Nuclear Inst. and Methods in Physics Research, A 924 (2019) 153–159

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Measurement of the charge collection in irradiated miniature sensors forthe upgrade of the ATLAS phase-II strip tracker
V. Cindro d ,∗, S.H. Abidi i , A.A. Affolder h , M. Arratia a , B. Ciungu i , K. Dette i , Z. Dolezal g ,C. Escobar c , V. Fadeyev h , C. García c , I.M. Gregor b , K. Hara j , L.B.A. Hommels a , Y. Ikegai e ,P. Kodys g , G. Kramberger d , J. Kroll k , C. Lacasta c , V. Latonova k , D. Madaffari c , I. Mandić d ,M. Mikestikova k , M. Mikuž d,f , M. Miñano c , K. Nakamura e , R.S. Orr i , D. Rodriguez c , E. Rossi b ,C. Solaz c , U. Soldevila c , J. Suzuki j , R. Teuscher i , Y. Unno e , S. Wada jaCavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United Kingdom
Deutsches Elektronen-Synchrotron, Notkestraße 85, 22607 Hamburg, GermanycInstituto de Física Corpuscular (IFIC) - CSIC-University of Valencia, Parque Científico, C/Catedrático José Beltrán 2, E-46980 Paterna, SpaindExperimental Particle Physics Department, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia
e IPNS, KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japanf Faculty of Mathematics and Physics, University of Ljubljana, Jadranska ulica 19, SI-1000 Ljubljana, Slovenia
g Faculty of Mathematics and Physics, Charles University, V Holesovickach 2, Prague, CZ18000, Czech RepublichSanta Cruz Institute for Particle Physics (SCIPP), University of California, Santa Cruz, CA 95064, USAi
Department of Physics, University of Toronto, 60 Saint George St., Toronto M5S 1A7, Ontario CanadajInstitue of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, JapankAcademy of Sciences of the Czech Republic, Institute of Physics, Na Slovance 2, 18221, Prague 8, Czech Republicb

ARTICLE
Keywords:
SiliconMicrostrip
SignalATLAS
UpgradeRadiation

INFO

ABSTRACT
Miniature sensors with external dimensions of 10 mm x 10 mm were produced together with full-size sensors
for the innermost ring (R0) of the end-cap part in the upgraded ATLAS inner tracker (ITk). AC- and DC-coupled
n-type strips with three different pitches (wide, default and narrow) were processed on high-resistivity p-type
FZ silicon substrates by Hamamatsu Photonics. The miniature sensors were irradiated with 70-MeV protons at
CYRIC, Tohoku University (Japan) and reactor neutrons at the Jožef Stefan Institute (Slovenia) to three different
1-MeV neutron equivalent fluences: 0.5, 1 and 2 x 1015 𝑛eq cm−2 . The upper fluence range exceeds the highest
anticipated in the innermost part of the ATLAS ITk-Strips over the HL-LHC lifetime (∼1.25 × 1015 neq cm2 ). The
charge collection in the test sensors was evaluated systematically using a 90 Sr 𝛽-source and an Alibava analogue
readout system at reverse-bias voltages up to 1000 V.

Contents
1.
2.
3.
4.
5.

6.
7.

Introduction .................................................................................................................................................................................................... 154
Samples .......................................................................................................................................................................................................... 154
Irradiations ..................................................................................................................................................................................................... 154
I/V and C/V measurements ............................................................................................................................................................................... 154
Measurements of the charge collection .............................................................................................................................................................. 155
5.1.
Experimental setup .............................................................................................................................................................................. 155
5.2.
Dependence of the charge collection on the bias voltage ........................................................................................................................... 156
5.3.
Annealing of the charge collection ......................................................................................................................................................... 157
Edge TCT measurements ................................................................................................................................................................................... 159
Conclusions ..................................................................................................................................................................................................... 159
Acknowledgments ............................................................................................................................................................................................ 159
References....................................................................................................................................................................................................... 159∗ Correspondence to: Jožef Stefan Institute, Jamova 39, 100 Ljubljana, Slovenia.
E-mail address: vladimir.cindro@ijs.si (V. Cindro).

https://doi.org/10.1016/j.nima.2018.10.007
Received 9 March 2018; Received in revised form 27 September 2018; Accepted 2 October 2018
Available online 15 October 20180168-9002/© 2018 Elsevier B.V. All rights reserved.

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Nuclear Inst. and Methods in Physics Research, A 924 (2019) 153–159

1. Introduction
The Large Hadron Collider (LHC) will be upgraded to operate at
higher luminosities (HL-LHC), with its peak luminosity expected to beabout L = 7.5 1034 cm−2 s−1 , having approximately 200 collisions per
bunch crossing. As the existing tracking systems of the ATLAS detector
cannot cope with this high multiplicity, the ATLAS collaboration [1] willcompletely upgrade its inner detector during Phase II (2024–26) [2].
The inner detector, which provides the track information about thecrossing particles, will be replaced by an all-silicon tracking systemwith strip sensors in the outer part and pixel sensors in the innerpart. The strip-detector part will consist of 4 barrel layers and 6 disks
in each end-cap (EC) region. A sensor type of an n-strip on a p-bulk
geometry [3] was chosen as the best solution since p-bulk does notinvert with radiation and the np junction remains on the same side,which allows a partially depleted operation. The collection of electrons
on the readout-strip electrodes has a further advantage compared tosensors with p electrodes, since the electrons drift faster and have longer
trapping distances with respect to the holes. Prototype sensors werealready produced and extensively studied [4,5]. The subject of this paper
is the measurements made on sensors produced in the prototype batch
for the end-cap region of the ATLAS detector.

Fig. 1. Temperature of silicon sensor measured with three different sensors during
irradiation at the full reactor power (250 kW). After 10 min sensor was pulled out of
the reactor core.

was then flushed around the samples to maintain a temperature of
approximately −15 ◦ C. The dosimetry of the protons was made with aluminum foils attached to the samples, with the fluence uncertainty being
approximately 10%. A more detailed description of the irradiations with
protons and scaling to equivalent fluences can be found in [4]. After theirradiation the samples were stored in a freezer to prevent annealing
before mounting in the experimental setup.
The irradiations with neutrons were performed by the insertion ofsmall polyethylene telephthalate (PET) containers with samples into
the reactor core. The samples were irradiated at the full reactor power(250 kW) with a uniform flux of neutrons (𝛷eq = 1.54 1012 neq cm−2 s−1 ).
The fluence was determined with the irradiation time being in therange from 330 s to 1320 s. The samples were not biased and were notcooled during the irradiation; however, they were put into the fridge
as soon as the activation dropped to an acceptable level (approx. ten
minutes after the irradiation). The temperature of the water in thereactor tank is about 20 ◦ C, while the temperature of the sample reaches
about 40 ◦ C during the irradiation (Fig. 1.). Taking into account theactivation energy for short-term annealing (1.1 eV [8]) we can estimatethat annealing at 40 ◦ C is about 20 times faster than at 20 ◦ C. Therefore,
the annealing during the irradiation is at a comparable level to annealing
during the assembly of sensors as part of the setup, which took about
1–2 h at room temperature. It is negligible compared to the 80 min
annealing on 60 ◦ C temperature.
The spectrum of reactor neutrons is continuous in the energy range
from thermal to several MeV [9]. The 1 MeV neutron-equivalent nonionizing loss fluences were determined with the measurements of leakage
currents in the silicon diodes [10] and verified with calculations [11].
The fluence uncertainty is approximately 10%.

2. Samples
The samples were miniature n-in-p strip sensors (ATLAS12EC-R0 or
shortly R0 sensors) delivered at the beginning of 2017 and produced
by Hamamatsu on the same p-type 6-inch wafers as the full-size annularsensors for the first ring (R0) of the ATLAS ITk end-cap strip detector [6].
The strip pitch of the full size end-cap sensors ranges from 70 μm to84 μm; therefore, miniature sensors with three different pitches (70 μm,
narrow; 75.5 μm, default; and 84 μm, wide) were produced to determine
how the properties of the sensors vary with the pitch of the strips.
The outer dimensions of the miniature sensors are 10 mm ×10 mm,
with each sensor having 104 8 mm-long readout strips. The strips arebiased through 1.5 MΩ polysilicon resistors. The readout electronics is
AC coupled to the n-type implant via aluminum metallization extending
over the implant. Regular measurements of the charge collection will
also be performed on a small number of these sensors during the
production for the ATLAS upgrade as a part of a Q&A procedure. Moredetails about the design of the sensor are presented in [7].
3. Irradiations
The sensors in ATLAS will be exposed to high fluences of charged
particles and neutrons [2]. The non-ionizing energy loss (NIEL) of
the charged particles will be dominant at the smaller radii, while
the contribution of neutrons becomes more important in the outerregion, where the strip detectors will be placed. To evaluate thedamage and degradation of the detectors’ performance, irradiations of
the sensors with charged particles and neutrons and measurements of
their properties are required. Therefore, we performed irradiations with
70 MeV protons at CYRIC (Cyclotron and Radioisotope Center, Tohoku
University, Japan) and with neutrons at the TRIGA reactor (Jožef Stefan
Institute, Ljubljana, Slovenia).
The maximum 1 MeV neutron equivalent fluences for the barrel andstrip end-cap detectors are predicted to be 2.8 1014 neq cm−2 and 8.21014 neq cm−2 , respectively [2]. In addition, a safety factor of 1.5 isrequired to exclude the effects of possible uncertainities in the simulation. We irradiated the miniature sensors to three different equivalent
fluences: 0.5 1015 neq cm−2 , 1 1015 neq cm−2 and 2 1015 neq cm−2 . The
highest fluence allows an interpolation of the measured values, evenfor fluences higher than we expect in ATLAS. All irradiations were
performed without bias on the sensors.
At CYRIC the holder with the samples was moved through the proton
beam to allow for uniform irradiation. Cold gas from liquid nitrogen

4. I/V and C/V measurements
The leakage current as a function of bias voltage was measured forthe non-irradiated and proton-irradiated samples (Fig. 2), with all threedifferent pitches (default, narrow and wide). It was measured with adelay of 10 s after setting the reverse-bias voltage. For the non-irradiatedsensors the IV characteristics were measured at 20 ◦ C up to 600 V,
while for the irradiated sensors it was measured at −30 ◦ C and up
to 1000 V. Before the measurements the samples were annealed for
80 min at 60 ◦ C. There were no observed onsets of micro-discharges
with respect to any of them. For all the tested detectors the leakage
current before irradiation was less than 4 nAcm−2 , well below thetechnical specification limit (100 nAcm−2 ). The smooth behavior of
the leakage current was also measured after the irradiation and nosystematic dependence on the strip pitch was observed.
The measured values of the leakage current per cm2 at 700 V
and after 80 min of annealing are compared with the measurements154

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Nuclear Inst. and Methods in Physics Research, A 924 (2019) 153–159

Fig. 2. Leakage current measured for proton irradiated sensors with default (75.5 μm), narrow (70 μm) and wide (84 μm) pitches as a function of the bias voltage. The current wasmeasured at 20 ◦ C for the non-irradiated (a) and at −30 ◦ C for the irradiated sensors (b). The values for the irradiated sensors are measured after annealing, and they are normalized to−20 ◦ C according to the scaling given in [12]. All fluences are in the units of neq cm−2 .

Fig. 4. Full depletion voltages measured in different batches of the ATLAS prototypes.
Measurements of the ATLAS12EC-R0 sensors are described in this paper, while themeasurements for the other sensors are from [13]. The drop in the full depletion voltage
after irradiation to 1014 neq cm−2 indicates acceptor removal in the p-type bulk.

Fig. 3. Leakage current measured on ATLAS12EC-R0 miniature sensors at 700 V bias
voltage and normalized to −15 ◦ C. Straight line represents the values calculated from theleakage current constant 𝛼 = 4.0 10−17 A/cm normalized to −15 ◦ C. The measurementson sensors ATLAS07, ATLAS12A and ATLAS12M miniature sensors are from [13].

voltages well above 1000 V should be applied, even at the lowest
fluence (5 1014 neq cm−2 ) to obtain reasonable curves from which thefull depletion voltage could be estimated.

on the miniature sensors produced during previous sensor-production
runs [13] (Fig. 3). The values are normalized to −15 ◦ C with scaling I 𝛼
T2 e−1.21 eV∕kT [12]. They all agree well with the value obtained from aleakage current constant 𝛼 = 4.0 10−17 A/cm [10]. Agreement is good,even at the highest fluence, indicating that at a bias of 700 V the wholedetector volume contributes to the leakage current.
The full depletion voltage of the annealed sensors was determinedfrom the fit of the straight lines to the measured 1/C2 plot. The
C-V measurements were performed at −10 ◦ C and at 1 kHz. It is
known that the measured values for irradiated detectors depend on thefrequency and temperature [14], and that the electric field in highly
irradiated silicon detectors is not linear, as would be expected in a
simple model [15]. However, this value was chosen to be compatible
with the measurements of previous batches. The results are summarized
in Fig. 4. The full depletion voltages calculated with a stable acceptor
creation rate 𝑔c are also shown as a comparison. There is no clear
evidence for the removal of the initial acceptors in the ATLAS12EC-R0
sensors. The initial acceptor removal can be observed at the low fluence(1014 neq cm−2 ) in previous measurements, which are also shown in thisplot. There are no measurements at low fluences for the ATLAS12EC-R0
sensors to show this effect that can be seen as a drop in the full depletion
voltage measured after an irradiation to 1014 neq cm−2.
The increase of the effective dopant concentration after the neutron
irradiation was too high to be measured with the C-V method since

5. Measurements of the charge collection
5.1. Experimental setup
The signals generated in the silicon sensors with the 90 Sr source
were recorded with the Alibava readout system [16]. The signals areamplified in the analogue frontend electronics with a 25 ns peaking
time. The amplifiers are integrated into a Beetle readout chip [17].
The Alibava system samples the analogue signals with a 40 MHz clock
frequency, while the scintillator set underneath the sample triggers
the readout cycle. Only the signals sampled in a certain time window(10 ns) of the delay measured relative to the clock signal are used in theanalysis. The spread of the signal over the strips is taken into accountwith a clustering algorithm that has a seed threshold of 3.5 times thechannel’s average noise level and a neighbor threshold of 1.5 times
the noise. A Landau function convoluted with a Gaussian was fitted tothe distribution of the measured signals and the most probable signal
obtained from the fit was defined as a measure of the collected charge.
The measurements were performed with three setups (Ljubljana, KEK,
Valencia), each using several boards with Beetle chips. Amplificationof the Beetle input stage depends on the control parameters; therefore,
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Nuclear Inst. and Methods in Physics Research, A 924 (2019) 153–159

Fig. 5. Most probable value of the collected charge as a function of the bias voltage measured after the irradiation with neutrons to 5 1014 neq cm−2 (a), 1015 neq cm−2 (b) and 2 1015 neq cm−2(c). Dashed lines represent measured values before annealing, and solid lines after 80 min of annealing at 60 ◦ C. As a comparison the values measured before the irradiation are shown(a).

the calibration procedure was applied to reduce the 10% board-to-board
variations. Since the internal calibration is sensitive to the fine-tuning of
the timing, we calibrated the setups and boards with measurements on
non-irradiated samples. We assumed full charge collection above thefull depletion voltage: 23100 e− for a sensor with a 300 μm active
depth [4]. All the sensors were measured at the same temperature,since amplification of the Beetle chip also depends on the temperature.
The noise of the system was between 800 e and 900 e for all themeasurements and is correlated with the leakage current of the sensor.

of 280–290 V. The strip width does not influence the size of the signal,
except at the highest fluence and high voltages above approximately 800
V. This can be explained by the onset of the charge multiplication [20].
Before annealing the effective dopant concentration is high enough tocause large electric fields close to the strips at high voltages in which
multiplication occurs. The weighting field is higher in the vicinity ofwider strips, since the distance between the strips is smaller compared to
the narrower strips. The effect of the multiplication which occurs close
to the strips is more evident there, compared to the narrower strips.
The results measured with the proton-irradiated sensors are shownin Fig. 6, where the results for the sensors with the default pitch arepresented before and after short-term annealing. As with the neutrons,
the short-term annealing improves the charge collection since theeffective dopant concentration drops.
For a comparison, the fluence dependence for the neutron- andproton-irradiated sensors is shown in Fig. 7 for two bias voltages: 500 V
and 700 V. The charge collection of the proton-irradiated sensors ishigher than that of the neutron-irradiated sensors. The reason for this is
the lower introduction rate of stable effects with protons than for neutrons [21]. The applied voltages were not high enough to establish theelectric field in the whole volume of the neutron-irradiated detectors,
even at the lowest fluence. Therefore, the electric field in the neutronirradiated detectors does not extend as deep as in the proton-irradiateddetectors and there is less charge generated in the active volume. Thiseffect was shown with edge TCT measurements [4]. The slightly higher
trapping of the charge carriers after the proton irradiation did not reduce
this difference.
The aim of this study was also to compare the measured chargecollection with the measurement on the previous batch of ATLAS sensors(A12) [4]. The voltage dependence for the non-irradiated as well as theneutron-irradiated A12 and R0 sensors is summarized in Fig. 8. Before
the irradiation the only difference is below the full depletion voltage,
which was slightly higher for the A12 (350 V) compared to the R0

5.2. Dependence of the charge collection on the bias voltage
The results of the voltage scans of the charge measurements forneutron-irradiated sensors are shown in Fig. 5, before and after 80 minof annealing at 60 ◦ C. This annealing time was chosen to minimize
the effects of the annealing on the results: short-term annealing of
the effective dopant concentration is completed and the effect of longterm annealing is still small [18]. Short-term annealing improves thecollection of charge as a result of two effects: the effective dopant concentration drops [18] and the trapping time for the electrons becomes
longer [19]. In the case of the irradiated n-in-p strip sensors the electrons
make the dominant contribution to the signal, since they drift faster thanthe holes and move towards the region of the larger weighting field close
to the n-strip. The temperature during the measurement was measured
close to the silicon sensor at a cooling block connected to a Peltier cooler.
The samples were cooled to −20 ◦ C.
The collected charge increases with the applied voltage and even at
the lowest fluence does not saturate at high voltages since larger electricfields increase the drift velocity and reduce the trapping. The resultsof the measurement with a non-irradiated detector are also shown in
Fig. 5a. After reaching the bias voltage of approximately 300 V, thecollected charge remains constant, in agreement with the measurementof the full depletion voltage with C-V measurements, which gave a value
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Nuclear Inst. and Methods in Physics Research, A 924 (2019) 153–159

Fig. 6. Most probable value of the collected charge of the proton-irradiated sensors before (a) and after annealing (b).

Fig. 7. Fluence dependence of the collected charge measured at 500 V (a) and 700 V (b).

Fig. 9. Collected charge at different voltages measured after annealing at 60 ◦ C. Measurements were taken at −20 ◦ C. The sensor was irradiated with neutrons to 2 1015 neq cm−2 .

Fig. 8. Comparison of the measured charge in the ATLAS12EC-R0 and A12 sensors.
Fluences are expressed in the units of neq cm−2 .

5.3. Annealing of the charge collection(290 V). The difference is due to the lower wafer resistivity at which
the A12 sensors were produced (3.1 kΩ compared to 3.5 kΩ). Abovefull depletion both sensors give an approximately 23 ke− signal, whichmeans that they have the same active depth.
At the highest fluence expected in the ATLAS strip detector(∼1.25 × 1015 neq cm2 ) the signal expected at 500 V after proton
irradiation is about 13 ke− and about 8 ke− after neutron irradiation. At
700 V these numbers are 15 ke− and 11 ke− . Assuming that the damages
after the proton and neutron irradiations are additive [22], with a 50%
mixture of protons and neutrons an approximately 10.5 ke− signal is
expected at 500 V and a 13 ke− signal at 700 V. This is in agreementwith the data obtained in the previous measurements [2,4].

We performed long-term annealing at 60 ◦ C on the ministrip sensor
with a default pitch and irradiated with neutrons to 2 1015 neq cm−2 .
The charge-collection measurements taken at −20 ◦ C during annealing
intervals for voltages between 500 V and 1100 V in 100 V steps areshown in Fig. 9. The longest annealing time was 164 h. At lower
voltages and shorter annealing times the measured charge first increases
with time and reaches its maximum after approximately 160 min,then it starts to decrease. This behavior is to be expected due tothe change of the effective dopant concentration. The slight shift ofthe maximum in time (160 min) compared to the measurements ofeffective dopant concentration (80 min) can be explained by the effectof annealing on the electron-trapping time. It increases over several
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Fig. 10. Velocity profile measured with the edge TCT method in the ATLAS12EC-R0 and A12 sensors for non-irradiated sensors (a), after irradiation with neutrons to 5 1014 neq cm−2(b) and 2 1015 neq cm−2 (c). The sum of electron and hole velocities is probed by the rise of the signal [23].

Fig. 11. Charge measured with the edge TCT method in the ATLAS12EC-R0 and A12 sensors for non-irradiated sensors (a), after irradiation with neutrons to 5 1014 neq cm−2 (b) and 21015 neq cm−2 (c).

hundred minutes at 60 ◦ C [21], with a longer time constant than the
short-term annealing of the effective dopant concentration [24]. At
higher voltages the effect of multiplication [20] can be seen. Shortterm annealing initially decreases the effective dopant concentrationand consequently the electric field close to the strips, which reduces
the multiplication and thus the measured charge. After approximately

1000 min the increase in the effective dopant concentration due to longterm annealing is high enough to stimulate the amplification in highelectric fields and the collected charge starts to increase. At the longest
annealing time and the highest voltage a charge larger than before theirradiation is measured.
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6. Edge TCT measurements

SC0010107; European Union’s Horizon 2020 Research and Innovation
programme under Grant Agreement no. 654168; Slovenian Research
Agency (research core funding No. P1-0135 and project ID PR-06802);
Ministry of Education, Youth and Sports of the Czech Republic comingfrom the project LM2015058 - Research infrastructure for experiments
at CERN. The authors would like to thank Dr. M. Ito and the accelerator
operation staff of CYRIC at Tohoku University for the proton irradiation,and the TRIGA reactor operation staff at Jožef Stefan Institute (JSI) for
the neutron irradiation.

Edge TCT measurements [23] proved to be a very useful tool for themeasurement of the electric field in the sensor bulk. Electron hole pairs
are generated at a certain depth of the sensor with a narrow (FWHM≈ 8 μm) beam of infrared light (𝜆 = 1060 nm) that penetrates into thedetector from the side. The velocity of the charge carriers is probed by
the rise of the signal and by scanning the detector with a beam, whichallows the velocity as a function of depth in the detector to be measured.
Measurements at 200 V for the A12 and R0 sensors with the default pitch
are shown in Fig. 10. The difference between the sensors comes from thelower resistivity of the A12 sensors. Therefore, the electric field in R0
expands more deeply than in A12. The measured values for the R0 sensor
are more scattered than those of the A12, because the edge of the R0
sensor was not polished; however, this does not affect the conclusions.
After irradiation to the highest fluence, this difference diminishes since
the concentration of the initial acceptors becomes small compared tothe concentration of the acceptors created by the irradiation.
The charge in the edge TCT measurements is defined as the time
integral over the signal. If the integration time (in our case 25 ns)
is longer than the drift time and there is no trapping like in a nonirradiated detector, the whole charge is measured. In the absence of
charge-carrier trapping the measured charge profile (Fig. 11) above
the full depletion is uniform along the detector depth. The scales werenormalized to give the same average charge in both detectors. These
measurements also confirm that the active depths of the A12 and R0
sensors are the same (≈ 300 μm). Before the irradiations the charge
profiles in the A12 and ATLAS12-R0 sensors are both uniform, sinceboth sensors are already fully depleted at 400 V and there is no trapping.
It is similar for the velocity profile, i.e., the difference in the irradiatedsensors diminishes at the highest fluence.

References[1] ATLAS Collaboration, The ATLAS experiment at the CERN large hadron collider, J.
Instrum. 3 (8) (2008).
[2] ATLAS-TDR-025. CERN-LHCC-2017.-005. https://cds.cern.ch/record/2257755.
[3] G. Casse, et al., Comparison of charge collection efficiency of segmented silicondetectors made with FZ and MCz p-type silicon substrates, Nucl. Instrum. Methods
Phys. Res. A 591 (2008) 178–180.
[4] K. Hara, et al., Charge collection and field profile studies of heavily irradiated strip
sensors for the atlas inner tracker upgrade, Nucl. Instrum. Methods Phys. Res. A
831 (2016) 181–188.
[5] R. Mori, et al., Evaluation of the performance of irradiated silicon strip sensors forthe forward detector of the atlas inner tracker upgrade, Nucl. Instrum. Methods
Phys. Res. A 831 (2016) 207–212.
[6] R. Hunter, et al., First bulk and surface results for the ATLAS ITk Strip stereo annulus
sensors - these proceedings.
[7] C. Lacasta, et al., Design of the first full size ATLAS ITk Strip sensor for the endcap
region – these proceedings.
[8] G. Lindstroem, et al., Radiation hardness of silicon detectors: a challenge from highenergy physics, Nucl. Instrum. Methods Phys. Res. A 426 (1999) 1–15.
[9] K. Ambrožič, et al., Computational analysis of the dose rates at jsi triga reactor
irradiation facilities, Appl. Radiat. Isot. 130 (2017) 1140–1152.
[10] M. Moll, et al., Leakage current of hadron irradiated silicon detectors - material
dependence, Nucl. Instrum. Methods Phys. Res. A 426 (1999) 87–93.
[11] D. Žontar, et al., Time development and fux dependence of neutron-irradiationinduced defects in silicon pad detector, Nucl. Instrum. Methods Phys. Res. A 426(1999) 51–55.
[12] A. Chilingarov, Temperature dependence of the current generated in si bulk, J.
Instrum. 8 (2013) P10003.
[13] M. Mikestikova, et al., Study of surface properties of atlas12 strip sensors and theirradiation resistance, Nucl. Instrum. Methods Phys. Res. A 831 (2016) 197–206.
[14] D. Campbell, et al., Frequency and temperature dependence of the depletion voltage
from cv measurements for irradiated si detectors, Nucl. Instrum. Methods Phys. Res.
A 492 (2002) 402-210.
[15] G. Kramberger et. al, Modeling of electric field in silicon micro-strip detectors
irradiated with neutrons and pions, J. Instrum. 9 (2014) P10016.
[16] R. Marco-Hernandez, et al., IEEE Trans. Nucl. Sci. 56 (2009) 1642–1649.
[17] M. Agari, et al., ALIBAVA: a portable readout system for silicon microstrip sensors,
Nucl. Instrum. Methods Phys. Res. A 518 (2004) 468–469.
[18] G. Lindstroem, et al., Radiation hard silicon detectors developments by the RD48
(ROSE) Collaboration, Nucl. Instrum. Methods Phys. Res. A (2001) 308–326.
[19] G. Kramberger, et al., Annealing studies of effective trapping times in silicondetectors, Nucl. Instrum. Methods Phys. Res. A 571 (2007) 608–611.
[20] M. Mikuž, et al., Study of anomalous charge collection efficiency in heavily
irradiated silicon strip detectors, Nucl. Instrum. Methods Phys. Res. A 636 (2011)
550–555.
[21] V. Cindro, et al., Radiation damage in p-type silicon irradiated with neutrons andprotons, Nucl. Instrum. Methods Phys. Res. A 599 (2009) 60–65.
[22] G. Kramberger, et al., Performance of silicon pad detectors after mixed irradiations
with neutrons and fast charged hadrons, Nucl. Instrum. Methods Phys. Res. A 609(2009) 142–148.
[23] G. Kramberger, et al., Investigation of irradiated silicon detectors by edge-TCT,IEEE Trans. Nucl. Sci. 57 (2010) 2294–2302.
[24] E. Fretwurst, et al., Radiation hardness of silicon detectors for applications in highenergy physics experiments, J. Optoelectron. Adv. Mater. 2 (5) (2000) 575–588.

7. Conclusions
We measured the charge collection of the ATLAS12EC-R0 sensors
with pitches in the range from 70 to 85 μm and fluences up to 2.21015 neq cm−2 . This range of pitches corresponds to the ATLAS end-cap
strip tracker, which will be a part of the future tracker system (ITk).
The charge collection is equal for all the pitches at bias voltages below
800 V. This voltage is 100 V higher than the highest voltage foreseen for
the ATLAS strip tracker. Above this voltage the multiplication of charge
was observed in sensors irradiated with neutrons to the highest fluence.
Multiplication is more evident in the sensors with a wider pitch.
Measurement of the charge collection during annealing at 60 ◦ C
showed that the multiplication becomes higher at long annealing times
when the effective dopant concentration increases.
We did not observe any breakdowns at biases up to 600 V before
irradiation and up to 1000 V after irradiation, in accordance withthe specifications for the ATLAS ITk sensors. The value of the stable
acceptor creation rate for protons (0.004 cm−1 ) is in agreement with themeasurements of previous batches produced for ATLAS by Hamamatsu.
Acknowledgments
The research was supported and financed in part by the Canada
Foundation for Innovation, the National Science and Engineering Research Council (NSERC) of Canada under the Research and Technology
Instrumentation (RTI) grant SAPEQ-2016-00015; the UK’s Science andTechnology Facilities Council; USA Department of Energy, Grant DE-

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