging power of different

radiation types, they are related to the damaging power of

1 MeV neutron.

H. Angerer et al. / Nuclear Instruments and Methods in Physics Research A 512 (2003) 229–238 231

(less than 0.2% of the beam outside, see Fig. 11).The wafer is 300 mm thick and can be read outboth from the p- and the n-side with orthogonalprojections. The design was optimized to standhigh radiation: the shape and material (polysili-con) of the bias resistors and the isolation on then-side with pþ-stops was chosen due to its lowsensitivity to radiation damage. A multiple guardring structure in combination with high resistivitysilicon allows very high bias voltages up to 600 V:For more details see [8].

The readout chip APV258 [9] was developed forthe CMS experiment at the Cern/LHC in 0:25 mmCMOS technology and thoroughly tested for itsradiation hardness. It has 128 analogue inputs onwhich the signals from the silicon are sampled witha frequency of about 40 MHz and sequentiallystored in a 192 cells deep buffer. On the arrival ofan external trigger, the corresponding bufferregion is serialized and transferred to the outsideon a single differential line. In the readout schemeused the chip sends out the signals from threeconsecutive buffer cells per channel. In this mode itallows a maximum of 10 triggers in 110 ms;corresponding to B90 kHz: As explained inSection 5.3, the information from the three buffercells can be used to extract timing informationfrom the signal.

The readout chips are very sensitive to noise onthe power- and grounding lines and start to

oscillate when the noise surpasses a certain level.An additional difficulty is introduced with thedouble-sided operation of the silicon: as thereadouts of the two projections are coupled toeach other via the silicon wafer, any noise from thepower lines of the readout chips of the oneprojection is passed through the silicon into theanalogue input of the chips of the other projection.

The power lines were therefore equipped withcapacitors and inductances at several positions forstabilization and frequency filtering. The valuesranged from a few mF right underneath thereadout chips to 200 mF mounted in parallel tothe silicon. With this, a stable operation at lownoise could be achieved.

The remaining baseline fluctuations are cor-rected actively on the ADC card (Common Mode

Noise Correction), which reduces the strip noise by38% from 1350 to 840 electrons Equivalent NoiseCharge (ENC).

The readout chips are mounted on two L-shaped 6-layer PCBs. As can be seen in Fig. 3, thewafer itself is sandwiched between these twoboards to accommodate the readout from the p-and n-side, respectively. The silicon is glued to thePCB with a single component silicone glue [10],whose elasticity is sufficient to absorb thermallyinduced mechanical stress.

The PCB not only provides the electricalconnections between the readout chips and thesilicon wafer, but also the cooling facilities neededfor cryogenic operation: thin Cu/Ni capillaries(diameter douter ¼ 1:6 mm; dinner ¼ 1:3 mmÞ aresoldered onto the back side of the PCB and canbe flushed with liquid nitrogen.

Through the copper layers of the PCB andadditional metalized holes in the PCB the heatflows from the silicon to the cold capillary. Inaddition the copper layers are arranged such thatthe (heat producing) readout chips are thermallydecoupled from the silicon and the capillary. Likethis, the heat flow from the readout chips to thesilicon is significantly reduced.

3.3. The readout chain

As explained in Sections 1 and 3.1, the mainchallenge for the readout is a high data throughput.

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cooling

capillary

readout chips

(backside)

(frontside)

readout chips

silicon wafer

Fig. 3. One Compass silicon module mounted inside its

cryostat.

8For the 2002 run the version APV25S0 was used, for further

detectors the successor APV25S1 will be taken.


It is desirable, however, to save as much informa-tion as possible from the passing particle toimprove space and time information.

To reduce the huge amount of data9 to thephysical relevant part already at the earliest stage,a special ADC card was developed, which digitizesthe analogue APV stream with 10 bit precision andat the same time performs pedestals subtraction,common mode noise correction and zero suppres-sion. Thus both amplitude and cluster sizeinformation can be kept, while the event size isreduced to 5%.10

A more detailed description of the implementa-tion of the ADC using reconfigurable logic chips(FPGA—Field Programmable Gate Array) andthe whole data flow from the APV25 through theADC to the data acquisition computer is given inRef. [11].

3.4. Mounting and cryogenics

A detector which is operated at a temperature of130 K has to be well insulated to prevent icing onthe surface of the structure. At the same time thematerial budget has to be kept low. Both require-ments can be met using a vacuum vessel with a lowmass density, which still is, however, a technicallychallenging project.

To get experience with the necessary techniquesfor vacuum stability and cryogenic operation, weconcentrated for the 2002 run on the siliconstations placed upstream of the target where theproblem of material density is limited to the beamarea. For this purpose a cryostat from stainlesssteel was build. It has two 80 mm big windows forthe beam made of 30 mm aluminized Mylar foilwhich are supported by Kevlar wires to givemechanical stability against the pressure.

The cryostat is the basis for a controlledcharacterization of the detectors: it provides an

environment for vacuum and cryogenic operationand enables implementation of electromagneticshielding.

With a size of 240 � 240 � 82 mm3 it providesenough space for the silicon detector plus safefeedthroughs for liquid nitrogen and electricalconnections as well as connectors for vacuumpump and gauge.

In the laboratory, liquid nitrogen is providedfrom a self-pressurizing dewar to ensure a constantflow during the operation. The flux of liquidnitrogen was monitored and controlled with flowmeters at the outlet of the cooling pipes measuringthe by then gasified nitrogen. The cryostat wasoperated successfully down to a pressure of10�6 mbar and a temperature of 77 K:

4. Tests with cooled detectors

4.1. Readout chips at low temperature

To minimize signal loss and input capacitancethe readout chips are placed as close as possible tothe silicon microstrips. For this reason operatingthe detector at cryogenic temperatures also impliesthat the readout chips have to be able to work atthis temperature. Originally the APV25 was notdesigned explicitly for these conditions. Therefore,a first characterization of the APV25 properties inthe temperature range from 77 to 150 K was done(cf. Ref. [12]).

Injecting a test pulse resembling the signal of aMinimum Ionizing Particle (MIP), the responsefunction of the APV25 was measured. Fig. 4 showsthe signals at 130 K and at room temperature. Onecan see that the signal rises 15% faster at 130 Kcompared to room temperature.

Another important requirement for the readoutchip is stability under temperature variation. Forthis purpose, noise, pulse shape and output dataformat were checked before and after 50 thermalcycles between room temperature and 77 K; noapparent degradation occurred.

Further studies on the behavior of the completesystem of APV25 and silicon detector at cryogenictemperatures are planned.

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9Three consecutive buffer cells, each 10 bit digitized, for 1000

strips gives 4 kBytes per projection per trigger: in the order of

100 MBytes per spill.10The reduction depends on the occupancy of the detector:

without beam, all data apart from noise and some header

information is suppressed; the more real hits, the less can be

suppressed.


4.2. Simulations of temperature distributions

In order to exploit the Lazarus effect, thetemperature of the silicon has to be well con-trolled. The temperature distribution of thecomplete set-up was simulated numerically undervarious conditions. The temperature dependenceof the thermal conductivity of the silicon crystalwas taken into account. The main heat source arethe APV25 chips, which dissipate 0:4 W each,giving a total power of about 7 W dissipateduniformly along the two sides of the detectorwhere the chips are mounted. Since the silicon isplaced in a vacuum cryostat, the heat exchangewith the environment at room temperature islimited to thermal radiation. A first check on a testpiece cooled down to 130 K has shown anagreement of simulations and measured datawithin 2 K:

Owing to the high thermal conductivity ofsilicon at room temperature ðB1 W=cm KÞ whicheven increases by a factor of about 10 at 100 K [13]it may be possible to cool the detector on twoedges only instead of all four, nevertheless reach-ing the operating temperature. This scenario wastested in the simulation assuming that two sides ofthe detector were uniformly at 130 K: the hottestpoint is, as expected, the one opposite of thecooling pipe and reaches a temperature of 137 K:Therefore, two capillaries running along the twosides of the wafer where the chips—the main heat

source—are placed are sufficient to cool thedetector.

4.3. Cooling facilities

The cooling facilities consist of capillaries whichrun parallel to the wafer edges and are solderedonto dedicated pads, as described in Section 3.2.

To optimize the heat transport, the pads on thePCBs reading the p- and n-side of the detector areelectrically connected to localized copper layers inthe PCBs. Since these layers may be connected tothe bias voltage of the silicon wafer, they must beelectrically insulated. In order to use one singlecooling pipe per detector, a special connector witha feedthrough fitting the external diameter of thecapillaries was made to measure. It is cast in a non-conductive epoxy material and is glued to thecapillaries with the same type of epoxy glue. Totest its reliability, the connector was operated at apressure of 10�6 mbar and flushed with liquidnitrogen equivalent to approx. 600 l=h of gaseousnitrogen, which we estimated to be about 1.5 timesthe flux needed to cool down the silicon to 130 K:No pressure variation was observed. After approx.10 cycles the connector was leak tested with heliumand no leaks were found.

4.4. Temperature measurements

The cryostat was evacuated to a pressure of1:5 � 10�4 mbar and the chips were operated in away that their power consumption was in total2:6 W homogeneously distributed along both sidesof the detector. To measure the temperature,PT100 thermometers were placed on three cornersof the wafer, two of them close to the capillariesand the third one on the opposite corner. Thethermometers were fixed on the silicon withconductive paste and read out using a Keithleymultimeter. With a flux of liquid nitrogen equiva-lent to 100 l=h of gas, the measured temperaturewas 127 K at the position of the capillaries and133 K on the corner ðDT ¼ 6 KÞ: Measurementsof the temperature distribution at different powerdissipations and nitrogen fluxes are continuing aswell as further checks of the simulation.

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time in ns0 100 200 300 400 500 600 700 800

AD

C c

ou

nts

0

20

40

60

80

100 130 KRT

Fig. 4. Signal shape of the APV25 at room temperature (solid)

and 130 K (dashed).


5. Results from the Run 2002

Two silicon stations, i.e. 8 projections in total,have been installed in the Compass beam and wereoperated in standard conditions. After a shorttuning period the detectors were running stably upto the end of the run. In total they were operated2100 h and observed 1014 muons. They collected adose of 1 kGy: Up to now no obvious degradationof the detection efficiency was found.

As the cooling with liquid nitrogen was not yetavailable, the power dissipated in the readoutchips had to be removed in an alternative way. Forthis the cooling tubes already present for liquidnitrogen were flushed with gaseous nitrogen,which kept the temperatures on the board belowþ45�C:

For the muon program the trigger rate is smallerthan for the hadron program. In the 2002 run atrigger rate of 4:5 kHz; resulting in 22 000 triggersper spill, was used. After zero suppression theremaining data rate of 4:5 MByte per spill andprojection could be handled by the readout chainand data acquisition.

As a first approach the readout chips wererunning with reduced speed: the analogue inputswere already sampled with the design value of40 MHz; but the data were multiplexed anddigitized with only 20 MHz:

Test runs showed that the resulting limit of a45 kHz trigger rate could be reached withoutproblems, assuming that reasonable event sizesare kept. The transition to 40 MHz multiplexingwill be done this winter, thus enabling a triggerrate up to 90 kHz: As the hardware was designedfor 40 MHz operation, no changes are neededthere, what remains is a careful tuning of thesynchronization between readout chip and ADCand a thorough check of the performance.

First estimates of the efficiencies of the detectorsgive values higher than 98%. In the followingsections some plots are presented which give apreliminary glimpse on the performance.

5.1. Signal amplitude

Fig. 5 shows the measured pulse height distribu-tion of the beam muons. For these measurements

the zero suppression was set to a threshold of 4s;i.e. the amplitude on every strip was tested againstits individual noise level and rejected when it wassmaller than 4 times this level.

The charge is summed up over the whole cluster.The cluster size was measured to be 1.5 strips,which is due to the relatively high cut from thezero suppression a number on the lower side.

The tail of the noise is visible at the left side, aswell as the cut due to the sparsification. The signalpeak is well separated from the noise.

The most probable value is at 15 000 electron/hole pairs. The average noise level is 840 electronsENC per strip. The ratio of cluster charge to noisein a single strip is 17.9.

5.2. Occupancy of the detectors

The data acquisition system has to be able todigest the data from detectors placed right in thecenter of a high intensity beam. The zero suppres-sion, which is necessary to reduce the amount ofdata to the physical interesting part was alreadyexplained in Section 3.3. Here, results from theCompass beam are shown. In Fig. 6 the number ofstrips above a threshold of 4 times the noiselevel ð4sÞ of the individual strip is plotted. Thepeak value is 19 out of 965 strips, whichcorresponds to an average occupancy of 2.0%and to a suppression of data by a factor 20, takinginto account headers. These values are well withinthe expectation and can be explained with the highbeam rate causing pile up in the detector readout.

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amplitude in 1000 electrons

0 10 20 30 40 50 60 70 80

nu

mb

er o

f en

trie

s

0

500

1000

1500

2000

2500

3000

3500

Fig. 5. Charge collected for muons with 160 GeV:


From the beam intensity one estimates one beammuon every 25 ns: As can be seen in Fig. 8, thesignal shaped by the readout chips has a time overthreshold in the order of 200–300 ns: One expectstherefore 8–12 particles traversing the detector inthe same readout gate, which are not correlated tothe trigger signal. With a cluster size of 1.5 stripsper particle, this results in 12–18 strips abovethreshold. The Gaussian noise passing the sparsi-fication for this threshold is negligible (0.1 strips).From Fig. 5 one can estimate the non-Gaussiandistribution with 0.4 strips to be of the same orderof magnitude. Fig. 7 shows the number of hits foreach strip separately, normalized to the totalnumber of events. The maximum is at B7%.The shape reflects the spatial profile of the beam.The vertical width of the beam extracted from thisplot is 7:8 mm Gaussian sigma.

5.3. Time resolution

To disentangle different events from each otherand to correlate hits from one detector with hitsfrom others, information on the timing of each hitis needed. As the readout chip initially wasdesigned for a collider experiment, where thebeam structure already gives a quite good timinginformation, Compass as a fixed target experimenthas to cope with a long spill of 5 s where theparticles are randomly distributed.

In Section 5.2 it was shown in Fig. 6 that due tothe high beam intensity and pile up several hits arepresent in one readout gate. In order to reduce pile

up already with the silicon data alone, specialefforts were made to improve the readout schemeand exploit the fast rise time of the signalsproduced in a silicon detector.

As was already mentioned, the readout chipprovides information from three consecutive buf-fer cells, which are separated by 25 ns each. In thisway, the shape of the signal is coarsely sampled(Fig. 8).

From the relative size of the three entries in theconsecutive buffer cells one can deduce the start ofthis signal within the 25 ns gate. Adding therelative phase between the sampling clock andthe asynchronous trigger signal, which has to bemeasured externally, one gets a very good estimateon the absolute time of the passage of the particle.

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strips above threshold0 20 40 60 80 100 120 140 160

# o

f en

trie

s

0

200

400

600

800

1000

Fig. 6. Number of strips above threshold.

strip number0 200 400 600 800 1000

hit

s p

er e

ven

t

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Fig. 7. Number of hits in each strip, normalized on the number

of events.

time in ns

-50 0 50 100 150 200 250 300 350 400

amp

litu

de

in a

rbit

rary

un

its

0

10

20

30

40

50

60

70

80

90

Fig. 8. Example for three consecutive sample points on the

shaped signal.


The optimum time resolution is obtained with thefirst two samples on the rising edge and the thirdsample at the peak of the signal. In Fig. 9 thedifference between the trigger time and the timecalculated from the three silicon samples is plotted.A clear correlation can be seen, which proves thatthe method works. The width of this peak is of theorder of 2–4 ns: In the current status of theanalysis the extracted values differ for differentprojections, thus a final number is not yetavailable.

With a cut on this timing, the pile up in thedetector is significantly suppressed. Combinedoffline with data from Scintillating Fibers, whichhave a very good time resolution with a stillreasonable space resolution ðB430 ps; 120 mm[14]), it should be possible to sort out all relevanttracks.

5.4. Correlation between p- and n-side

The p- and the n-side of a silicon detector see thesame charge cloud but with a different polarity. Itis possible to use this information to correlate theclusters from the two projections to improvetracking efficiency in regions with high multi-plicity.

In Fig. 10 the correlation of the signal ampli-tudes is shown. The two signals were joined bymeans of their timing behavior as explained above.The amplitudes on both sides are very similar, theratio is close to 1 (compare with the bisecting line).

Once the correlation is known, one can make ahit profile which gives a two-dimensional view ofthe beam (Fig. 11).

6. Conclusions

During the 2002 Compass run two stations with8 projections of double-sided silicon were operated

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time in ns

-80 -60 -40 -20 0 20 40 60 80

# o

f en

trie

s

0

2000

4000

6000

8000

10000

12000 Chi2 / ndf = 61.61 / 35 21.14 ±p0 = 1894 0.5531 ±p1 = -9.231 0.08307 ±p2 = -1.032 52.73 ±amp = 9933

0.01525 ±mean = -0.4364 0.01686 ±sigma = 3.134

Chi2 / ndf = 61.61 / 35 21.14 ±p0 = 1894 0.5531 ±p1 = -9.231 0.08307 ±p2 = -1.032 52.73 ±amp = 9933

0.01525 ±mean = -0.4364 0.01686 ±sigma = 3.134

Fig. 9. Difference of trigger time and time extracted from

silicon data.0

200

400

600

800

1000

1200

1400

cluster signal p-side in 1000 electrons0 5 10 15 20 25 30

clu

ster

sig

nal

n-s

ide

in 1

000

elec

tro

ns

0

5

10

15

20

25

30

Fig. 10. Correlation of hits in p- and n-side. The structure on

the right is from real particles, the structure on the left is noise

(see also Fig. 5).

0

10

20

30

40

50

60

70

80

x [cm]-4 -3 -2 -1 0 1 2 3 4

y [c

m]

-4

-3

-2

-1

0

1

2

3

4 RMS x: 1.10 cmRMS y: 0.79 cm

Fig. 11. Hits correlated between p- and n-side. The box

indicates the active area of the detector.


in a high intensity beam. The data are notyet completely analyzed, but preliminary resultsalready show that the requirements were met:the resolution of the detector in space (betterthan 16 mm) and time (of the order of 3 ns) issufficient to provide good tracking information.The electronics is able to cope with the high beamintensity via sparsification and buffering. Thewhole system proved its stability during therun.

The goal for the 2003 run is clearly to operatethe detectors at 130 K for improved radiationhardness. Tests in the laboratory were promising:the cryostat was successfully tested at its nominalconditions ðpB10�6 mbar; TB130 K). It wasconfirmed that the electronics readout still worksat cryogenic temperatures. The cooling schemewas shown to provide powerful and homogeneouscooling.

Acknowledgements

We are indebted to the HERA-B group at MaxPlanck Institute for Physics in Munich, in parti-cular I. Abt and S. Masciocchi, for the big helpgiven to us by supplying silicon detectors andgiving advice for handling them.

We owe many thanks to the bonding labs ofCern and Freiburg for their continuous help,often on short notice from our side.

We are grateful to U. Trinks and I. Altarev fortheir help with the cryogenic setup.

References

[1] The Compass Collaboration, Compass proposal, Cern/

SPSLC 96-14, SPSC/P297 (Geneva, March 1996); CERN/

SPSLC 96-30, Geneva, May 1996. and the web page:

http://wwwcompass.cern.ch/.

[2] A. Vasilescu, The NIEL scaling hypothesis applied to

neutron spectra of irradiation facilities and the ATLAS

and CMS, SCT—ROSE/TN/97-2.

[3] The ROSE collaboration, RD48 Status Report, CERN/

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ARTICLE IN PRESS


http://wwwcompass.cern.ch/

http://rd48.web.cern.ch/RD48/

http://rd48.web.cern.ch/RD48/

http://www.cern.rd39/

http://totem.web.cern.ch/Totem/

http://na50.web.cern.ch/NA50/

http://www.te.rl.ac.uk/med/

Present status of silicon detectors in C ompass

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