PROCEEDINGS OF THE 31st ICRC, ŁODZ 2009 1

The AMS-02 Silicon Tracker: Status and Performance Results

Alberto Oliva ∗ on behalf of the AMS Tracker Collaboration∗†

∗INFN Sezione di Perugia and Universita degli Studi di Perugia, I-06100 Perugia, Italy†Universite de Geneve, CH-1211, Geneve 4, Switzerland;

Massachusetts Institute of Technology, Cambridge, MA 02139, USA;University Politechnica (UPB), R-76900 Bucharest, Romania;

Laboratoire de Physique Theorique et Astroparticules, Universite de Montpellier II, France.

Abstract. At the beginning of 2008, the AMS-02Silicon Tracker has been successfully integrated andinstalled within the AMS-02 detector. An extensive setof cosmic-rays muons data were acquired, allowingfor a thorough study of the Si Tracker performancesin terms of calibration stability, signal and noisemeasurements as well as ladder efficiency. In absenceof the magnetic field, the straight muons tracks havebeen used to study the alignment of the Trackerelements. The alignment procedure has been usedto verify the Tracker stability against mechanicalstresses: no displacement above 3 µm was observed.

The AMS-02 Silicon Tracker main characteristicsin terms of spatial resolution, charge measurementand alignment accuracy both from beam test andcosmic-rays data taking results will be reviewed inthis report.

Keywords: Astroparticle physics, Space-borne in-struments, Magnetic Spectrometers

I. I NTRODUCTION

The Alpha Magnetic Spectrometer (AMS) is an high-energy physics experiment designed to operate in space.The AMS mission is devoted to the direct measurementof the energetic spectrum of almost all the chargedcosmic ray species, from the most aboundant (p, He,e−) to the most rare ones (D, nuclei up toZ = 26, e+,p, D), with the unique possibility of the discovery oftraces of primordial nuclear antimatter (He).

A preliminary version of the detector (AMS-01) wasflown on NASA space shuttleDiscovery in June 1998[1]. The final detector (AMS-02) is based on the AMS-01 design with: a) an improved particle distinction due tothe presence of additional sub-detectors (as the Electro-magnetic CALorimeter, the Transition Radiation Detec-tor and the Ring Imaging CHerenkov); b) a wider rigid-ity range thanks to the 0.8 Tesla magnetic field generatedby the toroidal superconducting magnet of diameter of∼ 1 m. Over, under and around the magnet a scintillatorsystem (Time-Of-Flight and Anti-Coincidence Counter)provide the trigger of the experiment. In the magnetbore, eight planes of Silicon detectors act as trackingdevice.

According to the NASA Shuttle manifest, AMS-02will start to operate aboard the International SpaceStation in 2010. Thanks to the very large acceptance

(0.5 m2 sr) and a data taking period of at least 3 years,AMS-02 will measure over1010 charged particles withenergies ranging from fractions of GeV/n up to fewTeV/n [3].

II. T HE AMS-02 SILICON TRACKER

The measurement of the curvature radius of thecharged particles bent trajectories in the magnetic field -through the precise location of the particle impact pointson the Si Tracker planes - allows for the computation ofthe particle rigidity and charge sign.

The AMS-02 Silicon tracker contains 2264, 41.360× 72.045 mm2 double-sided silicon microstrip sensors.Each sensor is made of a 300 µm thick high-resistivityn-doped Silicon substrate. On the two surfaces p+ andn+ doped microstrips are implanted along orthogonaldirections with implantation (readout) pitches of 27.5(110) µm and 52 (208) µm, respectively. The p-side stripsare used for the precise measurement of the bendingcoordinatey while n-side strips are used for thexcoordinate determination.

A variable number of sensors (7 to 15) are bondedalong they direction (leftmost picture of fig. 1) andconnected with the low noise, high dynamic range front-end electronics constituting the minimum readout unitcalled ladder. The 192 ladders are arranged in eightlayers on five support planes to fill the circular plane.The two outermost silicon layers are separated by adistance of 1.06 m and located on the first and lastsupport planes at both ends of the spectrometer. Two Silayers are disposed on both sides of the three supportplanes located inside the magnetic volume. The totalthickness of one internal plane including two siliconlayers and one support plane is about 1% of the radiationlength.

The performace of the AMS-02 Tracker have beenevaluated with a wide campaign of beam tests from2002 up 2007 exposing prototype and flight-model lad-ders to protons and helia [2], ions [4] [7], electronsand positrons of different energies. The complete AMSSilicon Tracker has been tested with cosmic muons atground during the AMS pre-integration in the first halfof 2008 [6].

2 A. OLIVA et al. AMS-02 TRACKER STATUS

Fig. 1: The AMS-02 Tracker Integration: a) microstrip sensors are bonded together; b) the bonded sensors arecoupled with the front-end electronics; c) the 192 readout units (ladders) are arranged over layers of diameter∼

1 m; d) the 8 layers are arranged over 5 Al honeycomb planes; e)the Tracker is inserted inside the magnet.

III. T HE TRACKER INTEGRATION

At the end of 2007 the Tracker as well as the otherdetector subsystems were integrated onto the AMS-02prototype vacuum case of the magnet (rightmost pictureof fig. 1). In 2008, from April to June, the entireTracker electronics was connected with the AMS DataAcquisition (DAQ) system.

The Tracker electronics consists of 8 independentcrates. All crate electronics is managed by a JINF boardwhich communicate with the AMS DAQ and is able tocontrol the 24 Tracker Data Reduction (TDR) boardsresponsible of the calibration and data acquisition, the2 Tracker Power Supplies (TBS) which furnish the biasvoltage of the Silicon and the 6 Tracker Power Suppliesfor the Front-end Electronics (TPSFE).

In the pre-integration phase the Silicon Tracker wasalso connected to a Groud Support Tracker ThermalControl System (GCTTCS) a two phasesCO2 coolingdevice able to mantain the front-end electronics temper-ature below 30◦C.

Several tests were performed during the pre-integration phase: a) electronic functionality tests; b)DAQ software debugging; c) hold delay time scan; d)mechanical stress study (by rotating the whole spectrom-eter). The behaviour of 190 ladders has been tested interms of functionality and measurement performancesand has been found compatible with what expected fromthe beam tests.

IV. T RACKER DATA RECONSTRUCTION

The ADC values of the readout strips include apedestal, a common noise, a strip noise and an eventualsignal. For each strip, the mean (pedestal) and the root

mean square (strip noise) of the common noise sub-tracted ADC value in absence of signal are determined inthe calibration run. Channels that exhibit a very low/highnoise level are flagged as dead/noisy channels and ex-cluded from subsequent signal reconstruction. Duringthe full test period the tracker calibrations showed verystable results. An average noise level of 2.71 (3.31) ADCcounts and a percentage of dead+ noisy strips of 0.04+2.11 (0.26+ 3.36) % approximately independent fromtime and temperature have been achieved respectivelyfor p (n) side.

In the case of a physical trigger generated by Time-Of-Flight (TOF) scintillator planes an online data re-duction procedure is performed: pedestal values aresubtracted from the raw data, the common noise con-tribution (in group of 64 channels) is evaluated andsubtracted. A channel with a signal over noise ratio (SN)above a given thresholdSNseed = 4 is called seedand triggers the creation of a cluster. All the channelsaround the seed with aSN over a second thresholdSNneig = 1 are associated to the cluster. The clusterhas an associated amplitude given by the sum of thestrip signals, and an associated coordinate estimated withthe Center-of-Gravity ofn stripsCoG(n) (for inclinedZ = 1 particlesn ≤ 3).

The offline reconstruction pairs the clusters on the twoopposite sides of the same ladder inNp ×Nn tridimen-sional hits. An ensemble of hits of different planes arecombined in a track candidate. All the possible hits com-binations of different layers with a minimum number ofalmost 3 are examined and fitted with a tridimensionalstraight trajectory. The hit combination with the bestχ2 is chosen as the particle track. About 274 millionevents of cosmic-ray tracks were reconstructed during

PROCEEDINGS OF THE 31st ICRC, ŁODZ 2009 3

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Fig. 2: Spatial resolution evaluated in beam tests: a) spatial resolution function of projected incident angle (σx

function θxz andσy function θyz); b) spatial resolution function of perpendicular incident ion of chargeZ.

the overall test period.

V. TRACKER SPATIAL RESOLUTION

On November 2007, 4 AMS-02 ladders and 2 refer-ence high-resolution ladders (50 µm readout pitch) wereplaced on a 400 GeV/c momentum proton beam line onthe H8 area at SPS, CERN. Definingz as the axis alongthe beam, one of the AMS-02 ladder was inclined inthe xz, yz andxz/yz planes with angles ranging from0 to 50 degrees. The track has been reconstructed fromthe fit of the two high resolution ladders. The variableinclination ladder residuals, i.e. the difference betweenthe estimated coordined (CoG(2) or CoG(3)) and theone by the interpolation of the track, have been describedwith the sum of two gaussian functions. Both gaussiansare centered on the axis origin and the narrow gaussianaccounts of most of the events.

The spatial resolution has been calculated deconvolv-ing the residuals with a GEANT3 simulation followinga technique described in [2]. This procedure is ableto take into account the different resolutions betweenthe AMS-02 and the reference ladders. In left plot offigure 2 (left) is presented the preliminary result of thisanalysis. The effective spatial resolution is defined asthe weighted mean of the widths of the two gaussiansfitted to the residuals. The optimal spatial resolution iscalculated considering only the narrowerσ. A spatialresolution below 20 µm in the bending coordinate hasbeen obtained.

A detailed study of the AMS Tracker response toperpendicular incident completely ionized nuclei of∼

158 GeV/n kinetic energy and atomic numberZ ≤ 12has been performed using heavy ions beam test datacollected in October 2003 at CERN SPS facilities [4].Signal collection on both sides of six AMS Silicondetector flight modules have been analyzed as a functionof the particle charge and entrance position. A non-lineardependence in the signal collection on the p-side as afunction of the energy deposit in the Silicon has been

observed and a set of corrections has been implementedto take into account this effect.

The spatial resolution has been calculated using gaus-sian fits to the residuals distribution. In right plot offigure 2 is presented the spatial resolution for per-pendicular incident particles ranging from protons toMagnesium. The spatial resolution on the p-side reachesis approximately 5 µm for high charges.

The spatial resolution obtained from the 2008 cosmicmuon data [6] are resulted compatible with the inclinedtrack beam test (2007) and with theZ = 1 straightparticle beam test (2002, 2003) [2] [7].

Full exploitation of the ladder spatial resolution in thetrack reconstruction requires the knowledge of ladderpositions within the assembled tracker with a precisionof few µm, whereas 100µm is the expected accuracy inthe mechanical assembly of ladders in the tracker planes.An An iterative procedure was developed to performalignment calibration, i.e. to evaluate the actual ladderpositions in the tracker with respect to the nominaldesign.

VI. T RACKER ALIGNMENT WITH COSMIC MUONS

The fitting residual - the difference between themeasured and interpolated coordinate - of each ladderincludes the intrinsic resolution of the sensors, the mul-tiple scattering, the track topology and the mechanicalprecision of the assembly. The first two make the theresidual width wider, the third is a constant term whilethe fourth one makes the residual mean shifted. From thestructure of the residual mean the alignment parametersfor each ladder can be estimated.

The single ladder alignment procedure consist inthe calculation of the p and n residual over a set oftracks selected with a loose cut on theχ2 of the trackfitting to avoid large scattering events. The 3 translationparameters are obtained directly by the residual meanof the n-side (alongx) and of the p-side (alongy) andwith their combination with the track inclination (along

4 A. OLIVA et al. AMS-02 TRACKER STATUS

Iteration 0 5 10 15

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Fig. 3: Development of the RMS of the the residual meandistribution for the 190 ladders at each iteration in thealignment process.

z). The other 3 rotation parameters are estimated fromthe residual the track inclination and the hit position onthe ladder. The procedure is applied iteratively using ateach iteration the estimated parameters to improve theprediction from the track fit.

In figure 3 is presented the RMS of the the residualmean distribution at each iteration. It is clearly shownthat the deviation is converging to 1 µm level. At theend of the iteration procedure the translationx and yparameter distributions are centered to zero and have thewidth of about 100 µm as expected from the mechanicalprecision of the assembly.

Set of 106 tracks are determined as sufficient foran alignment precision of 1 µm. The robustness ofthe alignment method and the stability of its resultshave been tested over 5 independent data samples. Nosignificant deviation in the alignment parameters wasobserved.

During 10-14 June 2008, the mechanical stabilityof the tracker structure was checked by rotating theAMS spectrometer along the y-axis at different angles(-90, -60, -30, 30, 60 and 90). The level of mechanicaldistortions possibly induced by the gravity was evaluatedby the analysis of the data sets recorded at each step ofthe rotation. For each step we evaluated the alignmentparameters and compared them with the reference onesobtained with the data taken prior to the rotation. Wecould not find any significant shift in the alignmentparameters within the statistical accuracy.

VII. T RACKER CHARGE PERFORMANCES

The cluster amplitude is roughtly proportional to theenergy loss of the traversing particle. From the Bethe-Block formula a signal increase proportional toZ2 isexpected and roughly observed [4]. However lookingin more detail several peculiar behaviors appear as afunction of the particle charge and impact point.

The 2003 ion beam test data have been used to studythe signal collection on both sides of six AMS-02 Silicondetector flight modules as a function of the particle

cluster amplitude (ADC)0 500 1000 1500 2000 2500 3000 3500 4000 4500

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Fig. 4: The n-side cluster amplitude distribution forcharges up toZ = 12. Charge peaks have been fittedwith Landau convoluted with gaussian (with an addi-tional exponential tail) functions.

charge and entrance position. In the left plot of figure4 is presented the signal response of a single ladder fordifferent charges (and for a specific impact point region)fitted with Landau convoluted with gaussians.

A single plane charge identification algorithm basedon a maximum probability test has been implement.An average efficiency of 82 (94) % and contaminations< 5 (2) % have been achieved for the bending (not-bending) plane for charges up to Fluorine. The chargeidentification algorithm has been extended to multiplemeasurement using the a combined maximum likeli-hood method. An overall charge discrimination powerof 10−7(10−5) for the combination of 8(5) planes ofthe AMS-02 Silicon Tracker has been estimated using aMonte Carlo model for charges uf toZ = 10.

VIII. A CKNOWLEDGEMENTS

We wish to thank our collegues in AMS-02 and inparticular: X. Cai, M. Capell, A. Kounine, V. Koutsenko,A. Lebedev and M. Vergain of the Massachusetts In-stitute of Technology; B. Verlaat of NIKHEF; and C.Gargiulo of INFN Roma. This work was partly fundedby the Agenzia Spaziale Italiana (ASI contract 035/07,AMS-02 - Fase E2 ed analisi dati), the Istituto Nazionaledi FIsica Nucleare (INFN), the Fonds National Suissepour la Recherche Scientifique, the Deutches Zentrumfur Luft und Raumfahrt (DLR). A complete list ofacknowledgements can be found on .

REFERENCES

[1] M. Aguilar et al. , Phys. Rep. 366 (2002) 331–404.[2] J. Alcarazet al. , Nucl. Instrum. Meth. A 593 (2008) 376–398.[3] A. Oliva, Nucl. Instrum. Meth. A 588 (2008) 255–258.[4] B. Alpat et al. Nucl. Instrum. Meth. A 540 (2005) 121-130.[5] G. Ambrosi, Nucl. Instrum. Meth. A 435 (1999) 215–223.[6] B. Alpat et al. , The performance of the AMS-02 silicon tracker

with cosmic-ray muonsin preparation.[7] B. Alpat et al. , The AMS-02 Silicon Tracker: performance results

for 2 < Z ≤ 12 completely ionized nucleiin preparation.[8] P. Zuccon, this conference.