Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Nuclear Instruments and Methods inPhysics Research A
journal homepage: www.elsevier.com/locate/nima

Charge collection and ﬁeld proﬁle studies of heavily irradiated strip
sensors for the ATLAS inner tracker upgrade
K. Hara x,y,n, P.P. Allport a, M. Baca a, J. Broughton a, A. Chisholm a, K. Nikolopoulos a,S. Pyatt a, J.P. Thomas a, J.A. Wilson a, J. Kierstead b, P. Kuczewski b, D. Lynn b, M. Arratia c,
L.B.A. Hommels c, M. Ullan d, I. Bloch e, I.M. Gregor e, K. Tackmann e, A. Troﬁmov e,
E. Yildirim e, M. Hauser f, K. Jakobs f, S. Kuehn f, K. Mahboubi f, R. Mori f, U. Parzefall f,
A. Clark g, D. Ferrere g, S. Gonzalez Sevilla g, J. Ashby h, A. Blue h, R. Bates h, C. Buttar h,F. Doherty h, T. McMullen h, F. McEwan h, V. O'Shea h, S. Kamada i, K. Yamamura i,
Y. Ikegami j, K. Nakamura j, Y. Takubo j, Y. Unno j, R. Takashima k, A. Chilingarov l, H. Fox l,
A.A. Affolder m, G. Casse m, P. Dervan m, D. Forshaw m,1, A. Greenall m, S. Wonsak m,
M. Wormald m, V. Cindro n, G. Kramberger n, I. Mandić n, M. Mikuž n, I. Gorelov o,
M. Hoeferkamp o, P. Palni o, S. Seidel o, A. Taylor o, K. Toms o, R. Wang o, N.P. Hessey p,
N. Valencic p, K. Hanagaki q,j, Z. Dolezal r, P. Kodys r, J. Bohm s, M. Mikestikova s, A. Bevan t,
G. Beck t, C. Milke u, M. Domingo u, V. Fadeyev u, Z. Galloway u, D. Hibbard-Lubow u,
Z. Liang u, H.F.-W. Sadrozinski u, A. Seiden u, K. To u, R. French v, P. Hodgson v,
H. Marin-Reyes v, K. Parker v, O. Jinnouchi w, K. Hara x,y, K. Sato x, K. Sato x,y, M. Hagihara x,
S. Iwabuchi x, J. Bernabeu z, J.V. Civera z, C. Garcia z, C. Lacasta z, S. Marti i. Garcia z,
D. Rodriguez z, D. Santoyo z, C. Solaz z, U. Soldevila zaSchool of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, United Kingdom
Brookhaven National Laboratory, Physics Department and Instrumentation Division, Upton, NY 11973-5000, USA
Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, United KingdomdCentro Nacional de Microelectronica (IMB-CNM, CSIC), Campus UAB-Bellaterra, 08193 Barcelona, Spaine
DESY, Notkestrasse 85, 22607 Hamburg, GermanyfPhysikalisches Institut, Universität Freiburg, Hermann-Herder-Str. 3, D-79104 Freiburg, Germanyg
DPNC, University of Geneva, 24, Quai Ernest-Ansermet, CH-1211 Geneve 4, Switzerlandh
SUPA- School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, United Kingdomi
Solid State Div., Hamamatsu Photonics K.K., 1126-1, Ichino-cho, Higashi-ku, Hamamatsu-shi, Shizuoka 435-8558, Japanj
Institute of Particle and Nuclear Study, KEK, Oho 1-1, Tsukuba, Ibaraki 305-0801, Japank
Department of Science Education, Kyoto University of Education, Kyoto 612-8522, Japanl
Physics Department, Lancaster University, Lancaster LA1 4YB, United KingdommOliver Lodge Laboratory, Department of Physics, University of Liverpool, Oxford St., Liverpool L69 7ZE, United Kingdom
nJožef Stefan Institute and Department of Physics, University of Ljubljana, Ljubljana, SloveniaoDepartment of Physics and Astronomy, University of New Mexico, MSC07 4220, 1919 Lomas Blvd. NE, Albuquerque, NM 87131, USAp
Nikhef, Science Park 105, 1098 XG Amsterdam, Netherlandsq
Department of Physics, Osaka University, Machikaneyama-cho 1-1, Toyonaka-shi, Osaka 560-0043, Japanr
Charles University in Prague, Faculty of Mathematics and Physics, V Holesovickach 2, Prague 8, Czech Republics
Academy of Sciences of the Czech Republic, Institute of Physics, Na Slovance 2, 18221 Prague 8, Czech RepublictSchool of Physics and Astronomy, Queen Mary University of London, London E1 4NS, United KingdomuSanta Cruz Institute for Particle Physics (SCIPP), University of California, Santa Cruz, CA 95064, USAv
Department of Physics and Astronomy, The University of Shefﬁeld, Hicks Building, Hounsﬁeld Road, S3 7RH Shefﬁeld, United Kingdomw
Institute of Science and Engineering, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japanx
Institute of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8751, JapanyCenter for Integrated Research in Fundamental Science and Engineering, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japanz
IFIC/CSIC-UVEG, Ed. Inst. Investigacion, PO Box 22085, 46071 Valencia, SpainbcnCorresponding author at: Center for Integrated Research in Fundamental Science and Engineering, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan.
E-mail address: hara@hep.px.tsukuba.ac.jp (K. Hara).
1
Now at Syracuse University.

http://dx.doi.org/10.1016/j.nima.2016.04.035
0168-9002/& 2016 Published by Elsevier B.V.

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art ic l e i nf o

a b s t r a c t

Article history:
Received 20 November 2015
Received in revised form
12 April 2016
Accepted 12 April 2016

The ATLAS group has evaluated the charge collection in silicon microstrip sensors irradiated up to a
ﬂuence of 1 × 1016 n eq /cm2, exceeding the maximum of 1.6 × 1015 n eq /cm2 expected for the strip tracker
during the high luminosity LHC (HL-LHC) period including a safety factor of 2. The ATLAS12, n þ -on-p
type sensor, which is fabricated by Hamamatsu Photonics (HPK) on ﬂoat zone (FZ) substrates, is the latest
barrel sensor prototype. The charge collection from the irradiated 1  1 cm2 barrel test sensors has been
evaluated systematically using penetrating β-rays and an Alibava readout system. The data obtained at
different measurement sites are compared with each other and with the results obtained from theprevious ATLAS07 design. The results are very consistent, in particular, when the deposit charge isnormalized by the sensor's active thickness derived from the edge transient current technique (edgeTCT) measurements. The measurements obtained using β-rays are veriﬁed to be consistent with themeasurements using an electron beam. The edge-TCT is also effective for evaluating the ﬁeld proﬁlesacross the depth. The differences between the irradiated ATLAS07 and ATLAS12 samples have been examined along with the differences among the samples irradiated with different radiation sources:
neutrons, protons, and pions. The studies of the bulk properties of the devices show that the devices canyield a sufﬁciently large signal for the expected ﬂuence range in the HL-LHC, thereby acting as precision
tracking sensors.
& 2016 Published by Elsevier B.V.

Keywords:
High luminosity Large Hadron Collider
Microstrip sensor
Charge collection
Edge transient current techniqueField proﬁle1. Introduction
The ATLAS experiment [1] at the Large Hadron Collider (LHC) is
due to undergo phased detector upgrades in accordance with theplanned accelerator upgrades. The instantaneous beam luminosity
after the Phase-II upgrade of the LHC, termed the HL-LHC (highluminosity LHC) [2], is expected to reach 5  1034 cm  2s  1 anddeliver a total of 3000 fb  1 of collisions at a center-of-mass energyof 14 TeV. The ATLAS inner detector is subjected to a major upgrade to cope with these signiﬁcant increases in the instantaneousand integrated luminosities. A new all-semiconductor type innertracker (ITk) [3–5] composed of pixel and microstrip layers will be
installed. As described in the Letter of Intent (LoI) [6] the microstrip detector is constructed of ﬁve barrel layers and seven discs ineach of the endcaps. The barrel strips in the three inner layers are
24 mm long while the outer layers are 48 mm long. The strip pitch
is 74.5 μm in both cases. The length of the endcap strips varies
from 17 mm to 59 mm depending on the radius. The strip length ischosen so as to maintain the average hit occupancy at less than 1%
at the expected maximum instantaneous luminosity.
The particle ﬂuence and total ionizing dose expected at the endof experiment's lifetime has been evaluated with a FLUKA simulation [7]: this simulation is based on the experience from thecurrent detector where the agreement between simulation andmeasurement is within 20% [7]. The estimated maximum lifetime
ﬂuence values [7] are 5.3  1014 n eq /cm2 in the 24 mm long barrel
strips, 2.9  1014 n eq /cm2 in the 48 mm long barrel strips, and8.1  1014 n eq /cm2 in the endcap. Here, the contributions of different kind of particles are translated into 1-MeV neutron
equivalent values by taking into account the non-ionizing energy
loss (NIEL) [8] factors in silicon. The particle composition (n,π,p)varies according to the location and is found to be (57%, 35%, 8%)
and (73%, 19%, 8%) at the points with the largest ﬂuence values in
the 24 mm and 48 mm long barrel strips [9].
We have extensively studied the radiation hardness of Hamamatsu n þ -on-p ﬂoat-zone (FZ) strip sensors to predict the performance at the end of lifetime and determine if the collectedcharge is sufﬁciently large. Previous studies [10] using the ATLAS07 layout sensors [11] have shown that after irradiation to the
HL-LHC ﬂuence the charge is reduced. However, the signal-tonoise ratio is more than 15 at 500 V bias and thus, the detector
remains as a precision tracker.
The ATLAS12A (A12A, for short) and ATLAS12M (A12M) [12]sensor layouts are the latest sensor designs implemented at thetime of writing (2016). A12 main sensors have the axial strips only
and A12M main sensors have the axial and stereo strips mixed.
Different bulk resistivity and further design evolution compared to
A07 sensors allow us to ﬁnd effects that depend on them andfurther assure that the resistivity range expected in production iswithin the required speciﬁcation. The bulk damage properties ofthese series were compared with those of the ATLAS07 (A07)
series, and the dependence of damage difference on the radiating
particle type (neutron, proton, pion) was investigated. The collected charge values were evaluated by seven groups using identical Alibava [13] readout systems utilizing penetrating β rays
emitted from 90Sr. We extended the study to a 4.4-GeV electron
beam to cross check if the β results are representative of the HLLHC experiment.
The magnitudes of damage due to neutrons, protons and pions
were found to be different for the same NIEL ﬂuence of 5  1014 n
2
eq /cm . The signal as a function of sensor depth was evaluatedusing an edge transient current technique (TCT) [14] to determine
the ﬁeld proﬁle after these irradiations.

2. Samples, irradiation, and charge collection measurements
2.1. Samples
Hamamatsu utilizes 6 in wafers for sensor fabrication. The
9.75  9.75 cm main sensor, which is surrounded by 24 pieces of
1.0  1.0 cm miniature sensors, is placed on each wafer. Only theminiature sensors are used in this study. The A07, A12A, and A12M
sensors are fabricated on the same p-type wafer category FZp,
where the resistivity was speciﬁed to be within 3–8 kΩ cm. In
practice, the typical sensor resistivity varied among the wafer
production lots. Consequently, the full-depletion voltages are typically 200–220 V (4.3–4.7 kΩ cm) for A07, 220 V for A12M, and
270–320 V (2.9–3.5 kΩ cm) for A12A.
2.2. Irradiation
The samples were irradiated with neutrons at Ljubljana TRIGA
Reactor [15], with 300-MeV pions at PSI, and with 23-, 27-, 70-,
and 800-MeV protons at Karlsruhe, Birmingham, CYRIC (Tohoku
University, Japan), and Los Alamos accelerators. The 1-MeV neutron equivalent ﬂuence values were calculated using NIEL hardness factors [8].

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Fig. 1. Sample box used in the irradiation experiment at CYRIC. The bottom-leftinset shows the sample holder to be inserted in the sample box.

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Fig. 2. (top) Collected charge vs. bias for non-irradiated samples measured at different sites. The charge is normalized to 23 ke  for a sample with an active
thickness of 300 μm. All samples are A12A except for one A07. (bottom) Average
subtracted collected charge in ke  for 12 A12A non-irradiated samples.

voltage to be
As an exemplary irradiation setup, the scanning box and sample holder used at CYRIC are shown in the photograph (Fig. 1).
There are ﬁfteen sample slots in the box. The holder with thesamples to be irradiated was pushed in remotely, and then the box
was moved laterally to allow for uniform irradiation of the samples with the proton beam. The box was thermally insulated andliquid nitrogen was ﬂushed through it to maintain a temperatureof approximately  15 °C. Aluminum foils were attached to thesamples to obtain dosimetry using Al27(p,3pn) Na24 spallation reaction. For ﬂuences in the range of 1012  1016 n eq /cm2 , samples
were irradiated from few minutes to six hours at a beam current
ranging from 10 nA to 1 μA. The ﬂuence uncertainty was approximately 10%, as determined from the uncertainty in theavailable spallation cross section. The irradiated samples werestored in a refrigerator immediately after a series of irradiation
was completed.
2.3. Charge collection measurement technique and calibration
All the measurements reported here utilized the Alibava system[13] which uses a Beetle analog readout chip with a ﬁeld-programmable gate array (FPGA)-based readout. The Alibava system
digitizes charge while recording the trigger arrival time with respect to the clock cycle. Since a scintillating counter set underneath the sample triggers on the penetrating β particles generated
through 90Sr decay, the trigger timing is not always optimal withrespect to the charge sampling timing. To be selected for the
analysis, events were required to be within 5 ns of the trigger
signal which resulted in the maximum collected charge. The collected charge of an event was determined by a clustering algorithm with a seed threshold of 3.5 times the channel's noise leveland neighbor threshold of 1.5 times the noise. The collected charge
for the sample at a given voltage was found by ﬁtting the cluster
charge distribution with a Landau function convoluted with aGaussian. The most probable cluster charge is deﬁned as the collected charge. Instead of relying on the internal calibration of the
Beetle, we have determined the calibration with non-irradiatedsamples assuming charge collection Q [e  ] above depletionQ=d[190 + 16.3 ln (d)]3.68

for the measured active thickness d (μm) of the device [16]. Notethat active thickness is typically 10 μm smaller than physicalthickness, which is explained in Section 4.
Fig. 2 shows the plot of collected charge vs. bias voltage measured for non-irradiated samples and at various measurementsites. Most of the curves are for A12A samples except for one
corresponding to an A07. As explained previously the full depletion voltage of A07 is lower than that of the others but all thesamples show identical collected charge above full depletion.
The average subtracted collected charge is plotted in Fig. 2 for
the A12A sensors. Larger variations are only seen when not fully
depleted: the above depletion agreement is very good. Since different sites used different samples, any resistivity and thickness
variations within the production lead to changes in the normalization. Since the observed variation is less than 1 ke  below full
depletion and at most 0.3 ke  above full depletion, these effects
are minimal.
The samples were measured at low temperatures, around 20 °C to suppress the leakage current induced by radiation. The
actual temperatures were different among the measurement sites
depending on the cooling system performance. Since the calibration of the Alibava system has a temperature dependence, charge
calibration relying on non-irradiated sensors of known thickness,
as described, enhances the reliability of charge measurement.

3. Results of charge collection measurements
3.1. Annealing
The annealing properties of the samples irradiated to ∼1015 n eq
/cm2 are quite different from those reported previously [17],especially at high bias voltages. Fig. 3 shows the collected chargeof the A12A and A12M sensors irradiated with protons to a ﬂuenceof 1015 n eq /cm2 measured as a function of annealing time at 60°C.

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Fig. 3. Examples of annealing properties of A12A (left) and A12M (right). The samples were irradiated with protons to a ﬂuence of 1  1015 n eq /cm2.

Fig. 4. Collected charge of neutron-irradiated samples measured at 500 V and
900 V. The lines connect the average data points separately for A07 and A12A
samples.

Fig. 5. Collected charge of proton irradiated A12A samples measured at 300, 500,
and 900 V.

The collected charge increases (beneﬁcial annealing) up to 80 min,and then decreases for both samples at lower bias voltages.
However, the collected charge of A12M stays almost constant atbias voltages of 900 V and 1000 V after 80 min. Similar trends as
for A12M have been observed in another study [18] conducted on
A07 sensors, where the samples were irradiated to 1015 n eq /cm2 by

Fig. 6. Comparison of collected charge of proton-irradiated A07, A12A, and A12M
samples, measured at (1) 300 V, (2) 500 V, and (3) 900 V. The A12A data points are
the averages of the data shown in Fig. 5 with variations represented as uncertainty.
The neutron data at 500 V and 900 V, as shown in Fig. 4, are also plotted.

neutrons and pions. Owing to radiation-induced defects, the depletion depth decreases with radiation. Since depletion develops
from the strip side and the applied bias voltage is sustained in thedepletion region, the ﬁeld around the strips increases with irradiation and bias, reaching a region where avalanche multiplication

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Fig. 7. Comparison of source (dashed) and beam (solid histograms) measurements of the non-irradiated (above) and 1 ×1015 n eq /cm2 irradiated samples (below). Comparison of charge distributions and cluster size distributions at 500 V.

Fig. 8. Comparison of collected charge values between beam (solid line)- andsource (dashed line)-measurements, shown as a function of bias. The samples are
non-irradiated or irradiated with neutrons to the ﬂuence denoted by the numbers
shown next to the curves (the unit is 1014 n eq /cm2). The curves 20 and 50 ×1014 n
2
eq /cm are for the data before annealing.

occurs.
The magnitude of initial beneﬁcial annealing is dependent on
irradiation conditions such as irradiation rate [19]. Therefore acertain controlled annealing is preferred prior to comparing various types of irradiation data. Also in real experiments where theradiation rate is much smaller and beneﬁcial annealing is taking
place, results evaluated with controlled annealing are expected torepresent actual characteristics in the real experiments. In thefollowing, the results of charge collection measurement after
controlled annealing for 80 min at 60 °C are presented.

Fig. 9. Collected charge vs. bias of A12A samples irradiated with neutrons, protons,
and pions to a ﬂuence of 5  1014 n eq /cm2.

3.2. Neutron and proton irradiation
Fig. 4 shows the collected charge at 500 V and 900 V as afunction of ﬂuence for all neutron-irradiated A12A and A07 samples. As expected from [20,21], there is a difference between A12A
and A07 sensors observed below 5  1014 n eq /cm2 due to the difference in the initial resistivity of the devices. At higher ﬂuences,
this effect is much reduced.
All the data points measured for proton irradiated A12A samples are plotted for three bias voltage settings, 300, 500, and
900 V, in Fig. 5. No noticeable dependence on the source of proton
irradiation or the charge collection measurement site was observed for these results.
In Fig. 6, the results of A12A are compared with those of A07

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K. Hara et al. / Nuclear Instruments and Methods in Physics Research A ∎ (∎∎∎∎) ∎∎∎–∎∎∎and six neutron-irradiated samples. Of the six samples, two were
measured before being subjected to controlled annealing. The
agreement between the two measurements is remarkable, although there is a slight tendency for the collected charge of theirradiated samples for the beam data to be 5–6% higher than thatfor the source data.
The results of this study with the electron beam veriﬁed thatmeasurements using a 90Sr β source can provide reliable values inthe estimation of collected charge for minimum-ionizing particles.
3.4. Comparison among proton, neutron, and pion irradiation

and A12M at 300 V, 500 V, and 900 V. Since the A12A data points,
as shown in Fig. 5, form speciﬁc bands, we take the averages if
multiple data points are available, with the variations represented
as uncertainty.
At 500 V, the difference among proton irradiated samples issubstantially reduced, compared to that observed at 300 V, as
expected for the different full depletion voltages. Moreover, thedifference to the neutron-irradiated samples becomes small at
900 V as shown in the ﬁgure.

As discussed in Section 3.2, the reduction in charge collection ishigher for neutrons than for protons at 500 V bias. A systematiccomparison of the collected charge was carried out for the A12A
samples of similar initial full depletion voltages irradiated withneutrons, 23-MeV protons (Karlsruhe), and 300-MeV pions (PSI) to
the same NIEL ﬂuence of 5  1014 n eq /cm2. From Fig. 9, we observethat the collected charge is larger for the pion-irradiated sample
than for the proton- or neutron-irradiated samples. For a bias of
500 V or less, the difference is noticeably large.
The results clearly suggest that NIEL hypothesis is not suitable
for the normalization of collectable charge at the concerned ﬂuence level, which is typically 1  1015 n eq /cm2.
The differences after irradiation are the main subject of thestudy presented in Section 4. We discuss the differences in theﬁeld proﬁle along the depth instead in the full depletion voltages,
since the irradiated sensors do not remain as a simple diode andcharacterization by the full depletion only is not appropriate.

3.3. Test beam evaluation of neutron-irradiated samples

3.5. Charge collection at 500 V

The charge collection from a few neutron-irradiated samples
was measured using a DESY 4.4 GeV electron beam. Two samples,
read out with the Alibava system, were placed in the beam in
between two beam tracker systems. The samples were cooled to−25 ± 3 °C by circulating cooled Silicone oil around them.
Fig. 7 compares the charge distributions of the non-irradiatedand 1  1015 n eq /cm2 irradiated samples between the source andbeam measurements. The distributions of cluster size, which isdetermined by the number of hit strips in a hit cluster, are alsoplotted. The charge distributions are consistent between thesource and beam. The cluster size distributions are slightly wider
for the source measurement, which can be explained by the incident angle and scattering effects of β rays.
The most probable charge values are compared in Fig. 8 as afunction of the detector bias for eight samples: two non-irradiatedFig. 10 plots the summary of neutron, proton, and pion irradiation measurements at 500 V including the measurements using
the electron beam. The maximum operation voltage at the HL-LHC,
expected in ATLAS, is 500 V, which is determined from the speciﬁcations of the available cables used in the present inner detector. At 500 V, all proton-irradiated samples, A07, A12A, andA12M show similar ﬂuence dependences, while the collectedcharge is reduced for neutron-irradiated samples in the ﬂuence
range of 0.5–2  1015 n eq /cm2 for A12A, and of 1–2  1015 n eq /cm2
for A07 samples.

Fig. 10. Summary of collected charge measurements at 500 V for A07, A12A and
A12M samples, shown separately for various irradiation sources.

4. Field proﬁle studies
In the transient current technique (TCT), transient currents on

Fig. 11. Velocity-sum proﬁles of non-irradiated A07 and A12A samples at 200 V (left) and 400 V (right).

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Fig. 12. Velocity-sum proﬁles of A07 and A12A samples at 700 V irradiated to a ﬂuence of 5 × 1014 (left) and n eq /cm2 2 × 1015 n eq /cm2 (right).

Fig. 13. Velocity-sum proﬁles of A12A samples irradiated with protons, neutrons, and pions to a ﬂuence of 5 × 1014 n eq /cm2 measured at 300 V (left) and 500 V (right).

readout electrodes are induced by charge released in the detector
by short laser pulses. The charge collection characteristics of thesilicon detector can be investigated from the induced current
pulses. The method, when the laser light is injected from the detector side, is effective in evaluating the active thickness and understanding the difference between the sensors and irradiation
sources.
In an edge-TCT [14] study, an infrared laser of 1060 nm wavelength and 300 ps duration was injected from a polished side edge
parallel to the strip direction. The induced current from one of the
strips was measured using a high-speed ampliﬁer and a 1.5 GHz
oscilloscope, in which neighboring strips were set to the samepotential as the readout strip to avoid disturbance of the electricﬁeld in the strip sensor. The laser was collimated to 8 μm (FWHM)
underneath the readout strip. By scanning through the depth of
the sensor the charge collection characteristics can be investigatedas a function of sensor depth.
The induced current time proﬁle is inﬂuenced by the number ofgenerated charge carriers, carrier velocities, and carrier trapping
along their paths. The current proﬁle right after the laser injection
can be expressed as [22]:I (y , t ∼ 0) ∼ qE w (y)[v¯e (y) + v¯h (y)]where y denotes the depth location in the sensor, Ew(y) is theweighting ﬁeld in Ramo's theorem, q is the elementary charge, andv¯e and v¯h are electron and hole velocities, respectively. The timet ∼ 0 condition in the equation is a result of the requirement thatthe current amplitude is measured immediately after carrier

generation before trapping and before charges move signiﬁcantly
away from the location of the laser beam. When the laser beam isperpendicular to the strips, the weighting ﬁeld is effectively constant because of the contributions of carriers drifting to theneighboring strips [14]. The current values at t¼ 0.6 ns was foundto be adequate for measurement of the velocity sum, v¯e (y ) + v¯h (y )[22].
Since velocities are expressed as the product of mobilities andthe electric ﬁeld, the electric ﬁeld can be extracted simply by using
I (y, t ∼ 0) ∼ v¯e (y ) + v¯h (y ) = [μe (E ) + μh (E )] E , provided that theelectric ﬁeld is not very high, as velocity saturates with increasing
electric ﬁeld. The detailed analysis has been reported in a previous
study [22].
The velocity sum, v¯e (y ) + v¯h (y ), is plotted in Fig. 11 as a functionof sensor depth for non-irradiated A07 and A12A samples at 200 V
and 400 V. Here, the vertical scale is arbitrary but the sample areasin the same plot are normalized, since integration of the electricﬁeld (∼v¯e (y ) + v¯h (y )) over the depth gives the bias voltage, which is
ﬁxed. Since resistivity is different, the depth proﬁle is different at
200 V, when A12A is not fully depleted. At 400 V both the sensors
are fully depleted. Note that the full depths are slightly different
between A07 and A12A. We conclude that the active depths are
10 μm smaller than the physical thickness of the A07 and A12A
samples, which are 310 and 320 μm, respectively.
The velocity-sum proﬁles of neutron-irradiated A07 and A12A
samples measured at 700 V are shown in Fig. 12. The ﬁeld near thebackside of A07 is substantially larger than that for A12A at
5 × 1014 n eq /cm2 as expected, because of the different initial

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8

resistivities. The difference, however, diminishes at 2 × 1015 n eq
/cm2 irradiation, which can be explained by the fact that radiationinduced space charge dominates over the initial difference. Notably, non-zero velocity proﬁles near the backside are observed forboth the samples at 2 × 1015 n eq /cm2 irradiation. Such a proﬁle is
known as “double peak electric ﬁeld proﬁle” [23,22], where thespace charge in the detector changes sign. The carriers released by
the generation current are trapped in defects, and subsequently,
holes ﬂow to the backplane, effectively changing the sign of thespace charge near the backplane of the p-type bulk. Therefore thespace charge concentration becomes zero at a certain depth. For
both the sensors, zero charge concentration occurs at a depth ofapproximately 180 μm at 700 V at 2 × 1015 n eq /cm2, and at approximately 250 μm for A12A only at 5 × 1014 n eq /cm2.
The velocity-sum proﬁles of protons-, neutron-, and pion-irradiated A12A samples, measured at 300 V and 500 V, are comparedin Fig. 13. At both the bias voltages, there are substantial non-zerocontributions near the backside for the pion-irradiated samples.
The proﬁle reveals that the sample irradiated by pions to
5 × 1014 n eq /cm2 is fully depleted at 500 V, while there is a substantial undepleted region in the neutron-irradiated sample at
500 V. The measured velocity-sum proﬁles agree with the chargecollection difference shown previously (Fig. 9).

5. Conclusions
We extensively studied the charge collection from Hamamatsu
p-bulk FZ sensors of 310–320 μm thickness using penetrating β
rays. Highly consistent results were obtained by the seven groups
that participated in the measurements. The results were further
veriﬁed by independent measurements using an electron beam.
The difference in the charge collection between different sensor types, which is due to different initial resistivity, diminisheswith the irradiation ﬂuence and bias voltage. The effect depends
on the particles used in the irradiation. For example, at 500 V, thesamples irradiated with protons showed a small difference incharge collection, while those irradiated with neutrons showed adifference below 1 × 1015 n eq /cm2.
The reduction in the charge collection is largest for neutron
irradiation followed by that for 23-MeV proton irradiation. Damage caused by 300-MeV pions is the least for the same NIEL
ﬂuence.
The carrier velocity proﬁles across the depth were evaluatedusing an edge TCT. The proﬁles differ for different irradiating
particles, neutrons, protons, and pions. The ﬁeld near the backsideis the largest for the pion-irradiated samples, which explains thelargest charge collection observed for these samples.
The expected signal-to-noise ratios after the HL-LHC ﬂuencecan be evaluated using the ENC noise values of the readout electronics with corresponding wire-bonded sensors. Typical ENC
noise values are 550e  for the barrel with 24 mm long strips
connected, 720e  for the barrel with 48 mm connected, and650e  for the endcap module [24]. Assuming the safety factor of
2 and the neutron damage dominance, we arrive at conservative
estimate for the lowest S/N value of 14. It is realized for endcap
location.
The present studies have veriﬁed that the Hamamatsu p-bulk
strip sensors are operational and provide precise particle tracking
in the high-radiation environment expected in the HL-LHC.

Acknowledgments
The irradiations were performed: with protons at the University of Birmingham MC40 cyclotron, supported by the H2020
project AIDA-2020, GA number 654168, and the UK's Science andTechnology Facilities Council, at Cyclotron and Radioisotope Center(CYRIC), Tohoku University, with Y. Sakemi, M. Ito, and T. Wakui, atthe Karlsruhe Institute of Technology (KIT) by A. Dierlamm, supported by the Initiative and Networking Fund of the Helmholtz
Association, contract HA-101 (Physics at the Terascale) and theEuropean Commission under the FP7 Research Infrastructures
project AIDA, Grant agreement no. 262025, and at the LANSCE
facility, Los Alamos National Laboratory; with neutrons at JSI
TRIGA reactor in Ljubljana supported by the H2020 project AIDA2020, GA no. 654168; with pions at Paul Scherrer Institut (PSI);
and with γ's at Brookhaven National Laboratory (BNL).
The research was supported and ﬁnanced in part by CONICYT
Becas Chile 72140349 and Cambridge Trust, the Ministry of Education, Youth and Sports of the Czech Republic (Grant no.
LG13009), the German Federal Ministry of Education and Research,and the Helmholtz Association, the European Social Fund and by
the Ministry of Science, Research and Arts, Baden-Wuerttemberg,
Germany, the Japan Society for Promoting Science KAKENHI-A
Grant number 20244038 and KAKENHI-C Grant number
20540291, the Ministry of Education, Culture, Sports, Science andTechnology-Japan, KAKENHI for Research on Priority Area Grant
number 20025007 and for Scientiﬁc Research on Innovative Areas
Grant number 23104002, the Slovenian Research Agency, theSpanish Ministry of Economy and Competitiveness through theParticle Physics National Program (ref. FPA2012-39055-C02-01 andFPA2012-39055-C02-02) and co-ﬁnanced with FEDER funds, theﬁnancial support of the State Secretariat for Education, Research,and Innovation, the Swiss National Science Foundation and theCanton of Geneva, Switzerland, the UK Science and Technology
Facilities Council (under Grant ST/M006409/1), and the UnitedStates Department of Energy, grant DE-FG02-13ER41983.

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Please cite this article as: K. Hara, et al., Nuclear Instruments & Methods in Physics Research A (2016), http://dx.doi.org/10.1016/j.
nima.2016.04.035i