Nuclear Instruments and Methods in Physics Research A 485 (2002) 67–72

Beam study of irradiated ATLAS-SCT prototypes
T. Akimotod, J. Bernabeub, L. Eklundc, J.E. Garc!ıab, K. Harad, Y. Ikegamie,
Y. Iwatah, Y. Katod, C. Kettererf, H. Kobayashid, T. Kohrikie, T. Kondoe,
T. Koshinoa, C. Lacastab, G. Llosa! b, D. Macinag, H. Masudah, T. Matuoa,
G. Moorheadi, I. Nakanoa, K. Norimatsua, T. Ohsugih, S. Shinmad,
R. Takashimaj, R. Tanakaa, N. Tanimotoa, S. Teradae, N. Ujiiee, Y. Unnoe,
M. Vosb,*,1, K. Yamanakah, T. YamashitaabaPhysics Department, Okayama University, Okayama 700-8530, Japan
IFIC, University of Val"encia, CSIC, P.O. Box 22085, E-46100 Paterna, Val"encia, Spainc
Department of Radiation Science, Uppsala University, S-75121, SwedendInstitute of Physics, University of Tsukuba, Tsukuba 305-8571, Japane
Institute of Particle and Nuclear studies, KEK, Tsukuba 305-0801, Japanf
Department of Physics, Albert-Ludwig University, D-79104 Freiburg, Germanyg
DPNC, Universite de Geneve, CH-1211 Geneve 4, SwitzerlandhPhysics Department, Hiroshima University, Higashi-Hiroshima 739-8526, Japani
School of Physics, University of Melbourne, Parkville, Victoria, AustraliajEducation Department, Kyoto University of Education, Kyoto 612-0863, Japan

Abstract
Prototypes of ATLAS-SCT modules with ABCD readout chips were tested in a 4 GeV/c pion beam at KEK’s proton
synchrotron. Of both SCT module geometries—barrel and forward—three identical modules were placed in the beam.
One module of each type had been irradiated to 3  1014 protons/cm2 in the CERN PS previous to the beam test. A
method has been developed to reconstruct the time-resolved shaper pulse from the binary hit information, allowing amore detailed study of the timing properties of the ABCD. The present results will be compared to a simulation of thecharge collection and Front End electronics response. r 2002 Elsevier Science B.V. All rights reserved.

1. Introduction
In December 2000 a 4 GeV/c pion beam line (p2)of KEK’s proton synchrotron was made available
for a beam test of module prototypes. Of bothbarrel and forward SCT geometries, three identical*Corresponding author.
E-mail address: vos@iﬁc.uv.es (M. Vos).
1
In collaboration with Twente University, The Netherlands.

modules were submitted to the beam test, one ofwhich had previously been irradiated in the PS at
CERN to a dose equivalent to that expected after
10 years of operation in ATLAS: 3  1014 protons/
cm2 [1].
All modules were built with the same version of
the SCT readout chip, the ABCD3T [2]. Eachchannel of this chip has a preampliﬁer, shaper andcomparator circuit. Comparison of the signal to asettable threshold produces the binary (hit/no hit)
output.

0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 0 5 3 3 - 8

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T. Akimoto et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 67–72

With the binary readout scheme, statisticalmethods must be developed to extract pulse height
and timing information. Here, the binary hit
information is combined with a measurement of
the incidence time of the particle to reconstruct the
time-resolved shaper pulse. Comparison of thepulse shape at different bias voltages in nonirradiated and irradiated modules provides information on the timing relations of detector signal
and Front End electronics.
A simulation of the time-resolved detector signal
was convoluted with the approximate transfer
function of the ABCD. The results will becompared to those obtained in the test beam.
Results of earlier SCT beam tests can be foundin Refs. [3,4]. The detailed results from the KEK
test beam have been submitted for publication at alater date [5].

2. Setup
Fig. 1 shows the arrangement of the six modules
under test in the beam line. Modules of the barrel
and forward geometry have quite distinct layouts
and use different hybrid technologies [6,7]. The
three KEK modules are of the barrel type, whereas
the forward modules were built by IFIC Valencia
and CERN-Geneva. One module of each type—
KEK03 and VAL166—had been irradiated to3  1014 protons/cm2.

The KEK-PS produces a continuous beam,
whereas in the LHC the bunch crossing andreadout clocks will be synchronous. Therefore,
the time of incidence of the particles with respect
to the system clock is measured using a time todigital converter. Off-line, the time information
can be used to minimise the effect of thecontinuous beam and evaluate the performanceof the prototypes in circumstances similar to thoseenvisaged for the LHC.
A beam telescope, made up of three pairs ofsilicon microstrip detectors with analogue readout,
is used to reconstruct the tracks of passing
particles. The three pairs of telescope planes areinterleaved with SCT modules inside the light tightclimate chamber to reduce the effect on trackreconstruction of multiple scattering.
Each of the DUTs was contained in a test frame
equipped with cooling contacts similar to thoseenvisaged for ATLAS and the possibility to cool
the modules using cold liquid from a chiller. The
temperature on the forward hybrids was found to
be considerably higher than that on the barrel
hybrids, at least partly due to the different hybrid
topology.
In the irradiated modules some of the ASICs
showed a problem in the receiver circuit that
handles token passing. Also, on some chips thetrim range was not set correctly when operated at
low temperature.2 Both the problems could be
avoided by raising the digital supply voltage andthus the temperature of the ASICs. VAL166 andFig. 1. Schematic setup of the setup of the December 2000 beam test.

T. Akimoto et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 67–72

KEK03 were supplied with a digital voltage of 5.1
and 4.8 V, respectively, where the nominal digital
supply voltage is 4 V.

3. Calibration
In order to relate the comparator thresholdvoltage to input charge, a full characterisation of
the Front End shaper-ampliﬁer is made using the
ABCD’s internal calibration circuit. In the various
calibrations performed the input charge was variedin the range between 0.5 and 8 fC. For each ofthese charges a threshold scan is performed. The
50% occupancy threshold corresponding to eachcharge is found from an error function ﬁt of the Scurve.
The absolute accuracy of the calibration depends critically on the accuracy of the various
components of the calibration circuit. Measurements on test structures from the same wafer [8]and on some of the modules [9] have found small
deviations from the nominal values. The analysis
uses a parameterisation of these measurements tocorrect the charge scale.

4. Analysis
To facilitate the analysis, the raw data from all
sub-systems are processed off-line into summaryﬁles that present all data in an accessible format.
Importantly, all DUT and telescope planes areinternally aligned at this stage and tracks arereconstructed from the space points measured by
the telescope modules. The off-line pre-processingof the data includes the reconstruction of the
tracks measured by the telescope.
The analysis compares the response of thebinary prototypes to the ‘‘known’’ position of
the tracks, extrapolated from the beam telescope
information. A detector plane is considered
efﬁcient if a cluster centre is located within
100 mm of the known track position. Traditionally2

The trim range problem has been solved in later versions of
the chips. Later batches do not show the token passing
problem.

69

only events in a narrow time slice were accepted in
the analysis in order to simulate a synchronised
readout. The present analysis tries to make a fulluse of the available information by explicitly
reconstructing the dependence of the signal on
the charge deposition time.3 The time axis isextended to cover 75 ns using the hit informationfrom the previous and next clock cycle, as
provided by the ABCD. For each 1 ns timeinterval, the variation of efﬁciency with comparator threshold (S-curve) is constructed. The mediancharge is then found as the threshold where 50%
efﬁciency is obtained (VT50).
Charge sharing between adjacent strips will leadto relatively low signals that can distort the pure
pulse shape. Also, this effect is hard to estimate in
the simulation. Therefore, only tracks ‘‘right on
the strip’’, where the extrapolated track position
lies within 20 mm of the nearest strip, are acceptedin the analysis.

5. Simulation
An algorithm originally developed by Leslie,
Seiden and Unno [10] was used to simulate thedetector signal. A discrete map of the electricalﬁeld in the silicon is obtained by solving Poisson’sequation on a two-dimensional mesh. A ﬁxed
charge density is assumed, related to the depletion
voltage by VFD ¼ ed 2 jrj=2e0 eSi In the non-depleted
pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
region, covering a fraction w ¼ 1 ¼ Vbias =VFD
of the detector width, the charge density is
assumed to vanish. The depletion voltage of nonirradiated detectors is set to 60 V in agreementwith wafer measurements.
The charge deposition by 4 GeV pions issimulated by generating a random charge according to a Landau distribution, convoluted with aGaussian [11], in small intervals along the particle’s track. The charge carriers are drifted throughthe silicon according to a parameterisation by
Muller and Kamins [12]. The induced signal on the
strips by moving charges in the silicon depends on
3

The relative charge deposition time is measured as the delay
between the raw trigger from the scintillators and the next risingedge of the 40 MHz system clock.

T. Akimoto et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 67–72

amount of charge, the drift velocity of the carriers
and a weighting ﬁeld, found by solving the Laplace
equation.
For the irradiated modules, it is assumed thattype inversion has taken place and the p–njunction forms at the backplane. Partially depleted
operation is still possible, but the signal has to be
induced across the non-depleted region. The
depletion voltage after beneﬁcial annealing will
be assumed to be 250 V, a rough estimate on thebasis of the charge collection versus bias voltage
behaviour measured in the same beam test, to be
published at a later date [5].
The total input signal to the electronics is taken
to be the sum of the currents induced by bothtypes of carriers. To obtain the ABCD front end’s
response to the simulated detector signals, the strip
current is convoluted with the response function ofa CR(RC)3 shaper with an extra differentiation
step.

6. Results
The pulse shape of six modules has beenreconstructed for a whole range of detector bias
voltages.
Fig. 2 compares the pulse shape obtained from a
non-irradiated forward module (VAL165) with thedetectors biased at 150 V to a simulated pulse
using the same voltage.

4.5
4

simulation: tp=24 ns
VAL165 150 V

3.5

charge (fC)

3
2.5
2
1.5
1
0.5
0
10

20

30

40

50

60
time (ns)70

Fig. 2. Pulse shape for a non-irradiated forward module.

4.5
4

75 V
100 V
150 V
250 V

3.5
charge (fC)

70

3
2.5
2
1.5
1
0.5
0
10

20

30

40

50

60

70

time (ns)Fig. 3. Pulse shapes for a non-irradiated barrel module.

A shoulder is found on the falling edge of thepulse that cannot be reproduced by the simulation.
It is probably due to feedback of the discriminatorswitching. Apart from this shoulder, the dependence of the pulse shape on bias voltage in nonirradiated modules is described rather well by thesimulation throughout the measured bias voltage
range (25–275 V). The pulse shape hardly changes
between 150 and 275 V. At lower voltages it isstretched due to ballistic deﬁcit of the shaper(Fig. 3).
For the barrel modules the best agreement
between simulation and data is obtained for asimulated peaking time of 22 ns, in agreement with
the ABCD speciﬁcation. A consistently longer
peaking time—best-ﬁt value 24 ns—is observed on
the forward modules. This difference is most likely
due to the higher ASIC temperature on theforward modules.
As becomes clear in Fig. 4, for the irradiatedmodules the deviation of the pulse shape from theprediction by the model, the shoulder, is more
pronounced. Moreover, irradiated modules arefound to have a longer pulse shape than nonirradiated modules of the same type. For theirradiated barrel module, the best ﬁt between dataand simulation is obtained for a peaking time of
24 ns. The increase in peaking time in the forward
module is even more pronounced. This increase isthought to be the result of the combination ofslower charge collection in the irradiated detectors

T. Akimoto et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 67–72
4.5
4

4.5
simulation: tp=24 ns
K3** 500 V

4

250 V
300 V
350 V
500 V

3.5

3

charge (fC)charge (fC)

3.5

2.5
2
1.5

3
2.5
2
1.5

1

1

0.5

0.5

0
10

71

20

30

40

50

60

70

0
10

20

30

40

50

60

70

time (ns)time (ns)Fig. 4. Pulse shape for an irradiated barrel module.

Fig. 6. Pulse shapes for an irradiated forward module.

4.5
4

250 V
300 V
350 V
500 V

charge (fC)

3.5
3
2.5
2
1.5
1
0.5
0
10

20

30

40

50

60

70

time (ns)Fig. 5. Pulse shapes for an irradiated barrel module.

and the degraded speed of the ampliﬁer after
irradiation.
Figs. 5 and 6 present the pulse shape in the two
irradiated modules for a number of bias voltages.

The peaking time of the shaper in non-irradiatedbarrel modules was found to be approximately
22 ns, in agreement with the design value. Forward
modules have slightly longer peaking times ofapproximately 24 ns. The diﬁerence is thought tobe an effect of the higher ASIC temperature on theforward modules. In either case, the pulse is notaffected signiﬁcantly by ballistic deﬁcit of the
shaper down to 150 V.
In modules irradiated to 3  1014 protons/cm2,
the pulse shape depends strongly on the bias
voltage. In the irradiated modules an increase inpulse length was observed with respect to the nonirradiated modules, even when the detectors arebiased to 500 V. This increase is attributed to theslower charge collection in the irradiated detectors
and the degraded speed of the ampliﬁer.

References
7. Conclusions
A statistical method to extract the pulse shape of
the ABCD Front End ampliﬁer/shaper has been
applied to test beam data taken at KEK.
Comparison of the reconstructed pulse shapes ina range of detector bias voltages to simulation
yields good agreement for non-irradiated modules.

[1] C.M. Buttar, et al., Nucl. Instr. and Meth. A 447 (2000)
126.
[2] The ATLAS collaboration, ABCD Chip Speciﬁcation
Version 1.1.
[3] A. Barr, et al., Beam test of prototype ATLAS SCT
modules at CERN H8 in June and August 2000, ATLASInternal Note, submitted for publication.
[4] J. Bernabeu, et al., Results of the 1999 h8 beam tests of
ATLAS-SCT prototypes, ATLAS Internal Note ID2000–
004.

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T. Akimoto et al. / Nuclear Instruments and Methods in Physics Research A 485 (2002) 67–72[5] Y. Unno, et al., Beamtest of non-irradiated and irradiated
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[6] Forward kapton hybrid, version KIII.
[7] Y. Unno, High density, low-mass hybrid and associatedtechnology, Proceedings of the LEB2000.
[8] W. Dabrowski, Measurements of ABCD3T calibration
capacitors, private communication.

[9] J. Bernabeu, et al., Measurements of the temperaturedependence of ABCD3T DACs, private communication.
[10] A. Seiden, J. Leslie, Y. Unno, Signal simulation for
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[12] R. Muller, T. Kamins, Device Electronics for Integrated
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