Beamtest of Non-irradiated and Irradiated ATLASSCT Microstrip Modules at KEK
Y. Unno, T. Matuo, T. Hashizaki, T. Akimoto, J. Bernabeu, Z. Dolezal, L. Eklund, K. Hara, Y. Ikegami, Y. Iwata,
Y. Kato, C. Ketterer, H. Kobayashi, T. Kohriki, T. Kondo, T. Koshino, J. Ludwig, T. Masuda, G. Moorhead,
I. Nakano, K. Norimatsu, T. Ohsugi, K. Runge, S. Shinma, R. Takashima, R. Tanaka, N. Tanimoto, S. Terada,
N. Ujiie, M. Vos, K. Yamanaka, and T. Yamashita
Abstract--Non-irradiated and irradiated ATLAS SCT barrel andendcap modules have been beamtested with 4 GeV/c pions. Pulse
shapes confirmed the peaking time of the amplifier to be 22 ns
with slight deterioration in the irradiated modules. Mediancharges saturated around 3.8 fC both in the non-irradiated andirradiated modules. Signal/Noise ratios, using the noise estimatesfrom the in-situ calibration, were >16 in the non-irradiated(>150 V), and >10 in the irradiated (>300 V) barrel modules. No
excess common-mode noise was observed.

I. INTRODUCTION
In order to investigate the origin of the mass of elementary
particles and search for new physics, a new accelerator, LHC(Large Hadron Collider), is being built at CERN which will accelerate and collide protons at the centre of mass energy of
14 TeV, with a collision luminosity of 1034 cm-2sec-1 and abeam crossing interval of 25 ns. In one of the LHC experiments, ATLAS, the detection and the measurement of momentum of charged particles will be made in the inner detector,
consisting of pixel (PIXEL), silicon microstrip (SCT), andtransition-radiation (TRT) detectors, inside a 2 Tesla solenoid
magnet of a diameter of 2 m [1].
The SCT provides high-precision tracking information witha position resolution of 23 µm per sensing plane. It comprisesa central barrel section of 4 cylinders and two endcap sectionsof 9 disks each. The cylinders or disks have two sensing planes
per layer, with total areas of silicon of 34 and 27 m2 in the barrel and the endcap sections, respectively. The silicon microstrip sensors will receive a cumulative fluence of 2x1014
1 MeV-neutron-equivalent particles/cm2 over 10 years of operation at the inner-most radius of 30 cm from the collision axis.
The silicon planes are implemented in detector units called"modules" made of silicon microstrip sensors and fast-shaping, on-off discriminating readout electronics, combined withelectrical, mechanical, and thermal structures [2]. After intensive development, a number of realistic SCT modules, fully
equipped with SCT-specified components, have become available, of which a significant number have been irradiated to thefull fluence of particles.
A number of beamtests to study individual components
have previously been carried out at KEK using pion beam of
4 GeV/c at the 12 GeV proton synchrotron [3]. Recently, two
beamtests were carried out of fully equipped SCT modules [4].
These were the first beamtests of realistic SCT modules.
II. SCT SILICON MICROSTRIP MODULES.Manuscript received Nov. 23, 2001. This work was supported in part by
the Grant-in-aid for Scientific Research of Japan Ministry of Education, Science, and Technology, Australian Research Council and Department of Industry, Science and Resources, German Ministry for Education and Research(BMBF), and Valencia University and the Spanish research council CSIC.
Y. Unno, T. Kohriki, T. Kondo, S. Terada, N. Ujiie are with the Institute ofParticle and Nuclear Studies, High Energy Accelerator Research Organization(KEK), Tsukuba 305-0801, Japan
T. Matuo, T. Hashizaki, T. Koshino, I. Nakano, K. Norimatsu, R. Tanaka,
N. Tanimoto, T. Yamashita are with the Physics department, Okayama University, Okayama 700-8530, Japan
C. Ketterer, J. Ludwig, K. Runge are with the Department of Physics,
Albert-Ludwig University of Freiburg, D-79104 Freiburg, Germany
Z. Dolezal is with the Institute of Particle and Nuclear Physics, Charles
University, CZ-180 00 Prague 8, Czech Republic
Y. Iwata, T. Masuda, T. Ohsugi, K. Yamanaka are with the Physics department, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
R. Takashima is with the Education department, Kyoto University of Education, Kyoto 612-0863, Japan
G. Moorhead is with the School of Physics, University of Melbourne,
Parkville, Victoria 3052, Australia
T. Akimoto, K. Hara, Y. Kato, H. Kobayashi, S. Shinma are with the Institute of Physics, University of Tsukuba, Tsukuba 305-8571, JapanL. Eklund is with the Department of Radiation Science, Uppsala University, S-75121 Uppsala, Sweden
J. Bernabeu, M. Vos are with the Instituto de Fisica Corpuscular, Univ.
Valencia/CSIC, E-46071, Valencia, Spain

A. ATLAS SCT silicon microstrip sensor
After intensive studies of the radiation damage in semiconductors and following the development of radiation-tolerant
designs, ATLAS SCT has chosen a type of silicon microstrip
sensors with p-implant strips in n-bulk silicon wafers,
so-called p-in-n sensors. Several geometries of otherwise similar detector technology have been implemented; one, essentially square, is used for the barrel modules, and five different
wedge shapes used for the endcap modules. The sensors aredesigned to have a mean strip pitch of 80 µm at the centre of amodule. The parameters of the barrel and the endcap sensors
used in the beamtest are summarized in Table I [5].
B. Frontend readout ASIC
The signals from the strips are read out by frontend circuitswith high gain (50mV/fC), unipolar fast shaping (20 ns peaking time), on-chip discrimination (binary readout), a digitalTABLE I
PARAMETERS OF THE ATLAS SCT P-IN-N SILICON MICROSTRIP SENSORS
ModuleSensor type

BarrelEndcapp-in-n, AC-coupled, Single-sided

Thickness

285 ± 15µm

Shapesquarewedge

Size (outer)(width×length)63.6mm x 64.0mm

71.8/64.7mm x 57.5mm
64.6/56.5mm x 65.5mm

Bulkn-type, high resistivity (about 4 kΩcm)

Strip typep+

Strip pitch

80 µm

85.95, 77.45 µm

Strip length

62 mm

55.5, 63.5 mm

Strip width

16 µm, (22 µm: Al metal)

Number of strips

768 readout + 2 shaping

Strip AC coupling

SiO2 + SiliconNitride, >20 pF/cm

Backside

Uniformly doped n+ layer

pipeline for the trigger decision delay of 3.3 µs duration, andan eight-event deep derandomising buffer to permit sustained
operation at trigger rates up to 105 per second without deadtime. The custom SCT design, the ABCD, is implemented as a
BiCMOS single chip application-specific- integrated-circuit(ASIC) in a radiation-tolerant technology [6]. Each ASIC has
128 channels with a common threshold for discrimination.
Following testing of the first generation ABCD, the second
generation, ABCD2T, implemented a 4 bit DAC adjustment to
the threshold at each channel to reduce excess threshold spread(trim DAC), and shielding the input pads for excess noise protection [7]. The third generation, ABCD3T, fixed poor matching in the circuit which generated the correction voltage for thediscriminator threshold, and added a 2-bit range for the step of
the trimDAC to cope with the deterioration in the trimming
offset spread after irradiation [8].
C. Barrel and endcap modules
The SCT modules are made from four silicon microstrip
sensors, glued in pairs on each side of a baseboard for total
strip lengths of 12 cm. The two over-lapping planes are slightly
rotated with respect to each other by an angle of 40 mrad. This
small stereo angle allows measurement of a space-point withcoarse longitudinal resolution while retaining the high transverse spatial resolution with two nearly parallel planes. Each
module is equipped with 12 ABCD chips on a hybrid, 6 on the
top and 6 on the bottom side. In the barrel modules, the hybrid
is placed near the centre of the module in a wrap-around configuration; in the endcap modules, the hybrid is placed at one
end of the module and is double-sided. The different designs
were driven principally by the difference in cylindrical anddisk geometries. Photographs of the barrel and endcap modules are shown in Fig. 1 and Fig. 2.
Thermal management is critical to extract the heat of the
ASICs (5 to 8 W) and of the sensors, especially after the sensors are heavily radiation-damaged(1 to 2 W). The baseboard

Fig. 1. ATLAS SCT barrel module

Fig. 2. ATLAS SCT endcap module, together with a module frame. A
black line in the sensors was a line on a transparent protection plastic
cover.

with a high thermal conductivity is a key element to preventthe thermal runaway of the sensors. The barrel module iscooled at one area at the wide tab, whereas the endcap module
has two small cooling areas: one at the junction of the sensor
and the hybrid, and the other at the far-end of the sensors.
Two modules, one barrel and one endcap, were irradiatedprior to the beamtest to the fluence of ~3x1014 protons/cm2 at
the 24 GeV proton synchrotron at CERN, equivalent to the~2x1014 1 MeV neutrons/cm2. The modules were irradiatedcold and then annealed immediately after the irradiation for 7
days at 25 °C to simulate the effects of the warm-up for the
maintenance in the real experiment. Additionally, the modules
were at room temperature for several days during the beamtest

preparation but otherwise kept cold (<-10 °C).
III. BEAMTEST
A. Setup
Two beamtests are reported here. In the first, three non-irradiated modules were tested: two barrel (k3103, k3104) and one
endcap (FR-k81), all with ABCD2T chips. In the second, six
modules were tested: two non-irradiated (011, 022) and one irradiated barrel (003), two non-irradiated (VAL-k3-165,
CG-k3-170), and one irradiated endcap (VAL-k3-166) modules, all with ABCD3T chips [9]. Since the results of the second beamtest supersede and expand on the first, only those of
the second test are presented in this paper. The characteristics
of modules in the second test are summarized in Table II.

was fixed in the more recent ABCD3TA chips.) The ranges
were stuck to 1 and 2 in mod3, and 2 and 3 in mod4, in the 4chips illuminated by the beam. The number of channels
masked due to this out-of-trimming was, however, small: 1 and4 channels in the mod3 and the mod4 modules, respectively.
C. Charge calibration
The relations of the threshold voltages and the charges were
calibrated, in situ, by injecting charges with the internal circuitry in the ABCD chips. The resulting calibration curves, averaged over the chips in the beam spot, are shown in Fig. 3.

500
mod1
mod2
mod5
mod6
mod3
mod4
mod1(fit)mod2(fit)mod5(fit)mod6(fit)mod3(fit)mod4(fit)

400

NAME, TYPE, ASIC, AND IRRADIATION CHARACTERISTICS OF THE MODULESIN THE SECOND BEAMTEST

Beamtest id

Name

Type

ASIC

Irradiation

mod1

011

barrelABCD3T

non-irrad

mod2

022

barrelABCD3T

non-irrad

mod3

003

barrelABCD3T

irrad

mod4

VAL-k3-166

endcap

ABCD3T

irrad

mod5

VAL-k3-165

endcap

ABCD3T

non-irrad

mod6

CG-k3-170

endcap

ABCD3T

non-irrad

Threshold [mV]TABLE II

300

200

100

0
0

1

2

3

4

5

6

7

Threshold [fC]

The modules were attached to aluminium module frames
and positioned in a thermo-box in the beamline. Three analogue readout "Si-telescopes" provided independent tracking
with a position resolution of about 5 µm in horizontal and invertical directions. Pairs of modules, mod2 and mod3, andmod4 and mod5, were sandwiched between telescope planes,
all spaced at 30 mm intervals. The smearing by multiple scattering was estimated to be negligible by interpolating the positions of the incident particles with two adjacent telescopes.
Readout was triggered by scintillators of 2 cm x 2 cm placed
upstream. The beam size (FWHM) was about 1 cm x 1 cm inthe modules.
The modules were cooled in two ways: the overall environment inside the thermo-box was cooled with a circulation ofcold air of about -20 °C, and, in addition, the irradiated modules were cooled with a liquid cooling of about -13 °C. The
temperatures of the barrel hybrid were about 0 °C.
The temperatures of the sensors were estimated to be about-10 °C. Typical leakage currents of the non-irradiated and theirradiated modules were 0.2 µA and 2 mA, respectively. Problems were encountered in cooling the endcap modules, with
their hybrids running at about 37 °C and 54 °C in the non-irradiated and the irradiated modules, respectively.
B. Trim range setting in the irradiated modules
After the irradiation, ABCD3T chips showed a problem insetting the trim DAC range of the trim circuitry. (This problem

Fig. 3. Calibrations of the threshold in mV and fC

The internal calibration circuitry required several corrections: the variation of the values of the charge injection capacitance (estimated to be ~7% from test structures), and variationof charge scales due to chip temperatures (0~5%) and radiation
damage (0~3%). These factors were determined separately foreach module and applied in the analysis presented below.
D. Data taking and analysis
Two parameters were varied during data-taking: the sensor
bias voltage, between 25 and 275 V for the non-irradiated and
150 and 500 V for the irradiated modules, and the threshold between 0.7 and 6 fC. In the following analyses, the conditions
taken for a typical setting were, otherwise mentioned, (1) bias
voltages of 150 V and 350 V for the non-irradiated and the irradiated modules, and (2) threshold at 1 fC. In the plots, thevoltages of the irradiated modules were corrected for the voltage drops in the series resistance in the bias supply lines.
IV. DATA ANALYSIS
A. Pulse shape reconstruction
The ABCD chip has, after amplification and shaping, discriminator circuitry. The output is sampled at a fixed phase of
the 40 MHz clock. Unlike at the LHC where the clock will be
synchronised with collision frequency, in the beamtest the ar-Median Pulse Height [fC]5
non-irrad
CRRC3
irrad
CRRC3

4

3

2

correction was made to the range in other bias voltages since
the move of the peak position was small.

50

Peaking time [ns]rival time of the triggering particle is random relative to theclock phase. This interval was measured using a TDC which,when combined with the threshold scans, allowed the time distribution of the median pulse heights to be reconstructed.
The resulting time distributions of the median pulse heights
are shown in Fig. 4 for the non-irradiated (average of mod1
and mod2) and for the irradiated (mod3) modules. In Fig. 4,
the offsets of the peaking were adjusted to be at 40 ns. The impulse response curves of a first-order differential and third-order integration circuitry, CR-RC3, were fitted to the rising partof the pulses between 20 and 45 ns. The peaking time of the
non-irradiated module was thus found to be 22 ns and that of
the irradiated 27 ns. The peaking time of the non-irradiatedchips is consistent with the ABCD specification. The peaking
time of the irradiated module was longer due to a slower
charge collection in the radiation-damaged sensors and a slower speed of the damaged amplifiers.
Another distinctive feature of the reconstructed pulse
shapes was the shoulders seen after the peaking. This shoulder
could not be reproduced in a simulation of the charge collection in the damaged sensors. Since this shoulder has not been
observed in similar chips without the discriminators [10], it
could be a physical feedback from the discriminator.

mod1
mod2
mod5
mod6
mod3
mod4

40

30

20
0

100

200

300

400

500

Bias voltage [V]Fig. 5. Bias voltage dependence of the peaking times

B. Position resolutions
Consecutive hit-strips were clusterized within each sensing
plane to define a hit-cluster with the geometrical centre as the
position of the "cluster centre" and the number of strips as the"cluster width". The trajectories of the incident particles were
defined by the two adjacent telescopes in order to minimize theeffect of multiple scattering.
The differences of the cluster centres from the projected hit
gave the position resolution of the sensing plane. The deviation
distributions of one module, mod1, is shown in Fig. 6, for all
events (circle) and for the cluster width greater than 1, the"multi-hits", (diamond). Also shown are the Gaussian functions fitted without error-weighting.

1
120
all events
multi-strip
fit(all events)
fit(multi-strip)

0
0

20

40

60

80

100

100

Fig. 4. Median pulse height distributions as a function of time between
trigger and clock phase: non-irradiated (open circle) and irradiated (filled
circle) modules, and the impulse responses of CR-RC3 circuitry with thepeaking times of 22 and 27 ns, respectively

The peaking times of the modules as a function of bias voltages were shown in Fig. 5. The error bars are statistical only,
from the CR-RC3 fits to the average pulse shapes. In the
non-irradiated modules, the pulses slowed down slightly, by
3 ns, below 150 V to 50 V. The endcap modules were slower
than the barrel modules, by 5 ns in the non-irradiated and 7 ns
in the irradiated modules. This deterioration might be causedby the higher temperature of the chips in the endcap hybrids.
In the following analyses, events have been selected inwhich time was in the peak region between 35 and 45 ns. No

Events / 2.5 µ m

Time [ns]
80

60

40

20

0
-200

-100

0

100

200

Deviation [µm]Fig. 6. Deviations of the cluster centre from the expected positions ofincident particles of the mod1 module: all events (circles) and the eventswith multi-hits (diamonds), with Gaussian fits without weighting

The distribution of all events deviates from Gaussian, is

34

30

mod1
mod2
mod5
mod6
mod3
mod4

RMS resolution [ µ m]

32
30
28
26
24
22
20
18
0

1

2

3

4

5

Threshold [fC]

Fig. 8. RMS resolutions as a function of threshold.

1.5
mod1
mod2
mod5
mod6
mod3
mod4

1.4

mod1
mod2
mod5
mod6
mod3
mod4

32

RMS resolution [ µ m]

34

Mean cluster widthcloser to a uniform (box) distribution as expected for discriminated binary readout with a strip pitch of 80 µm but affectedby the telescope resolution and noise hits. The distribution of
the multi-hits is much narrower due to the predominance of
two-strip hits where the tracks pass through a small inter-strip
region (two-strip-hit sensitive region).
The root-mean-square (RMS) resolution of the distribution,
being less affected by the telescope resolution and the noise
hits, was obtained by dividing the Gaussian sigmas by a factor
1.24 which is the ratio of the RMS and the Gaussian sigma ofa uniform distribution. The resulting RMS resolutions were22.5 µm and 10 µm for the all events and multi-hits events, respectively.
In analysing the endcap modules, the variable pitch of fan
geometry was corrected to the average strip pitch of 86 µm at
the beam spot. The RMS resolutions are shown in Fig. 7 andFig. 8 as a function of bias voltage and threshold. Those of themodules outside the telescopes were corrected for the multiple
scattering effect. The RMS resolutions of the barrel and theendcap modules were about 23 µm and 26 µm, consistent with
their pitch of 80 µm and 86 µm, respectively, although those of
the endcap modules were slightly worse than the expected.

1.3

1.2

1.1

1
0

28

100

200

300

400

500

Bias voltage [V]26

Fig. 9. Mean cluster widths as a function of bias voltage

24
22
20

100

200

300

400

500

Bias voltage [V]Fig. 7. RMS resolutions as a function of bias voltage.

C. Mean cluster width
The fractions of the multi-hits, parameterized as the meancluster width, are shown in Fig. 9 and in Fig. 10 as a functionof bias voltage and threshold, respectively. There seems to bea correlation between the decrease in the RMS resolutions andthe increase in the fraction of multi-hits, e.g., below 2fC in
Fig. 8 and Fig. 10. The better resolution of the multi-hit events
can be seen in Fig. 6.
D. Efficiency scans and Median charges
By counting hits with cluster centres within a window of theincident particles (full width of 500 µm in the analysis), the ef-

mod1
mod2
mod5
mod6
mod3
mod4

1.4

Mean cluster width

18
0

1.5

1.3

1.2

1.1

1
0

1

2

3

4

5

Threshold [fC]

Fig. 10. Mean cluster widths as a function of thresholdficiency at a threshold can be obtained. The pulse height(Landau) distribution is reconstructed as an efficiency curveby scanning the threshold. An example is shown in Fig. 11 forthe irradiated module, mod3, where with the bias voltage isalso scanned. The threshold of the 50% efficiency is the median charge of the pulse height (Landau) distribution.
In order to obtain the threshold of 50% efficiency, a modified error function, (1), was fitted to the efficiency scans,eff ( q ) = p 3 ( 1 – erf ( T ⋅ f ( T ) ) )(1)

f ( T ) = 1 + 0.6 ⋅ tanh ( – p 4 ⋅ T ) and(2)

T = ( q – p1 ) ⁄ ( 2 ⋅ p2 ) .

(3)

where

The function, erf, was the integral of the Gaussian distribution.
The function, f(T), is an empirical function to modify theGaussian to the Gaussian-convoluted Landau distribution. The
fitting parameters expressed the median (p1), the width (p2),the saturation (p3), and the skew (p4).

1.2
131V
178V
225V
276V
324V
376V
448V
471V
131V(fit)
178V(fit)
225V(fit)
276V(fit)
324V(fit)
376V(fit)
448V(fit)
471V(fit)1

Efficiencies

0.8

0.6

0.4

0.2

0
0

1

2

3

4

5

6

7

Threshold [fC]

Fig. 11. Threshold scan of the efficiencies at various bias voltages in theirradiated barrel module, mod3. The lines are fits to the empirical formulain the text

Median charge [fC]

4

3

2
mod1
mod2
mod5
mod6
mod3
mod4

0
0

100

200

300

400

E. Noise occupancies and ENC
Hits which were out of time (time < 15 ns) and out of trackwindow (twice the efficiency window) were counted as noise
hits due to electronics noise. The resulting number of hit strips
was divided by the number of strips outside the track window
to obtain the noise occupancy. The noise occupancies of themodules are shown in Fig. 13 as a function of threshold-squared.
The noise occupancies at 1 fC of both the non-irradiatedbarrel and endcap modules were well below, and the irradiatedbarrel module was about satisfying the requirement for the ATLAS SCT modules: 5x10-4 [1]. Those of the endcap modules
were an order larger than of the barrel modules which could be
attributed to the difference in chip temperatures.
The relation between the noise occupancy and the threshold-squared (th2) is a straight line in logarithmic scale of occupancy because the occupancy can be approximated as2

2

occupancy ( th ) ∝ exp ( – ( 1 ⁄ 2 ) ( ( th ) ⁄ σ ) )(4)

where σ is the equivalent-noise-charge (ENC) of theamplifier [12]. The ENCs (occuENC) were fitted in th2 below
3 fC2, in general, at low thresholds. The occuENC had little
bias voltage dependence. The average above 100 V (non-irrad)and 300 V (irrad) are summarized in Table III. The ENCs (calibENC) were obtained separately in the in-situ calibration,with an average of ENCs at 1, 2, and 3fC, including the corrections in the section III.C. In comparison, calibENC’s obtained
at high thresholds were systematically larger than occuENC’s
obtained at low thresholds.

5

1Fig. 12 as a function of the bias voltage. The median chargesof the non-irradiated modules saturate above 150V to thecharges between 3.8 fC and 4.0 fC. The saturations of the median charges of the irradiated modules were slower, reachingaround 3.8 fC at 500V. The full depletion voltages of the
non-irradiated and irradiated sensors were about 70 V and
300 V, respectively.
Usually the charge collection of the irradiated silicon sensors does not reach that of non-irradiated ones [11]. The correction of the charge scale due to radiation damage could beinsufficient. The median charges of the two non-irradiated barrel modules do not coincide, nor do those of the barrel and theendcap modules. These differences could be attributed to thechip-by-chip variation of the calibration capacitance and insufficient corrections on calibration scales due to temperature.

500

Bias voltage [V]Fig. 12. Bias voltage dependence of the median charges.

The median charges of the modules obtained are shown in

F. Signal-to-Noise ratios
One way of cancelling the uncertainty of the calibration capacitance is to take the ratio of the median charges to theENCs, the signal-to-noise ratios (S/N). The S/N ratio is also anindicator of the performance since a S/N > 9 is required inpractical use from experience. In order to calculate the S/N, theworse ENCs, calibENC, were used to be conservative. The resulting S/N ratios are shown in Fig. 14 as a function of bias

1 0-1

20
mod1
mod2
mod5
mod6
mod3
mod4
mod1(fit)mod2(fit)mod5(fit)mod6(fit)mod3(fit)mod4(fit)

Occupancy

1 0-3
1 0-4

mod1
mod2
mod5
mod6
mod3
mod4

16

12

S/N

1 0-2

8

1 0-5
4

1 0-6

0
0

1

2

3
2

4

0

5

100

2

200

300

400

500

Bias voltage [V]Threshold [fC ]Fig. 13. Noise occupancies as a function of (threshold)2. The lines are fits
to the function, exp(-1/2*(th/σ)2)

Fig. 14. Signal-to-noise ratios as a function of bias voltage. The noises
were averages of 1, 2, and 3 fC of the in-situ calibration

TABLE IIICOMPARISON OF ENCS OBTAINED FROM THE OCCUPANCY AND THE IN-SITU

1.2

CALIBRATION

occuENC[fC]

calibENC[fC]

Ratio

1

0.239

0.245

0.976

2

0.222

0.234

0.949

3

0.311

0.342

0.910

4

0.385

0.411

0.937

5

0.264

0.291

0.907

6

0.264

0.290

0.910

1

0.8

Efficiency

mod

0.6

0.4

1.07fC(non-irrad)1.04fC(irrad)3.21fC(non-irrad)

0.2

3.12fC(irrad)

voltage.
A good match of the S/N was seen between two barrel andtwo endcap modules, which supported the differences of themedian charges were caused by the chip-to-chip variation of
the calibration capacitance. The S/N > 16 was reached above
150 V in the non-irradiated barrel, and the S/N > 10 above
300 V in the irradiated barrel modules. The S/N of endcap
modules were considerably lower than those of barrel modules, which can be attributed to the higher chip temperatures.
G. Charge collection in the interstrip regions
Since the charges moving in the midway region between
two strips generate currents in both strips, the induced charge
on either strip is halved. This is seen in Fig. 15 where "eta" isthe normalized distance from the strip centres. Although degradation was seen in high thresholds, the efficiencies were stillhigh at 1 fC, the expected operating threshold. The ratios of themedian charges in the strip and the inter-strip region wereabout 0.95 and 0.85 in the non-irradiated and the irradiatedmodules. The thresholds where the loss of efficiency starts
could be estimated by applying these factors to the efficiency
curves, e.g., in Fig. 11.

0
0

0.1

0.2

0.3

0.4

0.5

Eta

Fig. 15. Efficiencies between the strips. "Eta" defines the distance fromthe strips: 0 at strip and 0.5 at midway.

H. Common-mode noise
In the case the electronics system has external noise pick-up
or an internal oscillation, excess hits appear across many strips
as common-mode noise. The distributions of number of hits inthe non-irradiated (average of mod1 and mod2) and irradiated(mod3) modules are shown in Fig. 16 at the threshold of
0.7 fC. No excess common-mode noise was observed. The distribution of the irradiated module is the Poisson distribution
with a mean occupancy consistent with the threshold, 768
strips (per side), and 3 time bins.
V. SUMMARY
Two beamtests were carried out by using 4 GeV/c pionsfrom the 12 GeV proton synchrotron at KEK for the ATLASSCT barrel and endcap modules fully equipped with the SCT
specification components, including modules irradiated to afluence of ~3x1014 protons/cm2.
A pulse shape analysis confirmed the peaking time of the

105

non-irrad
irrad

Events per module side

10000

1000

100

10

1

0

50

100

150

Number of strips per module side

Fig. 16. Distribution of the number of strips hit in the non-irradiated(average of mod1 and mod2) and irradiated (mod3) modules at 0.7fC
thresholds

amplifier to be 22 ns in the non-irradiated modules with deteriorated to 27 ns in the irradiated modules. The RMS position
resolutions were consistent with the uniform distribution of thepitch of 80 µm in the barrel and of 86 µm in the endcap at thebeam spot. The variations of the resolutions were correlated to
the fraction of the multi-hits events.
The median charges of the pulse height (Landau) distribution were obtained from discriminator threshold scans, and saturated around 3.8 fC in the non-irradiated (>150 V) andirradiated (500 V) modules. The noise occupancies of the
non-irradiated barrel and endcap and the irradiated barrel module satisfied the ATLAS SCT requirement: 5x10-4.
By using the median charges and the ENCs of in-situ calibration, the signal-to-noise ratios were obtained to be >16 in
the non-irradiated (>150 V) and >10 in the irradiated (>300 V)
barrel modules. The matching of the S/N of the non-irradiatedmodules indicated the spread of the median charges could becaused by the variation of the chip-by-chip calibration capacitance.
The endcap modules were worse in two respects: rise timesof the pulses, and noises. These could be attributed to higher
temperature of the readout ASICs and improvement is foreseen. No excess common-mode was observed in the beamtests.
VI. ACKNOWLEDGEMENT
The authors wish to acknowledge the beam channel crewsof the π2 beamline of the KEK PS, L. Sospedra of Univ. of Valencia for beamtest preparation, P. Kodys of Charles Univ. fordata analysis, and all the support from the ATLAS SCT collaboration.
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