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Nuclear Instruments and Methods in Physics Research A 565 (2006) 18–22

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Test of prototypes of the ALICE silicon pixel detectorin a multi-track environment

A. Pulvirentic,�, G. Anellia, F. Antinorid, A. Badalac, G.E. Brunob, M. Burnsa, I.A. Calia,b,M. Campbella, M. Caselleb, S. Ceresaa, P. Choculaa, M. Cinauserog, J. Conrada, R. Dimad,D. Eliab, D. Fabrisd, R.A. Finib, E. Fiorettog, S. Kapustaa, A. Klugea, M. Krivdag, V. Lentib,

F. Librizzic, M. Lunardond, V. Manzarib, M. Morela, S. Morettod, F. Osmica,G.S. Pappalardoc, V. Paticchiob, A. Pepatod, G. Preteg, P. Riedlera, F. Riggic, L. Sandorg,R. Santorob, F. Scarlassarad, G. Segatod, F. Soramelf, G. Stefaninia, C. Torcato de Matosa,

R. Turrisid, L. Vannuccig, G. Viestid, T. Virgilie

aCERN, Geneva, SwitzerlandbDipartimento di Fisica dell’Universita and INFN, Bari, Italy

cDipartimento di Fisica ed Astronomia dell’Universita and INFN, Catania, ItalydDipartimento di Fisica dell’Universita and INFN, Padova, ItalyeDipartimento di Fisica dell’Universita and INFN, Salerno, ItalyfDipartimento di Fisica dell’Universita and INFN, Udine, Italy

gLaboratori Nazionali di Legnaro, Legnaro, Italy

Available online 12 June 2006

Abstract

The silicon pixel detector (SPD) comprises the two innermost layers of the ALICE Inner Tracking System (ITS). It is instrumented

with arrays of hybrid pixels made out of 150mm thick ASICs, each containing 8192 readout cells, bump bonded to 200 mm thick silicon

sensors. The dimensions of the pixel cells are 50mm (rj)� 425 mm (z). Prototype assemblies have been tested in high-energy particle

beams at the CERN SPS. The results of measurements in a multi-track environment, from interactions of an In beam at 158AGeV on a

Pb target, are reported.

r 2006 Elsevier B.V. All rights reserved.

PACS: 29.40.Gx; 29.40.Wk

Keywords: ALICE; LHC; Silicon pixel detector; Efficiency

1. Introduction

ALICE [1] is one of the approved experiments at theLarge Hadron Collider (LHC). It has been designed tostudy both proton–proton and heavy ion collisions atultrarelativistic energy (Pb+Pb at 5.5ATeV), with theprimary aim of investigating quark-gluon plasma (QGP)formation. ALICE is composed of many detectors, in order

e front matter r 2006 Elsevier B.V. All rights reserved.

ma.2006.04.094

ing author. Tel.: +39095 3785286; fax: +39 095 3785231.

ess: alberto.pulvirenti@ct.infn.it (A. Pulvirenti).

to be able to reconstruct and identify particles in a widemomentum range.The innermost ALICE detector is the Inner Tracking

System (ITS). It consists of three different technologies ofsilicon detectors: silicon pixel detectors (SPD), silicon driftdetectors (SDD) and silicon strip detectors (SSD). TheSPD is essential in order to obtain precise trackinginformation close to the interaction point. It offers thevery high granularity required to handle the large numberof tracks from nucleus–nucleus interactions at the LHC,where the multiplicity is expected to reach up to 8000charged particles per unit of rapidity.

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2 Tracking planes(#3, #4)

1 Plane of study(#2)

2 Tracking planes(#0, #1)

Pb target

Beam

QuartzScintillator MWPC

Table

Fig. 1. Schematics of the 2003 beam test setup.

A. Pulvirenti et al. / Nuclear Instruments and Methods in Physics Research A 565 (2006) 18–22 19

The SPD allows the precise determination of the primaryvertex location. It will also allow one to identify secondaryvertices of charmed and beauty hadrons, which are of greatinterest in the study of de-confined matter. The SPD mainfeatures are reviewed in Section 2.

In 2002 through 2004, prototype SPD single assemblieshave been tested in high-energy particle beams at theCERN SPS in order to evaluate space resolution anddetection efficiency. In 2003, measurements were taken in amulti-track environment generated by a 158AGeV Indiumbeam hitting a lead target. The main objective of the testwas to evaluate the detector performance at multiplicitieshigher than those achievable in a proton beam. Theexperimental set up is described in Section 3.

Data analysis, the reconstruction algorithm and resultsare reported in the following sections.

2. The ALICE SPD

The SPD [2] consists of two instrumented barrel layers atradii of 3.9 and 7.6 cm, respectively. The inner layer coversa pseudo-rapidity range jZjp1:9, while the outer one coversjZjp0:9. The overall silicon area is 0.24m2.

The pixel readout ASIC has been developed in acommercial 0.25 mm CMOS process radiation hardenedby design. It contains a matrix of 32� 256 readout cells of50 mm (rj)� 425 mm (z) each. The chip dimensions are13.5� 15.8mm. The chip wafers are thinned to 150 mmafter bump deposition. The readout is binary.

The basic detector component is a ladder, consisting offive ASICs bump bonded to a 200 mm sensor. The basicdetector module is a half-stave, consisting of two laddersattached and electrically connected to an aluminum/polyimide multi-layer laminate (pixel bus) that carriespower and signals. Each half-stave is equipped with amulti-chip module (MCM) for control and readoutmanagement.

The SPD is organized in staves, each one composed oftwo half-staves along the z direction. The inner and outerSPD layers contain, respectively, 20 and 40 staves,mounted on carbon fiber supports with embedded coolingcircuits.

The total number of pixel cells is 9.8� 106.

3. The 2003 beam test setup

The test took place in the H4 beam line at the CERNSPS. The In ion beam was directed to a Pb target, 4mmthick, in the experimental area. The detectors were locatedoff the beam axis to detect interaction products, as shownin Fig. 1.

The trigger was generated by a quartz counter, placed�25 cm upstream, in coincidence with a 2� 2 cm scintilla-tion counter which detected interactions in the target.

The experimental set up [3] consisted of five planes with asingle assembly each. The planes were aligned with the longpixel cell side along the x-axis and the short one along the

y-axis. The z-axis is along the beam line. Planes #0 and #1and planes #3 and #4 were linked together and were usedto reconstruct the straight line track of the incidentparticles. Plane #2 (the central one) contained the assemblyunder test. The working point parameters of the pixel chipwere remotely adjusted by means of its internal program-mable 8-bit DACs. In particular, measurements were takenat different settings of the threshold DAC (Pre_VTHDAC). The threshold decreases with increasing DACsetting.

4. Tracking

4.1. Algorithm

The data collected are treated as a set of clusterscorresponding to groups of pixels which have fired whenhit by particles [4]. Each cluster defines a space pointcorresponding to its center. The uncertainty in thecoordinates of each point (sx and sy) is related to thepixel cell size.Each triggered event contains, in general, signals from

many particles. Track recognition is required to associateclusters into groups of 5 (1 per plane) that shouldcorrespond to the traversal of the planes by the sameparticle. In the absence of magnetic field, the expectedparticle trajectory is a straight line:

xðzÞ ¼ Ax þ Bxz

yðzÞ ¼ Ay þ Byz:(1)

If we select n points from the event data (one per plane)and calculate the residuals to a straight line fit, the qualityof the fit is measured by the parameter Q in the expression:

Q ¼Xn

i¼1

ðxi � xðziÞÞ2þ ðyi � yðziÞÞ

2

s2xiþ s2yi

. (2)

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A small value of Q corresponds to a good approxima-tion.

The tracking procedure is then performed in severalsteps:

1.

Three points are selected in planes #0, #1 and #4, inorder to use the most distant planes, plus a third planewhich is necessary to make a ‘‘preliminary’’ straight linefit.

2.

The projection of the fitted line is found in plane #3, anda point is selected in this plane, if it is close to theprojected point; this final 4-point candidate is fitted to astraight line and it is stored in a list sorted with respectto the Q parameter.

3.

When two candidates share at least one cluster, their Q

values are compared, and the track with the largest oneis rejected; at the end of this step, one obtains a list offour-point tracks;

4.

Fig. 2. Distribution of (x, y) track projection to the z nominal position of

the target. The circle represents the cut described in the text

The same procedure is applied in plane #2 (plane ofstudy): the projection is first calculated, and then a pointis selected if it lies close enough to this projection; if twodifferent four-point tracks are in competition for thesame cluster, it will be associated only to the closest one.At the end of this step, another list is obtained, with allfive-points tracks.

In order to evaluate the space resolution of plane #2, thefifth point is not used for a further refit of the track.

4.2. Cuts

To improve the performance of tracking, two cuts havebeen applied.

The first cut was determined by a distribution of the (x,y)projection of tracks at the z nominal position of the target.Such a distribution is shown in Fig. 2.

Looking at the distribution in Fig. 2, one can see a widearea with few or zero counts (light grey), and a more denseregion, where the distribution rapidly increases. Then, wehave chosen to reject all tracks whose projection to thetarget falls outside such dense region (dashed circlein Fig. 2).

Another cut was applied on the final Q parameter ofeach four-points track. We have chosen to reject all trackswhich had Q410. We did not notice any significantdifference in the obtained results when tightening this cut.

4.3. Alignment

An alignment procedure has been followed in order toadjust the spatial position of all five planes used fortracking.

This has been made by means of a study on the residualsdistributions, which allows to determine the small correc-tions on the plane positions and orientations in space. Theprocedure of tracking and residual comparison wasrepeated iteratively until it converged to a stable result.

5. Results

5.1. Cluster sizes

One important difference that arises when changing chipthreshold is in the typical cluster distribution. As Fig. 3shows, for higher thresholds (smaller value of Pre_VTHDAC), most of the clusters are made of single fired pixels,while at lower thresholds, 2-pixel clusters which share onelong edge start to dominate.This result is similar to the one obtained in the analysis

of cluster sizes performed with a proton beam in the samerun period [5].

5.2. Space resolution

The uncertainty in the x coordinate is affected by thelarge size of the pixel. This quantity has been preciselydetermined from measurements with a proton beam [5]. Inthis case the y coordinate only was studied in detail.The space resolution is calculated from the residual

distributions in plane #2, where we compare the position ofthe projection of the fitted four-point tracks with theposition of their fifth point found in plane #2.Fig. 4 shows an example of this distribution. As expected

after the alignment procedure, it is centered around zero,and it has a symmetric shape. We found that the obtaineddistribution can be well fitted by a Lorentzian function (w2/NDFo2). Then, we assumed as global resolution the full-width at half-maximum (FWHM) parameter of theLorentzian fit, which turns out to be �25 mm.This global resolution is not the true intrinsic resolution

of the device, because it contains a contribution due to thetracking error. Moreover, the tracking error is strongly

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Fig. 3. Cluster size distribution, as a function of the size in columns (x) and rows (y). A larger value of Pre_VTH corresponds to a smaller threshold.

Fig. 4. Residual distribution in plane #2. The continuous curve shows the

Lorentzian fit to the histogram.

Fig. 5. Efficiency vs. half-width D of the acceptance window.

A. Pulvirenti et al. / Nuclear Instruments and Methods in Physics Research A 565 (2006) 18–22 21

affected by multiple scattering (MS), whose effect dependson the particle momentum, which is not known in this case.This prevented us from disentangling all different con-tributions, but we can estimate the total tracking errorfrom both best-fit and MS.

As shown in the study with a proton beam [5] theintrinsic y resolution is estimated at �10 mm. Then, we canestimate the tracking error by simply subtracting theintrinsic resolution contribution from the total errorresulting from the fit:

s2tracking ¼ s2total � s2intrinsic. (3)

This gives a tracking error of �23 mm.

5.3. Detection efficiency

The efficiency is calculated as follows. We obtain first alist of four-points tracks, which are the groups of points inthe four tracking planes that are most likely to begenerated by the same particle. When a correspondingpoint in plane #2 is not found, it is assumed that it is due toan inefficiency of that plane.An estimate of the intrinsic efficiency is then the ratio of

five-points tracks over four-points tracks numbers:

eff ¼# of 5�points tracks

# of 4�points tracks. (4)

When doing this calculation, one must be careful aboutedge effects. If we look at the displacement of planes, weunderstand that it can happen, in fact, that a four-pointtrack does not cross plane #2 because of the relative spatialposition of all tracking planes. This effect is purelygeometrical, and has no relation to the intrinsic efficiencyof the device. Then, these tracks must be removed from thedenominator in Eq. (4).In order to do this a check for edge effects has been

introduced. Tracks are required to stay within anacceptance window, centered on the chip, with variable

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half-size D. The efficiency is calculated as a function of D,starting with D equal to the half-size of the chip, and thenreducing it progressively. The result is shown in Fig. 5.

A plateau is reached for Do5.8mm. The intrinsicefficiency at the plateau is 499%.

6. Conclusions

The spatial precision and detection efficiency of proto-type single assemblies of the ALICE SPD have beenstudied in a multi-track environment obtained frominteractions of an In beam at 158AGeV on a Pb target,and in a range of threshold settings.

An overall resolution of about 23 mm has been obtained;the intrinsic efficiency is higher than 99% in all cases.

References

[1] ALICE Collaboration, J. Phys. G 30 (2004) 1517.

[2] ALICE Collaboration, Technical Design Report of the Inner Tracking

System, CERN/LHCC/1999-12.

[3] The 2003 beam test apparatus was set up in the same way of 2002

beam test, which is described in the proceedings of the ‘‘PIXEL 2002’’

workshop: P. Riedler, et al., SLAC Electronic Conference Proceedings,

eConf C020909.

[4] Details on the clusterization software can be found in: J. Conrad, P.

Nilsson, ALICE-INT-2005-003.

[5] D. Elia, Proceeding in This Book.