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Nuclear Instruments and Methods in Physics Research A 538 (2005) 384–407
www.elsevier.com/locate/nima

Beam tests of ATLAS SCT silicon strip detector modules
F. Campabadala, C. Fletaa, M. Keya, M. Lozanoa, C. Martineza, G. Pellegrinia,
J.M. Raﬁa, M. Ullana, L. Johansenb, B. Pommerescheb, B. Stugub, A. Ciocioc,
V. Fadeyevc, M. Gilchriesec, C. Haberc, J. Siegristc, H. Spielerc, C. Vuc, P.J. Belld,
D.G. Charltond, J.D. Dowelld, B.J. Gallopd, R.J. Homerd, P. Jovanovicd,
G. Mahoutd, T.J. McMahond, J.A. Wilsond, A.J. Barre, J.R. Cartere,
B.P. Fromante, M.J. Goodricke, J.C. Hille, C.G. Lestere, M.J. Palmere,
M.A. Parkere, D. Robinsone, A. Sabetfakhrie, R.J. Shawe, F. Anghinolﬁf,
E. Chesif, S. Chouridouf, R. Fortinf, J. Grosse-Knetterf, M. Gruwef, P. Ferrarif,
P. Jarronf, J. Kaplonf, A. Macphersonf, T. Niinikoskif, H. Perneggerf, S. Roef,
A. Rudgef, G. Ruggierof, R. Wallnyf, P. Weilhammerf, W. Bialasg,
W. Dabrowskig, P. Grybosg, S. Kopernyg, J. Blockih, P. Bruckmanh,
S. Gadomskih, J. Godlewskih, E. Gornickih, P. Maleckih, A. Moszczynskih,
E. Staneckah, M. Stodulskih, R. Szczygielh, M. Turalah, M. Wolterh, A. Ahmadi,
J. Benesi, C. Carpentierii, L. Feldi, C. Kettereri, J. Ludwigi, J. Meinhardti,
K. Rungei, B. Mikulecj, M. Mangin-Brinetj, M. D’Onofrioj, M. Donegaj,
S. Moêdj, A. Sfyrlaj, D. Ferrerej, A.G. Clarkj, E. Perrinj, M. Weberj, R.L. Batesk,
A. Cheplakovk, D.H. Saxonk, V. O’Sheak, K.M. Smithk, Y. Iwatal, T. Ohsugil,
T. Kohrikim, T. Kondom, S. Teradam, N. Ujiiem, Y. Ikegamim, Y. Unnom,
R. Takashiman, T. Brodbecko, A. Chilingarovo, G. Hugheso, P. Ratoffo, T. Sloano,
P.P. Allportp, G.-L. Cassep, A. Greenallp, J.N. Jacksonp, T.J. Jonesp, B.T. Kingp,
S.J. Maxﬁeldp, N.A. Smithp, P. Sutcliffep, J. Vossebeldp, G.A. Beckq,
A.A. Carterq, S.L. Lloydq, A.J. Martinq, J. Morrisq, J. Morinq, K. Nagaiq,
T.W. Pritchardq, B.E. Andersonr, J.M. Butterworthr, T.J. Fraserr, T.W. Jonesr,
J.B. Laner, M. Postraneckyr, M.R.M. Warrenr, V. Cindros, G. Krambergers,
I. Mandićs, M. Mikužs, I.P. Duerdotht, J. Freestonet, J.M. Fostert, M. Ibbotsont,F.K. Loebingert, J. Patert, S.W. Snowt, R.J. Thompsont, T.M. Atkinsonu,
Corresponding author. School of Physics, University of Melbourne, Victoria 3010, Australia.

E-mail address: gareth.moorhead@cern.ch (G.F. Moorhead).
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2004.08.133

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385

G. Brightu, S. Kaziu, S. Lindsayu, G.F. Moorheadu,, G.N. Tayloru,
G. Bachindgagyanv, N. Baranovav, D. Karmanovv, M. Merkinev, L. Andricekw,
S. Bethkew, J. Kudlatyw, G. Lutzw, H.-G. Moserw, R. Nisiusw, R. Richterw,
J. Schieckw, T. Cornelissenx, G.W. Gorﬁnex, F.G. Hartjesx, N.P. Hesseyx, P. deJongx, A.J.M. Muijsx, S.J.M. Peetersx, Y. Tomeday, R. Tanakay, I. Nakanoy,
O. Dorholtz, K.M. Danielsenz, T. Husez, H. Sandakerz, S. Stapnesz,
P. Bargassaaaa, A. Reicholdaa, T. Huffmanaa, R. Nickersonaa, A. Weidbergaa,
G. Doucasaa, B. Hawesaa, W. Lauaa, D. Howellaa, N. Kunduaa, R. Wastieaa,
J. Bohmbb, M. Mikestikovabb, J. Stastnybb, Z. Broklovácc, J. Brožcc, Z. Doležalcc,
P. Kodyšcc, P. Kubı́kcc, P. Řeznı́čekcc, V. Vorobelcc, I. Wilhelmcc, D. Chrendd,
T. Horazdovskydd, V. Linhartdd, S. Pospisildd, M. Sinordd, M. Solardd, B. Sopkodd,
I. Stekldd, E.N. Ardashevee, S.N. Golovnyaee, S.A. Gorokhovee,
A.G. Kholodenkoee, R.E. Rudenkoee, V.N. Ryadovikovee, A.P. Vorobievee,
P.J. Adkinff, R.J. Apsimonff, L.E. Batchelorff, J.P. Bizzellff, P. Bookerff,
V.R. Davisff, J.M. Eastonff, C. Fowlerff, M.D. Gibsonff, S.J. Haywoodff,
C. MacWatersff, J.P. Mathesonff, R.M. Matsonff, S.J. McMahonff, F.S. Morrisff,
M. Morrisseyff, W.J. Murrayff, P.W. Phillipsff, M. Tyndelff, E.G. Villaniff,
D.E. Dorfangg, A.A. Grillogg, F. Rosenbaumgg, H.F.-W. Sadrozinskigg,
A. Seidengg, E. Spencergg, M. Wildergg, P. Boothhh, C.M. Buttarhh, I. Dawsonhh,
P. Dervanhh, C. Grigsonhh, R. Harperhh, A. Moraeshh, L.S. Peakii, K.E. Varvellii,
Ming-Lee Chujj, Li-Shing Houjj, Shih-Chang Leejj, Ping-Kun Tengjj,
Changchun Wanjj, K. Harakk, Y. Katokk, T. Kuwanokk, M. Minagawakk,
H. Sengokukk, N. Bingeforsll, R. Brennerll, T. Ekelöfll, L. Eklundll, J. Bernabeumm,
J.V. Civeramm, M.J. Costamm, J. Fustermm, C. Garciamm, J.E. Garciamm,
S. Gonzalez-Sevillamm, C. Lacastamm, G. Llosamm, S. Marti-Garciamm,P. Modestomm, J. Sanchezmm, L. Sospedramm, M. Vosmm, D. Faschingnn,
S. Gonzaleznn, R.C. Jarednn, E. CharlesnnaInstituto de Microelectronica de Barcelona, IMB-CNM, CSIC, Barcelona, Spainb
University of Bergen, Bergen, NorwaycLawrence Berkeley Laboratory and University of California, Berkeley, CA, USAdSchool of Physics and Astronomy, The University of Birmingham, Birmingham, UKeCavendish Laboratory, Cambridge University, Cambridge, UK
fEuropean Laboratory for Particle Physics (CERN), Geneva, Switzerlandg
Faculty of Physics and Nuclear Techniques, AGH University of Science and Technology, Cracow, Polandh
The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Cracow, Polandi
Fakultät für Physik, Albert-Ludwigs-Universität, Freiburg, GermanyjSection de Physique, Universite de Geneve, SwitzerlandkDepartment of Physics and Astronomy, University of Glasgow, Glasgow, UKl
Department of Physics, Hiroshima University, Higashi-Hiroshima, Japanm
Institute of Particles and Nuclear Studies (IPNS), KEK, Tsukuba, Japann
Kyoto University of Education, Fukakusa, Japano
Department of Physics, Lancaster University, Lancaster, UKp
Department of Physics, Oliver Lodge Laboratory, University of Liverpool, Liverpool, UK

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F. Campabadal et al. / Nuclear Instruments and Methods in Physics Research A 538 (2005) 384–407
qDepartment of Physics, Queen Mary and Westfield College, University of London, London, UKr
Department of Physics and Astronomy, University College London, London, UK
s
J. Stefan Institute and Department of Physics, University of Ljubljana, Ljubljana, Sloveniat
Department of Physics and Astronomy, University of Manchester, Manchester, UKu
University of Melbourne, Parkville, Victoria 3052, Australiav
Moscow State University, Moscow, Russiaw
Max-Planck-Institut für Physik, München, Germanyx
NIKHEF, Amsterdam, The NetherlandsyPhysics Department, Okayama University, Okayama, Japanz
University of Oslo, Oslo, Norway
aa
Department of Physics, Oxford University, Oxford, UK
bb
Academy of Sciences of the Czech Republic (ASCR), Prague, Czech Republic
ccCharles University, Prague, Czech RepublicddCzech Technical University, Prague, Czech RepubliceeInstitute of High Energy Physics (IHEP), Protvino, Russia
ff
Rutherford Appleton Laboratory, Chilton, Didcot, UK
gg
Santa Cruz Institute for Particle Physics, University of California, Santa Cruz, California, USAhh
Department of Physics, University of Sheffield, Sheffield, UK
ii
University of Sydney, Sydney, Australia
jjInstitute of Physics, Academia Sinica, Taipei, Taiwan
kkInstitute of Physics, University of Tsukuba, Tsukuba, Japanll
Uppsala University, Department of Radiation Sciences, Uppsala, Swedenmm
Instituto de Fisica Corpuscular (IFIC), Universidad de Valencia-CSIC, Valencia, Spainnn
Department of Physics, University of Wisconsin, Madison, Wisconsin, USA
Received 18 August 2004; accepted 31 August 2004
The ATLAS Semiconductor Tracker Collaboration
Available online 14 October 2004

Abstract
The design and technology of the silicon strip detector modules for the Semiconductor Tracker (SCT) of the ATLAS
experiment have been ﬁnalised in the last several years. Integral to this process has been the measurement andveriﬁcation of the tracking performance of the different module types in test beams at the CERN SPS and the KEK PS.
Tests have been performed to explore the module performance under various operating conditions including detector
bias voltage, magnetic ﬁeld, incidence angle, and state of irradiation up to 3  1014 protons per square centimetre. A
particular emphasis has been the understanding of the operational consequences of the binary readout scheme.
r 2004 Elsevier B.V. All rights reserved.
PACS: 29.40
Keywords: ATLAS; Silicon; Micro-strip; Detector; Beam; Test

The Semiconductor Tracker (SCT) will form theintermediate layers of the ATLAS [1] and Inner
Detector [2]. It will provide four three-dimensional
space-points on tracks at radial distances1 in therange 27–56 cm. Good spatial resolution is required
to provide a precise transverse momentum measurement in the bending direction of the 2 T magnetic
ﬁeld of the Inner Detector solenoid. The SCT
consists of a central barrel consisting of fourcylinders and two endcaps each consisting of nine1
The ATLAS geometry is best described in terms of acylindrical coordinate system with its origin at the interaction(footnote continued)
point and its z-axis parallel to the beam line.

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disks, covering a rapidity range up to jZjo2:5 with ahalf-length of 279 cm. The four barrel layers are
149 cm long covering the rapidity range up tojZjo1:0 with a transition to the endcaps by jZjo1:5:
The barrel and endcaps are populated with a
total of 4088 silicon micro-strip detector modules.
The modules are constructed from two pairs of
micro-strip sensors glued back-to-back with a
40 mrad stereo-angle providing a two-dimensional
measurement. The sensors of the barrel modules
are rectangular with strips nearly parallel to thebeam axis. The endcap sensors are fan-shaped withstrips nearly radial away from the beam axis. In
both of these geometries the azimuthal angle f—essential to the transverse momentum determination—is measured with high precision. The barrel
modules also provide a measurement with limited
resolution in the coordinate z along the beam axis,
and the endcap modules in the radial distance R
from the beam axis. In both cases the third
coordinate is given by the sensor position. The
barrel modules are all identical whereas the radial
coverage of the endcaps requires three geometrically different versions [3,4].
All the silicon sensors are of 285 mm thick n-type
h1 1 1i material with a resistivity of 4–8 kO cm:
Each sensor has 768 active micro-strips formed by
implanting pþ material. The strips on the barrel
sensors have a pitch of 80 mm; while the strip pitchon endcap modules varies from 54.4 to 94:8 mm
with the fan geometry. A detailed description of
the sensors is given in Ref. [5].
The pþ implant is AC coupled to a metallized
strip on the sensor surface which is connected to achannel of the readout electronics. The SCT frontend integrated circuit—the radiation-hard
ABCD3T [6]—has 128 channels, and there are
six chips on each side of the module. Each channel
has preampliﬁer and shaper stages with a total
shaping time of 20 ns. The SCT readout isbinary: the ampliﬁed signal is discriminated on
the chip, after which only one bit (on/off) of digital
information remains. The digital information fromall channels is sampled at the LHC clock
frequency of 40 MHz and remains in a 128 bit
wide pipeline on the chip during the trigger latency
period of 3:2 ms: On receipt of a level one accept(L1A), the hit pattern in the relevant pipeline cell is387

shifted into an output buffer, compressed, anddespatched serially to the data acquisition system.
The tracking performance of the SCT modules
has been measured systematically in beam tests
throughout their development. In this article wesummarise beam measurements over the last
several years during the ﬁnal stages of development and qualiﬁcation. These measurements havetypically occurred several times a year at the H8beamline of the Super Proton Synchrotron (SPS)at CERN. In 1999 and 2000, additional tests werecarried out at the p2 beamline at the 12 GeV
Proton Synchrotron at KEK. In the H8 beamline
several secondary beam types and energies areavailable, the most commonly used for this work
being 180 GeV charged pions. In the KEK tests a
3–4 GeV pion beam was used.
Modules are tested in the H8 beamline inbatches of 10–12, mounted in-line along the beamaxis. Modules of all four SCT geometries aretested together. The modules are contained inindividual test boxes to simplify safe mounting andcooling, and surrounded by a common environmental enclosure. Extraneous material in thebeamline is minimised. A separate set of silicon
micro-strip detectors is used to provide reference
tracking. A pair of scintillation counters provide afast trigger signal which is delayed to the pipeline
length of the SCT modules.
A beam test typically consists of a large numberof short runs during which the front-end discriminator threshold is scanned in steps through the
expected signal range, resulting in an efﬁciency ‘‘scurve’’ from which the collected charge distribution can be recovered. The 12 or 16 runs of thethreshold scan are repeated for each combinationof the operating parameters of interest such as thesilicon sensor bias voltage, the applied magnetic
ﬁeld, the angle of incidence on the silicon, thetrigger timing, the position of the beam around themodule, and the operating modes of the ABCD3T.
The effect of a strong magnetic ﬁeld is tested byinserting the complete setup in the superconducting 1.56 T Morpurgo magnet. This magnet provides a vertical ﬁeld over a volume large enough tocontain the entire assembly. The modules aremounted with strips parallel to the ﬁeld andnormal to the incident beam, emulating theARTICLE IN PRESS
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conﬁguration of barrel modules at Z ¼ 0 inside the
2 T Inner Detector solenoid. The deviation of thecharge carriers due to the Lorentz effect has beenstudied in detail yielding a precise measurement of
the Lorentz angle.
A major design concern for the SCT is radiation
tolerance during the lifetime of the experiment in
the LHC. Modules have been systematically
irradiated at the T7 proton irradiation facility at
the CERN Proton Synchrotron [7–9] to ﬂuences
up to 3  1014 protons=cm2 ; well in excess of that
expected in 10 years of LHC operation. The
performance of most of these irradiated modules
was then studied extensively in the beam test
program.
Recent testbeam results have been published ininternal notes and conference proceedings [10–17].
Results from test beams prior to 1999 can befound in Refs. [18–22]. In this article, we describe
the experimental set-up (Section 2), the ofﬂine
analysis including the deﬁnition of the mostimportant single-module tracking performance
characteristics (Section 3), and the main results(Section 4.)

2. Set-up
The CERN 400–450 GeV SPS provides secondary beams to the H8 beamline. The usual beam for
SCT tests is 180 GeV pþ with a small fraction ofmþ : Particles are extracted into the beamline
during a spill of several seconds in a repeating
cycle some 14–17 s long.2 The intensity is roughly
constant during the spill. The beam is parallel
through the length of the assembly, and is about
one centimetre in diameter.
A typical arrangement, that of August 2002, isillustrated in Fig. 1 consisting of a scintillator
trigger, a beam telescope and environment chamber containing the modules under test. The
telescope and modules under test are mounted on
2

In recent years the SPS has been operated at 400 GeV. This
reduced energy has allowed the total cycle length to be
increased from 14.4 to 16.8 s with the spill increased from2.58 to around 4.8 s, signiﬁcantly reducing the integrated deadtime between spills.

a granite table sitting on a trolley on a set of rails
permitting insertion into the active volume of themagnet.
2.1. Trigger and timing
Two scintillator detectors sensed by photomultiplier tubes are used to detect the passage ofbeam particles and trigger the readout system. The
trigger signal timing jitter is around 1–2 ns. The
acceptance of the scintillators is 2  2 cm2 ; large
enough to contain typical beam spots and comparable in size to the acceptance of the beam
telescope. As the SCT modules have a sensitive
area around 6  12 cm2 only a small region istested at any one position of the module in thebeam.
In ATLAS the readout of the detectors will be
synchronised with the bunch crossings of the
LHC. The SCT timing will be carefully optimised
to ensure that the signal is sampled at the peak of
the charge response curve. In most test beams,
however, the beam and readout clock are notsynchronised.3 The beam particles and therefore
the trigger signal arrive with random phase withrespect to the 40 MHz readout clock. Sinceelectronics designed for the LHC has typically ashort rise-time comparable to the 25 ns clock
period, this random arrival time would have asevere effect on the apparent performance. Therefore the time between the raw trigger from thescintillators and the next rising edge of the clock ismeasured using a Time to Digital Converter(TDC),4 allowing the dependence of the performance on trigger phase to be evaluated off-line.
2.2. Beam telescope
A beam telescope is used to measure thetrajectories of beam particles independently ofthe devices under test. The telescope modules
consist of two perpendicular silicon micro-strip3
In certain periods the SPS has been operated with an ‘‘LHClike’’ bunch structure which has been used to test SCT modules
under more stringent timing conditions. In this case the readout
clock was synchronised to the accelerator RF bunches using the
LHC timing distribution system.
4
CAEN model V488.

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389

Fig. 1. Arrangement of SCT prototype modules in the H8 beamline during the August 2002 beam test.

sensors providing an X–Y space point. The sensors
have a strip pitch of 50 mm and are read out by
VA2 analogue multiplexer chips with a peaking
time of around 1 ms: The modules are rigidly
mounted on the same massive granite table as themodules under test, two before and two after theenvironment chamber. A spatial resolution better
than 5 mm is obtained. The analogue signal isdigitised by ﬂash ADC VME modules which also
perform pedestal subtraction and cluster ﬁnding
using on-board DSPs, signiﬁcantly reducing thedata volume. The dead time of this system,13 ms, is the dominant limitation to the total
event rate.
While the beam telescope provides a track withgood spatial resolution compared to the SCT
modules under test, it does so with a much longer
shaping time. In order to ensure that a track
detected in the telescope corresponds in time withthe much shorter sensitive time window of the SCT
modules, one of the SCT modules is also used toprovide a reference hit. This ‘‘anchor’’ module
does not undergo threshold or bias scans but is
kept at constant settings selected for high efﬁciency and low noise occupancy. The requirementthat the anchor module records a hit matching thetelescope track also effectively eliminates residual
inefﬁciencies due to clock and readout glitches,and since it is located among the modules under
test, helps eliminate badly reconstructed or widely
scattered tracks.
2.3. Control, readout and power supply
The SCT modules are powered, controlled andread out during the test beam measurements by asuite of custom VME cards which has beendeveloped speciﬁcally for small-scale SCT test
systems [23–26].
A clock and control card (CLOAC) provides acommon 40 MHz system clock to all detector
modules and to the readout system. It is also used
to distribute ‘‘fast’’ commands—resets and triggers—which are simultaneously transmitted to all
modules during active data-taking, thus providing
the global synchronisation. An important functionof the CLOAC is to delay the prompt trigger
signal by an amount equal to the pipeline length
less any cable and gate delays, and then to issue alevel-one accept synchronised to the next clockedge.
Slow command generator cards (SLOG) areused to assemble the long bit strings unique toeach module which conﬁgure the many registers of
the 12 ABCD3T chips. These ‘‘slow’’ commands
are transmitted outside active data-taking. The
SLOG modules also fan-out the clock and the fastcommands to the individual modules, six modules
per unit.
‘‘MuSTARD’’ cards receive the serial bit stream
output data from the modules. Each MuSTARD
reads out the two data links of six modules, 12
links in total. It performs header and trailer
detection, making individual event data available
over VME to the data acquisition system. It also
provides fast on-board hit pattern decoding andoccupancy histogramming with error detection.
In ATLAS the clock, command and data signals
to and from the SCT will be carried by opticalﬁbres. However, in the beam tests, optical readout
has not been used. Instead, the LVDS electricalsignals on the module are buffered nearby andtransmitted by 25 m shielded twisted-pair cables tothe control room.

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SCT low voltage VME cards (SCTLV3) provide
the high-current analogue and digital power to thefront-end chips on the modules as well as several
low-current control voltage and temperaturemonitoring channels. The high-current voltage
levels are compensated for ohmic voltage dropson the cable by a four-wire sensing scheme. EachSCTLV3 powers two modules.
SCT high voltage VME cards (SCTHV) provide
the silicon detector bias voltage, up to 500 V at
5 mA, extreme values which may be required after
high radiation doses. Each SCTHV supplies four
modules.
2.4. Module calibration
A full characterisation of the module front-end
parameters such as gain and noise is made in situ
prior to the beam test using the ABCD3T’s in-built
calibration circuit [6]. A voltage step across thecalibration capacitor (Ccal) results in accumulated
charge being injected into the front-end of thechip. The amplitude of the calibration charge iscontrolled by a DAC which can be conﬁgured
through the range 0–13 fC. The response to thecalibration charge is obtained by scanning thediscriminator threshold, resulting in an occupancy‘‘S-curve’’ histogram for each channel. A complementary error function is ﬁtted to this histogram
yielding the median 50% point efﬁciency ðvt50 Þ andfrom the Gaussian spread the output noise. This
process is repeated for several calibration charge
amplitude settings from 0.5 to 8 fC covering the
expected signal range. The graph of vt50 thresholdsettings versus calibration charge amplitude describes the response of each channel. The response
is summarised at 2.0 fC by the gain, determinedfrom the slope, the offset from the intercept, andthe input noise determined by dividing the output
noise by the gain.
The discriminator threshold setting is common
to all channels within one chip. Any spread in the
response among the different channels of a chip
results in a spread of the efﬁciency and noise
occupancy which degrades effective performance,
particularly after irradiation. The ABCD3T has
therefore been designed with threshold trim
adjustments for each channel to allow the spreadto be minimised. This is a lengthy procedure whichmust be carried out before the ﬁnal response canbe measured.
In addition to the standard electrical proceduredescribed above, further optimisation is requiredfor irradiated modules as described in Ref. [27].
Modelled as a transimpedance ampliﬁer with abipolar input transistor, the front-end circuit ofunirradiated ASICs have nominal values forcollector (pre-ampliﬁer) and shaper currents of
220 and 30 mA; respectively. As a consequence ofthe b factor degradation, the optimum values ofthese currents decrease with irradiation. Two 5-bit
DACs are therefore implemented in the ABCD3T
to adjust the pre-ampliﬁer and shaper currents [6].
Each chip is independently optimised. Typicalvalues for irradiated modules are around 120 or
140 mA for the input transistor current and 27 or
30 mA for the shaper bias. In 2002, beam tests ofirradiated modules with different current conﬁgurations were performed to study the variation of
signal-to-noise with respect to pre-ampliﬁer andshaper current.
Digital timing performance has also been
observed to degrade with irradiation. It has beenfound that the number of slow channels can besigniﬁcantly reduced by increasing the analogue
voltage, V cc ; supplied to the chips. Set-up effects
such as these are avoided by running the ﬁnal
response curve calibration in situ under the same
conditions as for the beam measurements. For
example, the temperature differences of the chips
may also have a 5% effect on the noise measurement [28].
2.5. Cooling
The modules under test are mounted in a lighttight, thermally insulating chamber. The chamber
is ﬂushed with cold nitrogen gas to ensure a dry
atmosphere. Each of the modules is contained in
its own aluminium test box to which it is thermallycoupled through its designed cooling contact
surfaces. The test boxes, of various designs
according to the particular module type, arecooled by water/ethanol chilled to around
15 C: Each module dissipates up to 7 W power.
The resulting temperatures, measured by thermis-

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tors mounted on the electronics hybrid of themodule, are monitored continuously. Hybrid
temperatures range between 5 and 5 C: The
detector leakage current of heavily irradiateddetectors is in the range from 1 to 3 mA, dependingon the cooling which is not as efﬁcient as expectedin ATLAS. Non-irradiated modules have currentsof the order of 1 mA:

3. Off-line and analysis
During the beam test, data from all subsystems—telescope modules, SCT binary modules,
TDC—are assembled into events and written todata ﬁles. To facilitate the analysis, these raw data
are processed off-line into summary ﬁles. These
present the data in an easily accessible n-tuple
format. Off-line processing includes the internalalignment of telescope and module planes and thereconstruction of tracks using the measurementsof the beam telescope.
3.1. Alignment
The beam telescope provides four very precise
measurements in two perpendicular space coordinates. The reconstruction of the track and theinterpolation to the plane of the module under test
require a precise knowledge of the relative alignment of all detectors in the set-up.
The positions of telescope and SCT modules,
though stable, are only approximately ﬁxed by themounting frames. A precision measurement of therelative alignment of all planes is obtained using
the tracks in a large number ð10,000) of events.
The alignment procedure minimises the alignment
error by iteratively varying the alignment parameters.
As a ﬁrst step in the alignment procedure, theposition of the global telescope frame is ﬁxed by
identifying X - and Y -axis with, respectively, thehorizontal and vertical coordinates measured by
the ﬁrst telescope module, and the Z-axis with thebeam direction. Once the global system is deﬁned,
the remaining telescope detectors are aligned byminimising the distance of their hits to tracks
reconstructed from the outer two modules. At this391

stage only two space points are used to deﬁne a
track. Ambiguities due to noise hits are effectively
avoided by rejecting events in which all four planes
under consideration do not have exactly one hit.
After the internal alignment of the telescope, full
three or four space point information from thetelescope can be used to reconstruct tracks
projected through the modules under test. The
alignment parameters of each of the two planes of
the test modules are varied until the difference
between the interpolated track position and themeasurement by the plane converges to a minimum. The two planes of each module are aligned
independently.
When a magnetic ﬁeld is applied, the tracks havea signiﬁcant curvature. Moreover, the Lorentz
effect leads to a displaced position measurement.
For data taken in the magnetic ﬁeld the alignment
procedure described above—with straight tracks—
is nevertheless applied. The effects of the magnetic
ﬁeld are thereby absorbed in the effective module
positions. This procedure allows the interpolationof the track to the modules because all tracks haveclosely the same momentum.
The alignment procedure was repeated eachtime the set-up is physically accessed or whenever
the magnetic ﬁeld is changed.
3.2. Track reconstruction
The off-line pre-processing of the data includes
the reconstruction of the tracks using the spacepoints measured by the telescope.
The measurements of the two perpendicularplanes of each telescope module are combined into
three-dimensional space points. Track segments
are constructed by combining the space points
from the most upstream and most downstream
modules. The track is then reﬁned by including thespace points from the intermediate telescope
modules if the distance between the track and thespace point is within 50 mm: Events are included if
the track contains at least three space points after
this cut.
A number of track quality indicators areavailable to select a sample of tracks with apredeﬁned efﬁciency and purity. The most important are: the number of space points in the

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track ﬁt, the w2 of the ﬁt, the track gradients withrespect to the beam axis, and whether a hit isrecorded in the anchor module. In the H8 beam
tests the position where the tracks are incident oneach module is interpolated from the telescope hitswith a precision of approximately 5 mm including
the alignment error. The dominant error is due tomultiple scattering within the module array.
3.3. Event selection
A run consists of the order of 10,000 events
collected under a ﬁxed set of operating conditions
including discriminator threshold. The ﬁrst step in
the analysis is the selection of the event sample. To
select a sample of clean, unambiguous events, anumber of selection criteria are applied:
Single track events. In a small fraction of events(depending on the beam intensity, generally ofthe order of 5%) more than one track isreconstructed from the telescope information
due to its relatively long shaping time. As theintegration time of the binary modules is much
shorter the second track is generally notdetected in the binary system. However, toavoid ambiguities, events with more than one
telescope track are rejected from the analysis, as
are events where no telescope track has been
accepted.
Track quality. From the remaining sample, only
those events are accepted where the telescopetrack satisﬁes quality criteria. Most analyses
require that the track quality criteria listed
above be fulﬁlled. Moreover, a hit close to thetrack in the anchor module is required. This
combination of cuts efﬁciently removes faketracks and tracks suffering a large deﬂection due
to multiple scattering. The loss of statistics inthis stage depends on the values of the cut, but isin general of the order of 10–30%.
Masked channels. During the characterisation of
the modules faulty channels, for example dead,stuck or particularly noisy channels, are identiﬁed. These channels are conﬁgured to bemasked in the ABCD3T chips so that they arenever read out. Events in which the track points
to one of these ‘‘bad’’ channels or their

immediate neighbours are rejected from theanalysis of that particular module. In recent
modules the fraction of masked channels is
below 1%. The loss of statistics in this step istherefore small.
Time window. A ﬁnal cut removes those events
where the random trigger phase falls outside an
optimum window. This window is optimised
separately for each module to account for small
cable length differences. The window is normally set to 10 ns incorporating the peak
response time, making it particularly expensive
in terms of statistics as only 40% of the events
are retained. In the special LHC-like bunch
structured beams this cut is not necessary as inthat case the non-random trigger timing isadjusted into phase with the 25 ns clock.
A further subtlety is implicit in the last
mentioned cut. The binary data output by the
ABCD3T actually takes into account hit information from the preceding and succeeding time bins
as well as that centred on the trigger time, threebits in total. The compression algorithm can be
conﬁgured to operate in one of several modes ofincreasing severity, reducing noise occupancy but
also potentially impacting efﬁciency. In ‘‘anyhit’’mode, a hit in any of the three time bins (1XX,
X1X or XX1) results in the three bits being readout. In ‘‘level’’ mode, a central hit (X1X) is
required. In ‘‘edge’’ mode, a central hit precededby the absence of a hit (01X) is required. It is
expected that the SCT will normally be operated in
one of the latter modes. However, in the beam
tests the anyhit mode was normally used tomaximise the recorded information. The time
window cut, however, requires a hit in the central
time bin, effectively simulating level mode operation of the chips.

4. Results
4.1. Efficiency and noise occupancy
The most important benchmarks for SCT
binary module performance are the efﬁciency andthe noise occupancy at the operating threshold,ARTICLE IN PRESS

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Noise Occupancy

Efficiency

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0.95
10-3
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Noise Occupancy

Efficiency

F. Campabadal et al. / Nuclear Instruments and Methods in Physics Research A 538 (2005) 384–407

Fig. 2. Results under the reference conditions of the efﬁciency (left axis) and noise occupancy (right axis) versus corrected threshold in
fC in the region near the nominal 1.0 fC operating point. The horizontal dashed lines indicate the module speciﬁcations for efﬁciency(4 99%) and noise occupancy ðo5  10 4 Þ: The vertical lines indicate the threshold interval where both speciﬁcations are met: theoperating region. The leftmost ﬁgure corresponds to a non-irradiated module, the rightmost ﬁgure to a module uniformly irradiated to
the reference ﬂuence of 3  1014 protons=cm2 :nominally 1 fC. A detector plane is considered
efﬁcient if a binary cluster centre is located within
150 mm of the interpolated telescope track position. Typical residual distributions are shown in
Section 4.2. Note that in our deﬁnition, theefﬁciency of good strips is measured. The inefﬁciency due to dead sensor or electronics channels is
not accounted for, nor is the inactive area between
sensors (see Section 4.6 for the latter source of
inefﬁciency).
The noise occupancy is measured by counting
all hits in special ﬂagged events taken in the
periods between accelerator spills, during which
there is no beam occupancy. As these events areinterleaved with beam events the operating conditions are identical. The noise occupancy is deﬁned
as the probability to ﬁnd a hit due to noise in one
channel in one clock cycle.
Fig. 2 shows the efﬁciency for a number ofthreshold settings around the design operating
threshold of 1 fC. The noise occupancy corresponding to each threshold is shown on thelogarithmic axis on the right of the same ﬁgure.
The results correspond to a non-irradiated barrel
module and a module of the same type irradiated
up
to
the
reference
ﬂuenceof31014 protons=cm2 : All modules are operated in
the same test beam reference conditions: thedetector bias voltage is set to 150 V for the nonirradiated modules and to 450 V for the irradiatedmodules, the beam is incident perpendicularly on
the modules and no magnetic ﬁeld is applied.
When the modules are operated with sufﬁcient
over-depletion, there is a long plateau where
efﬁciencies over 99% are obtained. The noise
occupancy, on the other hand, is a rapidly falling,nearly Gaussian, function of threshold, independent of bias voltages high enough to deplete thedetector. The occupancies are found to coincide
rather accurately with the expectation from theequivalent noise charges measured during thecharacterisation. After irradiation, the noise occupancy is considerably higher for a given threshold,in agreement with the increased noise charge. The
efﬁciency for irradiated modules is seen to fall off
at a lower threshold than for similar nonirradiated modules.
The SCT speciﬁcations require over 99% tracking efﬁciency5 and a noise occupancy of less than
5  10 4 : Both requirements are indicated asdashed lines in the ﬁgure. The operating range isdeﬁned as the range in thresholds where the
efﬁciency and noise occupancy speciﬁcations areboth met. For non-irradiated modules, of all fourgeometrical types, the operating range extends
5
The efﬁciency measurement and speciﬁcation discussion
here are limited to ‘‘good’’ strips. The maximum tolerable
number of defective channels is deﬁned in a separate speciﬁcation.

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efficiency

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2.0 fC
2.5 fC
3.0 fC
4.0 fC

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0.4
0.2
0

1.0 fC
1.5 fC
2.0 fC
2.5 fC
3.0 fC
4.0 fC

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

interstrip position (pitch)

interstrip position (pitch)

Fig. 3. Efﬁciency as a function of the inter-strip position of the incident particle for a non-irradiated (a) and irradiated barrel module(b).

from 0.8 up to 1.5 fC. After irradiation, theoperating range is signiﬁcantly narrower for all
modules. The lower limits range from 1.0 to 1.2 fC,
whereas the upper limit is generally around 1.2 fC.
In detectors with analogue readout the charge
on multiple consecutive strips is measured and theposition is determined from the properties of thecluster. The clustering procedure yields a quasicontinuous position measurement. In the binary
readout scheme, however, the signal in most cases
is over threshold on only one strip for tracks atnormal incidence. Thus, only a discrete measurement is available. This could lead to nonuniformities in the response depending on theposition of the track with respect to the two closest
strips, particularly in the inter-strip region.
Fig. 3 show the efﬁciency versus inter-strip
position for perpendicularly incident tracks on a
non-irradiated and an irradiated barrel module.
The position is expressed in units of the read-out
pitch: inter-strip position 0.5 corresponds totelescope tracks crossing the detector at equal
distances from the two strips and inter-strip
positions of 0 and 1 correspond to tracks thatare incident on the centre of the readout strip. In
the central region diffusion of the charge carriers
leads to increased charge sharing. The lower signal
on each of the strips is reﬂected in the reduced
efﬁciency at high threshold. However, at theoperating threshold the efﬁciency is uniformly
over 99% for all inter-strip positions. Note thatthe distribution is smeared by the uncertainty inthe position of the interpolated track that becomessigniﬁcant at this level.

4.2. Spatial resolution
The spatial resolution is obtained as the width of
the residual distribution—the distribution of thedifference between the extrapolated track position
and the centre of cluster.6
Typical residual distributions are shown in Fig.
4. The outline histograms show the result using
only single strip clusters: the residuals form a
uniform distribution from p=2 to p=2; where p is
the readout pitch, characteristic for binary readout. The spatial resolution due to single-strip
clusters is given by the RMS of a uniform
distribution
pﬃﬃﬃﬃﬃ with a width equal to the pitch:s ¼ p= 12 23 mm: The contribution from twostrip clusters is shown in the ﬁlled histograms. As
the multi-strip clusters predominantly occur in anarrow region in the central region between two
strips the resolution of the multi-strip clusters is
better than that of single-strip clusters.7 Note thatthe residuals distributions are smeared by theuncertainty on the interpolation of the telescopetracks, approximately 5 mm:
6For endcap modules the residuals are rescaled by the ratio
between the barrel pitch ð80 mmÞ and the local pitch to facilitatecomparison.
7
See the discussion in the next section.

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160

160

140

140

120

120

100

100

80

80

60

60

40

40

20

20

0
-200 -150 -100 -50

050 100 150 200

residual (um)0-200 -150 -100 -50

0395

50 100 150 200

residual (um)Fig. 4. Single plane residuals with respect to the position interpolated from the beam telescope measurements. The outline histograms
correspond to single-strip clusters, the ﬁlled histogram to two-strip clusters. Two modules irradiated to 3  1014 protons=cm2 : a barrel
module (left) and an endcap outer module (right). The discriminator threshold is set to 1 fC. For the endcap module the residuals havebeen rescaled to the equivalent of a pitch of 80 mm:

For tracks of normal incidence the fraction ofmulti-strip clusters is small and the resolution isclose to 23 mm, independent of the operating
parameters. Also, no signiﬁcant change is observedin the residuals at a threshold of 1 fC after
irradiation. The effect of the bias voltage and theincidence angle on charge sharing are discussed in
Sections 4.4 and 4.5, respectively.
The residual distribution of Fig. 4 is a measurement of the spatial resolution of a single detector
plane. The stereo angle of 40 mrad between thetwo detector planes of the SCT module allows thedetermination of a two-dimensional space point.
The intersection of two 80 mm wide strips deﬁnes arhombus with short axis p= cos a=2 and long axis
p= sin a=2; where p is the readout pitch and a is thestereo angle. The short axis corresponds to the Xaxis of the global coordinate system in the test
beam, while the long axis is identiﬁed with the Yaxis. Projecting the uniform distribution in therhomb on both axes yields the triangular distributions of Fig. 5. A Gaussian ﬁt yields widths of16 mm in X and 850 mm in Y. More detailed
results on the single module resolution arereported in Ref. [12].
4.3. Median charge
In the binary readout scheme, it is necessary toscan the discriminator threshold through the

charge distribution in order to reconstruct the fullsignal distribution. The curve of efﬁciency as afunction of threshold, the ‘‘s-curve’’, measures theintegrated charge distribution. The collectedcharge distribution is described by the Landau
distribution of the deposited charge convoluted
with the Gaussian noise distribution and the effectof charge sharing between neighbouring strips.
The median charge is measured as the charge
corresponding to the threshold where 50% efﬁciency is obtained. In practice, the threshold isexpressed in equivalent charge and the value forthe median charge is obtained directly from a ﬁt
with a skewed error function, as in Ref. [13]: e xx exx
 ¼ max f x 1 þ 0:6 xx
(1)
e
þ exx
where f denotes the complementary error function.
The median charge m; the width s; the skew x andthe maximum efﬁciency
pﬃﬃﬃ max are free parametersand x ¼ ðqthr mÞ= 2s: This parameterisation
describes the data quite accurately.
In Fig. 6 the ‘‘s-curves’’ of a non-irradiated andan irradiated module are shown for reference
operating conditions. Each point corresponds tothe efﬁciency measured on a single run of 15,000
events. The lines represent the ﬁt with the functionof Eq. (1).
The error on the median charge is dominated by
the uncertainties in the calibration. The variation

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396

120
250

100

200

80

150

60

100

40

50

20
0

0
-0.01

-0.005

0

0.005

0.01

single module X residual (cm)

-0.2

-0.1

0

0.1

0.2

single module Y residual (cm)Fig. 5. Residuals of the space point reconstructed on a single, irradiated barrel module with respect to the position interpolated fromthe beam telescope measurements: X, perpendicular to the long axis of the module (left) and Y, parallel to the long axis of the module(right). The discriminator threshold is set to 1 fC.

1

Efficiency

0.8
0.6
0.4
0.2
0

0

1

2

3

4

5

6

7

Threshold (fC)
Fig. 6. Typical efﬁciency versus threshold ‘‘s-curves’’ for a nonirradiated (solid) and an irradiated (dashed) module underreference conditions (deﬁned in Section 4.1).

of the calibration from one chip to the next isevaluated in the beam test by pointing the beam toareas of the detector read out by different chips
and measuring the median charge of each chip
separately. For non-irradiated modules the chipto-chip spread in the median charge is of the orderof 4%. After irradiation the spread increases tonearly 10% on some modules [28]. Correcting thecharge of each chip by measurements of thecalibration DAC output improves the uniformity,suggesting that the spread in median charges is due
to variations in the components of the calibration
circuit [29].
The random error of a single median charge
measurement is therefore of the order of 0.1 fC for

non-irradiated modules and 0.3 fC for irradiatedmodules. In practice, comparison between different modules is greatly facilitated by the use of thesignal-to-noise ratio, the ratio of median charge
and the equivalent noise charge.
The average median charge of a signiﬁcantnumber of recent unirradiated barrel and outer
endcap modules is 3:5  0:1 fC at a bias voltage of
300 V. Division of the median charge by theequivalent noise charge, which is measured as partof the electrical characterisation of the module,
yields a signal-to-noise ratio of 14. For inner
endcap modules with shorter, 6 cm strips the lower
capacitive load at the ampliﬁer leads to an
improved noise performance, yielding a S=N of
20.
A theoretical expectation for the signal height isobtained from a GEANT4 simulation8 of theenergy deposition by 180 GeV charged pions in
285 mm of silicon. The number of electron–holepairs created along the track is obtained by
dividing the deposited energy by the averageenergy required to produced an electron hole pair,3.63 eV [30]. From a detailed simulation of thecharge transport and signal induction on the strip
the electron contribution to the signal is expected
to be small [10,31]. Ignoring the electron contribution, the signal induced on the readout strip istaken to be the sum of the charges of the holes.

8
GEANT4 is expected to give a rather accurate prediction forthe median deposited energy. For details, see Ref. [10].

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4.4. Detector bias voltage
The bias voltage of the detectors is an important
operating parameter. In ATLAS, the maximum
voltage is limited by the detector breakdown
voltage (greater than 500 V for SCT detectors),
the limit of the power supplies (500 V), and thelimit imposed by the cooling system. The bias
voltage dependence of the performance has therefore been studied in detail.
Fig. 7 shows the typical dependence of thesignal-to-noise ratio on the detector bias voltage
for modules before and after irradiation. For nonirradiated modules, of all types, the median signalto-noise ratio is nearly constant, S=N 13, above

14

Signal/Noise

12
10
8
6
4
2
0
0

50

100

150

200

250

300

350

400

Bias Voltage (V)14

Signal/Noise

12
10
8
6
4
2
0
150

200

250 300 350 400
Bias Voltage (V)

450

500

200

250

450

500

14
12
Signal/Noise

Under this assumption the median signal is
expected to be 4.0 fC [10,31].
The observed median charge is clearly lower.
Less than 0:1 fC of this difference can be attributed
to the measurement procedure, indicating a
discrepancy between the deposited and collectedcharge.
Charge shared between neighbouring strips
cannot be re-clustered in the binary readout
scheme as it is predominantly below threshold.
The effect of different charge sharing mechanisms
on the median collected charge sharing is investigated in Ref. [10]. Diffusion of the charge carriers
while they drift toward the readout plane can bestudied using the position predicted by thetelescope. Test beam measurements indicate thatthe loss of median charge due to diffusion is
0.13–0.16 for all module types. Cross-talk between
neighbouring channels is expected to be responsible for a charge loss of 6%, i.e. 0.26 fC. Finally,
the charge carried to neighbouring strips by delectrons can be quite substantial. However,
GEANT4 model calculations show that eventhough this effect leads to a less pronounced
high-energy tail in the charge distribution, themedian charge is not signiﬁcantly affected.
The total charge loss due to the three charge
sharing mechanisms is thus expected to amount to0.4 fC. Thus, charge sharing accounts for most of
the discrepancy between the observed mediancharge, 3.5 fC, and the expected charge deposition,
4.0 fC.

397

10
8
6
4
2
0
150

300

350

400

Bias Voltage (V)Fig. 7. Median signal-to-noise ratio versus bias voltage. Top
ﬁgure: non-irradiated module (all types are similar.) Central
ﬁgure: irradiated barrel module. Bottom ﬁgure: irradiatedendcap outer module. In all ﬁgures, the round and square
markers correspond, respectively, to the readings on the front
and back wafers of the module. In cases where the area of thesensor hit by the beam was read out by more than one chip, theresults of each chip are indicated separately.

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150 V. The charge difference between 150 V andthe highest voltage, 300 V, is 0.15 fC. As the bias
voltage is decreased towards the depletion voltage(around 75 V in these detectors) the signal
gradually decreases. This can be explained as aballistic deﬁcit of the shaper circuit, i.e. theintegration window of the front-end electronics is
no longer large compared to the charge collection
time of the detector, leading to charge loss. The
drift time of the holes becomes comparable to thepeaking time of the shaper at a bias voltage ofaround 100 V. For a detailed discussion of chargecollection times, see Refs. [32,33].
The relation between signal and bias voltage forirradiated modules is more complicated. The
actual bias voltage on the silicon is lower than
the supplied voltage by up to 30 V due to thevoltage drop in the hybrid bias resistor, in turn due
to the much higher, and strongly temperaturedependent, leakage current. The horizontal axes inFig. 7 have been corrected for this effect. After
irradiation the depletion voltage has signiﬁcantly
increased, estimated to be around 250 V for thesedetectors. Above the depletion voltage the collected charge continues to rise slowly. The signal
collected at a bias voltage of 350 V is approximately 90% of that at the maximum bias voltageof nearly 500 V.
Due to the increased noise, the signal-to-noise
ratio of irradiated modules does not reach the preirradiation value, even at the highest available bias
voltage. Barrel and endcap modules irradiated to
1:6  1014 protons=cm2 ; around half the reference
ﬂuence, have a median signal-to-noise ratio of
11  0:5 at 500 V. For modules irradiated to the
maximum ﬂuence, 3  1014 protons=cm2 ; the measurements are scattered between values 7 and 11.
The large spread between modules is not completely understood but may partly be due to dose
uncertainty. The endcap modules tend to havesignal-to-noise ratios that are in the lower part ofthe distribution, but the limited statistics does not
exclude systematic effects due to ABCD batch
variations.
Various explanations exist for the high overdepletion needed to approach full charge collection. Measurements by the ROSE collaboration[34] indicate that there is considerable charge loss

due to trapping of the charge carriers in radiationinduced lattice defects. Assuming a constant
trapping probability per unit time, the effect isbias voltage dependent through the charge collection time. For small over-depletion, the different
ﬁeld distribution across the silicon in type-inverted
bulk material may lead to a ballistic deﬁcit of the
shaper.
In irradiated, type-inverted pþ n detectors thejunction forms at the backplane, between the nþmaterial of the backplane and the p-bulk. Thus, if
the bias voltage is lower than the depletion voltage
the depleted region does not reach the readout sideof the detector. A small signal is neverthelessinduced on the strips across the dead region.

4.5. Incidence angle and magnetic field
In ATLAS the SCT modules will be situatedwithin a 2 T magnetic ﬁeld. Also, tracks will notgenerally be perpendicularly incident on thedetector plane. Therefore, an important subjectof study is the inﬂuence of the magnetic ﬁeld andincidence angle on the performance of themodules.
In the August 2001 beam test [12] thresholdscans were performed at various angles, bothwithout magnetic ﬁeld and in the 1.56 T ﬁeld of the
Morpurgo magnet. In this section the dependenceof the most important parameters—efﬁciency atthe operation threshold, median collected charge
and spatial resolution—on the incidence angle andﬁeld is discussed.
Two orthogonal orientations have been studied.
First, the simpler case is considered where theprojection of the particle trajectory on the readout plane is parallel to the strips,9 the longer path
length through the silicon leads to an increase of
the median charge with 1= cosðaÞ: This behaviour
has been conﬁrmed in beam tests up to angles of
16 in 2001 [12] and in a second measurement at
35 in 2002 [28] (see the leftmost ﬁgure of Fig. 8).
9In the SCT this angle is determined by the detector
geometry. The angle reaches quite large values (up to 70o )for the most forward modules on the barrel and for the disk
closest to the interaction point.

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2002 test beam
B053

Q/Q(0)

1.2
1.15
1.1
1.05

2001 test beam
B029

1
5

10 15 20 25 30 35 40
degrees

Median Charge (fC)

F. Campabadal et al. / Nuclear Instruments and Methods in Physics Research A 538 (2005) 384–407

3.5

399

2001 test beam
B018

3
2.5
2

2002 test beam
KB-105

1.5-20 -100

10 20
degrees

30

40

Fig. 8. Leftmost ﬁgure: median charge versus incidence angle in the R–z orientation. The points are normalised with respect to the 0
result. The continuous line is the function 1= cos a corresponding to the simple geometric behaviour expected from the increased path
length. Rightmost ﬁgure: median charge versus incidence angle in the R–f plane. The continuous line corresponds to a ﬁt with aparabola. In both ﬁgures, the 35 point is a result from the 2002 test beam, while the other measurements refer to a barrel module
measured in 2001. The detector bias voltage is 150 V.

In the second orientation the projection of theparticle trajectory is perpendicular to the strips.
For a particle from the ATLAS interaction point
non-perpendicular incidence arises from the deﬂection of particles in the solenoidal magnetic
ﬁeld, the tilt of the module, and the range of angles
which the module subtends. The incidence angle inthis orientation is generally smaller: the tilt angleof the barrel modules is 10 ; of the same order as
the deﬂection angle of a 1 GeV particle.
The longer path length again leads to a small
increase of the signal. But, in this orientation the
projection of the particle trajectory on the readout
plane is perpendicular to the readout strips. For
large incidence angles the projected distancebecomes signiﬁcant compared to the pitch andcharge sharing between neighbouring strips becomes important. The net effect is a decrease of themedian collected charge on a single strip as shownat right in Fig. 8.
In a strong magnetic ﬁeld the Lorentz force
deviates the drifting carriers. Fig. 9 shows how theLorentz force is equivalent to a rotation by a small
angle YL ¼ mH B ¼ rH mB; where mH the Hall
mobility, the conduction mobility m multiplied by
the Hall scattering factor rH : In pþ n type sensors,
the signal is predominantly due to holes and theLorentz angle is of the order of a few degrees.
In August 2001 the combined effect of incidence
angle and magnetic ﬁeld was studied in an

Beam

Beam

P+
+ +
+ +
+ + + +
+

α

+
+

+

B

+
+
+
+

ΘL

+
+
+
+

+
+

+

n+

Fig. 9. Representation of the effect of the incidence angle andthe application of a magnetic ﬁeld. The angle a is the incidence
angle of the beam particle, yL is the Lorentz angle of thedrifting carriers.

orientation representative for barrel modules at
Z ¼ 0: The modules were biased to nominal bias
voltages: 150 V for non-irradiated modules, and350 V after the full expected dose. The modules arenaturally classiﬁed in two groups: irradiated andnon-irradiated modules. The results are presented
as averages over all (non-) irradiated barrel
modules, the errors representing the variation
between different modules.
A sensitive measure of charge sharing is theaverage cluster size. Here, the cluster size dependence on angle in a magnetic ﬁeld is used todetermine the Lorentz angle. Noise clusters are

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1.5

1.5

1.4

1.4

avg. cluster size

avg. cluster size

400

1.3
1.2
1.1
1

-15 -10 -5

0

5

10 15

incidence angle (degrees)

1.3
1.2
1.1
1

-15 -10 -5

0

5

10 15

incidence angle (degrees)Fig. 10. Cluster size versus angle for non-irradiated modules at 1 fC (left) and irradiated at 1 fC (right). The ﬁlled markers represent
measurements with a magnetic ﬁeld of 1.56 T, the open markers correspond to the same measurement without ﬁeld. Both sets ofmeasurements have been ﬁt with a parabola (continuous line for the data with magnetic ﬁeld, dashed line without ﬁeld) to guide theeye.

excluded by placing a 200 mm window around thetrack position predicted by the telescope. Fig. 10shows the average cluster size as a function ofincidence angle (ﬁlled markers). The open markers
show the same measurement in a 1.56 T magnetic
ﬁeld.
The increase of cluster size at non-perpendicularincidence reﬂects the increased charge sharing. The
effect of the magnetic ﬁeld is a shift of the
distribution by the Lorentz angle.
The value of the Lorentz angle is determined as
the minimum of the cluster size versus angle curve.
The measurements from all modules in bothgroups have been combined. The statistical error
is taken to be the standard deviation of themeasurements of a group. As a cross check, theﬁt results for data taken without magnetic ﬁeld
have been calculated. The result Y0 ¼ 0:4  0:2
reﬂects the overall uncertainty in the angle scale.
The Lorentz angle obtained for both groups, nonirradiated and irradiated modules, is then:YL ð150 V; non-irradiatedÞ ¼ 3:3  0:3(2)

YL ð350 V; irradiatedÞ ¼ 2:1  0:4 :(3)

As the irradiated modules are biased at a higher
voltage, the observed difference could be due to anelectric ﬁeld dependence of the Lorentz angle or achange in the properties of the charge carriers due
to irradiation. In the literature, the effect of proton

irradiation on the mobility and thus Lorentz angleof holes is found to be compatible with no effect[32,35,36]. The difference in voltage, i.e. theelectric ﬁeld inside the detector, has an effect on
YL through the electric ﬁeld dependence of thecarrier mobility m [35,37–39]. Applying the theoretical model of Ref. [39] to beam test conditions(silicon temperature T ¼ 261 K; thickness d ¼
285 mm and ﬁeld B ¼ 1:56 T) a prediction of
YL ð150 VÞ ¼ 3:3 and YL ð350 VÞ ¼ 2:4 is obtained. The predictions agree within errors with
our measurements.
In the range of angles ½ 14 ; 16  studied
in the beam test no signiﬁcant effect on the
efﬁciency is observed at a threshold of 1 fC. For
irradiated modules a small effect becomes visible
when the discriminator threshold is raised to
1.2 fC. Still, the efﬁciency is over 97% for all
angles.
As discussed in Section 4.2 the spatial resolution
with binary readout depends on the number ofstrips in the cluster. An increase in the number ofmulti-strip clusters is expected to lead to a better
resolution. In Fig. 11 the effect of the incidence
angle on the spatial resolution of non-irradiated(leftmost ﬁgure) and irradiated modules (rightmost ﬁgure) is shown. The difference between theworst (0 ) and the best ð15 Þ resolution is around
2 mm: Again the magnetic ﬁeld causes a shift by the
Lorentz angle.

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0

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10 15

incidence angle (degrees)

25
24.5
24
23.5
23
22.5
22
21.5
21
20.5
20

-15 -10 -5

0

5

401

10 15

incidence angle (degrees)Fig. 11. Resolution in mm versus angle for non-irradiated modules at 1 fC (left) and irradiated at 1 fC (right). Filled markers aremeasurements in a 1.56 T magnetic ﬁeld, open markers without ﬁeld.

The deviation of the drifting carriers due to theLorentz effect leads to a shift of the apparent
position by several mm. A good description of thebias voltage dependence of the Lorentz angle
allows for a precise correction of this effect.

x
80 um
-1
Passive Strip

0

1

2

Active Strips

4.6. Edge measurements
In the SCT there will be a small overlap between
neighbouring modules. This will allow theirrelative alignment to be calculated from tracks
which pass through the edge of overlapping
modules. It is therefore important that thedetectors remain efﬁcient at their edges, and thatthe residuals are not signiﬁcantly distorted by edge
effects. The numbering convention used in thissection (shown in Fig. 12) is f0; 1; 2; 3; . . .g for theactive strips. The very edge strip is not connectedto the readout ASICs. This passive strip is assigned
the number 1: Immediately outside strip 1 isthe bias ring.
To investigate the behaviour of the modules
near their edges, some of the barrel modules wereoffset so that the beam passed through the cornersof the detectors or through the gap between thedetectors.
Fig. 13a shows the residuals as a function of the
projected perpendicular position of the track fromthe centre of the ﬁrst active strip. No deviation
signiﬁcantly greater than 1 mm is observed for the
tracks centred on strip #1 or beyond. No
signiﬁcant difference was observed when the bias

Fig. 12. Diagram demonstrating the numbering convention forstrips (not to scale).

is varied, as was also found for the irradiatedmodules.
Those tracks in the bin with projected positions
40oxo40 mm (strip #0) show a 2 mm average
shift towards the centre of the module. Such a shift
is to be expected, since the charge sharing towards
the edge of the module will not be detected by thepassive strip. From plots like Fig. 4 it is found thatabout 7% of clusters have more than one hit. We
would therefore expect to ﬁnd a systematic shift in
the residuals for the last active strip of about
7%=2  40 ¼ 1:4 mm; where the factor of one halfcomes from half of the charge being shared in eachdirection.
The fall-off in efﬁciency near the edge of thedetector is shown at right in Fig. 13. It can be seenthat the module remains fully efﬁcient to the centreof the last read-out strip, beyond which the
efﬁciency drops rapidly to almost zero over a

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1

4

0.8

2
Efficiency

Residual / um

3

0053

1
0
-1
-2
-3
-4
-5

0.6
0.4

0053
0034*

0.2
150 V
350 V
0
200
400
600
800
Distance from center of last strip / um

0-200 -100 0 100 200 300 400 500
Distance from center of last strip / um

Fig. 13. Residuals (left) and efﬁciency (right) at the edge of the detector as a function of the projected distance of the track from thecentre of the ﬁnal read-out strip. A positive residual means that the average hit position is towards the centre of the module relative tothe projected track position. Grey lines indicate active strips. Data taken at normal incidence in the absence of an applied magnetic
ﬁeld. In the rightmost ﬁgure the bias on the irradiated module number 0034 was 460 V. Data from both planes have been plotted
together to increase the statistics.

0035 (200 V)

physical gap

distance about 40 mm: This is what one might
expect since beyond this point the majority of thecharge will fall in the inactive region.
Each plane of a module consists of pairs of
62 mm long strips wire-bonded together to obtainan effective 124 mm strip length. The module isobviously not expected to be efﬁcient in the gap
between the two detectors. The width of this ‘‘deadarea’’ should be close to 2090 mm; the physicaldistance between the ends of the pþ implant on theadjacent detectors. The detectors themselves are
precision-placed to within a few microns.
The width of this dead area was investigated in
the beam test. Efﬁciency was measured as afunction of the projected position of the track inthe direction parallel to the strips. The size of theregion was found by ﬁtting a step function,
smeared with a Gaussian to each edge. The fullwidth at half-maximum of the ﬁtted gap was foundfor each side of three modules. The size of the gap
was found to lie in the range 2038–2082 mm; with aweighted mean of 2057 mm (see Fig. 14), indicatingthat part of the region between the implant andbias ring is efﬁcient.
With the measured gap width the average loss in
efﬁciency for normally incident particles caused by
the dead region will be about 2 mm/
124 mm= 1.6% for a single plane. The effect on

0020* (400 V)
0029 (200 V)

2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

gap / µm
Fig. 14. Fitted full-width at half-maximum of the gap region
for both sides of each of three modules. The values are offset
vertically for clarity, the lower of each pair being the front side.
Data taken with no magnetic ﬁeld, and at normal incidence.

a whole module can be calculated from thegeometry. Due to the 40 mrad stereo angle
between both detectors, the gap between detectors
on one side of the module will be partially coveredby those on the opposite side. There will remain
four triangular regions each of size 12ð61:44=2Þ2 sinð0:04Þ ¼ 19 mm2 in which one can expect a hiton only one detector—corresponding to 1% of thetotal area. The fractional area left uncovered byany of the module’s four detectors is a further
1.1%.

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There is no metalisation over the end of the strip
on the far detector because of the presence of apolysilicon biasing resistor. To investigate whether
this might cause a decrease in efﬁciency near theend of the strips, the asymmetry in the sharpnessof the rise- and fall-rates was measured. No
signiﬁcant asymmetry was observed at the 90%
conﬁdence level, either in the irradiated or theunirradiated modules.
4.7. Timing measurements
In the conventional beam test analysis describedin the previous sections events from a narrow
trigger phase window are selected to optimise thesampling time with respect to the charge deposition time and thus evaluate the module performance in an environment similar to the expected
operation in ATLAS. An alternative approach is
to explicitly reconstruct the dependence of collected charge on the charge deposition time. In thissection the method is outlined. A summary of thisstudy was published in Ref. [14].
The pulse shape reconstruction takes advantageof the fact that test beams are normally continuous, i.e. the sampling time varies randomly overthe length of a clock cycle—25 ns. The relative
charge deposition time is measured as the delay
between the raw trigger from the scintillators andthe next rising edge of the 40 MHz system clock.
The measured time range is extended to cover75 ns using the hit information from the previous
and next clock cycle provided by the ABCD.
Combining the efﬁciency versus time dependence for all runs of a threshold scan provides a
two-dimensional map of efﬁciency versus time andthreshold. For each 1 ns time slice, the variation of
efﬁciency with comparator threshold forms an Scurve. As in the previous analysis, the threshold isexpressed as the equivalent charge injected to thefront-end obtained from the response curve. The
median charge for each time slice is found as thethreshold where 50% efﬁciency is obtained(VT50). The ﬁt function of Eq. (1) is replaced by
a complementary error function to ensure a robust
ﬁt of the out-of-time bins.
The resulting charge versus time curve represents the median time-resolved input of thediscriminator. Fig. 15 shows the resulting reconstructed pulse shapes of two modules. The shapeof the non-irradiated module (leftmost ﬁgure) is
described fairly well by a parametrisation of the
expected pulse produced by the ABCD shaper.
The rise time of the pulse agrees with the design
value of 20 ns.
The result on the irradiated modules clearly
deviates from the expected shape: the experimental
pulse shows a shoulder. This shoulder persists up
to the highest bias voltage in the beam test (500 V).
This effect is also observed in measurements withthe fast pulse from the calibration circuit [12]. The
best ﬁt of the expected pulse shape to the data is

4.5

4

4

3.5

3.5
Charge (fC)

Charge (fC)

4.5

3
2.5
2
1.5

3
2.5
2
1.5

1

1

0.5

0.5

0

403

0

10 20 30 40 50 60 70
Time (ns)00

10 20 30 40 50 60 70
Time (ns)Fig. 15. The reconstructed pulse shape due to beam particles in a non-irradiated barrel module with a detector bias of 150 V (leftmost
ﬁgure) and an irradiated barrel module biased at 500 V. The measured points are ﬁt with a parametrization of the shaper response for
non-irradiated modules (see Ref. [28]).

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now obtained for a signiﬁcantly larger peaking
time (approximately 24 ns). The increase of thepeaking time after irradiation is conﬁrmed by time
walk measurements using the charge injection
circuit on many modules.
These observations suggest that the different
response after irradiation is not due to a detector
effect, but a property of the front-end. A possible
explanation is an extra delay in the discriminatorfor signals with a small over-drive (i.e. signals thatare close to the threshold).
The implications of the increased rise time of thefront-end on the operation of the SCT detector
have been studied in detail. In particular, inefﬁciencies due to out-of-time hits and the occupancy
load of these hits in the consecutive event (ghosthits) have been studied.
A too slow response could lead to out-of-timehits that have the time-stamp of the consecutive bunch crossing. These (ghost) hits cannot
be assigned to the track and thus are asource of inefﬁciency. Moreover, they add to theoccupancy of the consecutive event. The ghost
hit occupancy depends on the signal occupancy
and the probability that a track leaves aghost hit in the subsequent clock cycle. As the
maximum signal occupancy is about 0.6% [2], aghost hit probability of the order of 1%
yields a maximum occupancy of 6  10 5 ; insigniﬁcant compared to the allowed noise occupancyð5  10 4 Þ:
A special feature of the discriminator—theedge sensing circuit—effectively limits the signal
to the time bin where the rising edge of thesignal crosses the threshold. The leftmost ﬁgure of
Fig. 16 shows the efﬁciency dependence on thearrival time of the particle in level-sensing
discrimination. In level sensing mode (ﬁlled
circles) hits are recorded over a time interval much
longer than the 25 ns clock cycle. The effect of theedge sensing circuit is shown with open circles inthe same ﬁgure: the range where the module is
efﬁcient is limited to 25 ns. Virtually no hits related
to the track are found in the consecutive clock
cycle.
The edge sensing circuit effectively rejects
ghost hits due to the tail of the signal inducedon the strip. Some charge, however, is shared

between two strips through dispersion of
charge in the silicon or cross talk due to theinter-strip capacitance. Small charges due tocharge sharing are likely to give a late discriminator response (time walk) and can thus
appear in the clock cycle subsequent to thetriggered cycle.
The effect of irradiation on the timing characteristics is illustrated in the right-most ﬁgure of
Fig. 16. The irradiated module shows clearlyslower rising and falling edges. As a consequence,the efﬁciency plateau is much narrower. Assumingthat the clock and trigger jitter in ATLAS is of theorder of 1 ns the efﬁciency for optimal timing isdetermined as the average in a window ½tc
1:5; tc þ 1:5: The maximum efﬁciency found withedge sensing discrimination is found to be wellover 99%.
The probability of a ghost hit in the subsequentbunch crossing is read from Fig. 16 as the‘‘efﬁciency’’ in a similar window centred on tc þ
25 ns: The ghost hit probability is found to be
below 1% for all modules. The ghost hit rate inirradiated modules can be reduced by raising theanalog supply voltage of the chips ðV cc Þ to 3.8 V(the nominal value is 3.5 V) or the discriminator
threshold.
The timing in ATLAS will be optimised moduleby-module, i.e. it relies on the similarity of the
response of all chips on the same module. As ameasure for the variations in response between
chips, the responses (rising edges of the signal) of
two chips on the front and the back of the module
are compared. For irradiated modules the averagetime difference between the rising edges of two
randomly chosen chips on the front and back of
the module is 2  1 ns: Non-irradiated modules
show much smaller differences in timing: 0:4 
0:3 ns:
The width of the distribution—the difference
between the times corresponding to rising andfalling edge—is found to be 25:1  0:1 ns for all
modules.10 No systematic differences are observed
between modules.

10
Measured at 1.2 fC. At 1.0 fC the distribution is slightly
broader: 25:3  0:1:

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F. Campabadal et al. / Nuclear Instruments and Methods in Physics Research A 538 (2005) 384–407

0.6
0.4
0.2

405

0.6
0.4
0.2

0
0

10 20 30 40 50 60 70
trigger time (ns)00

10 20 30 40 50 60 70
trigger time (ns)Fig. 16. Efﬁciency versus arrival time of the particle. Leftmost ﬁgure: a non-irradiated endcap module at a detector bias of 250 V and adiscriminator threshold of 1 fC. The discriminator is operated in level sensing (ﬁlled circles) and edge sensing mode (open circles).
Rightmost ﬁgure: an irradiated endcap module. The bias voltage and discriminator threshold are set to V bias ¼ 450 V and Qthr ¼ 1 fC:

5. Conclusions
Repeated beam tests of ATLAS SCT modules
have yielded much information on the performance of a binary readout scheme for silicon
micro-strip detectors. The measurements discussed
here demonstrate that the various geometricaltypes satisfy the ATLAS detection efﬁciency andnoise speciﬁcations. The measured spatial
pﬃﬃﬃﬃﬃ resolution is in agreement with the pitch/ 12 expectation for single strip clusters.
For non-irradiated modules at the reference
detector bias voltage of 150 V, the charge collection efﬁciency is over 95%. Modules of the barrel
and endcap types have similar charge collection
results. An analysis of the charge loss mechanisms
in the binary readout scheme explains the difference between the measured median collectedcharge and the (GEANT4) expectation for the
median deposited charge. The median signal-tonoise ratio of modules with 12 cm detectors is 13.
For the shorter inner endcap modules the improved noise performance leads to a S=N 20.
For modules irradiated to 3  1014 protons=cm2
and operated at a detector bias voltage of 350 V,
the collected signal is approximately 90% of thatcollected at the maximum bias voltage of nearly
500 V. The noise increases signiﬁcantly as a resultof the irradiation. The median signal-to-noise ratio
decreases to values in the range 7–11 at 500 V. The
reduced signal-to-noise ratio leads to a narrow
operating range where the detection efﬁciency

ð499%Þ and noise occupancy ðo5  10 4 Þ speciﬁcations for ATLAS are met. Modules at half thisdose have median signal-to-noise around 11.
The deviation of drifting charge carriers in amagnetic ﬁeld has been measured on several
modules. For a 1.56 T magnetic ﬁeld and a
detector bias voltage of 150 V a Lorentz angle of
3:3  0:3 is found. At higher voltage with
irradiated modules, the Lorentz angle decreases
signiﬁcantly: yL ¼ 2:1  0:4 at 350 V. The measurements are in good agreement with the modelof Ref. [39].
The tracking efﬁciency and spatial resolution
remain unaffected over the full active area of thesensor. Only the last read-out strip has a slightly
lower efﬁciency (by a few %) and shifted residuals(by a few mm) due to asymmetric charge sharing.
Measurements of the width of the inefﬁcient gap
between two daisy-chained sensors show that it issmaller than the physical distance between the endof the implants by approximately 30 mm:
Measurements of the time response of the frontend yield a value in agreement with the design risetime of the shaper (20 ns) for non-irradiatedmodules. For heavily irradiated modules the risetime becomes signiﬁcantly larger. The effect on the
SCT performance—efﬁciency, ghost hit occupancy—is however negligible.
Importantly, the performance of SCT modules
is—to a large extent—understood. Beam test
results have been incorporated into the digitisationof the GEANT4 simulation of the ATLASARTICLE IN PRESS
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F. Campabadal et al. / Nuclear Instruments and Methods in Physics Research A 538 (2005) 384–407

detector [40]. A realistic description of the bias
voltage and incidence angle dependence of the
response is implemented for non-irradiated modules. The model is currently being updated using
more recent beam test data [41].

Acknowledgements
We thank the technical staff at CERN and KEK
for the operation of the accelerators and thebeamline facilities.
We acknowledge the support of the funding
authorities of the collaborating institutes including
the Australian Research Council; the Australian
Department of Education, Science and Training;
the Polish State Committee for Scientiﬁc Research;
the Ministry of Education, Science and Sport of
the Republic of Slovenia; the Spanish National
Programme for Particle Physics; the Particle
Physics and Astronomy Research Council of theUnited Kingdom; the United States Departmentof Energy; and the United States National Science
Foundation.

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