Radiation tolerance studies ofthe double sided silicon strip sensor(両面ストリップ型シリコンセンサーの放射線耐性評価)
By
Fumiaki Otani
男谷文彰

A Master Thesis
修士論文

Submitted to
The University of Tokyo
Department of Physics, Graduate School of Science
On 2023/01/27
Thesis Supervisor: Takeo Higuchi 樋口 岳雄

ABSTRACTThe B meson decays can be studied precisely and compared with the standard model (SM) to search
for new physics of elementary particles. The Belle II experiment, which has an accelerator and detector
in Tsukuba, Ibaraki, Japan, is also called the “B factory” experiment, and produces many B mesons
and can precisely measure their decays.
Currently, eﬀorts are underway to bring the beam luminosity of the SuperKEKB collider to 6 × 1035 cm2 /s
for the Belle II experiment. While an increase in beam luminosity increases the number of B produced
and allows for more precise measurements, the beam background also increases with beam luminosity.
Therefore, radiation damage to the detector is also expected to worsen. In particular, the innermost
layer of the Silicon Vertex Detector (SVD), which plays an important role in the search for new physics,
is simulated to receive 20 kGy of radiation after 10 years of target luminosity operation. We decided
to investigate whether there would be any problems with the operation of the SVD after receiving thisradiation dose through the electron beam irradiation test.
In order to evaluate the radiation hardness of the SVD sensor, a measurement system was developedthat allowed for the remote measurement of the leakage current-voltage characteristics (IV), capacitancevoltage (CV), and interstrip capacitance-voltage (cint) of the sensor. The system was used to focus
on the stepwise changes in IV, CV, and Cint with a radiation dose up to 100 kGy for the TID and3 × 103 neq 1/cm2 for the NIEL. Additionally, changes in noise and charge collection eﬃciency were
measured before and after irradiation of the sensor by utilizing a signal readout module equipped witha readout ASIC that was attached to the sensor.
The evaluation results conﬁrm that our SVD has no operational problems even after 10 years of
cumulative radiation dose under beam luminosity 6 × 1035 cm2 /s.

Contents

1 Introduction
1.1 Physics motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.1 New physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.2 Belle II experiment and TDCPV method . . . . . . . . . . . . . . . . .
1.2 Motivation of this thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1
1
1
1
2

2 Belle II detector
2.1 Overview of the Belle II detector . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Silicon Vertex Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Double-sided strip detectors . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2 Front-end readout ASIC: APV25 . . . . . . . . . . . . . . . . . . . . . .

4
4
6
7
9

3 SuperKEKB and the radiation dameges
10
3.1 SuperKEKB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.1 Overview of SuperKEKB . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.2 Future luminosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2 Beam background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3 Woriking principle of double-sided readout silicon strip detector . . . . . . . . . 16
3.3.1 Silicon semiconductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.3.2 PN junction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.3.3 Reverse bias voltage and depletion layer . . . . . . . . . . . . . . . . . . 17
3.3.4 Signal generation and collection mechanism of DSSD by particles passage 18
3.4 Possible radiation damages of the SVD . . . . . . . . . . . . . . . . . . . . . . . 19
3.5 Operational concerns due to radiation damage . . . . . . . . . . . . . . . . . . 22
3.5.1 Type inversion and VFD increase after Type inversion . . . . . . . . . . 22
3.5.2 Leakage current increase . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.5.3 Noise increase and charge collection eﬃciency deterioration . . . . . . . 23
3.5.4 Annealing eﬀect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4 Measurement methods of the irradiation damage in sensor
4.1 The sensors for the irradiation tests . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Devices and methods of IV, CV, Cint V measurement . . . . . . . . . . . . . . .
4.2.1 IV measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.2 CV measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.3 Cint V measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 The conﬁguration of remote measurement system . . . . . . . . . . . . . . . . .
4.3.1 Switching board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
iii

25
25
26
26
27
28
29
29

4.3.2 Remote control system . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The beam facility and beam condition, dose estimation . . . . . . . . . . . . . .
The measurement system placement and radiation shielding . . . . . . . . . . .
The detailed structure around sensor . . . . . . . . . . . . . . . . . . . . . . . .
4.6.1 The design of the switching board . . . . . . . . . . . . . . . . . . . . .
4.6.2 The design of the sensor board . . . . . . . . . . . . . . . . . . . . . . .
Annealing eﬀect measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . .

29
30
32
33
34
34
36

5 The results of the irradiation tests
5.1 Beam current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 The results of the irradiation test . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1 The results of IV, CV, Cint V measurement of mini sensor . . . . . . . .
5.2.2 The results of IV, CV, Cint V measurement of HPK large sensor . . . . .
5.2.3 IV measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.4 CV measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.5 Cint V measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Analyis and Disscution of IV, CV, Cint V results . . . . . . . . . . . . . . . . . .
5.3.1 Deﬁnition of the full depletion voltage . . . . . . . . . . . . . . . . . . .
5.4 Analysis of the type inversion . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.1 The type inversion results just after irradiation . . . . . . . . . . . . . .
5.4.2 The type inversion with annealing eﬀect . . . . . . . . . . . . . . . . . .
5.5 Analysis of leakage current increase . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.1 The damage factor and noise just after irradiation . . . . . . . . . . . .
5.5.2 The damage factor and noise of with anneal eﬀect . . . . . . . . . . . .

39
39
40
40
48
48
48
49
56
56
63
63
64
66
66
70

6 Measurement of Noise and Charge collection eﬃciency change
6.1 Irradiation system for SBW module . . . . . . . . . . . . . . . . . . . . . . . .
6.2 The measurement setup of Charge collection eﬃciency and noise . . . . . . . .
6.3 Measurement procedure of calibration data and charge collection . . . . . . . .
6.4 The result of measurement without 90 Sr before and after irradiation test . . . .
6.5 Charge collection measurement . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.5.1 Particle traversed position ﬁnding algorithm (Cluster ﬁnding algorithm)
6.5.2 Result of measurement with 90 Sr before and after irradiation test . . . .

74
74
75
78
79
84
84
84

7 Conclusion

94

References

96

4.4
4.5
4.6

4.7

iv

Chapter 1
Introduction
1.1
1.1.1

Physics motivation
New physics

The Standard Model (SM) of particle physics explains the experimental results to date welland without contradiction. However, there are still many things that the SM cannot explain.
For example, it cannot explain what has become the imbalance between matter and antimatterin this world (matter-dominated universe). Therefore, the SM is not believed to be yet theultimate theory “theory of everything” that can explain all particle behaviors of the universe.
Therefore, we search for a new theory of particles by precisely measuring their reactions andlooking for deviations from the standard theory.
In particular, decays involving the b → sγ processes are sensitive to the new theory.
The b → sγ process is allowed in the SM only through W boson one loop as Fig. 1.1. Because
the parity breaking occurs here, the gammas that appear in the ﬁnal state are predominantly
left- or right-handed, as shown by the pair of solid arrows in Fig. 1.2; the direction of thedashed arrow is strongly suppressed in the SM. Therefore, B 0 and B 0 decay into Xs γ are
independent and the CP asymmetry is supressed. For example, the CP asymmetry of theprocess B 0 → KS0 π0 γ mediated by the b → sγ transition is theoretically expected to be
SCP ≈ −0.023 ± 0.016 suppressed by (2ms )/mb [6].
On the other hand, if there are new physics in this loop that can contribute the b → sγ
process regardless of whether the photon is left- or right-handed, the percentage of events in the
direction of the dashed arrow in Fig. 1.2 will increase, and CP asymmetry is no longer zero andcan be signiﬁcant. From the above, it is possible to look for new physics by actually measuring
the CP asymmetry SCP by precisely measuring the events involving b → sγ. To experimentallymeasure the CP asymmetry, the Time Dependent CP Violation (TDCPV) method is used.
1.1.2

Belle II experiment and TDCPV method

The Belle II experiment is a high energy experiment, uses the SuperKEKB collider and BelleII detector at the High Energy Accelerator Research Organization in Tsukuba, Ibaraki, Japan.
Electron and positron beams produced at the SuperKEKB accelerator collide, and particle
reactions are recorded by the Belle II detector. The collision energy of the beams is set tothe Υ (4S ) resonant mass energy and the SuperKEKB can produce a large amount of Υ (4S )mesons by its high beam luminosity. Υ (4S ) then decays into B B pair. In other words, theBelle II experiment is provided with a large amount of B meson by SuperKEKB and its decay
1Figure 1.1: b → sγ diagram

Figure 1.2: Photon polarization in B meson decay. γL (γR ) represents Left-(Right-) handed
photon.
events can be observed, thus, the Belle II experiment is well suited for the precise measurementof b → sγ.
Since the Xs γ state is the CP-speciﬁc decay mode for B and B , theoretically, the CP
asymmetry is seen in the diﬀerence between the time-dependent decay widths ΓB →X CP γ (t) ofs

B and B as the following equation [15],Asym. =

Γ 0

CP

γ

Γ 0

CP

γ

B →Xs
B →Xs

(t) − Γ 0CPγ(t) + Γ 0

CP

γ

B →Xs
B →Xs

(t)
(t)

= ACP cos(∆mt) + SCP sin(∆mt)

,

(1.1)

so, we can obtain the CP asymmetry by measuring the time-dependent decay widths of Xs γeach of B and B .

1.2

Motivation of this thesis

In the experiment, the decay time t of ΓB →X CP γ (t) is calculated from the decay point. Theres
fore, the precise determination of the decay vertices of the B mesons is essential to precisely
measure the of the CP asymmetry, and in the Belle II experiment, the silicon vertex detector(SVD) plays an important role in the measurement of the decay vertices of the B mesons.
2

In the Belle II experiment, it is planned to increase the peak luminosity of the SuperKEKB
collider to 6 × 1035 cm2 /s. This increase in luminosity will result in a higher number of events
produced, thus providing a more accurate measurement of btosγ. However, it is also anticipatedthat the increased luminosity will lead to a higher beam background. Consequently, it isimperative to estimate the eﬀect of the radiation damage caused to the SVD by the higher
beam background will pose a problem for SVD operation. In particular, the amount of radiation
received during 10 years of operation at the target luminosity (≈ 20 kGy) will be used as anindicator to conﬁrm that the operation will not be aﬀected by the amount of radiation received.
For conﬁrmation, we decide to actually irradiate electron beams to the SVD to inversticate
the eﬀects from the NIEL damage mainly.
The previous studies have investigated the eﬀects of Total Ionizing Dose (TID) damage on
SVD up to 100 kGy using 60 Co [24] radiation, however, no studies have been conducted on
Non-Ionizing Energy Loss (NIEL) eﬀects. Despite this, the Silicon Vertex Tracker (SVT) [9]used in the Babar experiment has similar characteristics to our SVD and based on the results of
SVT irradiation tests [21], it can be inferred that our sensor also possesses suﬃcient radiation
tolerance to withstand NIEL eﬀects in practical use, and its has been used to date.

3Chapter 2
Belle II detector
2.1

Overview of the Belle II detector

Figure 2.1: Overview of the Belle II detector [22]The Belle II detector Fig. 2.1 [10] has a near cylindrical shape with a diameter of approximately 8 meters. At the center of the detector is the interaction point (IP) of the electron
and positron beams produced by SuperKEKB (detail in Sec. 3.1), with various sub-detectors
covering the IP in an onion-like structure.
This thesis focuses on the Silicon Vertex Detector (SVD) [2] and brieﬂy introduces theother sub-detectors, presented in order of proximity to the IP:
Pixel Detector (PXD) The design consists of two layers of PXD sensors, with 8 sensors in
the inner layer and 12 sensors in the outer layer, placed 14 mm and 22 mm from the IP,
respectively. The sensors are made of silicon and each sensor has 250 × 768 pixels of size
50 × (55 ∼ 85 µm2 ). Each pixel serves as a signal readout channel. The sensors are used4

in a fully depleted state, similar to the SVD. When a particle passes through, the nearest
pixel collects the charge generated by the passing particle and identiﬁes the position of
the particle as a signal. Together with the SVD, the system is called the VeteX Detector(VXD) and uses the positions of the particles passing through the PXD two layers andthe SVD four layers to reconstruct the trajectories of the passing particles and estimate
the point of generation of the particles (the point of decay of the parent particle)
Silicon Vertex Detector (SVD)

See Sec. 2.2

Central Drift Chamber (CDC) The CDC is a cylindrical gas-chambered detector withan inner diameter of 32 cm and an outer diameter of 226 cm, which exists in the outerlayer of the SVD. The inside of the detector is lined with a large number of gold-plated
tungsten wires of 30 µm diameter and aluminum alloy wires of 126 µm diameter and isﬁlled with a mixture of helium and ethane gas. The aluminum alloy wires are called
ﬁeld wires and are not voltage-applied, whereas the gold-plated tungsten wires are called
sense wires and are voltage-applied. One sense wire is surrounded by eight ﬁeld wires,
which are the cells of the CDC measurement. When charged particles pass through thiscell, they ionize the gas, producing electrons. These electrons move toward the sensewire due to the voltage diﬀerence in the cell, causing an electron avalanche and then asignal will be generated.
It measures the trajectory of the charged particles and uses the obtained trajectory tomeasure the momentum of the charged particles and to generate trigger signals. In
addition, the dE/dx of the particles in the gas can be measured, and the particles canbe identiﬁed.
Time Of Propagation counter (TOP) The TOP counter is installed outside the CDC.
It consists of a quartz radiator 2.7 m long, 45 cm wide, and 2 cm thick, and a MicroChannel-Plate Photo-Multiplier Tube (MCP-PMT) placed at the end. When a particle
passes through the radiator of the TOP counter and its velocity exceeds c/n, Cherenkov
light is generated. (c is the speed of light and n is the refractive index of the radiator)
The Cherenkov light reaches the MCP-PMT and becomes a signal while being totally
reﬂected inside the radiator.
Using the fact that the emission angle of Cherenkov light is larger for particles with thesame momentum but smaller mass, we can identify particles based on the diﬀerence inthe propagation time of Cherenkov light. It is particularly good at identifying π and K.
Aerogel Ring Imaging CHerenkov counter (ARICH) ARICH is installed near the end
cap portion of the CDC on the right side of Fig. 2.1. The ARICH is composed of a radiator composed of a silica aerogel with an outer diameter of 1.1 m, an inner diameter of
0.4 m, and a thickness of 40 mm, as well as a Hybrid Avalanche Photo-Detector (HAPD)
with a thickness of 30 mm. The HAPD and radiator are separated by a distance of
160 mm.
Similar to the TOP, when charged particles pass through the radiator, Cherenkov light is
produced. The Hybrid Avalanche Photo-Detector (HAPD) receives this produced lightand performs particle identiﬁcation (of π and K) by reconstructing the Cherenkov ring.
Electromagnetic Carolimeter (ECL) The ECL is placed outside of the ARICH and the
TOP counter to enclose detectors and other detectors mentioned above (White area in5

Fig. 2.1). A set of cmthallium-doped caesium iodide CsI(Tl) crystals of approximately
30 cm × 5.5 cm × 5.5 cm size combined with a PIN-PD (PhotoDiode) is used as a scintillation detector. This scintillation detector set is used 8736 sets in the ECL. As the
gamma rays and electrons pass through the ECL crystal, an electromagnetic shower isgenerated, and the particles lose energy in the electromagnetic shower. On the other
hand, scintillation light is emitted in proportion to the energy lost. By measuring thescintillation light, the total energy of each particle can be calculated.
K-Long-Muon detector (KLM) The KLM is the Belle II detector outermost detector.
14 steel plates about 4.7 cm thick are spaced about 4.4 cm apart. The detector is placed
between them. The detector is basically a Resistive Plate Counter (RPC), and a part ofthe detector (the end cap and the innermost two layers of the barrel section) uses a strip
plastic scintillator as a detector.
The charged particles that reach the KLM are occupied by muons or KL0 , and the KLM
distinguishes between these two types of particles. Muon penetrates the steel plates in
the KLM and leave trails on the detector, while KL0 interacts with iron and form hadron
showers. In this way, the particles are distinguished by the diﬀerence in their tracks.

2.2

Silicon Vertex Detector

The SVD is arranged in four cylindrical layers that wrap around the PXD as shown in Fig. 2.2.
Each layer consists of a ladder with a diﬀerent shape in each layer as shown in Fig. 2.3. Eachof the innermost layer 3, as well as layers 4, 5, and 6, consists of 7, 10, 12, and 16 ladders,
respectively. An example of the ladder structure is shown in Fig. 2.4.

Figure 2.2: The Belle II vertex detector, Constructed with two layers PXD and four SVD
layers.

6

Figure 2.3: Ladders for each layer
2.2.1

Double-sided strip detectors

The DSSD is an SVD sensor and one of the most important parts of the SVD to capture
the position of the particles as they pass through. A more detailed principle of operation is
described below (Sec. 3.3). The structure and overview are described here.

Figure 2.5: The strip structure of the DSSD, SVD sensor.
DSSD is a semiconductor detector, and the bulk of the sensor is an n-type semiconductor.
7

Figure 2.4: Example of ladder structure, the 6th layer ladder
It has a strip-like structure on both surfaces of its bulk, as shown in Fig. 2.5. The individual
strips are also semiconductors, but they are much more highly doped with impurities than thebulk semiconductor; one side (called P side) uses P+ strips with high acceptor concentrationand the other side (called N side) uses N+ strips with high donor concentration.
The entire sensor can be depleted by applying a voltage between the P+ and N+ strips.
When a particle passes through this depleted DSSD, the P+ and N+ strips near the pointof passage react; since the P+ and N+ strips are orthogonal, the point of passage can be
estimated in two dimensions. Each strip on both sides is coupled to an aluminum strip thathas undergone a SiO2 oxide coating. Through this aluminum strip, signals can be read out tothe outside of the sensor (to ASIC; APV25). In addition, the Belle II DSSD has a P+ strip(called P+ stop) between the N+ strips, which does not have a small readout, and the space
between the N+ strips is wider than the P side, and the sensor bulk is N-type bulk, whichmakes it diﬃcult to apply voltage. Therefore, depletion is diﬃcult to occur, and the P+ stopis placed to completely isolate the strips from each other.

8Hereafter, P+ and N+ strips will be referred to simply as P strips and N strips, respectively.
DSSDs are incorporated into the SVD ladder as shown in Fig. 2.4. Each layer has a diﬀerent
number and diﬀerent geometries of DSSDs in a single ladder. The DSSDs showed in table 2.1.
As can be seen in Fig. 2.3 and Fig. 2.4, there is an angled fabricated section on the rudder
after layer 4. That section also has a DSSD, which is a trapezoidal DSSD. That part we call
SVD Forward (SFW) and the other end we call SVD Backward (SBW).

Readout strips P -side
Readout strips N -side
Readout pitch P -side (µm)
Readout pitch N -side (µm)
Sensor active area (mm2 )Sensor thickness (µm)
The number in layer3 ladder
The number in layer4 ladder
The number in layer5 ladder
The number in layer6 ladder

Small rectangular

Large rectangularTrapezoidal

768
768
50
160
122.90 × 38.55
320
2
0
0
0

768
512
75
240
122.90 × 57.72
320
0
2
3
4

768
512
50 ∼ 75240
122.76 × (38.42 ∼ 57.59)
300
0
1 (for SFW)
1 (for SFW)
1 (for SFW)

Table 2.1: The properties of three types of double-sided strip detector

2.2.2

Front-end readout ASIC: APV25

The DSSD strip is connected to the readout ASIC; APV25 [11], and the signal induced on the
strip is shaped and ampliﬁed by APV25 and sent to the subsequent stage. One chip has 128
readout channels. The APV25 has fast shaping time 50 ns, and high radiation tolerance; It
has been conﬁrmed that the system works ﬁne even after 100 Mrad irradiation [16].

Figure 2.6: Block diagram of the one channel of the APV25

9Chapter 3
SuperKEKB and the radiation dameges
3.1
3.1.1

SuperKEKB
Overview of SuperKEKB

The SuperKEKB collider [3] (Fig. 3.1) is a circular lepton collider with a circumference of
3 km located at the High Energy Accelerator Research Organization (KEK) in Tsukuba-city,
Ibaraki-Pref., Japan. The Belle II detector measures the events produced by collisions of 7 GeVelectrons and 4 GeV positrons. The Belle II experiment precisely tests the Standard Modelof particle physics by observing a large number of events. In order to increase the number ofphysical events produced by the collider, the luminosity of the collider must be increased. The
luminosity L is proportional to the following parameters in SuperKEKB;
L ∝ γ±

I± ξy±βy ∗ ±(3.1)

where γ: Lorenz factor, I: beam current, ξy : vertical beam-beam parameter, βy ∗ : vertical beta
function at the beam collisions point. The subscript ± indicates electron (−) and positron (+).
The target values for those values are summarized in table. 3.1. The ξ parameter represents
the electromagnetic eﬀect from the current or the particles of one beam to the other, andwhile an increase in ξ is desirable in order to enhance luminosity, it has been observed throughempirical evidence that there exists an upper limit (0.1∼ 0.2) beyond which improvement is
not possible [29]. The beta function is an envelope function (for particle trajectories; related
to beam size) determined by the collider optics (magnetic ﬁeld) design. Compressing the beta
function can increase luminosity, but if the beta function becomes narrower than the beamsize, the beam bunch will take an hourglass shape, with a narrow center and bulges at thefront and back. And, particle collisions within the ﬁnite area before and after the collision
point also contribute to luminosity, so due to the bulging of the beam bunch, The luminosity
can not be improved. In other words, the lower limit for the beta function is roughly equal
to the beam size (hourglass eﬀect). In order to reduce the achievable beta while taking into
account the hourglass eﬀect, a method of colliding beams at an angle ϕ with each other wasconsidered [19], and was adopted in the SuperKEKB collider.
The SuperKEKB can be divided into two main parts: the main ring and the linac section
which accelerates the beams. In the Linac section, electrons are ﬁrst produced by a photocathode electron gun using the ﬁeld emission eﬀect. They are then accelerated through aRadio-Frequency (RF) cavity. A tungsten target for positron generation is placed in the middle
10

Figure 3.1: SuperKEKB colliderof the linac, and positrons are generated by striking the target with electrons. Electrons areproduced at the photocathode, so the beam emittance (beam size and momentum spread) issmall. Positrons, on the other hand, are produced and accelerated by pair production usingtungsten as a catalyst to produce low-energy positrons. Therefore, the positron beam emittance is large. A positron damping ring was installed to reduce positron emittance. Emittancecan be reduced by radiation damping when positrons are traveling in the damping ring. The
electrons and positrons in the low emittance state are then injected into the main ring. Sinceelectrons and positrons have very low mass, acceleration is completed in the linac section, andbeam velocity is maintained and stored in the main ring. Then, the beams collide with eachother at a suﬃcient timing.
Table 3.1: The designed parameters of the SuperKEKB collider [10]Energy [GeV]
Beam current I [A]ξyβy ∗ [mm]e− beam (HER)e+ beam (LER)

7.0
2.62
0.088
0.41

4.0
3.60
0.090
0.27

11

3.1.2Future luminosity34
2
On June 8, 2022, the peak∫ luminosity L reached a value of L = 4.65 × 10 cm /s, and theintegrated luminosity L = Ldt accumulated through the operation of the SuperKEKB collider
was L = 424 fb−1 . The Belle II experiment is currently in the ﬁrst long shutdown period
for upgrading and is scheduled to resume in 2023. After that, SuperKEKB will continue tobe enhanced, and continuous improvement of the instantaneous luminosity is planned. The
current goal is to reach the peak luminosity of L = 6 × 1035 cm2 /s by 2034. In addition to thisgoal, one benchmark is SuperKEKB’s original design Luminosity Ldesign = 8 × 1035 cm2 /s.

Figure 3.2: The Luminosity projection of SuperKEKB [1], The red line represents the planned
peak luminosity, and the blue line represents the integrated luminosity

3.2

Beam background

Beam background refers to the phenomena described below that cause beam particles to deviate
from the beam orbit and strike the beam pipe, producing electromagnetic showers, neutrons,
etc. These are called the beam background. The background is not only noise to the physicalevent signal, but also damaging to the detector. In particular, because of the proximity of the
SVD to the beam pipe at the point of impact, it is necessary to understand the damage causedby the background. The background can be classiﬁed into two types: Single beam backgrounds
and luminosity-related backgrounds.
• Single beam background: Background due to reactions between particles in the beam
bunch or with residual gas inside the beam ring.
Touschek eﬀect : Scattering by particles in a bunch of beams. Signiﬁcant in positronring (with low energy). The rate of occurrence increases in proportion toI± 2
σy ± nb± E± 3
12

(3.2)

where σy is the beam size, nb is the number of beam bunch, and E is beam energy.
Therefore, the rate of the Touschek eﬀect occurring in the HER and the LER is
diﬀerent, and it is higher in the LER due to E+ < E− .
Coulomb scattering : Elastic Scattering of beam particles with residual gas particles
in the beam pipe, which changes the direction of the particles; scattering of residual
particles with nuclei: Rutherford scattering, scattering with electrons outside nuclei:
Møller scattering. The rate of occurrence can be represented as follows
rate = σCoulomb Nbeam Nnucl. ∝ σCoulomb Nnucl. I±(3.3)

where σCoulomb is the cross-section of the Coulomb scattering, Nbeam is the numberof the beam particles in one circle of the ring, and Nnucl. is the number of nuclei
seen from the beam particles per barn per second. The rate increases in proportion
to the beam current, so the rate at LER is higher due to I− < I+ .
Bremsstrahlung (Brems) : Inelastic scattering caused by the electric ﬁeld of an
atomic nucleus. This scattering produces electromagnetic waves that reduce theenergy of the beam particles. The particles with reduced energy are deﬂected oﬀ
the ideal beam orbit by the deﬂector magnet. This inﬂuence is the strongest of thethree.
The rate of the Bremsstrahlung can be obtained by rewriting σCoulomb in Eq. 3.3 as
the cross-section of Bremsstrahlung σBremss. . This means that the Bremsstrahlung
also increases proportionally to the beam current, meaning that LER has a higher
rate.
• Luminosity related background: Background caused by beam-to-beam collisions.
Radiative Bhabha scattering e+ e− → e+ e− γ, By emitting a gamma after beam
impact, the electrons or positrons that have lost energy and hit the beam pipe,
creating a background. The emitted gamma may also produce neutrons by hitting
nearby magnets.
Two-photons processes e+ e− → e+ e− (γγ) → e+ e− e+ e− , Photons mediate the creation of new low-momentum electron-positron pairs, which become radiation to the
VXD.
Increased luminosity is expected to increase luminosity related background especially. To date,
the amount of beam background, around the SVD, generated during operation at designed luminosity Ldesign = 8 × 1035 cm2 /s has been studied and reported using Monte Carlo simulation(MC) [2] [27]. Simulation results predict that the innermost (third) layer of the SVD will suﬀer
the most radiation damage, exceeding 2 kGy in one snowmass year (smy; typical operation timeof an collider facility in one year; 1 × 107 sec) with the designed luminosity, refer to Fig. 3.3.
The type of background radiation was also investigated in the simulations in the sameway. Fig. 3.4 shows the results of weighting the amount of each particle with respect to thekinetic energy by the degree of damage each particle inﬂicts on the interior of the sensor in the
innermost layer of the SVD in order to consider the damage to the sensor. In the high kinetic
energy region (above 10 MeV), damage by electrons and positrons is dominant, while neutrons
are higher in the region below 1 MeV.

13

Figure 3.3: The simulation of beam background around the SVD with designed luminosity
Ldesign = 8 × 1035 cm2 /s, ‘RBB’, ‘RHWide’, and ‘BHWide LA’ are generally included in the
Radiative Bhabha scattering and are divided by the scattering angles of an electron θe ; the‘BHWide LA’ is deﬁned as events that θe ≥ 1◦ , the ‘BHWide’ is deﬁned as events that 1 >θe ≥ 0.5◦ , the ‘RBB’ is deﬁned as events that θe < 0.5◦ . ‘HER’ represents the electron beamline side, ‘LER’ represents the positron beam line side. The coeﬃcients in the ﬁgure arecorrection coeﬃcients obtained by comparing the diﬀerence between the MC results and theactual measured values during the same period using the radiation measurement data during
actual operation in our past period. [27].

14

6

10

2

Neutron Flux [neq/cm /smy]

All Components
ee+

5

10

n
p
π+

4

10

πγ

3

10

102
10
1
−1

10

−2

10 −5

−4

−3

−2

−1

0

1

2

3
4
log(K/(1 MeV))

Figure 3.4: Converted neutron ﬂux vs kinetic energy. The vertical axis uses the NIEL factor(below for details, Sec. 3.4), converted to neutron ﬂux, and the damage is evaluated as NIEL
damage; damage in the bulk of a sensor bulk. K represents kinetic energy [25]. The distributionof all background elements (two-photon, RBB,...) has a peak at 1 ∼ 10 MeV, but only RHWide
LA also has a small peak near 10 GeV. The peak around 10 GeV in this ﬁgure is the contribution
from the electromagnetic shower caused by RHWide LA.

15

3.3
3.3.1

Woriking principle of double-sided readout silicon strip detector
Silicon semiconductor

Silicon is an atom with four electrons in its outermost shell. It forms crystals by covalently
bonding its outermost electrons with other silicon atoms. A pure silicon crystal is an intrinsic
semiconductor and does not conduct current easily. By adding a small amount of a pentavalentatom (e.g., phosphorus) to a silicon crystal, one extra electron of phosphorus becomes a carrier
and moves through the silicon crystal. This makes it easier for the current to ﬂow. On theother hand, if a small amount of trivalent atom (e.g., boron) is added, one electron pair ofsilicon that should be ﬁlled is not ﬁlled, and it becomes a carrier called a hole, which also
facilitates current ﬂow. A semiconductor to which a pentavalent atom is added is called an N
type semiconductor, and the added pentavalent atom is called a donor. On the other hand, asemiconductor to which trivalent atoms are added is called a P type semiconductor, and theadded pentavalent atoms are called acceptors. Fig. 3.5 shows an example of N type silicon
semiconductor.

Figure 3.5: Bond model of Phosphorus doped silicon [7]3.3.2

PN junction

In semiconductors, doped atoms (dopants) create new levels in the band gap. The new leveloccurs close to the conduction band EC in P type semiconductors and close to the valence band
EV in N type semiconductors. Carriers in each type of semiconductor can easily transition
from the new level to the corresponding immediate band at room temperature. This causes achange in the Fermi level (quasi-Fermi level); the Fermi level of a P type semiconductor drops
to near EV and rises to near EC at the Fermi level of an N type semiconductor, as shown in theleft diagram in Fig. 3.6. When P-type and N-type semiconductors are joined (PN junction), theFermi levels of the two semiconductors are aligned, and a built-in potential ∆E is spontaneously
generated near the junction due to the diﬀerence in EC of the two semiconductors. At thistime, the carriers are in thermal equilibrium and no net carrier current exists at the junction.
The region where the carriers are in equilibrium is called the depletion layer.

16

Figure 3.6: Band structure of P type and N type semiconductors (left) and after PN junction(right) [7]Figure 3.7: Reverse bias [7]3.3.3

Reverse bias voltage and depletion layer

If a further voltage is applied in the direction of the built-in electric ﬁeld E in the right diagramof Fig. 3.6, the potential barrier at the junction to the carrier becomes e(∆E + V ) as shownin the right diagram of Fig. 3.7. Therefore, carrier migration becomes more diﬃcult and very
little current occurs in the depletion layer itself. This tiny current is due to electron-hole
generation caused by thermal ﬂuctuations in the silicon crystal. For this reason, the current
has a temperature T dependence, which is written by the following relation;
( )2
(
)]
[
Eg (T ) Eg (T0 )
T
I
1
=
,
(3.4)
exp −
−
I0
T0
2kB
T
T0
kB : Boltzmann constant, Eg (T ): energy gap of silicon, Eg (T ) ≈ Eg (T0 ) ≈ 1.12 eV typically.
In this thesis, the current is called leakage current. Since leakage current leads to noise,
DSSD is used for its small leakage current and low noise.
Considering the boundary condition that there is no electric ﬁeld in the region outside
the depletion layer, the depth of the depletion layer widens as the voltage is added to make a17smooth connection of the potential with the region where there is no electric ﬁeld. the depletion
layer width d can be written using the applied reverse bias voltage Vbias as the equation [28];√√2ϵSi (∆E + Vbias ) NA + NDd=·∝ Vbias ,
(3.5)
e
NA ND
NA : acceptor concentration, ND : donor concentration, ϵSi : dielectric constant, e: elementarychange.
In the case of our double-sided readout sensor, the P side sensor strip corresponds to theP side semiconductor in the discussion here, and the N type bulk corresponds to the N side
semiconductor here. However, the depletion layer spreads in the bulk because the carrier
concentration in the P-side sensor strip (NA ) is much higher than that in the bulk (ND ). In
addition, the above equation becomes√
2ϵSi Vbias √d=∝ Vbias ,(3.6)
eND
basically, the applied reverse bias voltage is much higher than the built-in voltage. The upper
limit of d for a typical DSSD is the thickness dsensor of the DSSD, and when d = dsensor , thesensor is said to be Full depleted, and the bias voltage at that time is called Full depletion
voltage VFD .
In this paper, the “reverse bias voltage” will simply be referred to as the “bias voltage”.
3.3.4

Signal generation and collection mechanism of DSSD by particles passage

The mean energy required for an electron-hole pair production in silicon is ϵ = 3.63 eV. On theother hand, charged particles impart energy dE/dx to silicon as they pass through it. Thus,
when a charged particle passes through silicon, it produces ρSi dsensor (dE/dx)/ϵ electron-holepairs, ρ is silicon density.
A bias voltage is applied to the DSSD when the DSSD is used, and a steady electric ﬁeld
is generated between the P and N side strips. The surrounding steady-state electric ﬁeld isdisturbed by the movement of an electron or a hole generated by charged particles that passthrough it. The disturbed electric ﬁeld is instantly transmitted around the strips near where
the electron-hole pairs were generated, and an induced current is generated in those strips,
which is processed as a signal by the ASIC (APV25).
Thus, an electric ﬁeld is required to receive signals from charged particles. Since the electricﬁeld is generated only in the depletion layer, DSSD is used by depleting the entire sensor (calledfull depletion). In addition, charge collection per strip cannot be done correctly because thestrips may be electrically connected to each other if the sensor is not fully depleted. This
makes it diﬃcult to determine the correct location of particles passing through.

18

Figure 3.8: Electron hole pair generation in the DSSD by particle passage [7]3.4

Possible radiation damages of the SVD

The damage caused to SVD silicon sensors by background radiation falls into two main categories; Total Ionizing Dose (TID) and Non-Ionizing Energy Loss (NIEL).
TID: TID is sensor surface damage caused by charged particles (mainly electron, positron,and proton) and photons through ionization. These can cause electron-hole generation
within the oxide ﬁlm (SiO2 ), with electrons going to the electrode but holes being transported and captured as a ﬁxed positive charge near the interface between the oxide ﬁlm
and silicon. This creates new levels near the interface (Si-SiO2 ) and increases leakage
current near the surface, causing interstrip capacitace and other surface properties tochange. Fig. 3.9.

NIEL : NIEL is damage inside the silicon sensor (sensor bulk) caused primarily by neutrons
or high-energy charged particles. As shown in Fig. 3.10, for a silicon crystal lattice,radiation can bounce oﬀ the silicon atoms that form the lattice, creating lattice defects.
These defects act as acceptors. In addition to Schottky defects, where silicon is repelled
out of the lattice, there are cases where the repelled silicon remains in the lattice, as in
the case of Frenkel defects. The excess silicon in the lattice has unpaired electrons, which
act as acceptors. This increase in acceptors causes silicon bulk to change from N type to
P type. This is called the type inversion.
The NIEL is evaluated in terms of the number of 1 MeV neutrons passing per unit. The
amount of damage in various momentum regimes for diﬀerent particles is compared tothe amount caused by 1 MeV neutrons and investigated to obtain a scaling factor to
1 MeV neutrons, these parameters are shown in Fig. 3.11, and Fig. 3.4 was used theseparameters on scaling.

19

Figure 3.9: The scheme of Total Ionizing Dose

Figure 3.10: The scheme of Non-Ionizing Energy Loss20

Figure 3.11: NIEL scaling parameter of each particle, D(1 MeV) = 95 MeVmb [14]

21

3.5
3.5.1

Operational concerns due to radiation damage
Type inversion and VFD increase after Type inversion

As described in Sec. 3.4, NIEL damage can change the carrier concentration and cause typeinversion, in which the N-type bulk changes to the P type. It is necessary to understand thedose that causes such a drastic change in the properties of the bulk. The variation of carrier
concentration (charged carrier concentration called eﬀective carrier concentration Neﬀ ) with
irradiation dose can be considered in the following equation [17];Neﬀ (Φ) = ND e−cΦ − βΦ

(3.7)

where Φ is equivalent neutron ﬂuence, β is rate of the generated acceptors, c is donor removal
cross section. The type inversion occurs when Neﬀ decreases due to radiation and becomes 0
and When Neﬀ < 0, the acceptor concentration is more, indicating that the bulk is the P type.
As the carrier concentration changes, VFD also changes. Rewriting Eq.( 3.6) for the full
depletion, changing ND to |Neﬀ |, and rearranging for VFD , we obtain
VFD (Φ) =e|Neﬀ (Φ)|
dsensor 2
2ϵSi

.

(3.8)

Fig. 3.12 is an example of VFD behavior. We expect that the VFD we measure will have astructure with the type inversion as shown in the ﬁgure.
Based on the above, it is necessary to understand the type inversion and to examine how
the VFD rises after type inversion. The way to ﬁnd out the type inversion is to follow the VFD
value for each irradiation dose. And there are two ways to evaluate the VFD .
1. CV measurement, The capacitance of DSSD can be calculated as a parallel plate
capacitor. Taking the area of the sensor as Ssensor , the capacitance C is related to theapplied voltage by the following equation;()
Ssensor −22ϵ V−2C = ϵSi ϵ0= (ϵSi ϵ0 Ssensor )−2 Si bias ∝ Vbias(3.9)
de|Neﬀ |
Capacitance is inversely proportional to the depletion layer width spread. In other words,
capacitance works in the region where V ≤ VFD , while for V > VFD , the depletion layer
does not expand any further, so capacitance remains constant. Thus, we can measure
the CV of the sensor and follow the plot of C −2 V to consider VFD as the point where theslope of C − 2 reaches a plateau.
2. IV measurement, The leakage current I can be written using the electron-hole pairgenerate rate U and the depletion layer width, the bias voltage and the following equation
I 2 = (eU dSsensor )2 = (eU Ssensor )22ϵSi (Vbias )∝ Vbiasq|Neﬀ |,

(3.10)

the leakage current follows this formula until the voltage at which d < dsensor , i.e.,V ≤ VFD , and then it is expected that the leakage current will not increase. This leakage
current is considered only within the sensor bulk, but other currents occur on surfaces
and other areas of the actual sensor. However, those currents are small compared to thebulk current, so these current can be ignored and the leakage current behavior of theentire sensor follows Eq. 3.10. This means that we can estimate VFD by making an IV
measurement and plotting I 2 V , looking for where the slope of I 2 V reaches a plateau.
22Thus, we estimate the VFD for each irradiation dose through CV and IV measurements. We
then look for type inversion based on the structure of VFD by irradiation dose, and estimate
the degree of increase in vfd after type inversion.

Figure 3.12: A example of VFD change with Type inverison [14], Udep equivalent of our VFD

3.5.2

Leakage current increase

It is known that there is a proportional relationship between leakage current and radiationdose (equivalent neutron ﬂuence Φ) as in the following equation [14].
I/vsensor = αΦ(3.11)

where vsensor is sensor volume, and α is a coeﬃcient that called damage factor.
Increased leakage current leads to increased signal noise. In addition, it may exceed theallowable current range of the power supply module that is providing the bias voltage to the
DSSD. For this purpose, we perform IV measurements for each irradiation dose and calculate
the damage factor to determine how much the current increases with radiation and up to what
irradiation dose our SVD module can withstand.
3.5.3

Noise increase and charge collection eﬃciency deterioration

The main source of noise in DSSD is the interstrip capacitance of the sensor strip. In our ASIC,
APV25, the noise ENCC is investigated to vary depending on a capacitance C connected to
APV25 as follows [20];
ENCC = 270 + 38 × C(3.12)
23

Therefore, the increase in interstrip capacitance due to radiation leads to an increase in noise.
In fact, interstrip capacitance and the resulting increase in noise on the other group’s DSSD
have also been reported. [4] with 12 GeV proton beam test.
The change in interstrip capacitance is actually measured by measuring the interstrip capacitance and the applied voltage.
On the other hand, charge collection eﬃciency (CCE) is also considered to be degraded byradiation. The reason is that if the time required for carriers generated by transit particles
to be trapped and re-emitted (detrapping) due to silicon crystal defects caused by radiation
damage is longer than the shaping time of the ASIC (50 ns for APV25), the trapped carriers
will not become a signal.
Noise and CCE are measured using the SBW module, which is capable of reading out
signals connected to DSSD and APV25.
3.5.4

Annealing eﬀect

Thermal vibration of silicon crystals can repair defects in the crystal. This is called theannealing eﬀect. This annealing eﬀect depends on the elapsed time after irradiation and theenvironmental temperature, and we will investigate how much radiation damage is recoveredby the annealing eﬀect by focusing on the change over time of the measurement from Sec. 3.5.1
to Sec. 3.5.3.

24

Chapter 4
Measurement methods of the irradiation damage insensor
IV, CV, Cint V are useful ways to evaluate the irradiation damage in sensors as described in
the previous chapter described. To measure changes in these properties during and after, wedevelope a remote measurement system for these tests.

4.1

The sensors for the irradiation tests

Three diﬀerent types of sensors are prepared for the irradiation tests. Their geometries arelisted in Tab. 4.1.
Table 4.1: The speciﬁcation of irradiation sensor samples

thickness [µm]sensor area [cm×cm]
number of strips P side/N side
strip pitch P side/N side [µm]strip length P side/N side [cm]number of irradiated sensor
irradiated area*2

HPK large sensor

HPK mini sensor

320
12.492 × 5.964
768/512*1
75/240*1
12.29/5.772
1
16 mm wide along
P strips at center

320
2.12 × 2.12
192/192
100/100
1.92/1.92
10whole sensor*1 HPK large sensor has readout strips and non-readout strips (ﬂoating strips), thedescribed value of number and pitch are for readout strips only.
*2 Refer to Fig. 4.7

The HPK large sensor and the HPK mini sensor were manufactured from the same siliconwafer provided by Hamamatsu Photonics K.K. (Fig. 4.1). The HPK large sensor is used forthe DSSD sensor in the operating SVD modules, thus the properties of the sensor bulk can
be expected as the same as the operating DSSD sensor, and the characteristic of the bulk of
the HPK mini sensor can be expected to be close to the HPK large sensor due to both cut
from the same wafer. On the other hand, the layout of the surface including the strips aroundthe mini sensor is diﬀerent from the large sensor, this diﬀerence would appear in the Cint V
measurement.

25

Figure 4.1: The silicon wafer layout. One large sensor and four mini sensors can be taken froma wafer, a large sensor is in the center and four mini sensors are arranged to surround the large
sensor.

4.2

Devices and methods of IV, CV, Cint V measurement

The used meter of each measurement and conceptual drawings of the connection to the sensor
are shown here.
4.2.1

IV measurement

A source meter is able to apply bias voltage to a sensor and measures the leakage current of
the sensor at the same time. The positive and negative two terminals are directly connectedwith the corresponding N (+ terminal) or P (− terminal) side of a sensor. The connection to
the sensor is through wire bonding to the bias pad of the N or P side bias ring.
We used the Keithley2614B (Fig. 4.2) source meter for the irradiation tests, it can apply
the bias voltage from 0 V up to 200 V and can measure the leakage current range in 0.1 fA to
10 A order. Considering the range of change in leakage current before and after irradiation(expected from nA to mA region), the source meter capability is suﬃcient. The voltage range
is also suﬃcient because the VFD before irradiation was expected to be around 65 V, and the
VFD will be increased proportional to dose, but it will occur after the type inversion in thesensor so that voltage range is suﬃcient to and conﬁrm the type inversion in the sensor andmake sensor bulk full depleted up to a certain high dose after the type inversion.

26

Figure 4.2: Source meter; Keithley2614B
4.2.2

CV measurement

We used an LCR meter with a protection circuit to measure the bulk capacitance of the sensor.
The circuit diagram of the protection circuit is shown in Fig. 4.3. By using this protection
circuit, measurements can be made while an external DC power supply provides voltage up
to 200 V to the sensor, and it is based on 4-terminal pair (4TP) method that can typicallyeliminate an ill eﬀect from the probes own resistance or conduct resistance, and can minimize
the inﬂuence come from measuring current induced magnetic ﬁeld [23].
We used KeysightE4980AL (Fig. 4.4) as an LCR meter, which has four measuring terminals
for 4TP, and the measurement signal amplitude is arbitrarily set to the range of 0 to 2 Vrms ,the measurement signal frequency range is from 20 Hz to 300 kHz. The source meter to be
used for IV measurements is also used for the external DC power supply here.
Regarding those connections, the source meter should be connected to the symbol marked
Ex in the ﬁg. 4.3, four terminals marked Hp, Hc, Lp, Lc should be connected to the corresponding LCR meter’s terminal, and the sensor should be connected to the symbol of a capacitor
marked Cx with wire bonding like the IV measurement.
In addition, To perform measurement more precisely, the so-called OPEN/SHORT correction should be done around the point attached to the sensor before the measurement to reduce
the residual impedance of the measurement circuit and adjust the oﬀset on the impedance
measurement.

27

Figure 4.3: 4TP method with external DC bias circuit, in the principal, can be used formeasuring frequency up to 2 MHz, external bias voltage up to 200 V [23]. We handmade it.
(used a 100 µF electrolytic capacitor for C1)

Figure 4.4: LCR meter; KeysightE4980AL
4.2.3

Cint V measurement

The measurement setup of Cint V is similar to CV measurement, but in this measurement, theexternal DC bias power supply (source meter) should be directly connected to the bias ring;
remove Ex and 1 kΩ resistor in Fig. 4.3, not needed via the protection circuit because two
measurement terminals (Cx in Fig. 4.3) will be connected to the strips, and there are no needs
to apply bias voltage.
The Cint was expected to be small as pF order, hence by measuring the combined capacitance28

(> Cint ) consisting of two interstrip capacitances and averaged by dividing by 2 to obtain amore accurate interstrip capacitance. Therefore, we arranged the connection of Cx to strips
like Fig. 4.5 and we deﬁned the interstrip capacitance as the value of measured capacitance
Cmeasured divided by 2; Cint ≡ Cmeasured /2.

Figure 4.5: Example of the wire bonding for Cint measurement, wire bonded on the DC pad ofeach strip. There are three strips as one pair to measure Cint , the center strip and two adjacentstrips are connecting diﬀerent signal terminals via the protection circuit leading to an LCR
meter. Thus, the equivalent electrical circuit is two capacitors (two interstrip capacitances)connected in parallel (right ﬁgure).

4.3

The conﬁguration of remote measurement system

he system is designed based on the concepts of our remote measurement system are:• can optionally switch IV, CV, Cint measurement circuit• Minimize the eﬀect come from the devices are irradiated in results except for the sensor• Can operate the system through the outside of an irradiation chamber
The time available for measurements at the irradiation facility is limited, so users must complete
their operations quickly. Our system reduces the time needed for measurement, allowing us
to perform measurements between irradiation steps and quickly observe changes in sensor
properties just after each irradiation.
4.3.1

Switching board

To satisfy the concepts above, We designed a switching board with a function to switch the circuit between IV, CV, Cint V measurement circuits, and all measurement meters were connectedto this switching board. To prevent radiation damage from the beam to the board, we decided
to create a switching board using an FR-4 PCB substrate, and for the switching function, we
decided to use a mechanical relay (SANYU SWITCH Co., Ltd JS-105Z; drive voltage DC 5 V,
maximum switching voltage DC 200 V). The detailed design can be found on Sec. 4.6.
4.3.2

Remote control system

What needs to be remotely controlled is the operation of meters (source meter, LCR meterand data logger) and circuit switching. The used source meter and LCR meter, the datalogger (GL840) supporting the Virtual Instrument Software Architecture (VISA) which is aused standard in instruments, We utilized the VISA standard to send commands and receive
responses to and from the meters via USB communication with PC.
29

The circuit switching; mechanical relay control was used a digital 5 V output control board(Interface Corporation PEX-234104; open collector sink type, photocoupler output), it had
been installed in the PC, and provides a GUI operating screen to control output channels.

4.4

The beam facility and beam condition, dose estimation

The irradiation test was performed using the electron beam facility at ELPH, Tohoku University. The beam conﬁguration is summarized in table. 4.2, The beam current is variable,
and we typically operated it at 140 nA. To calculate the irradiation dose in a beam spot perirradiation time D from the beam current I, we used the following relationships;
D1 electron =
D=

qe dE
(dE/dx)ρdqe=;
Sbeam ρdSbeam dx

(4.1)

I
I dE
D1 electron =qeSbeam dx

(4.2)

where D1 electron : dose by one electron, dE/dx: energy deposit in silicon per length; 1.6 MeV/(g/cm2 )in minimal ionizing particle energy [26], ρ: density; 2.3 g/cm3 , e: electron charge, Sbeam : beam
spot area on a sensor; measured to be around 1 mm2 . In particular, when the beam current is
140 nA, D = 2.2 kGy/s
Table 4.2: Beam status
Amplitude
Beam current
Beam spot size

90 MeV140 nA (70 nA)∼ 1 mm2

In order to cause radiation damage to the entire irradiation area (refer to Table 4.1, Fig. 4.7),
we moved the sensor position with two-axis stage (Fig. 4.8) to achieve uniform irradiation on thesensor; the beam sweeps in the irradiation area on the sensor like Fig. 4.6. We use “one routine”
which means a single sweep of the entire irradiation area to estimate irradiation dose and timein operation. We adjust the beam sweeping speed to 1 mm/s during 1 routine, This allows anirradiation dose of 2.2 kGy to be given to the entire region in 1 routine when I = 140 nA, andthe irradiated time tirradiation can be simply calculated as tirradiation = Dtarget /D × Sirradiationfrom target dose Dtarget , irradiated area Sirradiation (this comes from beam sweep speed 1 mm/s).

30

Figure 4.6: Beam sweep on a sensor. The red circle represents the beam spot, and the graysquare represents a sensor. The beam sweeps on the sensor like a stroke as a red line

Figure 4.7: Irradiated area; pinkish area, we used radiochromic ﬁlm to crosscheck the irradiateddose and area. As the irradiated dose increases, the color darkens. The radiation dose survey
is currently being requested by a collaborator to a company.

31

Figure 4.8: The 2-axis stage; consisted from SGSP20-85 (small stage) and OSMS33-500 (largerone) from SIGMAKOKI CO., LTD. An equerre was used to connect them, and the larger stage
was ﬁxed on the breadboard which was provided from the facility side. The substrate carrying
the sensor is mounted on the smaller stage and moves in a plane perpendicular to the beam.
These stages are also can be operated remotely via PC.

4.5

The measurement system placement and radiation shielding

A 2-axis stage was placed on the beamline. As mentioned above, a sensor and a switchingboard are attached to this 2-axis stage. The switching board is connected to each measurement
meter and protection circuit with LEMO cables, and the connection to the data logger andrelay power supply is provided by lead wires. The instruments were connected to the operating
PC with USB cable.
Since radiation such as electrons and neutrons come from the beamline direction, polyethylene blocks for neutrons and lead blocks and lead sheets for electrons were prepared for shielding. The source meter and other precision instruments were placed in an aluminum-framed
enclosure, and the enclosure was shielded with lead and Polyethylene blocks around the beamline direction. The operating PC is located at least 10 m away from the beamline to avoid
radiation eﬀects. The PC is connected to another PC outside of the beam radiation hall via a
LAN cable and can be operated from outside of the radiation hall.

32

Figure 4.9: The measurement system placement

4.6

The detailed structure around sensor

In the irradiation test, two diﬀerent types of boards were designed: one for sensor placement
and the other for the switching circuits for connecting to measurement instruments. The
switching board can be connected to a source meter and an LCR meter, allowing for theswitching between IV, CV, and Cint measurement circuits. Additionally, both the sensor board
and switching board feature MIL connectors, which can be connected to each other throughthe use of ﬂat cables to complete the measurement circuit. Moreover, multiple sensor boards(up to three in this particular test) were stacked on top of one another to expose the sensors
to irradiate like skewered. The ﬁrst and second layers of sensor boards, when viewed from thebeam port, can be connected to the switching board, enabling each sensor to independently
measure IV, CV, and Cint by switching circuits.
A radiochromic ﬁlm holder is positioned in front of the ﬁrst layer of sensors and between theﬁrst and second layers. A radiochromic ﬁlm, such as the one shown in Fig. 4.7, is placed in theholder. The ﬁlm serves as a protective layer for the sensors to protect sensor from accidental
touch to the sensor, as well as providing conﬁrmation of the irradiation position and amount.
Those boards are ﬁxed to the 2-axis stage by board holders as shown in Fig. 4.10 and move ina plane perpendicular to the electron beam.
Moreover, the environmental temperature is an important input for the measurement, we
prepared a Resistance Temperature Detector Pt100 around the sensor, which is connected toa Data logger and uses the fact that resistance changes at a constant rate with temperature
to obtain the temperature.
33

By separating the sensor section and the switching circuit section, the majority of themeasurement circuit can be shared, allowing for consistent measurement conditions even whensensors are replaced. Including a MIL connector also allows for quick measurement of diﬀerent
types of sensors, with only minor adjustments to the sensor design required.

Figure 4.10: The structure around the sensor board and switching board. Each of the two
1 MIL connectors connect to each other and ○2 connect to each other with 10 cm ∼boards ○20 cm MIL cables.
4.6.1

The design of the switching board

The actual switching board is shown in Fig. 4.11.
A source meter and an LCR meter are connected to the corresponding LEMO connectors
via LEMO cables. By switching the relays of the switching circuit to IV, CV and Cint V
measurement circuits, the respective measurements can be performed. Cint measurements canbe performed for 3 channels for each sensor.
To minimize leakage current and parasitic capacitance caused by the circuit board, we
created a circuit using jumper wires on a universal board. Considering the damage to therelay to ensure the reliability of the system, we also included a socket for the relay, allowing it
to be easily replaced if necessary.
4.6.2

The design of the sensor board

The board consists of a sensor and a MIL connector, with pads for wire bonding only. The
sensor is connected to the MIL connector via wire bonding between the sensor and the pads,
allowing for measurements to be taken.

34

Figure 4.11: The designed switching board for the test.
Fig. 4.12 is for a large sensor, but the only diﬀerence is the geometry, and the other kind of
sensors are based on a similar design. An epoxy adhesive Araldite 2011∗ , which has a proventrack record of radiation tolerance, was used as the adhesive for bonding the sensors. The
substrate behind the sensor was cut, leaving a tiny adhesive area.

∗

If the adhesive adheres to the pad on the sensor board, it cannot be bonded to that pad, so it is necessary
to cover the pads with masking tape when attaching the sensor.

35

Figure 4.12: An example of the sensor board.

Figure 4.13: Wire bonding (aluminum) to bias pad. The Pad on the board is made of electrolessnickel immersion gold (ENIG) as a material for wire bonding. It is known that the successrate of wire bonding will be much decreased when gold is not included in.

4.7

Annealing eﬀect measurement

Annealing is a process that occurs after irradiation. We investigated the annealing eﬀect overtime after irradiation. The study will be evaluated by looking at changes over time in IV, CV,
and Cint V values.
The annealing eﬀect is inﬂuenced by the sensor temperature (Since there is no substancenear the sensor that can be a heat source this time, the environmental temperature near
the sensor should be investigated.), so we needed to monitor the temperature changes in theenvironment where the sensors were stored. To do this, we use two types of thermometers
shown in Fig. 4.14
36

Figure 4.14: Thermometer for checking environment temperature around sensor: Pt100 (left),
TR-76Ui by T&D Corporation (right)
Fig. 4.15 shows the temperature and elapsed time since just after irradiation. The sensors
were stored at ELPH until August 17th, 2022, and then transported by car to KEK Tsukuba
experiment hall on the day. The sensors were stored at temperatures ranging from 17 ∼ 20 ◦ C
at ELPH and 23 ∼ 27 ◦ C at KEK. IV, CV, and cint measurements were performed at thetime corresponding to the green vertical line in the ﬁgure. For the annealing measurement,
the switching board, the sensor board, each measurement meter, and the control PC, whichconstitute the measurement circuit used in the irradiation test, are used. A lightproof box isused to shield the sensors from light.

37

Figure 4.15: Envioment temperature changes in annealing process38Chapter 5
The results of the irradiation tests
5.1

Beam current

As described in Sec. 4.4, the irradiation dose is calculated from the beam current. The current
value of the beam is monitored (an example: Fig. 5.1) by the facility and is used in our calculation. The current values were obtained without any problems during the entire irradiation
period.

Figure 5.1: An example histogram of beam current during an irradiation period provided by
the beam facility side. The unit of the horizontal axis is Ampere.

39

5.2

The results of the irradiation test

Irradiated dose on sensors
Fig. 5.2 shows the relationship between the irradiated sample and the irradiation dose to thesensor calculated from the current values using the method of Sec. 4.4. In the ﬁgure, the dots
represent the doses at which IV, CV, and (Cint V) measurements were taken. The star symbol
and accompanying number denote the dose at the end of the irradiation. At this irradiation
test, 10 samples of HPK mini sensor and 1 HPK large sensor were irradiated.

Figure 5.2: The samples with irradiated dose. In the vertical axis labels, ‘mini’ represents the
HPK mini sensor, and ‘large’ represents HPK large sensor. The number beside it is the serial
number of a sensor to identify.
In order to investigate annealing eﬀects occurring in sensors with diﬀerent irradiation doses,
the diﬀerent dose at the end of irradiation was intentionally chosen for each sensor.
5.2.1

The results of IV, CV, Cint V measurement of mini sensor

The results of the IV, CV, and Cint V measurements are presented here. The results are shown
here for each irradiation dose of mini822 as a representative example of mini sensors, where
the results of the other mini sensors show similar behavior.
The mini sensor has a diﬀerent surface strip structure from the SVD sensor currently inoperation, which makes it diﬃcult to directly feedback with the information obtained from themini sensor’s Cint V measurement into the SVD operation. As a result, the number of Cint V
measurements taken with the mini sensor was reduced and used for irradiation time instead.
The results are shown from Fig. 5.3 to Fig. 5.6. The order from the top to the bottom is
IV, CV, and Cint V measurements, and to ﬁnd the VFD , we plotted in I 2 V for IV and C 2 V for
CV. On the other hand, as mentioned in Sec. 4.2.3, the Cint is the measured value divided by
2. The plot shows ‘NO DATA’ for dose steps where Cint V measurement was not performed.
The Dose where the measurement was performed is displayed above each plot.
40

41

Figure 5.3: The IV, CV, Cint V results of mini822 0 kGy ∼ 5.1 kGy

42Figure 5.4: The IV, CV, Cint V results of mini822 8.5 kGy ∼ 21.2 kGy

43Figure 5.5: The IV, CV, Cint V results of mini822 25 kGy ∼ 40 kGy

44Figure 5.6: The IV, CV, Cint V results of mini822 60 kGy ∼ 100 kGy

IV measurement summary
From the plots, A shoulder shape can be seen clearly on the I 2 V curve (Henceforth referred
to as IV curve) in the irradiation dose range from 1.7 kGy to 60 kGy. At 0 kGy, Observing
the shoulder accurately is diﬃcult because slight leakage currents (≈ 100 nA) from externalsources (most likely due to the switching board) have contaminated the measurement. However,a point can be identiﬁed at which the slope of IV curve changes around 65 V. The same CV
measurement of 0 kGy shows a plateau from around 65 V Thus at 0 kGy measurement, theabsolute values of the measurements are unreliable, but we can roughly estimate that the VFD
is around 65 V; Locating the VFD is not an issue.
On the other hand, it is known that the leakage current of the sensor increases along with
the increase of the irradiation dose, and this is supported by our results. While external leakage current was a problem at 0 kGy, it is thought that the external leakage current does not
signiﬁcantly increase with increasing irradiation dose because the rest of the sensor area isnever irradiated. Therefore, the external leakage current is expected to be at most approximately 100 nA, which is a small fraction (around 2%) of the shoulder current value at 1.7 kGy
irradiation. This value is considered to be insigniﬁcant, and even more so for measurements
at higher irradiation dose steps.
The position of the shoulder of the IV curve shifts towards lower voltage with increasing
irradiation dose up to 20 kGy, but beyond that point, it turns to shift towards higher voltage,
To better visualize the changes in the shoulder’s position, Fig. 5.7 shows each measurement
scaled by the maximum value of the measurement. This behavior can be considered as a changein VFD . From here it means that with this measurement we were able to conﬁrm the typeinversion, a detailed analysis will be given later.

45

Figure 5.7: IV curves scaled by its maximum value. The unit of vertical axes is [arbitrary unit].
above: 0 kGy ∼ 23.5 kGy, below: 19.8 kGy ∼ 93.2 kGy. Only for 75.0 kGy and 93.2 kGy, data
were taken up to 200 V of applied voltage because the shoulder of IV curves are at higher
voltages. In the ﬁgure above, we can see the shoulder shifting more and more to the left until
19.8 kGy, In the lower ﬁgure, the shoulder appears to shift more and more to the right as theirradiation dose increases from 19.8 kGy.

46

CV measurement summary
Measurement frequency of LCR meter was set to 1 kHz and signal amplitude is 0.1 V.
The C −2 V curve () is expected that the capacitance would be like as C −2 ∝ V before VFD
and then plateau. This trend is visible for 0 kGy and 1.7 kGy in this result Fig. 5.3, however,other irradiation doses show bulging in areas that were expected to be linear. Thus, we hard
to ﬁnd out the VFD from the CV curve, the VFD is expected to be a point that separates thetwo areas in the CV curve, but we can conﬁrm that the angle between the rising edge of theCV curve and the horizontal axis tends to increase in proportion to the irradiation dose anddecreases after a certain irradiation dose. This trend is consistent with that observed in the
IV measurement.
Cint V measurement summary
Measurement frequency of LCR meter was set to 100 kHz and signal amplitude is 0.1 V.
Basically, the structure around the strip surface of the mini sensor is diﬀerent from that of
the current SVD or HPK large sensor, so it is not possible to provide direct feedback from theCint V measurement of the mini sensor, but we would like to see how it changes before andafter the type inversion.
Cint V results were summarized in Fig. 5.8, we can conﬁrm the Cint increases overall voltage
region; near 0V after irradiation is nearly 2.3 pF, but can be reduced by about 1 pF when abias voltage is applied beyond VFD (star symbol in the ﬁgure), which is evaluated from theIV curve described below.

Figure 5.8: Cint V changes, 1 channel is picked up

47

5.2.2

The results of IV, CV, Cint V measurement of HPK large sensor

The results of IV, CV, Cint V of the HPK large sensor (large845) are shown here as the minisensor results. It is important to note here that unlike the mini sensor, which was irradiatedover its entire surface, the irradiated area for the HPK large sensor was limited to a 16 mm
wide area along the Pstrip at the center of the sensor as shown in Fig. 4.7, thus it is possiblethat type inversion not occur in the non-irradiated areas of the HPK large sensor.
5.2.3

IV measurements

The change in the position of the shoulder (VFD ) of the IV curve and the increase in leakage
current with irradiation dose like a mini sensor can be conﬁrmed. However, the leakage currentof the large sensor is larger than that of the mini sensor due to its larger area.
5.2.4

CV measurements

The Cint V measurement were performed with the same settings as for the mini sensor. We
summarized the results of CV measurements in Fig. 5.9. According to the ﬁgure, C −2 decreases
as the irradiation dose increases. This indicates that C is increasing. However, the shouldersof the CV curves for 0 kGy and 1.7 kGy were around 50 V. As the irradiation dose increased,a peak appeared in that location. The position of the peak shifts slightly to the lower voltage
side with increasing irradiation dose, but the overall location remains largely unchanged.
Based on the information provided, we can conclude that the HPK large sensor has diﬃcultyin accurately estimating VFD (voltage-for-discharge) using CV measurement. We speculatethat this is due to the fact that the irradiated and non-irradiated regions are on the same
sensor, and that the diﬀerent behaviors of the two regions are mixed, and it appears on the
results.

Figure 5.9: CV measurements of the HPK large sensor just after the irradiation
48

5.2.5

Cint V measurement

The Cint V measurements were performed with the same settings as for the mini sensor. A plotof Cint V vs. irradiation dose obtained for one irradiated Cint measurement channel is shownin ﬁg. 5.10.

Figure 5.10: Cint V measurement of the HPK large sensor just after the irradiation
From this ﬁgure, it can be conﬁrmed that Cint increases in proportion to the irradiationdose over the entire bias voltage range as Cint of the mini sensor; the maximum Cint was 6 pF
before irradiation, but by irradiated, it is conﬁrmed that the Cint is more than 10 pF when nobias voltage is applied and gradually decreases with the applied bias voltage increasing.
After 29.1 kGy irradiation, a shoulder-like area appeared on the Cint V curve, which shifted
toward a higher bias voltage with increasing irradiation dose. The position of the shoulder isclose to VFD , and the decrease in Cint appears to decrease after the bias voltage exceeds VFD. From this, it can be inferred that the behavior of Cint has some relationship with VFD .

49

50

Figure 5.11: The IV, CV, Cint V results of large845 0 kGy ∼ 4.9 kGy

51

Figure 5.12: The IV, CV, Cint V results of large845 7.8 kGy ∼ 12.2 kGy

52

Figure 5.13: The IV, CV, Cint V results of large845 14.3 kGy ∼ 18.5 kGy

53

Figure 5.14: The IV, CV, Cint V results of large845 20.7 kGy ∼ 24.9 kGy

54

Figure 5.15: The IV, CV, Cint V results of large845 29.1 kGy ∼ 60.2 kGy

55

5.3
5.3.1

Analyis and Disscution of IV, CV, Cint V results
Deﬁnition of the full depletion voltage

As seen in Fig. 5.7, the irradiation changes the shoulder position of the IV curve, that is, thechange in VFD can be conﬁrmed. This change indicates that type inversion is occurring. In
this section, in order to conﬁrm the behavior of type inversion more precisely, we deﬁne the
VFD quantitatively as much as possible and check the relationship between the VFD and theirradiation dose. This allows us to see at which irradiation dose type inversion occurs, andthen by how much the VFD varies with irradiation dose.
First, as seen in Eq. 3.10, I 2 is theoretically considered to increase proportionally to thevoltage before the applied voltage reaches VFD , and not much after it reaches VFD . From
this point of view, we believe that there is an inevitable change in the slope of the increase in
I 2 , and we consider this from the derivative of I 2 V . The central diﬀerence method was usedbasically to compute the derivative f ′ (x) and forward diﬀerence or backward diﬀerence wasused on the edge of data.;
f (x + h) − f (x − h), (x ̸= xmin , xmax )


2h


f (xmin + h) − f (xmin )(5.1)
f ′ (x) ≈, (x = xmin )

h




 f (xmax ) − f (xmax − h) , (x = x )maxhf (x): raw data of IV, h: voltage step; 1 V. An example of the calculated result on each dose isfrom Fig. 5.18 to Fig. 5.21, from top to bottom, they are the results of the zero-order derivative,ﬁrst-order derivative, and second-order derivative of IV, and the bottom ﬁgure is d log I 2 /dV(described later).
The results conﬁrm that there is a valley-like structure in all second-order derivative results.
The minimum of the valley and the shoulder of the IV curve visually coincide in most of theirradiated dose regions. Also, a structure similar to that shown in Fig. 5.17 appears in the
IV curve and its derivatives; the Step-like structure in the ﬁrst derivative visually coincides
with the minimum of the second-order derivative and the shoulder of the IV curve. Thus, theStep-like structure in the ﬁrst derivative and the minima of the valleys in the second derivative
appear to be good indicators.
However, starting at 12.6 kGy, the position of the minimum of the valley of the secondorder derivative gradually begins to deviate from the position of the shoulder of the IV curvethat we can visually conﬁrm and the Step-like structure in the ﬁrst derivative become hard toobserve. This trend continues until 27.3 kGy. This region has a low VFD and it is consideredthat the value of the ﬁrst derivative changes immediately and abruptly starting from 0V, andit hides the Step-like transition structure seen in Fig 5.17. The second-order derivative valley
associated with this structure has also shifted accordingly. Therefore, we need to consider
methods other than deﬁning VFD from the Step-like structure of the ﬁrst derivative or theminima of the second derivative troughs for regions with low VFD , especially for regions where
the second derivative minima is < 25 V.
As a solution for the region of the second derivative minima is < 25 V, we focused on theslope of the IV curve at VFD . The slope here is not the ﬁrst derivative of the IV curve, but it56

is the ﬁrst derivative of the IV curve scaled by 1/I 2 ;1 dI 2d log I 2=dV
I 2 dV,(5.2)

since I 2 increases in proportion to the irradiation dose, normalizing the slope of the IV curveby this value allows for comparison across diﬀerent irradiation doses.
Fig. 5.7 shows that the slope of the IV curve is almost constant just after the shoulder. As
VFD becomes smaller, the angle between I 2 and the horizontal axis increases until it reaches
VFD Ṫhis means that the slope at VFD is also expected to increase as VFD becomes smaller.
Based on this fact, we took the method of ﬁnding the relationship between the slope and VFD
in the region where the minima of the second-order derivatives are greater than 25 V, i.e.,
where VFD coincides with the minima of the second-order derivatives, then extrapolating it to
the region where the minima of the second-order derivative are less than 25 V (unclear region).
Since there is no diﬀerence in the behavior of each sensor, data from multiple sensors were used
for ﬁtting and extrapolation. The ﬁgure is shown in Fig. 5.22. using linear function ﬁtting andextrapolating to the unclear region resulted in the slope of ∼ 0.01 in the unclear region. In
this time, d log I 2 /dV of the IV curve was calculated from Eq. 5.2 (The Results are includedin from Fig. 5.18 to Fig. 5.21), and the place where the value of d log I 2 /dV is 0.01 is deﬁned
as VFD .
To summarize our deﬁnition of VFD ,1. Calculate dI 2 /dV and ﬁnd minima.
2. if minima ≥ 25, Deﬁne VFD at the position of the minima.
3. if minima < 25, Calculate d log I 2 /dV and ﬁnd a position where the value equal 0.01,
and we deﬁne VFD at that position.
The deﬁned VFD results are thus included as green vertical lines in the top ﬁgure in Fig. 5.18
through Fig. 5.21. It can be also conﬁrmed that it matches the visual shoulder well.
The discussion that follows uses this deﬁnition of VFD .

57

Figure 5.17: Structures related to VFD that appear in the plot of IV and its derivatives

58

59

Figure 5.18: Results of the diﬀerentiation of IV curve, 0 kGy ∼ 4.9 kGy

60

Figure 5.19: Results of the diﬀerentiation of IV curve, 8.1 kGy ∼ 19.8 kGy

61Figure 5.20: Results of the diﬀerentiation of IV curve, 23.5 kGy ∼ 38.6 kGy

62Figure 5.21: Results of the diﬀerentiation of IV curve, 57.1 kGy ∼ 93.2 kGy

Figure 5.22: extrapolation

5.4
5.4.1

Analysis of the type inversion
The type inversion results just after irradiation

Using the VFD deﬁned in the previous section, Fig. 5.23 plots the relationship between VFD andradiation dose. From the result, looking at mini822 as an example, we can observe a gradualdecrease in VFD up to near 20 kGy, and then an increase again after that. This behavior canbe considered as exactly the behavior of type inversion; up to 20 kGy, the sensor bulk was
n-type, but then it changed to p-type. Because As mentioned in Sec. 3.5.1, Neﬀ decreases
monotonically with irradiation dose, and type inversion occurs when Neﬀ = 0. As |Neﬀ | goesup again after a further decrease, VFD follows this trend, decreasing until before the typeinversion and then increasing.
Looking at the other sensors, it can be seen that the minimum value of VFD deviates slightly
from sensor to sensor, but in general the behavior is the same.
We compared the value of VFD to the supply voltage (upper limit of 200 V). The innermost
SVD will receive a cumulative radiation dose of 20 kGy over 10 years, and at this dose, VFD
was observed to be between 15 ∼20 kGy (equivalent neutron ﬂuence 4.5 ∼ 6.0 × 1012 neq /cm2 ),which is below the upper limit of the supply voltage. Even when the SVD received a doseof 80 kGy, which is four times the cumulative dose for 10 years, VFD did not exceed 150 V,
remaining below the supply voltage limit.
In summary, from this irradiation test, we conﬁrmed the type inversion of the SVD sensor
will occur at an irradiation dose of 15 ∼ 20 kGy. In addition, even when the SVD received aradiation dose of 80 kGy, which is four times the cumulative dose for 10 years (a safety factorof 4), VFD does not exceed the allowable range of voltage supplied to the SVD.

63

Figure 5.23: The result of VFD changes just after irradiation. This result can be interpreted
as the appearance of the type inversion occurring at 15 ∼ 20 kGy. HPK845 means large845
5.4.2

The type inversion with annealing eﬀect

The type inversion we have discussed so far is the result immediately after irradiation, but theannealing eﬀect can repair radiation damage, causing changes in VFD over time. Fig. 5.24 shows
the temporal evolution of VFD to see the annealing eﬀect. The annealing eﬀect is considered
to be completed at 2 × 102 hours (≈ 8 days) from Fig. 5.24 since there is no change in VFD
for most of the sensors after 200 hours. In the ﬁgure, it can be seen that the VFD gradually
decreases for sensors with an initial point VFD greater than 35 V, while for sensors lower than
35 V, the VFD increases (mini197, 199, 205)∗ or does not change much.
The primary aim of Fig. 5.25 is to examine how the type inversion changes with the annealing eﬀect, as shown by the plot of VFD versus irradiation dose for various sensors. The
data for this analysis were collected after 1 × 103 hours from the irradiation test. The valleylike structure in this ﬁgure corresponds to the type inversion behavior and indicates that theannealing eﬀect restores radiation damage to the sensor. This causes the position where typeinversion occurs to shift to a higher dose; it changed from 15 ∼ 20 kGy to 50 ∼ 80 kGy withthe annealing eﬀect. Furthermore, we can also conﬁrm that the VFD does not exceed 100 Von the sample 100 kGy irradiated. Therefore, we can expect that we can apply a bias voltage
suﬃciently to exceed the VFD with 2 × 102 hours ∼ 8 days (room temperature) annealing eﬀect
even if 100 kGy was irradiated.

∗

The rising voltage suggests that the annealing process has been completed and the reverse annealing process
has begun before the data for the ﬁrst point was collected. Further measurements will be taken to determine if
the voltage continues to increase, which would conﬁrm this conclusion

64Figure 5.24: VFD vs elapsed time

Figure 5.25: The result of type inversion with annealing eﬀect (after 1 × 103 hours)

65

5.5
5.5.1

Analysis of leakage current increase
The damage factor and noise just after irradiation

The fact that the leakage current increases in proportion to the irradiation dose was explainedin Sec. (4.TBD). First, we plotted the leakage current vs. irradiation dose at VFD for the minisensor. The ﬁgure is shown in Fig. 5.26.

Figure 5.26: The leakage current at VFD per sensor volume vs. irradiated dose, the result justafter irradiations
We performed a linear function ﬁtting the results by considering it to be an equation;
I/V = αΦﬂuence where Φﬂuence [cm−2 ] is equivalent neutron ﬂuence which we deﬁned to be
3 × 1011 [cm−2 ] = 1 [kGy], V is sensor volume. The calculated damages factors are summarizedin Table. 5.1. Now that we have calculated the damage factor, which is a proportional constant,
Table 5.1: The damages factor of sensor just after irradiation
sensordamage factor α [A/cm]

slope [µA/cm2 /Mrad]temperature [◦ C]

mini822mini208mini195
mini198
mini203

4.1 × 10−17
3.8 × 10−17
4.0 × 10−17
3.6 × 10−17
4.3 × 10−17

3.9
3.7
3.8
3.5
3.8

17.0 ± 0.2
17.0 ± 0.2
17.0 ± 0.2
17.0 ± 0.2
17.0 ± 0.2

66we can estimate how much the leakage current increases with irradiation dose. The question
for the SVD is whether the increased current will exceed the allowable current (10 mA) of thepower supply to the SVD. A power supply module is the one responsible for an area equivalent
to one small sensor (active area: 3.84 × 12.29 cm2 ) on the innermost layer. Therefore, assuming
the sensor area to be that area, and using a damage factor of 4.1 × 10−17 in the calculations,
it was obtained that irradiation doses up to 75 kGy (≈ 2.3 × 1013 neq /cm2 ) would not exceed
10 mA. This dose amount is equivalent to 3.75 times (safety factor 3.75) the 10-year cumulative
radiation dose received by the innermost SVD.
Comparison of the results of the slopes (table 5.1) from our measurement with the results
obtained from the SVD operation is shown in Fig. 5.27. This ﬁgure shows that our measure-

Figure 5.27: The histogram of the slope of leakage current and irradiation dose from operating
SVD modules [12]ments resulted in smaller values, but they were relatively close to the values of the operating
SVD modules. The discrepancies in the measurements could be caused by diﬀerences in theaccuracy of the radiation dose estimates in the operating SVD and the ambient temperature.
Since the leakage current is small before irradiation, the noise contribution from the leakage
current is small. However, since the leakage current increases with irradiation dose, it isnecessary to estimate how much the noise increases with irradiation dose. Thus, the next
step is to determine how much the noise (ENCI ) current ﬂowing into APV25 increases withincreasing leakage current. The following Eq. 5.3 is used to calculate the noise from the leakage
current;√
e ileak tinteg.
ENCI =
,
(5.3)
2
qe

67

e: Euler’s number, tinteg. : shaping time of APV25; 50 ns [11], qe : elementary charge. Here, forcomparison and to consider the situation of the innermost SVD, the leakage current of each
irradiated sensor sample is scaled to the volume of the innermost SVD small sensor.
1
V′
ileak = Ileak ××V#P RO-Stripssmall

,

(5.4)

V ′ : the volume of the innermost SVD small sensor, V : irradiated volume of each sensor,#P RO-Strips: the number of P side readout strips of the small sensor. The result is shownas Fig. 5.28

Figure 5.28: Calculated ENCI from leakage current scaled to SVD innermost layer small sensor.
HPK845 represents large845
On the other hand, noise ENCC from interstrip capacitance can be calculated from interstrip capacitance of large845 using A + B × C that C is capacitance, A and B are ASIC-speciﬁcvalues. These values for our ASIC, APV25 are already examined and reported on [20]; it isEq. 5.5, the calculated result of large845 using Eq. 5.5 is shown in Fig. 5.29.
ENCC = 270 + 38 × C(5.5)

Comparing these two results, We can conﬁrm that the noise source aﬀecting the APV25 is
the noise due to the interstrip capacitance of the SVD sensor when not irradiated and in low
68Figure 5.29: The noise ENCC from interstrip capacitance of the large845
radiation environments, but the noise ENCI due to leakage current increases to the comparable
level or more after irradiation.
Assuming further that the noise in ENCI and ENCC is occupied in the ASIC external noise
ENC, ENC can be calculated from ENCI and ENCC as a quadratic sum;√
ENC = ENC2I + ENC2C(5.6)
The calculated result of large845 is Fig. 5.30. The results show that noise increases 1.4-foldfrom pre-irradiation to 18.5 kGy irradiation and approximately doubles with 60 kGy irradiation.
Although the increase in noise from interstrip capacitance alone could be suppressed to someextent by applying more voltage, the addition of leakage currents reduces the noise reductioneﬀect.

69

Figure 5.30: Noise ENC calculated from the quadratic sum of leakage current and interstrip
capacitance of the large845, the results just after irradiation
5.5.2

The damage factor and noise of with anneal eﬀect

To see changes in the damage factor with annealing eﬀect as in Sec. 5.25. The data at 1 × 103
hours are presented here; there was no change compared to the data at 2 × 103 hours. It
is shown in Fig. 5.31 and the damage factor is calculated by ﬁtting points between diﬀerent
sensors.

70

Figure 5.31: Leakage current vs dose with annealing eﬀect
The results show that the annealing eﬀect reduced the damage factor by nearly half; theradiation damage increased the leakage current, but the annealing eﬀect restored the radiation
damage and reduced the increased leakage current.

Table 5.2: The damages factors with other studies
sensordamage source

α [×10−17 A/cm]

T1 *1

t*2

T2 *3

Our result w/o annealing eﬀect
with annealing eﬀect
RD48, Rose collab. [13]BaBar SVT [18] [21]ATLAS SCT [8]90 MeV e−
90 MeV e−
24 GeV p
900 MeV e−
24 GeV p4 (5)2.2 (1.6)(3.99 ± 0.03)
2.7*4
0.23 (6.01)

17.0 ± 0.2 (20)24.0+0.2−0.3 (20)20.0
23−9 (20)—1 × 103
1.3———
17 ∼ 2760——◦*1 T1 : measurement temperature [ C]
*2 t: elapsed time [hours]◦*3 T2 : storage temperature [ C]
*4 There is a two-month interruption during the irradiation period. Annealing eﬀects cannot be neglected.

The results with and without annealing eﬀects are summarized in Table 5.2, and since there
have been many investigations into the damage factor of silicon sensors, we add some resultsfrom other studies to the table. In comparison, our results are close to the other results.
Here ENC was calculated from the leakage current as in Fig.5.28. As a reference, the results
71

of the large845 sensor without the annealing eﬀect are plotted. From this ﬁgure, it does notappear that annealing reduces noise much at 20 kGy, but at higher doses, for example around
60 kGy, noise can be reduced by about 200e− . Next, look at how the Cint V and noise ENCC

Figure 5.32: The ENCI at VFD vs dose with the annealing eﬀect (after 1000 hours)of the large845 have changed due to the annealing eﬀect. The results are shown in Fig 5.32
Comparing Cint immediately after irradiation and after 1 × 103 hours, it can be conﬁrmed that
Cint has indeed gone down. On the other hand, after another 1 × 103 hours, the Cint at 2 × 103
hours is lower in the low voltage region, but after 30 V, the Cint at 2 × 103 hours is higher. Thisbehavior also can be seen in other Cint measurement channels. We can conﬁrm the position of
the curved mountain-like structure and VFD position coincide. Next, we should conﬁrm thechange of ENC due to the annealing eﬀect by taking a quadratic sum of ENCI and ENCC asin Fig. 5.30.
It can be seen that ENC is reduced by about 150 ∼ 250 e− due to the annealing eﬀect. On
the other hand, in the region of higher voltage than VFD , there is no signiﬁcant diﬀerence
between 1 × 103 hours and 2 × 103 hours.
Also, although 2 × 103 hours is indeed lower ENCC at lower voltages than VFD this does
not mean that SVD should be operated at lower voltages than VFD but other factors such as
charge collection eﬃciency should be checked.

72

Figure 5.33: The Cint V (right vertical axis) and ENCC (left vertical axis) of large845 changes
with annealing eﬀect

Figure 5.34: Noise ENC calculated from the quadratic sum of leakage current and interstrip
capacitance of the large845 with annealing eﬀect73

Chapter 6
Measurement of Noise and Charge collection
eﬃciency change
Here we discuss the changes in noise taken by the readout ASIC and charge collection eﬃciency
due to radiation damage. Sec. 6.1 and Sec. 6.2 describe the setup for irradiation and the setup
for charge collection using a beta source, and Sec. 6.3 describes the procedure for obtaining
correction data and charge collection data. Sec. 6.4 describes the changes in the measurement
results of the correction data including noise, and Sec. 6.5 describes the changes in the actual
charge collection performed using the beta source, and discussion on Charge collection eﬃciency. Here, data acquisition is performed using the Sub-assembly module, Backward (SBW),
which is a readable module that connects the DSSD and readout ASICs. Refer to Fig. 2.4

6.1

Irradiation system for SBW module

To measure the noise and the Charge Collection Eﬃciency (CCE), it is necessary to have aa
readout ASIC (APV25) connected to the sensor and acquire the raw signal output from it.
Therefore, to investigate changes in the noise and the CCE before and after irradiation, it isnecessary to irradiate the module to which the APV25 and sensor are connected. We prepared
the SBW module for irradiation. Irradiation of the SBW module (hereinafter called SBW) was
done in parallel with the large845 sensor as shown in Fig. 6.1. For the beam hole, the large845
sensor was set on the ﬁrst layer and the SBW is the second layer. The large845 sensor and
SBW have the same geometry of the sensor part and the same irradiation time, so the given
irradiation dose is the same 100.1 kGy and the same irradiated area; a 16 mm wide area along
the Pstrip at the center of the sensor. Therefore, The central Pstrips were irradiated uniformly
and all N side strips were irradiated non-uniformly. N side of the sensor facing the beam hole.
In addition, during the irradiation test, we did not perform any measurements and did not
apply any bias voltage on the SBW module, only irradiation to the SBW module.

74

Figure 6.1: The irradiation system for SBW module

6.2

The measurement setup of Charge collection eﬃciency and noise

To obtain the data output from the SBW before and after irradiation, the system shown in
Fig. 6.2 is prepared. The APV25s on the N and P sides of the SBW are connected to thecorresponding junction board ports by hybrid cables, and through this junction board, they
are connected to a Flash Analog to Digital Converter (FADC). The FADC converts analog
signals to digital signals, which are output as digital signals to a readout PC at the latterstage.
The power supply that supplies the drive voltage (LV) of the APV25 and bias voltage (HV)to the sensor is also connected to the SBW via the junction board. The SBW is placed inside
a light-shielding box.
With the setup up to this point, we can obtain samples of the pedestal, noise, and calibration
curves.
Pedestal Samples the average of raw signals of each strip without the sensor hit [2]; thesignal oﬀset. unit:[ADC].
Noise

The root mean square of raw signals subtracted by the pedestal of each strip. unit:[ADC].

Calibration curve 1 MIP (24000e− ) signal is input to the chip using an internal calibration
circuit inside the APV25 chip. The peak height (Cal PEAK) and width of the approximate ideal RC-CR waveform output can be obtained. 1 [ADC] to [e− ] can be converted75Figure 6.2: The overview of the measurement setup of SBW moduleusing Cal PEAK;

1 MIP = 24000 [e− ]
[e− ]
=
[ADC]
Cal PEAK [ADC]

It is mainly related to the value of the interstrip capacitance.
Information on these basic properties of the APV25 is used to calibrate the signals from thesensors.
Next, we describe the setup for the CCE measurement. In addition to the setup describedso far, we added a beta-ray source 90 Sr (3.33 MBq) to generate charge in the SBW and asystem that detects charges when the beta-ray passes through the SBW and triggers the FADC
board. The system that detects and sends a trigger signal when the beta-rays pass consistsof a plastic scintillator and a Photomultiplier Tube (PMT), which converts the scintillation
light emitted from the scintillator into an electrical signal when beta-ray passes through thescintillator, ﬁxed in a holder as shown in Fig. 6.3. During the measurement, 1300 V is applied
to the PMT using a separately provided power supply, and the obtained signal is input to the
NIM crate through the NIM cable. Since the rate of the signal put into the NIM crate was toohigh and unacceptable for the FADC as a trigger signal, the rate was adjusted to about 150 Hz
in the NIM crate by taking the coincidence of the input signal and the signal width widened
to 300 ns by a gate generator in the coincidence module. When the FADC receives this trigger
signal, the signal stored in the ring buﬀer of the APV25 is sent to the FADC, processed by the
FADC as a measurement signal, and the data is sent to the PC.
The holder holding the 90 Sr and scintillator can be positioned in the two positions (center,edge) shown in Fig. 6.4 concerning the SBW. The geometry relationship of scintillator-SBW90
Sr is written in Fig. 6.5. To conﬁrm the uncertainty to the path length of SBW that comesfrom its geometry, the area of the scintillator is w2 ≈ 10 × 10 mm2 , The window of 90 Srsource is 10 mm diameter circle. Distance between them is L = 70 mm. Since it is not known
76

Figure 6.3: The placement of SBW and 90 Sr in the light-shielding box

Figure 6.4: The placement of SBW and 90 Sr (top view)77Figure 6.5: Geometry relationship of scintillator-SBW-90 Sr
whether 90 Sr is actually somewhere in the Sr source window, the calculation is based on theassumption that this is where the maximum peek angle (θ = arctan(w/L) = 0.142) to thescintillator is located. The path length l that maximizes the path length in SBW with thissetting is l = d cos(θ) ≈ 0.99. Therefore, the upper bound of uncertainty for this geometry is(l − d)/d = 0.01, which is negligible.

6.3

Measurement procedure of calibration data and charge collection

1. Take a calibration run without an irradiation source at several HV steps; 0.2 to 100 in
10 V increments for this time. Calibration run: (ADC delay, FIR only for HV = 0.2 V),
pedestal, noise, internal calibration (Cal PEAK), performed in this order.
ADC delay ﬁnd the optimum delay time of the FADC system concerning the timing ofsampling clock frequency (31.75 MHz for this time). This value will not be aﬀectedby the HV changes, it is done only for the ﬁrst measurement (HV = 0.2 V)
FIR Finite Impulse Response, obtain the coeﬃcient of a digital ﬁlter; FIR ﬁlter tocompensate the distortion in the frequency dependence function loss of cable andsuppress the eﬀect of reﬂection in the cable due to the impedance changes. It will
occur in the transmission between APV25 and FADC. Same as ADC Delay, only
performed for the ﬁrst time (HV = 0.2 V)
Pedestal, Noise, Internal calibration As mentioned in the previous section. (Internal calibration refers to calibration curve), performed every HV step.
2. Set the radiation source and trigger system as shown in Figure ﬁg 6.3 and make thetrigger signal can be input to the FADC. Here, since the timing of the trigger and thetiming of the acquired waveform may diﬀer, the latency time of APV25 should adjust sothat the beta ray hit can be included in the data frame.
3. Check the hit map and adjust the relative positions of the sensor and scintillator with90
Sr . Take data at HV steps; 0.2 to 100 in 10 V increments for this time, and the78calibration data for the respective corresponding voltages obtained in step.1 is applied
to the measurement data.

6.4

The result of measurement without
test

90

Sr before and after irradiation

Changes in data for the calibration data without source are shown here. The estimated VFDof SBW: 53 V (before), 51 V (after). These values were estimated in the same way as for thelarge845 sensor. The measurements in this section and the next section were taken after 1×103
hours after irradiation, so the results include the annealing eﬀect.
Fig. 6.6 shows the noise of SBW changes before and after irradiation. The P side strips
between 250 ∼ 500 show higher noise than before irradiation. This area coincides with theirradiated area, suggesting that the noise increased by ≈ 400 [e− ] due to the radiation damage.
On the other hand, on the N side, noise is generally lower after irradiation in the regionof V < VFD , and conversely higher before irradiation in V > VFD . In addition, for V > VFD ,both sides plateau with respect to V , irrespectively to whether it is before or after irradiation,refer to Fig. 6.7 which picked up one strip to see the noise vs. bias voltage. When comparedto the region of the plateau, the after irradiation region is 300 e− higher.
We can observe that one of the N side strips has a very large noise after irradiation, which
was not observed before. We suspect a pinhole is occurring in that strip and are currently
investigating. A pinhole is a breakage of the AC decoupling capacitor between the implant
and the readout strip aluminum inside the sensor bulk, it leads to some unexpected current ﬂow
into the APV25 and the ampliﬁer possibly cannot work correctly; becomes a single defective
strip on the sensor.
Make a comparison with the ENC calculated from the leakage current and the interstrip
capacitance of the large845 as the previous chapter calculation. The before and after results
for both SBW and large845 are summarized in Fig. 6.8. Although the two sensors are diﬀerent,
they use the same type of large sensor, so their properties and behavior are expected to be
similar. The ﬁgure illustrates that the noise measured before and after irradiation is close
to the large 845 noise calculated using leak current and Cint . The leak current and Cint are
expected the primary causes of the noise. The discrepancy between the calculated noise oflarge845 and the noise measured by SBW is not only due to the fact that the two sensors are
diﬀerent. The diﬀerence between the two values may be due to the fact that the large845’s
noise includes not only interstrip capacitance but also parasitic capacitance and resistancefrom other parts (pitch adapter, etc.), whereas the large845’s measurement does not consider
these eﬀects except the leakage current and the Cint . The diﬀerence between the two values ispossibly due to this fact.
On the other hand, there is no change in the pedestal as shown in Fig. 6.9. The pedestal
is due to the DC oﬀset on the APV25 input, which is not changed by radiation damage, as
expected. The results of the Cal PEAK change are summarized in Fig. 6.10. The Cal PEAK
on the P side has a slightly lower value after irradiation when V < VFD , but by increasing
the voltage, it approaches the value before irradiation. Especially when V > VFD , there is no
signiﬁcant diﬀerence in value. For Cal PEAK on the N side, when V < VFD , the value after
irradiation is slightly higher than the value before irradiation, but by increasing the voltage inthe same way as on the P side, the value approaches the value before irradiation.
We will primarily focus on the results of the Noise and Cal PEAK measurements. We ﬁrst

79

P-side Noise (V bias scan)N-side Noise (V bias scan)

7000
6000

Noise [e-]8000

Noise [e-]

8000
0.2V

60.0V

10.0V

70.0V

20.0V

80.0V

30.0V

90.0V

40.0V

100.0V

7000
6000

50.0V

5000

4000

4000

3000

3000

2000

2000

1000

1000
100

200

300

400

500

60.0V

10.0V

70.0V

20.0V

80.0V

30.0V

90.0V

40.0V

100.0V

50.0V

5000

0
0

0.2V

600

0
0

700

P-side Strip

100

200

300

400

500

N-side Strip(a) before

P-side Noise (V bias scan)N-side Noise (V bias scan)

7000
6000

Noise [e-]8000

Noise [e-]

8000
0.2V

60.0V

10.0V

70.0V

20.0V

80.0V

30.0V

90.0V

40.0V

100.0V

7000
6000

50.0V

5000

4000

4000

3000

3000

2000

2000

1000

1000
100

200

300

400

500

60.0V

10.0V

70.0V

20.0V

80.0V

30.0V

90.0V

40.0V

100.0V

50.0V

5000

0
0

0.2V

600

0
0

700

P-side Strip

100

200

300

400

(b) after

Figure 6.6: The Noise changes; P strips (left), N strips (right). Colored with applied HV bias
voltage.
begin, to examine the results obtained before irradiation, when the depletion layer spreads
from the P-side to the N-side of the sensor. At this stage, the P-side strip is prone to depleted
around the interstrip at low bias voltages, regardless of the VFD . As a result, we expect thenoise and Cal PEAK values on the P side to plateau at low bias voltages. In contrast, thedepletion layer does not reach the N side strip until the applied bias voltage equals the VFD .
Therefore, the Noise and Cal PEAK values on the N side are expected to plateau at V = VFD .
Our observations are consistent with these expectations. Speciﬁcally, we found that on the Pside, the Noise and Cal PEAK values plateaued at bias voltages of 10V and above. On the Nside, we observed that these values plateaued at a bias voltage of 60 V, which is above the VFD
value.
The IV measurement of irradiated SBW suggests that the type inversion has occurred in
the SBW irradiated region after irradiation, i.e., the depletion layer is spreading from the Nside to the P side as the N type bulk becomes P type. Therefore, we expect that the Noise

80

500

N-side Strip

P-side Noise (V bias scan): Strip#400

N-side Noise (V bias scan): Strip#300
Noise [e-]6000

Noise [e-]6000

before5000

before

5000

after

4000

4000

3000

3000

2000

2000

1000

1000

0
0

2

4

6

8

10

12

14

V bias [V]

0
0

after

2

4

6

8

10

12

Figure 6.7: The comparison of the noise of one picked-up strip from the irradiated region before
and after the irradiation.

Figure 6.8: The comparison of ENC calculated from Large845 leakage current and Cint , and
ENC measured at SBW

81

14

V bias [V]P-side Pedestal (V bias scan)N-side Pedestal (V bias scan)
1000

900
800
700

0.2V

60.0V

10.0V

70.0V

20.0V

80.0V

30.0V

90.0V

40.0V

100.0V

Pedestal [ADC]

Pedestal [ADC]

1000

50.0V

900
800
700

600

600

500

500

400

400

300

300

200

200

100

100

0
0

100

200

300

400

500

600

0
0

700

P-side Strip

0.2V

60.0V

10.0V

70.0V

20.0V

80.0V

30.0V

90.0V

40.0V

100.0V

50.0V

100

200

300

400

500

N-side Strip(a) before

P-side Pedestal (V bias scan)N-side Pedestal (V bias scan)
1000

900
800
700

0.2V

60.0V

10.0V

70.0V

20.0V

80.0V

30.0V

90.0V

40.0V

100.0V

Pedestal [ADC]

Pedestal [ADC]

1000

50.0V

900
800
700

600

600

500

500

400

400

300

300

200

200

100

100

0
0

100

200

300

400

500

600

0
0

700

P-side Strip

0.2V

60.0V

10.0V

70.0V

20.0V

80.0V

30.0V

90.0V

40.0V

100.0V

50.0V

100

200

300

400

(b) after

Figure 6.9: The pedestal changes
and Cal PEAK values on the N side will plateau immediately, while on the P side they willplateau at V=Vfd. However, it is worth noting that in this study, only a portion of the N side
strip was irradiated, so the type inversion only occurred in that portion while the unirradiatedportion remained unchanged. This means that the observations regarding the N side describedabove are impossible to be seen.
Examination of the Noise vs bias voltage results after irradiation revealed an unexpected
trend on the P side, where there was no discernible structure related to the VFD . Instead, thevalues are expected to gradually decrease and approach the plateau from the outset. On the Nside, the plateau was observed at the same location as before irradiation, which we attributeto the inﬂuence of the unirradiated region.

82

500

N-side Strip

N-side Cal. Peak (V bias scan)Cal. Peak @ 24,000 e- [ADC]

Cal. Peak @ 24,000 e- [ADC]

P-side Cal. Peak (V bias scan)
100
90
80
70
60
50
40

0.2V

60.0V

30

10.0V

70.0V

20.0V

80.0V

20

30.0V

90.0V

10

40.0V

100.0V

0
0

50.0V

100

200

300

400

500

600

100
90
80
70
60
50
40

0.2V

60.0V

30

10.0V

70.0V

20.0V

80.0V

20

30.0V

90.0V

10

40.0V

100.0V

50.0V

0
0

700

P-side Strip

100

200

300

400

500

N-side Strip(a) before

N-side Cal. Peak (V bias scan)Cal. Peak @ 24,000 e- [ADC]

Cal. Peak @ 24,000 e- [ADC]

P-side Cal. Peak (V bias scan)
100
90
80
70
60
50
40

0.2V

60.0V

30

10.0V

70.0V

20.0V

80.0V

20

30.0V

90.0V

10

40.0V

100.0V

0
0

50.0V

100

200

300

400

500

600

100
90
80
70
60
50
40

0.2V

60.0V

30

10.0V

70.0V

20.0V

80.0V

20

30.0V

90.0V

10

40.0V

100.0V

50.0V

0
0

700

P-side Strip

100

200

300

400

500

N-side Strip(b) after

N-side Cal. Peak (V bias scan): Strip#300
Cal. Peak @ 24,000 e- [ADC]

Cal. Peak @ 24,000 e- [ADC]

P-side Cal. Peak (V bias scan): Strip#400
100
90
80
70
60
50
40
30

before

20

after

10
0
0

100
90
80
70
60
50
40
30

before

20

after

10
2

4

6

8

10

12

14

V bias [V]002(c) Picked one strip up from irradiated region

Figure 6.10: The CalPEAK changes

83

4

6

8

10

12

14

V bias [V]

6.5
6.5.1

Charge collection measurement
Particle traversed position ﬁnding algorithm (Cluster ﬁnding algorithm)

The word ‘Cluster’ is used for the hit of the beta passage; the Cluster ﬁnding algorithm isequivalent to searching for the signal due to the beta ray passage. The following is a descriptionof this ﬁnding algorithm. First, to discriminate between noise and signal, we look for a strip
with Signal-to-Noise Ratio (SNR) > 5; where the strip signal is 5 times the strip noise (called
cluster seed). Then, we search for left and right from an adjacent strip of the seed to ﬁnd
strips with SNR > 3 or higher. An event that meets this condition is called one cluster, andthe number of strips plus one seed strip deﬁne as the cluster size. Fig. 6.11 is an illustrationof cluster size. This algorithm is also used for Belle II SVD oﬃcial clustering.
In addition, Cluster charge is deﬁned as the total amount ofcharge of the cluster, and√∑−2 −Cluster SNR is deﬁned as Cluster SNR ≡ (Cluster charge [e ])/(i∈cluster Noisei [e ]).

Figure 6.11: The image of Cluster ﬁnding, cluster size is 3 in this case

6.5.2

Result of measurement with 90 Sr before and after irradiation test

Next, we present the measurement results for the cluster event using 90 Sr as the β ray source.
The data were collected using a sample size of 2.6 million events at a nominal bias voltage of
100 V, and 300 thousand events were acquired at each 10 V increment from 0.2 V to 100 V forthe before irradiation and up to 160 V for the after irradiation. Since the counting distributionof the√ number of events can be assumed to be Poisson distribution, the√error can be considered
as Number of events, and in this case, since Number of events ≫ Number of events, theerror in the number of events can be ignored. The following ﬁgures without descriptions ofvoltages are measured at nominal voltage; 2.6 million events. First, To review the hit positions

84

P-side Strip

2-Dimensional Cluster Position
500

700
600

400

500
300

400
300

200

200
100

100
0
0

100

200

300

400

500

0

N-side Strip(a) before irradiation

P-side Strip

2-Dimensional Cluster Position
500

700
600

400

500
300

400
300

200

200
100

100
0
0

100

200

300

400

500

0

N-side Strip(b) after irradiation

Figure 6.12: 2-D Cluster position (hit position) map with β − ray. Axis unit: strip number.
Colored with the hit count85

and entries at the location before and after irradiation, shown in Fig. 6.12. Using the collectedcharge Qi and strip position xi of each strip as in the center-of-gravity method, we deﬁne thehit position as∑i∈(#strips) xi Qi
hit postion = ∑.
(6.1)
j∈(#strips) Qj
The location where the hit count is high coincides with the location where the source isplaced. In the after irradiation results, there is a single white line present, which is considered
to be the pinhole mentioned in the previous section. Data cannot be obtained from this strip
due to the suspected presence of a pinhole. Furthermore, both before and after irradiation,
there are horizontal lines indicative of low-count areas in the 200 ∼ 300 and 500 ∼ 600 regions.
This part corresponds to the ribs that support the large sensor of the SBW (see Fig. 6.13).
The rib is made of polymeric foam (AIREX R82.60) sandwiched by a carbon ﬁber sheets.
The black part of the rib in Fig. 6.13 corresponds to the carbon ﬁber sheet. Check the number

Figure 6.13: SBW 3D model and hit position map overlayof clusters. Here, all clusters, the clusters on the N and P sides alone are plotted in Fig. 6.14.
2400

2200

before

2000

after

2400

before

2200
2000

1800
1600
1200
800
600
400
200
1

2

3

4

5

6

7

8

9

10

Number of Clusters

after

1400
1200
1000

800
600

800
600

400
200

400
200

0
0

before

1800
1600

1200
1000

1000

Number of n-side clusters

×103

2200
2000

after

1800
1600
1400

1400

0
0

Number of p-side clusters

×103

EntriesNumber of all clusters

×103

Entries

Entries

2400

1

2

3

4

5

6

7

8

9

10

Number of Clusters

0
0

1

2

3

Figure 6.14: number of clusters changes before and after irradiation

86

4

5

6

7

8

9

10

Number of Clusters

Let i, j = 0, 1, 2, . . . , k = 1, 2, . . . be the number of clusters respectively, the relationshipbetween the hit entries of all clusters allk , P clusters pi , and N clusters nj can be expressed asfollows;∑
allk =
ni + pj , ni + pj ̸= 0
(6.2)
k=i+j

The results show that the entries are dominant at i, j = 1, 2 before and after irradiation.
i, j = 2 is possible because two particles passed through at the same time in the same data
frame.
The number of cluster distributions on the P side did not change before and after irradiation,but on the N side, n0 increased slightly and n1 decreased by 500 [entries]. Since this change inentries occurs only on the N side, it can be attributed to the aforementioned pinhole issue. In
conjunction with this, as N1 decreased, all1 increased and all2 decreased.
Next, now that the number of clusters has been conﬁrmed, we check the distribution of
their cluster size. The cluster size distribution change of each side of the sensor is shown in
Fig. 6.15.
N-side Cluster Size
Entries

EntriesP-side Cluster Size
6

before10

106

before

after

after
105

105
104

103
104
102
0

1

2

3

4

5

6

7

8

9

10

0

Cluster Size

1

2

3

4

5

6

7

8

9

10

Cluster Size

Figure 6.15: Cluster size changes before and after irradiation
The results show that the number of clusters with cluster size ≥ 3 on both sides is decreasing. This trend is more pronounced on the P side, while the number of clusters withcluster size = 1 and 2 is conversely increasing.
This trend of the cluster size distribution can be conﬁrmed by focusing on the irradiatedarea and the area with the highest β rays hit entries; the area where the vertical incidence ofbeta rays can be expected, to exclude the eﬀect from the non-irradiated area. See Fig. 6.16,
the selected area: ; P side strip ∈ [368, 432), N side strip ∈ [160, 240).
Since there is no signiﬁcant diﬀerence in the total number of clusters, it simply means thatthe number of clusters with large cluster size is decreasing. There are two possible reasons forthis;
1. The decrease in CCE of each strip has reduced the number of adjacent strips that matchthe cluster ﬁnding algorithm.
2. The increase in noise for each strip no longer satisﬁes the SNR > 3 that the cluster
ﬁnding algorithm imposes on adjacent strips.

87

N-side Cluster Size
Entries

EntriesP-side Cluster Sizebefore105

before

105

after

after
104

104

368<=P-strip<432

103

368<=P-strip<432

160<=N-strip<240

160<=N-strip<240

102
3

10

10

0

1

2

3

4

5

6

7

8

9

1
0

10

Cluster Size

1

2

3

4

5

6

7

8

9

10

Cluster Size

Figure 6.16: Cluster size in selected region changes before and after irradiation

Figure 6.17: The cluster size of P side changes, the result of after Irradiation with after
irradiation noise data (left, as shown in the previous ﬁgure), assuming same noise calibration
data; after irradiation noise data (right; both use same noise)
The increase in the noise has already been seen in Fig. 6.6. To check how much the cluster size
is reduced by this increase in noise, we assume that the noise before irradiation is the same as
the noise after irradiation and calculate the cluster size again; using the same noise calibration
data to compare. The result of the P side is shown in Fig. 6.17.
The results indicate that altering the noise to the noise data before irradiation signiﬁcantly
diminished the diﬀerence in cluster size distribution between before and after irradiation. This
also can be conﬁrmed in the selected area of the SBW. In addition, it suggests that there ispossibly no change signiﬁcant in the CCE.
On the other hand, it is diﬃcult to discuss the N side at this time because it is non-uniformly
irradiated on the N side strips and a pinhole issue is suspected.
Since the cluster size of one, two and three are dominant in the cluster size distribution,
we examined the cluster charge (collected charge) distribution included in each cluster size (1,2, all cluster sizes). The distribution changes are shown in Fig. 6.18. Here we focus on the88

Figure 6.19: The cluster charge distribution of P side in the selected area with diﬀerence
voltages before (left) and after (right) irradiation (cluster size = 1)

Entries

Entries

aforementioned selected. The distribution of the sensor as a whole is close.

P-side Cluster Charge
All clusters (before)
Size = 1 (before)
Size = 2 (before)
Size = 3 (before)
All clusters (after)
Size = 1 (after)
Size = 2 (after)
Size = 3 (after)

6000
5000

N-side Cluster Charge
All clusters (before)
Size = 1 (before)
Size = 2 (before)
Size = 3 (before)
All clusters (after)
Size = 1 (after)
Size = 2 (after)
Size = 3 (after)

7000
6000
5000

4000
4000

368<=P-strip<432

3000

368<=P-strip<432

160<=N-strip<240

3000

2000

160<=N-strip<240

2000

1000

1000
×10
100

3

0
0

10

20

30

40

50

60

70

80

90

0
0

Cluster Charge [e-]

10

20

30

40

50

60

70

80

90

Cluster Charge [e-]Figure 6.18: The cluster charge distribution in each cluster size before (dashed line) andafter irradiation (solid line), left: P side, right: N side, selected area: P side strip ∈[368, 432), N side strip ∈ [160, 240)
All peaks of cluster charge distribution are near 20 ke− , The peak of the distribution withcluster size = 1 on both the N and the P sides decreased after irradiation, from 19 ke− to17 ke− for the P side, from 19 ke− to 18 ke− for the N side. While the peak of the distributionof the N side with cluster size ≥ 2 became higher after irradiation.
The following shows the cluster charge distribution measured at diﬀerent voltages. Correction data measured at the same voltage independently before and after irradiation are used to
89

×103
100

Figure 6.20: The cluster charge distribution of P side in the selected area with diﬀerence
voltages before (left) and after (right) irradiation (cluster size = all)correct the measured data for each voltage. Here we shall discuss the P side. In addition tothat, the discussion is focusing on the cluster size = 1 and ‘all cluster’ events. The results ofthe cluster size = 1 are shown in Fig. 6.19 and Fig. 6.20.
Although the number of entries diﬀers, it can be conﬁrmed that the peak of the cluster
charge distribution is proportional to the voltage, even when the cluster size is 1 or ‘all’.
Furthermore, in the region where V exceeds VFD , the peak shift is slower.
Meanwhile, the distribution below 50V after irradiation was cut oﬀ by zero suppression(SNR > 3; cut to reduce the amount of data in the event, the criteria is the same as Belle IIoﬃcial one.) because the noise increased due to irradiation, which is either not visible or not
visible at all. The peak is clearly visible after 60 V, which is the region beyond VFD . However,the position of the peak has shifted signiﬁcantly compared to before irradiation, moving about
10 ke− from 60 V to 100V.
Here, ﬁtting is performed for each distribution in order to quantitatively evaluate the peaks.
The clustered charge collected by the silicon strip sensor is related to the energy loss of the β
ray traversed. It can be evaluated with Gaussian convoluted Landau distribution function [5].
Thus we used the function to ﬁt the cluster charge distribution with ROOT; a software widelyused in the high-energy physics ﬁeld for analysis. Fitting parameters were adjusted visually.
The peak of the ﬁtted curve is called the ‘Most Probable Value (MPV)’.
The Charge MPV vs bias voltage is shown in Fig. 6.21 for cluster size = 1 and Fig. 6.22 forcluster size = ‘all’. As mentioned above, there is a voltage that does not peak after irradiation
due to the zero suppression cut. The MPV of that voltage is not considered in the plot.
The results of both cluster sizes conﬁrm that the MPV increases in proportion to voltage
and the maximum value of MPV decreased from 20 ke− to 18 ke− (10%) for cluster size = 1,
from 22 ke− to 20 ke− (10%) for cluster size = ‘all’ before and after irradiation. The voltage
at which the MPV changes to a plateau with respect to voltage is 70 V before irradiation andnearly 100 V after irradiation. However, the MPV after irradiation still appears to be slightly
increasing.
90

We should note that results in Fig. 6.21 and Fig. 6.22 cannot be immediately evaluated aschanges in CCE, as mentioned in Fig. 6.17, it is possible that the peak of the cluster charge
distribution, i.e., MPV, may change after irradiation due to an increase in noise, which may
be cut by the SNR criterion. To investigate this, we calculate the MPV assuming the same
noise as in Fig. 6.17. Fig. 6.23 and Fig. 6.24 show the results of calculations assuming thesame noise before and after irradiation.
Comparing Fig. 6.23 and Fig. 6.24, calculated with the same noise, and Fig. 6.21 andFig. 6.22, calculated with the diﬀerent noise, respectively, there is still a diﬀerence in MPV
before and after irradiation comparing the same voltage before and after irradiation, but thediﬀerence is smaller when the noise is aligned. For example, if we look at cluster size=1,
100 V, the diﬀerence is 3 ke− (20 ke − 17 ke− ) when the noise is each own noise, but 2 ke−(19 ke− −17 ke− ) when the noise is the same. Also, when comparing the maximum MPV values
for each, the diﬀerence is less than 500 e− . This is conﬁrmed by the fact that the SNR criteria
were tightened due to increased noise, which resulted in many charges not being collected asclusters. In addition, since there is no signiﬁcant diﬀerence in the number of hits when noiseis aligned from Fig. 6.17 for any cluster size, Fig. 6.23 and Fig. 6.24 can be considered as
the diﬀerence in the CCE. And while CCE is indeed lower at the same voltage before andafter irradiation, by applying a suﬃciently high voltage, CCE recovers to the pre-irradiationlevel. The CCE before irradiation has a voltage near VFD and the change plateaus but after
irradiation the voltage plateaus at a much higher position (∼ 100 V) than the expected VFD ,and still increased slightly.
We have a possible interpretation: It is thought that the sensor bulk after 100kGy irradiation undergoes type inversion and becomes P type bulk. Therefore, the depletion layer
expands from the N side to the P side. The VFD deﬁned by our current IV curve is thought
to pick up the structure of the IV curve that appears when the depletion layer touches the Pside strip, and it is thought that there is still a part of the depletion (especially between strips)
around the P side strip. As a result, at that point, the P strip has a mixture of depleted andundepleted regions and generates a complex electric ﬁeld, causing the CCE to decrease. As
the voltage is increased, the undepleted region decreases and the electric ﬁeld becomes orderly,
gradually increasing the CCE.

91

MPV [e-]P-side Charge MPV (Smeared Landau Fit)
22000
20000
18000

size = 1

16000

368<=P-strip<432

14000
12000
10000
8000
6000
4000

before

2000

after

0
0

2

4

6

8

10

12

14

V bias [V]Figure 6.21: Most provable value change in the selected area before and after irradiation(cluster size = 1)

MPV [e-]P-side Charge MPV (Smeared Landau Fit)
35000
30000
25000
20000

size
= all
368<=P-strip<432

15000
10000
before

5000

after

0
0

2

4

6

8

10

12

14

V bias [V]Figure 6.22: Most provable value change in the selected area before and after irradiation(cluster size = ‘all’)92

MPV [e-]P-side Charge MPV (Smeared Landau Fit)
22000
20000
18000

size = 1

16000

368<=P-strip<432

14000
12000
10000
8000
6000
4000

before

2000

after

0
0

2

4

6

8

10

12

14

V bias [V]Figure 6.23: Most provable value change in the selected area before and after irradiation, withusing same noise calibration data (cluster size = 1)

MPV [e-]P-side Charge MPV (Smeared Landau Fit)
35000
30000
25000
20000

size
= all
368<=P-strip<432

15000
10000
before

5000

after

0
0

2

4

6

8

10

12

14

V bias [V]Figure 6.24: Most provable value change in the selected area before and after irradiation, withusing same noise calibration data(cluster size = ‘all’)93Chapter 7
Conclusion
In pursuit of understanding the fundamental nature of the universe beyond the Standard
Model (SM), precision measurements of B meson decays are being performed by the Belle IIexperiment. To enhance these measurements, eﬀorts are currently being made to improve theinstantaneous luminosity of the SuperKEKB collider. The target luminosity is 6 × 1035 cm2 /s.
Simulations predict that, after 10 years of operation at this luminosity, the innermost layerof the Belle II silicon vertex detector (SVD) will have received a cumulative radiation dose of
20 kGy. As the SVD plays a crucial role in the detection of new physics, an evaluation was
conducted by irradiating SVD sensors with electron beams to assess the potential operational
impacts of this level of radiation exposure.
The following points should be understood in terms of the potential eﬀects of radiation
damage on the operation of the SVD:
1. The amount of radiation that causes type inversion of the sensor.
2. The increase of the full depletion voltage VFD of the sensor.
3. The increase in the leakage current of the sensor as a rate of radiation (damage factor).
4. The increase in the interstrip capacitance.
5. The increase in the noise due to the increased leakage current and the interstrip capacitance.
6. The degradation of charge collection eﬃciency (CCE).
To evaluate these items, we performed IV, CV, and Cint V measurements and, noise, and CCE
measurements.
A measurement system was developed for an electron irradiation test to sensors and it
was utilized. The measurement system enabled remote switching of measurement circuits for
IV, CV, and Cint V measurements for each irradiation dose, this is to acquire data during thetesting period. To examine the changes in noise and CCE, we connected the sensor to the
APV25 chips (SBW module) and acquired noise and collected change data using the beta-ray
before and after irradiation.
Our conclusions corresponding to the enumerated items above is
1. The type inversion occurred in 15 ∼ 20 kGy just after irradiation. The irradiation
damage is restored with the annealing eﬀect of 1 × 103 hours, and we can conﬁrm thatthe type inversion shifts to even higher doses 50 ∼ 80 kGy.
94

2. The VFD was conﬁrmed not to exceed 150 V at 80 kGy just after the irradiations. 80
kGy is 4 times the radiation dose that the innermost layer of the SVD receives in 10
years of operation of the target luminosity; safety factor 4. Additionally, through theannealing eﬀect of 1 × 103 hours, it can be conﬁrmed that the VFD does not exceed 100
V even when a dose of 100 kGy is irradiated. On the other hand, The maximum voltagethat can be supplied by the current SVD power supply is 200 V, therefore, Therefore,
a bias voltage exceeding the SVD sensor’s VFD can be applied with the current power
supply without any problem.
3. While the allowable current of our power supply module is 10 mA, It was conﬁrmed thatthe leakage current does not exceed 10 mA up to 75 kGy immediately after irradiation.
The damage factor was estimated to be 4 A/cm. On the other hand, the annealing eﬀect
recovered the radiation damage of the sensor and reduced the leakage current. This
reduced the damage factor to 2.2 A/cm, and the power supply module is expected towithstand current increases up to 130 kGy. Therefore, there is no problem with the SVD
operation in 10 years of target luminosity operation.
4. It was observed that as irradiation dose increased, Cint also increased. After 30 kGy
irradiation, the maximum value of Cint is more than 2.6 times higher than that of unirradiated Cint , but this value can be reduced to about 1.5 times by applying suﬃcient
voltage beyond VFD . The annealing eﬀect also reduced Cint in the whole bias voltage
range, i.e., the sensor surface damage was also recovered.
5. Measurements using the SBW module showed that the noise increased ≈ 400e− before
and after 100 kGy irradiation. This increase was found to be consistent with the increasein noise calculated from the increase in leakage current and the interstrip capacitanceseen in the large sensor with the same dose, so these two factors can be attributed to thenoise increase.
6. The MPV of the collected charge was found to drop by about 2 ke− after irradiation. This
is not because the CCE of the sensor itself has decreased, but rather because the thresholdof the signal that can be collected as a cluster has increased due to the higher noise, andthe amount of charge that was not collected as a signal has increased. Assuming thesame noise before and after irradiation, the calculated value of the MPV of the collectedcharge did not diﬀer signiﬁcantly before and after irradiation, thus the CCE was hardly
lowered.
In conclusion, based on the results of the evaluation, it was determined that the SVD is capableof functioning eﬀectively even when exposed to radiation doses equivalent to SuperKEKB’s
target luminosity for 10 years of operation.

95

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98

Acknowledgements
I ﬁrst would like to express my sincere gratitude to my supervisor, Takeo Higuchi (樋口岳雄) for their unwavering guidance, encouragement and support throughout my studies. His
dedicated mentorship has helped me to achieve my goals.
I would like to express my deep appreciation and gratitude to my co-researcher of this
irradation test, Yo Sato (佐藤瑶), for not only his invaluable contributions to our campain andalso his kind advise to my research life throughout his experience.
I also would like to express my deep appreciation and gratitude to Katsuro Nakamura (中村克朗) for his invaluable contributions and guidance. Without his expertise, dedication andhard work, I would not have been able to overcome the diﬃculties I faced during my research.
I would like to express my heartfelt thanks to Koji Hara (原康二), Toru Tsuboyama (坪山透), onuki Yoshiyuki (小貫良行) and Ishikawa Akimasa (石川 明正) for their immense supportand kind advises.
I would also like to thanks to the SVD groups for their insightful and productive discussions.
I would like to thank Hikaru Tanigawa (谷川輝), Yuma Uematsu (植松佑真), Zihan Wang
(王子涵), Tomoyuki Shimasaki (島崎奉文), Ping Ni (倪平), Xiaodong Shi (師暁東), Yuxin Liu(劉宇鑫), Hiroto Sudo (須藤裕人) who all have made my life in KEK enjoyable.
Thank you again to the member who participating in the radiation test.