Development of 50 cm Diameter
Hybrid Photo-Detector for Hyper-Kamiokande
Tianmeng Lou
Department of Physics, University of Tokyo
January 2017

Abstract
We have developed 50 cm Hybrid Photo-Detector (HPD) as a candidateof photodetectors for Hyper-Kamiokande (HK) which is proposed as a next
generation underground large water Cherenkov detector.
An HPD consists of a photocathode glass and a silicon avalanche diode (AD)instead of metal multiple dynodes which are used in a PMT. It is expectedthat an HPD has better charge resolution because of the high gain at the first
amplification stage, and better timing resolution due to its small fluctuationof the drift path in the AD compared to a PMT. In addition, its simpler andaxially symmetric structure makes the cost lower, the magnetic field tolerance
better and the afterpulse rate lower.
A large diameter AD is required for a large aperture HPD to realize a suﬃcient collection eﬃciency. A 50 cm diameter HPD with a 20 mm diameter AD
has been selected as our target design. One of the diﬃculties to develop thistype of HPD is the large junction capacitance because of the large area of AD.
It worsens the signal to noise ratio. We developed new ADs and preamplifiers
to solve this problem and succeed to separate single photoelectron signal andbaseline noise.
Using newly developed ADs and preamplifiers, we evaluated the performanceof a 50 cm HPD. An HPD has fast enough rise time of 13.4 ns, the charge
resolution of 17.7% and timing resolution of 2.7 ns in TTS (FWHM). They
are better than other candidate photodetectros. The dark rate of the HPD is∼10 kHz at room temperature and it is expected to reach ∼4 kHz at watertemperature in HK operation. The detection eﬃciency is better than that of
SK PMT and the same level of box and line dynode PMT. The output is linear
up to 1 MHz and the rate tolerance is suﬃcient to detect any supernova burst
events, while the preamplifier limits the performance. In order to confirm thewaterproof assembly of an 50 cm HPD for future long operation in water, we dothe waterproof HPD test and it works stably for two weeks with applying highvoltage.
The basic configuration of a 50 cm HPD is confirmed as the inner photodetector of HK and after the R&D of practical application, a 50 cm HPD will beready for use in a water Cherenkov detector.

Contents
Abstract

1

Contents

2

List of Figures

5

List of Tables

6

1 Introduction
1.1 Physics motivation . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.1 Neutrino Oscillation . . . . . . . . . . . . . . . . . . . . .
1.1.2 Mass hierarchy . . . . . . . . . . . . . . . . . . . . . . . .
1.1.3 Leptonic CP violation . . . . . . . . . . . . . . . . . . . .
1.1.4 Nucleon decay . . . . . . . . . . . . . . . . . . . . . . . .
1.1.5 Neutrino astronomy . . . . . . . . . . . . . . . . . . . . .
1.2 Current neutrino experiments . . . . . . . . . . . . . . . . . . . .
1.2.1 Super-Kamiokande . . . . . . . . . . . . . . . . . . . . . .
1.2.2 T2K experiment . . . . . . . . . . . . . . . . . . . . . . .
1.3 Hyper-Kamiokande project . . . . . . . . . . . . . . . . . . . . .
1.3.1 Overview of HK . . . . . . . . . . . . . . . . . . . . . . .
1.3.2 Detection principle . . . . . . . . . . . . . . . . . . . . . .

7
7
7
8
9
10
10
11
11
12
13
13
15

2 Development of new photodetectors for Hyper-K
18
2.1 R&D motivation and requirements for Hyper-K photodetectors . 18
2.2 Photodetector candidates for Hyper-K . . . . . . . . . . . . . . . 20
2.2.1 Box&Line PMT . . . . . . . . . . . . . . . . . . . . . . . 20
2.2.2 Hybrid Photo-Detector . . . . . . . . . . . . . . . . . . . 21
2.2.3 Goals of this thesis . . . . . . . . . . . . . . . . . . . . . . 22
3 Development of 50 cm HPD with 20 mm diameter avalanche24
diode
3.1 Diﬃculty of using 20 mm diameter AD . . . . . . . . . . . . . . . 24
3.2 Improvement of AD . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2.1 Development of low capacitance AD . . . . . . . . . . . . 25
3.2.2 Application of segmented AD . . . . . . . . . . . . . . . . 25
1

3.3

3.4

Improvement of preamplifier . . . . . . . . . . . . . . . . . . . . .
3.3.1 Diﬃculties in preamplifier development . . . . . . . . . .
3.3.2 Sequence of preamplifier development . . . . . . . . . . .
3.3.3 Developed preamplifiers . . . . . . . . . . . . . . . . . . .
Summary of developed HPD types . . . . . . . . . . . . . . . . .

27
27
28
30
30

4 Performance evaluation of 50 cm HPD
33
4.1 Setup for performance evaluation . . . . . . . . . . . . . . . . . . 33
4.2 Waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.3 Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
4.4 Charge resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.5 Timing resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.6 Dark rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.7 Photon detection eﬃciency . . . . . . . . . . . . . . . . . . . . . 44
4.8 Afterpulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.9 Rate tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
4.10 Viability test of an 50 cm HPD in water . . . . . . . . . . . . . . 47
5 Conclusion
50
5.1 Summary of performance evaluation . . . . . . . . . . . . . . . . 50
5.2 Remaining issues . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
5.3 Future prospect . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
Acknowledgements

54

A Long term measurement of dark rate of Hyper-K photodetectors
55
A.1 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
A.2 Result from October 2015 to September 2016 . . . . . . . . . . . 57
A.3 Result from August 2016 to November 2016 . . . . . . . . . . . . 58

2

List of Figures
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9

Schematic view of neutrino mass hierarchy. . . . . . . . . . . . .
A schematic view of Super-Kamiokande. . . . . . . . . . . . . . .
Inside of Super-Kamiokande. . . . . . . . . . . . . . . . . . . . .
Configuration of the detectors of T2K experiment. . . . . . . . .
View of the T2K experiment . . . . . . . . . . . . . . . . . . . .
Oﬀ-axis fluxes and oscillation probabilities. . . . . . . . . . . . .
The schematic view of near detectors. . . . . . . . . . . . . . . .
The schematice view of Hyper-Kamiokande detector. . . . . . . .
The example of neutrino interactions. The left (right) diagramshows the charged (neutral) current interaction of neutrino-nucleon
scattering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.10 Image of the emitted Cherenkov light. . . . . . . . . . . . . . . .
1.11 Image of the Cherenkov ring. . . . . . . . . . . . . . . . . . . . .
1.12 The diﬀerence of the Cherenkov rings in SK. Right shows thering of a muon and left shows that of an electron. The ring of anelectron is more indistinct than that of muon due to the multiple
scattering in water. . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1
2.2
2.3
2.4
3.1
3.2
3.3
3.4
3.5
3.6

Venetian blind PMT / Super-Kamiokande PMT (SK PMT) /Hamamatsu R3600 NQE. . . . . . . . . . . . . . . . . . . . . . .
Box&Line PMT (B&L PMT) / Hamamatsu R12860 HQE. . . . .
The principle of an HPD amplification and typical values of the
50cm HPD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Summary of HPD development. . . . . . . . . . . . . . . . . . . .
The principle of current noise increase due to the large junction
capacitance of an AD and a trans-impedance amplifier. . . . . . .
Schematic view of segmented AD and readout system. . . . . . .
The AD junction capacitance dependency of the noise level inpulse height (left) and in charge (right). . . . . . . . . . . . . . .
Description of problems to develop the preamplifier of 50 cm HPD
with 20 mm diameter AD. . . . . . . . . . . . . . . . . . . . . . .
Left (right) is the image (circuit diagram) of the preamplifier test
board. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The output signal waveform of a preamplifier at the pre-test. . .

3

9
12
12
13
13
13
13
14

16
17
17

17

20
21
22
23

25
27
27
28
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29

3.7

Comparison of waveform and charge distribution between type I
and II preamplifier. . . . . . . . . . . . . . . . . . . . . . . . . . .
3.8 The comparison of old and new preamp in waveform. . . . . . . .
3.9 The comparison of old and new preamp in charge distribution. .
3.10 Circuit diagram of type I preamplifier. . . . . . . . . . . . . . . .
3.11 Circuit diagram of type II preamplifier. . . . . . . . . . . . . . .
The overview of the whole system to operate an HPD and dataacquisition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 A picture of the socket assembly of HPD. . . . . . . . . . . . . .
4.3 Pictures of the HV-LV control power supply. . . . . . . . . . . .
4.4 Waveform of each photodetector. . . . . . . . . . . . . . . . . . .
4.5 Measured total gain of 1ch HPD with preamplifier as a functionof the HV with fixed AD bias voltage of 520 kV. . . . . . . . . .
4.6 Measured total gain of 1ch HPD with preamplifier as a functionof the AD bias voltage with fixed HV of 8 kV. . . . . . . . . . . .
4.7 1 p.e. charge distribution of 5ch, 2ch, 1ch HPD by single channel
readout (left) and sum-amp readout (right). . . . . . . . . . . . .
4.8 Definition of µ1 p.e. , µped , σ1 p.e. , σped , peak, valley in charge distribution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.9 Definition of transit time. . . . . . . . . . . . . . . . . . . . . . .
4.10 Single photo-electron transit time spread (TTS) of each photodetectors. Left (right) figure contains TTS of HPD with the bias
voltage ∼520 V (∼570V). . . . . . . . . . . . . . . . . . . . . . .
4.11 Setup of the timing resolution measurement. . . . . . . . . . . . .
4.12 The TQ-map (time charge map) of 1ch HPD. . . . . . . . . . . .
4.13 Multi photo-electron transit time spread (TTS) of each photodetectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.14 The dark rate of 1ch and 2ch HPD as a function of the threshold.
The doted red line shows the dark rate of each channel of 2ch
HPD and the solid red line shows the sum of tow doted red line.
4.15 The measured relative total detection eﬃciency (= relative collection eﬃciency (CE) × quantum eﬃciency (QE)), QE measured
by Hamamatsu photonics and calculated relative CE. . . . . . . .
4.16 The charge distrubition and trigger eﬃciency of an HPD and a
B&L PMT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.17 The afterpulse rate of each photodetector between -5 to 50 µs
time windows from main pulse. . . . . . . . . . . . . . . . . . . .
4.18 The output signal stability of HPD in charge and pulse height. .
4.19 The schematic view of EGADS tank (a 200-ton water tank). . . .
4.20 The extend elevation of EGADS tank. Green circles show theplaces planning to install 50 cm HPDs. . . . . . . . . . . . . . . .
4.21 The waterproof HPDs and the picture of operation in water. . .
4.22 1 p.e. and multi p.e. signal of a waterproof 50 cm HPD. . . . . .
4.23 The multi p.e. charge distribution of a waterproof 50 cm HPD
and charge of each number of photo-electron peaks. . . . . . . . .

31
31
31
32
32

4.1

4

34
34
35
36
37
37
38
39
40

40
41
42
42

43

44
45
46
47
48
48
48
48
49

4.24 The variation of dark rate and water temperature during continuous operation of a waterproof HPD. . . . . . . . . . . . . . . . .
4.25 The relationship of a temperature and a dark rate of a waterproof
HPD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1

49
49

The shape of the glass bulb. . . . . . . . . . . . . . . . . . . . . .

52

A.1 Image of setup in dark room. . . . . . . . . . . . . . . . . . . . .
A.2 The setup of DAQ system for long measurement. . . . . . . . . .
A.3 The time variation of gain (upper) and dark rate (lower). The
left column shows the whole history of gain and dark rate andthe right column shows the history after May 2016. . . . . . . . .
A.4 The time variation of the dark rate after installing newly developed B&L PMTs . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.5 The temperature dependency of PMT dark rate. . . . . . . . . .
A.6 The high voltage and trigger threshold level dependency of thedark rate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

56
56

5

57
58
59
60

List of Tables
1.1

The parameters of Hyper-Kamiokande. . . . . . . . . . . . . . . .

14

2.1

Minimum requirements for the Hyper-K inner photodetectors,
based on the performance of SK PMT(R3600) and HK. . . . . .
The performance summary of SK PMT and B&L PMT. . . . . .

19
21

Summary of AD types and typical values. 1ch AD means notsegmented AD. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sammary of HPD types and evaluation using each type of HPD.
Summary of developed preamplifiers. . . . . . . . . . . . . . . . .

26
30
31

2.2
3.1
3.2
3.3
4.1
4.2
4.3
4.4
5.1

Summary of rise time, fall time and FWHM of each photodetector. 35
Summary of 1 p.e. resolution, noise level, peak to valley ratio of
each photodetector. . . . . . . . . . . . . . . . . . . . . . . . . . . 39
The summary of single photoelectron timing resolution for eachphotodetectors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
The rate of afterpulse to mainpulse for each photodetector. . . . 46
The summary of the evaluated performances of the 50 cm HPDs
and comparison with SK PMT and B&L PMT. . . . . . . . . . .

51

A.1 Summary of the type of measured photodetectors and averaged
dark rate after stabilization. WPASSY is short for waterproof
assembly. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

58

6

Chapter 1

Introduction
In this chapter, we describe the physics targets which can be approached by alarge water Cherenkov detector in Section 1.1. Then, we introduce the current
experiments in Section 1.2 and the future project in Section 1.3.

1.1

Physics motivation

1.1.1

Neutrino Oscillation

A neutrino is one of the elementary particles interacting only via weak force
and gravity. In the standard model, there are three types of neutrinos, electron neutrino(νe ), muon neutrino(νµ ), tau neutrino(ντ ), and their masses are
assumed to be zero. However, In 1998, neutrino oscillation was discovered via
the measurement of the atmospheric neutrinos in Super-Kamiokande (SK). Thisimplies that neutrinos have non zero masses. By exploring the properties of neutrino in detail, it may be possible to construct the physics theory beyond thestandard model.
Neutrino oscillation is a phenomenon that a neutrino with a specific lepton
flavor (e, µ, or τ ) can be obsearved as a neutrino with a diﬀerent flavor after
a spatial propagation. Assuming that neutrinos have non zero masses, threegeneration neutrino mixing can be described as∑|να ⟩ =
Uαi |νi ⟩(1.1)
iwhere |να ⟩ (α = e, µ, τ ) are eigenstates of weak interaction, |νi ⟩ (i = 1, 2, 3) aremass eigenstates, and Uαi is an element of 3 × 3 unitary matrix called Maki-

7

Nakagawa-Sakata (MNS) matrix [1], given as




c13
0 s13 e−iδCPc12 s12 0100 −s12 c12 0
010
U = 0 c23 s23  iδCP
0 −s23 c23
0
0 1−s13 e0c13c12 c13c13 s12s13 e−iδCPc13 s23  (1.2)= −c12 s13 s23 eiδCP − c23 s12 c12 c23 − s12 s13 s23 eiδCP
iδCP
iδCP
−c12 c23 s13 e+ s12 s23 −c12 s23 − c23 s12 s13 e
c13 c23
where cij = cos θij , sij = sin θij , θ is a mixing angle, and δCP is a CP violatingphase.
In vacuum, the oscillation probability between two flavors after traveling the
distance L is described as
P (να → νβ ) =

∑

m2
i

∗
Uαi
Uβi e−i 2Eν Li= δαβ − 4∑(∗∗Re(UαiUαj Uβi Uβj) sin2

i>j

+2

∑

(

∗
∗
) sin
Uαj Uβi Uβj
Im(Uαii>j∆m2ij L2Eν∆m2ij L
4Eν
)

)

(1.3)

where E is energy of the neutrino and ∆m2ij = m2i − m2j .
As described above, neutrino oscillation between three-generation dependon six parameters: two mass-squared diﬀerences (∆m221 , ∆m232 ), three mixing
angles (θ12 , θ23 , θ13 ) and one CP phase (δCP ). At present, θ12 and ∆m221 havebeen measured by solar [2] and reactor [3, 4] neutrino experiments. θ23 and|∆m232 | (absolute value) have been measured by atmospheric [5, 6], accelerator[7, 8, 9] ad reactor [10] neutrino experiments. θ13 has also been measured byaccelerator [11] and reactor experiments [10]. The sign of ∆m232 and the valueof δCP are left to be determined.

1.1.2

Mass hierarchy

Neutrino oscillation experiments measure the mass square diﬀerences (∆m221
and ∆m232 ) instead of the absolute masses. In vacuum, only the absolute valueof the mass square diﬀerences can be measured. The sign can be measured inmatter because the electrons in matter change the neutrio oscillation probabilities by Mikheyev-Smirnov-Wolfenstein mechanism [12, 13].
Currently, the sign of ∆m221 is determined by the measurement of solar
neutrino, however that of ∆m232 is still unknown. Therefore, the ordering of
m1 , m2 and m3 is not determined. This is called the neutrino mass hierarchy
problem. We can consider two patterns of the mass ordering: normal hierarchy(NH), e.g., m3 ≫ m2 > m1 and inverted hierarchy (IH), e.g., m2 > m1 ≫ m3
as shown in Fig. 1.1.
8

It is expected that the neutrino mass hierarchy will be determined by future neutrino experiments, e.g., accelerator-based long-baseline experiments like
NOvA, DUNE and T2HK, atmospheric neutrino experiments like Hyper-K andPINGU and reactor neutrino experiments like JUNO.

Figure 1.1: Schematic view of neutrino mass hierarchy.

1.1.3

Leptonic CP violation

Why our universe mainly consists of not antimatter (antibaryons) but matter(baryons) is one of the unsolved problems in physics. To create the baryonicasymmetry, the Sakharov conditions [14] must be satisfied, which contains theexistence of CP violation. The CP violation in quark sector has been observedin the experiments of K [15] or B [16] mesons. However, it is not suﬃcient
to explain the matter-antimatter asymmetry. The CP violation in lepton sector is proposed as the other source of the baryonic asymmetry and neutrino
experiments aim to observe the CP violating phase.
The oscillation probability from νµ to νe is expressed, to the first order of9the matter eﬀect,
P (νµ → νe ) = 4c213 s213 s223 · sin2 ∆31+ 8c213 s12 s13 s23 (c12 c23 cos δCP − s12 s13 s23 ) · cos ∆32 · sin ∆31 · sin ∆21− 8c213 c12 c23 s12 s13 s23 sin δCP · sin ∆32 · sin ∆31 · sin ∆21+ 4s212 c213 (c212 c223 + s212 s223 s213 − 2c12 c23 s12 s23 s13 cos δCP ) · sin2 ∆21aL(1 − 2s213 ) · cos ∆32 · sin ∆31− 8c213 s213 s223 ·4Eν
a+ 8c213 s213 s223
(1 − 2s213 ) · sin2 ∆31 ,
(1.4)
∆m231
where ∆ij is ∆m2ij L/4Eν , and a[eV2 ] = 7.56 × 10−5 × ρ[g/cm3 ] × Eν [GeV]. The
probability from ν̄µ to ν̄e is obtained by replacing δCP → −δCP and a → −a.
If δCP is non-zero, the third term of Eq. 1.4 flips its sign between ν and ν̄
due to CP asymmetry (the last two terms also flip its sign due to the mattereﬀect). Therefore, the eﬀect of the CP phase can be observed by measuring∆Pµe ≡ P (νµ → νe ) − P (ν̄µ → ν̄e ).
To determine the CP phase, we need more statistics of neutrino events thanthat expected with T2K. A water Cherenkov detector with larger volume thanSK and higher intensity neutrino beam are required.

1.1.4

Nucleon decay

Grand Unified Theories (GUTs) are proposed models in which the three fundamental interactions of the Standard Model, i.e, the electromagnetic, weak, andstrong interactions, are integrated into one single force and quarks and leptons
are unified. Most GUTs predict baryon number violation, and nucleon decays
as a consequence.
Nucleon decays have never been observed experimentally and only lifetime
limits are set mainly by SK, the largest detector for the observation of nucleon
decay in the world. SK has been searching for nucleon decay events since 1996.
The current lower limits of proton lifetime are 1.6 × 1034 years for p → e+ + π 0 ,7.7 × 1033 years for p → µ+ + π 0 [17] and 5.9 × 1033 for p → ν̄ + K + [18].
In order to extend the sensitivity beyond these limits, a much larger numberof nucleons than SK and suﬃcient reconstruction ability to extract signals andsuppress backgrounds are necessary.

1.1.5

Neutrino astronomy

Neutrino astronomy is one branch of astronomy using neutrinos as the probe toobserve astronomical objects. The main sources of neutrinos observed in SK or
HK are the sun and supernovae.
The neutrinos from the sun are created with the nuclear fusion reactions,that are called the pp-chain and the CNO cycle. These processes are describedwith the standard solar model (SSM). Because of its rare reaction rate, neutrinos
10can probe the internal state of the sun in real time. Therefore, we can test theSSM predictions by observing the solar neutrinos. Another motivation is thestudy of neutrino properties themselves. The neutrino oscillation parameter θ12 ,∆m212 are determined by observing the solar neutrinos.
The supernova, the last process in the evolution of massive stars (> 8M⊙ ),will emit numerous neutrinos called the supernova burst neutrinos. From SN1987A,
the Kamiokande, IMB, and Baksan experiments observed 25 neutrino events andthis discovery starts neutrino astronomy. No supernova burst neutrino has been
observed since SN1987A, but recently it is said that Betelgeuse might explodeand emit neutrinos.
Another observation is the supernova relic neutrinos (SRN), produced by
all past supernova explosions since the beginning of the universe and diﬀused.
SRN contains the information of the star formation rate, energy spectrum of
supernova burst neutrinos, and black hole or neutron star formations.
To detect the supernova neutrinos, we need a large detector which can observe MeV order neutrino. In addition, the multi-messenger observation withvisible light, gamma-ray, x-ray and gravitational wave will also reveal the supernova explosion in details.

1.2

Current neutrino experiments

1.2.1

Super-Kamiokande

Super-Kamiokande (SK) is a large water Cherenkov detector located at 1,000
meter underground in the Kamioka-mine, Hida-city, Gifu, Japan. Fig. 1.2 showsa schematic view of SK. The detector consists of a cylindrical tank, 39.3m diameter and 41.4m tall, filled with 50,000 tons of ultra pure water. Fig. 1.3
shows the inside view of SK. Eleven thousand and two hundred of 50 cm photomultiplier tubes (PMTs) are used for the inner detector and one thousand
one hundred and eighty five of 20cm PMT are used for the outer detector toveto the cosmic muons. It measures the multiple kinds of neutrinos and nucleon
decay by detecting Cherenkov light emitted by a charged particle produced inwater. SK is characterized by its wide energy range between MeV to TeV andhas accomplished many significant achievements, most notably the observationof neutrino oscillation and the measurement of neutrino oscillation parameters.

11

Figure 1.2: A schematic view of SuperKamiokande.

1.2.2

Figure 1.3:
Kamiokande.

InsideofSuper-

T2K experiment

T2K (Tokai to Kamioka) experiment is a long baseline neutrino oscillation experiment, started from 2009 in order to measure θ13 , θ23 , ∆m232 and δCP . Theνµ (ν̄µ ) beam produced by J-PARC in Tokai, Ibaraki, Japan is detected by neardetectors in J-PARC and SK in Kamioka, which is 295 km away, as a far detector to measure the probability of νµ → νe (ν̄µ → ν̄e ) appearance and νµ → νµ(ν̄µ → ν̄µ ) disappearance.
To create the J-PARC neutrino beam, 30 GeV protons from the synchrotron
accelerator strike a graphite target and produce pions. They are focused by theelectromagnetic horns, decay in a 96 m decay volume and produce a neutrino
beam. The beam is directed 2.5◦ degrees away from the far detector to optimize
the neutrino energy spectrum which has a peak at the first oscillation maximum,around 0.6 GeV as shown in Fig. 1.6.
Near detector consists of on axis and oﬀ-axis detectors as shown in Fig. 1.7.
The on axis near detector, INGRID and Proton Module, measure the neutrino
event rate and the direction of the neutrino beam. They also measure the neutrino cross section to study the neutrino interaction. The oﬀ-axis near detector,
ND280 (Near Detector at 280 m from production target), measures the neutrino
event rate and the cross section of the neutrino interaction.

12

Figure 1.5: View of the T2K experiment
Figure 1.4: Configuration of the detectors of T2K experiment.

Figure 1.7: The schematic view of near
Figure 1.6: Oﬀ-axis fluxes and oscilla- detectors.
tion probabilities.

1.3

Hyper-Kamiokande project

1.3.1

Overview of HK

Hyper-Kamiokande (HK) is a next generation underground large water Cherenkov
detector, based on the SK [19]. Fig. 1.8 shows the schematic view of HK and theparameters of HK is summarized in Table 1.1. HK consists of two cylindricalwater tanks which are 60 m in height and 74 m in diameter. Its fiducial (total)
mass is 0.37 (0.52) Mtons, which is 17 times of that of SK. Forty thousand ofphotodetectors with 50 cm diameter will be used in the inner detectors and six
thousand and seven hundred of photodetectors with 20 cm diameter will be used13

in outer detectors to veto cosmic-ray muons.
HK aims to measure rich physics targets with much higher statistics andsensitive detectors than SK, i.e., precise study of neutrino oscillation (determination of mass hierarchy and θ23 octant) and the measurement of CP violationin lepton sector using beam and atmospheric neutrinos, search for proton decay to verify grand unified theory, observation of astrophysical neutrino andneutrino geophysics.
The candidate site is Kamioka mine, Gifu, Japan because of much water,
hard bedrocks and the limitation of oﬀ axis angle from J-PARC neutrino beam.
The measurement with the first tank will start from 2025 and that with thesecond tank will start from 6 years after the beginning of that with the firsttank.

Figure 1.8: The schematice view of Hyper-Kamiokande detector.

Table 1.1: The parameters of Hyper-Kamiokande.
Parameter
Value of 1 tank Value of 2 tank
Diameter of water tank [m]7474Height of water tank [m]6060
Total volume [Mton]
0.258
0.516
Fiducial volume [Mton]
0.187
0.374
Number of inner photodetectors
40,00080,000
Number of outer photodetectors
6,700
13,40014

1.3.2Detection principle

Neutrino interaction
There are two kinds of neutrino interaction as shown in Fig. 1.9. One is charged
current (CC) interaction which exchange a W ± and a neutrino converts into
a corresponding charged lepton like νe + N → e + N ′ . The other is neutral
current (NC) interaction which exchange a Z 0 and a neutrino does not convert
like ν + N → ν + N ′ . Only energy and momentum are exchanged.
In the accelerator or atmospheric neutrino experiments, which energy regimeis O(1 GeV), neutrino-nucleus interactions become dominant. These interactions can be described by two steps. First, a neutrino interacts with a nucleon
in a nucleus by exchanging weak bosons and produces secondary particles. Then,these particles interact with the nucleons in nucleus. Neutrino-nucleus interactions can be divided into four classes on the basis of the interaction product:• quasi-elastic scattering (ν + N → l + N ′ )• single meson production via baryon resonances (ν + N → l + N ′ + m)
• coherent pion production(ν + A → l + A + π)
• deep inelastic scattering (ν + N → l + N ′ + hadrons)
where N and N’ are nucleons (proton or neutron), l is a lepton, m is a meson
and A is a nucleus. For a neutrino which energy is less than 1 GeV like T2K
neutrino beam, CCQE interaction becomes dominant.
In HK, the informations of the initial neutrino can be reconstructed by detecting the Cherenkov light emitted by the charged particles produced in water.
For example, assuming a charged current quasi-elastic (CCQE) interaction of
the accelerator neutrino, the neutrino energy (Eνrec ) is reconstructed from theenergy of the final state charged lepton (El ) and the angle between the neutrino
beam and the charged lepton directions (θl ) asEνrec =

2(mn − V )El + m2p − (mn − V )2 − m2l2(mn − V − El + pl cos θl )(1.5)

where mn , mp , ml are the mass of neutron, proton, and charged lepton, pl is themomentum of charged lepton and V is the nuclear potential energy (27 MeV).

15

Figure 1.9: The example of neutrino interactions. The left (right) diagramshows the charged (neutral) current interaction of neutrino-nucleon scattering.
Cherenkov light
When a charged particle passes through a dielectric medium faster than thephase velocity of light in that medium, Cherenkov light is emitted as shown in
Fig. 1.10. The angle between the direction of the traveling charged particle andthe Cherenkov light, called the Cherenkov angle (θ), is written ascos θ =

1
nβ

(1.6)

where β is the velocity of the charged particle in unit of c (β = v/c) and n is
the refractive index. In water, n is 1.33 and the Cherenkov angle becomes ∼41
degrees in extreme relativistic limit.
Following Eq. 1.6 and 0 ≤ cos θ ≤ 1, the lower limit of the velocity to emit
Cherenkov light becomes β = 1/n and the corresponding energy is
Ethr = √mc2
1 − β2

mc2=√≃ 1.5 mc2 (in water)
1 − (1/n)2(1.7)

where m is the mass of the charged particle. For electron, Ethr becomes 0.77
MeV. The particle which energy is below Ethr cannot be observed.
As shown in Fig. 1.11, when the charged particle stops inside the water tank,
Cherenkov light projected on the wall forms a donut shape called the Cherenkov
ring and its shape depends on the particle type (Fig. 1.12). The Cherenkov lightis detected by photodetectors installed on the wall and each photodetector sends
its timing and intensity information. The ring image is reconstructed by theseinformations and the vertex point, event time, direction, type and energy of thecharged particle and neutrino can then be determined.
The number of Cherenkov photons dN generated per unit distance dx andunit wavelength dλ of the charged particle is written as()
d2 N
2παz 212παz 2=1−=
sin2 θ
(1.8)
2
2
2
dxdλ
λ
n β
λ2
where α is the fine structure constant (∼1/137) and z is the electric charge ofincident particle. By integrating the wavelength over 300 ∼ 600 nm, which is16the sensitive range of a photodetector for HK and considering the velocity ofβ ≃ 1, the number of emitted Cherenkov photons per unit of distance becomesdN≃ 340z 2 [photons/cm].
dx

(1.9)

Considering an electron with the energy of 3 MeV, which is a required lower
energy threshold in HK, an electron loses ∼2 MeV/cm in water from the BetheBloch equation and the range to reach Ethr becomes ∼1 cm. Then, the total
emitted Cherenkov photons are ∼340. Considering the water transparency,
the remaining number of photons becomes 340/2.7 = 126. Assuming a 40%
photo-coverage and a 30% quantum eﬃciency of a photodetector, the numberof photodetector which can observe Cherenkov photons is ∼15 (∼5 hit photodetertors/MeV). Therefore, the performance of photodetectors becomes criticalespecially for the study of low energy physics and proton decay.

Figure 1.10: Image of the emitted Figure 1.11: Image of the Cherenkov
Cherenkov light.
ring.

Figure 1.12: The diﬀerence of the Cherenkov rings in SK. Right shows the ringof a muon and left shows that of an electron. The ring of an electron is moreindistinct than that of muon due to the multiple scattering in water.

17

Chapter 2

Development of new
photodetectors for Hyper-K
2.1

R&D motivation and requirements for HyperK photodetectors

The performances of photo-detectors are one of the most influential factors todetermine the performance of a water Cherenkov detector. In Super-K, 50 cm
Venetian Blind dynode type PMT (SK PMT) is used as photo-detectors and its
performance is well confirmed. However, SK PMT is not suﬃcient for use in HK
because the bigger size of tank than SK will increase the path of light and thelight attenuation. Additionally, HK aims to increase the sensitivity of targeted
physics, for example low energy physics like supernova relic neutrino or proton
decay especially for p → ν̄K + mode and the newly developed reconstruction
algorithm like neutron tagging compared to SK. Moreover, high performance
photodetectors extend the possibility to develop new analysis methods. Therefore, it is necessary to develop new higher performance photodetectors than SK
PMT.
The requirements for photodetectors are summarized in Table 2.1. They are
determined by the physics sensitivity aimed in HK. The value of each requirement is evaluated based on the performance of SK and SK PMTs.
Better charge resolution is necessary for better energy resolution. Especiallyfor detecting a low energy particle, because several photons are yields and eachphotodetector detects one photon, the single photon counting performance becomes important.
Better timing resolution is required for better vertex reconstruction. Theinteraction vertices of particles are reconstructed in water by the shape of
Cherenkov image and the timing information of Cherenkov light detected in eachphotodetectors. Less timing jitter makes better reconstruction performance.
Low dark rate is important for suppressing the background of low energy

18event and the amount of DAQ data. Afterpulse also becomes the backgroundof delayed particle like Michel electron and contribute to the increase of dark
rate. It is desirable to reduce afterpulse.
Rate tolerance is important to measure supernova explosions. In HK, O(105 )Hz event rate in low energy is expected if supernova explosion occurs at 10
kpc from the earth. Therefore, it is necessary for the photodetectors to have
suﬃcient rate tolerance.
Higher detection eﬃciency is needed because the tank size of HK becomesbigger than SK and the attenuation of light increase. Detection eﬃciency becomes important especially for detecting a low light event. On the other hand,wide dynamic range is essential to observe a high energy event which yields
numerous number of photons.
The mechanical robustness of photodetectors is also required. In order toreduce the influence of terrestrial magnetism, photodetectors must have magnetic field tolerance. Low failure rate and long lifetime is necessary because
photodetectors will be operate in water for 20 years in HK. The high waterpressure tolerance is needed because the depth of HK tank is 80 m while thatof SK is 40 m.
Table 2.1: Minimum requirements for the Hyper-K inner photodetectors, basedon the performance of SK PMT(R3600) and HK.
Requirements
Value
Conditions
Gain
107 ∼ 108
Typ.
Signal window
200 nsMax. More than 95% of a total signal area
Charge resolution
50%
Typ. σ of p.e.
Timing resolution
2.4 nsTyp. FWHM of single photoelectron (p.e. ) 1
Dark rate
4 kHz
Typ. For 1/4 p.e. threshold
Afterpulse rate
15%
Max. For single p.e. , relative
to the main pulse
Rate tolerance
10 MHzMax. single p.e. pulse, within
10% change of gain
Detection eﬃciency
16%
Typ. Quantum eﬃciency ×Collection eﬃciency
Dynamic range
2 photons/cm2 Max. Per detection area on
wall
Magnetic field tolerance 100 mG
Typ. Within 10% degradation
Lifetime
20 years
Typ. Less than 10% dead rate
Pressure rating
0.8 MPa
Max. Static, load in water

1 The original requirement uses σ of single photoelectron (p.e. ), but in this thesis we use
FWHM because the transit time distribution is not gaussian but asymmetrical distribution.

19

2.2

Photodetector candidates for Hyper-K

2.2.1

Box&Line PMTIn SK, 50 cm venetian blind PMT (Hamamatsu R3600 NQE (normal QE),
called Super-Kamiokande PMT (SK PMT)) is used as a photodetector for 20
years. Fig. 2.1 shows the structure of SK PMT. SK PMT uses the venetianblind structure as dynodes and it allows to increase the size of photocathodeeasily. However, there is a room to improve the performance. Fig. 2.2 shows theperformance of SK PMT. Multiple photo-electron paths due to the structure ofvenetian blind dynodes worsen its timing resolution, and a low probability to
hit the first stage of dynodes results in worse charge resolution and detection
eﬃciency.
In order to improve the performance of the photodetector, Box&Line PMT(B&L PMT) is newly developed as a successor of SK PMT for use in HK. A
B&L PMT has the large first dynode of box shape and dynodes of line shape forthe rest. As shown in Fig. 2.2, box dynode maintain high collection eﬃciency
and line dynodes make uniform path length of photoelectrons. It is result in agood timing and charge resolution as shown in Fig. 2.2.
When the performance evaluation of HPD, the main theme of this thesis,
it becomes important to compare with the performance of these SK PMT and
B&L PMT.

Figure 2.1: Venetian blind PMT / Super-Kamiokande PMT (SK PMT) / Hamamatsu R3600 NQE.

20

Figure 2.2: Box&Line PMT (B&L PMT) / Hamamatsu R12860 HQE.

Table 2.2: The performance summary of SK PMT and B&L PMT.
SK PMT B&L PMT
Rise Time [ns]10.6
7.6
Fall Time [ns]
13.2
16.0
FWHM [ns]18.8
12.2
Charge resolution [%]53
35
Timing resolution (TTS in FWHM) [ns]
7.0
4.0

2.2.2

Hybrid Photo-Detector

A Hybrid Photo-Detector is a photodetector using a silicon avalanche diodeinstead of metal dynodes which are used in a PMT. Fig. 2.3 shows the methodof a signal amplification in an HPD. First, photo-electrons produced at thephotocathode are accelerated towards the AD by a high voltage electric field(∼8 kV), result in multiplication by bombardment. Second, electron-hole pairs
are created inside the AD, where a bias voltage of 400∼500 V is applied toamplify electrons by a factor of ∼50 in avalanche multiplication. The gain of an
HPD alone becomes ∼ 105 and a preamplifier is employed to obtain the total
gain equivalent to a SK PMT (∼ 107 ).
It is expected that an HPD would have many advantages compared to a
PMT. An HPD has better charge resolution because of the high gain at thefirst amplification stage, and due to its small fluctuation of the drift path in the
AD the timing resolution of a HPD is better. The simpler and axial symmetricstructure makes lower costs, better magnetic field tolerance and lower afterpulse.
Fig. 2.4 shows the history of the development of HPDs. First, as a prototype
of a large aperture HPD, a 20 cm HPD with a 5 mm diameter (ϕ) AD wasdeveloped [20]. The prototypes are now under a proof test in a 200 ton watertank where they showed good performances [21]. In order to test the productionof large photocathode HPD, a 50 cm HPD with 5 mm ϕ AD is developed as asimple extended version of 20 cm HPD with 5 mm ϕ AD. While it is confirmed
21

that it has good performances as well [22], the collection eﬃciency is low. To
improve collection eﬃciency (CE), the large area of AD is necessary. According
to the simulation result from Hamamatsu, a 20 mm AD can reach to CE∼93%
while a 5 mm AD reach to ∼10%. Therefore, a 50 cm HPD with 20 mm ϕ AD
becomes our target design. However, it is accompanied with a large noise bya large 800 pF junction capacitance, increased from 50 pF in the 5 mm ϕ AD.
This is described in detail in next section.

2.2.3

Goals of this thesis

In this thesis, we report R&D of a 50 cm HPD which can be used as innerphotodetector of HK. To increase a collection eﬃciency of a 50 cm diameter
HPD, a large area AD is necessary, but a large junction capacitance of a large
area of AD increases the noise level of the output signal as explained in Section
3.1. We developed new AD (Section 3.2) and preamplifiers (Section 3.3) to solve
this problem and succeed to separate single photoelectron signal and baseline
noise.
Then we evaluate the performance of a 50 cm HPD. We describe the resultof evaluation in Section 4.

Figure 2.3: The principle of an HPD amplification and typical values of the
50cm HPD.

22

Figure 2.4: Summary of HPD development.

23

Chapter 3

Development of 50 cm HPD
with 20 mm diameteravalanche diode3.1

Diﬃculty of using 20 mm diameter AD

To increase the collection eﬃciency (CE) of a 50 cm diameter HPD, a large area
AD is necessary. We developed a 50 cm diameter HPD with a 20 mm diameterAD based on a 50 cm diameter HPD with a 5 mm diameter AD. However, alarge junction capacitance of a large area of AD increases the noise level of theoutput signal [23].
Fig. 3.1 shows the principle of the noise increase due to a large junction
capacitance of an AD and a trans-impedance amplifier (a current-to-voltage
converter). If the feedback impedance is fixed and the input capacitance of
preamplifier C = Cd Ci /(Cd + Ci ) increases, the input impedance decreases by
1/ωC and the noise of preamplifier increases. Then, a current noise is amplified
by a feedback and the signal to noise ratio (S/N) becomes worse. In current
situation, Cd is much larger than Ci , therefore it is necessary to decrease thejunction capacitance of an AD.
To solve this problem, following improvements are attempted:• AD improvement– Development of new AD with low capacitance– Development of segmented (or multichannel) AD• Preamplifier improvement– Tuning parameters of trans-impedance amplifier– Introduction of decoupling transformer

24

– Bootstrap circuit– Non-feedback amplifier
In this thesis, new AD with low capacitance, segmented (or multi-channel)
AD and tuning parameters of trans-impedance amplifier are applied. In thefollowing sections, we describe them in detail. Other improvements are also
evaluated. For example, the application of decoupling transformer is tried in[24].

Figure 3.1: The principle of current noise increase due to the large junction
capacitance of an AD and a trans-impedance amplifier.

3.2

Improvement of AD

3.2.1

Development of low capacitance AD

The development of new AD with low capacitance is achieved by increasing
the thickness of an AD. The junction capacitance of the original AD is 800 pF
and that of a new AD is reduced by half (400 pF). The bias voltage of a new
low capacitance AD increases compared to the original AD in order to obtainthe same gain. Accordingly, the coupling capacitance inside the base blockand a guard voltage are changed. The typical values of each type of AD aresummarized in Table 3.1.

3.2.2

Application of segmented AD

By dividing an AD into multiple channels, we can reduce the junction capacitance per channel. We have developed two types of segmented ADs: 5ch AD
and 2ch AD. A 5ch AD has one circular AD at the center and four surrounding arc-like ADs. A 2ch AD has one large circular AD at the center and one
surrounding doughnut-like AD. Each channel is designed to have the same area
so that the junction capacitance becomes equal for all channels. Currently, the25AD cathode is common between channels. In the future, the AD cathode willbe also separated into each channels.
Fig. 3.2 shows a schematic view of a segmented AD and its readout system.
The signal of each channel of an AD is amplified by a preamplifier. There are
two method of the final read-out of signal output. One is to read out eachchannel of an AD individually. We have the signal outputs with the same
number of segmentations and each signal is amplified by a preamplifier. Second
is to read out by using a sum-amplifier with one signal outputs. The individual
read-out scheme can suppress the noise at the price of increased number ofchannels. On the other hand, the sum-amp read-out scheme has less number ofchannels
but larger noise. The total noise level of sum-amp read-out becomes√∑(noise from each channel)2 .
Fig. 3.3 shows the AD junction capacitance dependency of the noise levelof the output signal in pulse height and in standard deviation of charge. We
measure the output signal of a raw AD by connecting each channel of 5ch old
AD or 5ch low capacitance AD in parallel and varying the value of a junction
capacitance. The noise level of an HPD with 5ch old AD is also measured inorder to confirm the consistency between the measurements using AD alone oran HPD equipped with an AD. It is confirmed that the noise level of an HPD
output signal is suppressed by reducing the AD capacitance.
Table 3.1: Summary of AD types and typical values. 1ch AD means notsegmented AD.
Bias Voltage
Number of Thickness of Total Capac- CapacitanceSegment [ch] AD
itance [pF]ofEach [V]
Channel[pF/ch]
5
thin
800
160
∼400
5
thick
400
80
∼500
2
thin
800
400
∼400
2
thick
400
200
∼500
1
thin
800
800
∼400
1
thick
400
400
∼500

26

Figure 3.2: Schematic view of segmented AD and readout system.

Figure 3.3: The AD junction capacitance dependency of the noise level in pulse
height (left) and in charge (right).

3.3

Improvement of preamplifier

3.3.1

Diﬃculties in preamplifier development

There are three issues in the development of a preamplifier for a 50 cm HPD witha large diameter AD as shown in Fig. 3.4. First is the large junction capacitanceas described in previous sections. It increases the output noise and worsen S/N.
Second is the floating current source due to the high voltage between an AD
27

and ground. Because of first and second reasons, we cannot use the preexisting
preamplifier and have to develop new one. Third is the non-negligible delay line
between an AD and a preamplifier which generates the phase rotation in highfrequency. In the development, we mainly try to solve first and second problems
and third one had not been recognized until August 2016. As described later
section, the development of a preamplifier is accomplished by solving third one.

Figure 3.4: Description of problems to develop the preamplifier of 50 cm HPD
with 20 mm diameter AD.

3.3.2

Sequence of preamplifier development

The development of a preamplifier is carried out as follows.
First, a preamplifier is made at ICRR, the University of Tokyo or Hamamatsu photonics and pre-tested. Fig. 3.5 shows the test board of a preamplifier
used in ICRR. To simulate the structure of an HPD, diﬀerential inputs and acapacitor are equipped. A square wave and inverted one enter the two inputs
and 8fC charge is generated, which is equivalent of the raw output of an HPD.
The capacitor is exchangeable and we can test with various size of capacitors.
After the pre-test and clear S/N (= pulse height of signal / root meansquare(RMS) of baseline noise level.) > 10σ, we evaluate the performance ofa preamplifier using an HPD at Kamioka or Kashiwa. We check that there no
oscillation, whether 1 p.e. signal can be triggered by 0.25 p.e. threshold andwhether 1p.e. signal can be separated in charge distribution.

28

Figure 3.5: Left (right) is the image (circuit diagram) of the preamplifier test
board.

Figure 3.6: The output signal waveform of a preamplifier at the pre-test.

29

3.3.3Developed preamplifiers

In this section, we overview the developed preamplifiers. We can divide preamplifiers into three types by developed period and its characteristic (summarizedin Table 3.3).
First type is simple transplantation of a trans-impedance preamplifier used
for an HPD with 5 mm diameter AD. Its parameters are tuned for an HPD with
20 mm diameter AD, but the S/N is low and there are large ringings.
Second type is developed in January 2016. It increases the feedback of thepreamplifier and strengthen its ground to suppress the overshoot of the output
signal and noise level. Fig. 3.7 shows the comparison of waveform and charge
distribution between type I and II preamplifiers. Type II had better S/N, but
the rise time becomes long and the gain becomes large. Despite of its slow
response, we can detect a single photo-electron signal, so we used this typeof preamplifier for the performance evaluation of charge resolution, dark rate,
afterpulse and rate tolerance.
Third type is developed to solve the third problem described in Section 3.3.1.
It decreases the open loop gain of preamplifier especially in high frequency region
and improves S/N and signal response. This type of preamplifier is the final
version and used in the evaluation of timing resolution. Fig. 3.8, 3.9 compare theoutput signal waveform and charge distribution of type II and III preamplifiers.
The rise time of type III is faster than type II keeping the same charge resolution.

3.4

Summary of developed HPD types

Following the solutions described in previous sections, we have developed three
types of HPDs: HPD with 5ch AD (160 pF/ch), HPD with 2ch half capacitanceAD (200 pF/ch), HPD with 1ch half capacitance AD (400 pF/ch). Fig. 3.2
shows the usage of each type.
Table 3.2: Sammary of HPD types and evaluation using each type of HPD.
Serial No. TypeUsage
THD0034 HPD with 5ch high capaci- 1 p.e. resolution, position sentance AD
sitivity
THD0056 HPD with 2ch low capacitance 1 p.e. resolution, dark rate,
rate tolerance
THD0058 HPD with 1ch low capacitance 1 p.e. resolution, dark rate, afterpulse, detection eﬃciency
THD0061 waterproof HPD with 1ch low waterproof test
capacitance

30

Type
I
II
III

Table 3.3: Summary of developed preamplifiers.
Developed Period
Characteristic
2015
low S/N, large ringing
the beginning of 2016 high S/N, small ringing, long rise time
August 2016high S/N, small ringing, short rise time

Figure 3.7: Comparison of waveform and charge distribution between type I
and II preamplifier.

Figure 3.8: The comparison of old and Figure 3.9: The comparison of old andnew preamp in waveform.
new preamp in charge distribution.

31

Figure 3.10: Circuit diagram of type I preamplifier.

Figure 3.11: Circuit diagram of type II preamplifier.

32

Chapter 4

Performance evaluation of
50 cm HPD
4.1

Setup for performance evaluation

Fig. 4.1 shows the overview of the whole system to operate an HPD and collectdata. An HPD is located in a dark box. Photons are emitted from the picopulse laser and sent to the HPD via an optical fiber. The signal is read-out fromthe cable derived from the socket assembly of an HPD and processed by a DAQ
system depending on each measurement.
An HPD designed for R&D can be divided into two parts. One is the glass
bulb with a photocathode and an AD. The other is the socket assembly consistedof a HV module, a base-block and preamplifier(s) (+ sum-amp) as shown in
Fig. 4.2. The HV module is used to apply two kinds of voltage, the one between
the photocathode and the AD (typically ∼8 kV), and the bias voltage of AD(400∼500 V). The output of HV module is controlled by an external HV-LV
control power supply shown in Fig. 4.3. In the base-block, decoupling capacitors
are built-in to transmit the high voltage from the HV module to the HPD, andread-out the raw signal of the HPD. The preamplifier amplifies the raw signalof HPD. If an HPD has a segmented AD, additional preamplifiers and sumamplifier are implemented. The socket assembly is detachable mainly in orderto test preamplifiers. In the final version of an HPD, the parts of socket assemblyand glass bulb will be united.

33

Figure 4.1: The overview of the whole system to operate an HPD and dataacquisition.

Figure 4.2: A picture of the socket assembly of HPD.

34

Figure 4.3: Pictures of the HV-LV control power supply.

4.2

Waveform

Fig. 4.4 shows each photodetector’s waveform normalized by its pulse height.
Table 4.1 shows the rise time, fall time and FWHM of each photodetector. Risetime (fall time) is defined as the time required for the response to rise (fall)from 10% (90%) to 90% (10%) of the maximum pulse height. The intrinsic
HPD(which means HPD without preamplifier) has the best rise time compared
to SK PMT or B&L PMT and less ringing. While the response of HPD withthe old preamp becomes slower than others, new preamp succeeds to bring out
the potential of HPD.
Table 4.1: Summary of rise time, fall time and FWHM of each photodetector.
PD Type
Rise Time [ns] Fall Time [ns] FWHM [ns]HPD (with old preamp)
37.2
111.9
107.2
HPD (with new preamp)
13.4
59.2
41.3
HPD (without preamp)
6.3
47.7
28.7
SK PMT
10.6
13.2
18.8
B&L PMT
7.6
16.0
12.2

35

Figure 4.4: Waveform of each photodetector.

4.3

Gain

The gain of photodetector is defined as the output charge of 1 p.e. signal divided
by the elementary charge. The gain of HPD is proportional to HV as shown in
Fig. 4.5 and depends on the AD bias voltage with the followning function:
gain =a1 − (Bias/b)c(4.1)

as shown in Fig. 4.6. The gain of HPD also depends on the temperature because
the AD gain increases as the temperature decreases with the same AD bias
voltage.

36

Figure 4.5: Measured total gain of 1ch HPD with preamplifier as a function of
the HV with fixed AD bias voltage of 520 kV.

Figure 4.6: Measured total gain of 1ch HPD with preamplifier as a function of
the AD bias voltage with fixed HV of 8 kV.

37

4.4

Charge resolution

We measure the 1 p.e. charge distribution of 5ch, 2ch or 1ch HPD by ADC
with the gate width is 200 ns. Fig. 4.7 shows the result of single channel andsum-amp readout. The detection capability of single photo-electron (p.e.) isevaluated by calculating 1 p.e. resolution, noise level, peak to valley ratio (P/V)
defined asσ1 p.e.
µ1 p.e. − µped
σped
noise level =µ1 p.e. − µped
Peak
peak to valley ratio (P/V) =Valley1 p.e. resolution =

(4.2)
(4.3)
(4.4)

where µ1 p.e. , µped , σ1 p.e. , σped , peak and valley are evaluated by fitting charge
distribution by a composite function of two Gaussian and two error functions
as shown in Fig. 4.8. The error functions are used to take into account thecontamination of photo-electron backscattering on the AD which forms the flat
regions between pedestal and 1p.e. or 1p.e. and 2p.e. peaks. Table 4.2 shows
the summary of evaluated 1 p.e. resolution, noise level, peak to valley ratio of
each photodetector.
Single channel readout of 5ch HPD or 2ch HPD has three times better 1
p.e. resolution than B&L PMT or SK PMT. Sum-amp readout of 5ch HPD has
worse 1 p.e. resolution and noise level because of common mode noise, however
sum-amp readout of 2ch HPD and single channel readout of 1ch HPD has still
twice better 1 p.e. resolution than B&L PMT or SK PMT.

Figure 4.7: 1 p.e. charge distribution of 5ch, 2ch, 1ch HPD by single channel
readout (left) and sum-amp readout (right).

38

Figure 4.8: Definition of µ1 p.e. , µped , σ1 p.e. , σped , peak, valley in charge distribution.

Table 4.2: Summary of 1 p.e. resolution, noise level, peak to valley ratio of eachphotodetector.
PD Type1 p.e. resolution [%] noise level [%] P/V
5ch HPD (Single)
10.0
7.9 10.1
5ch HPD (Sum)
36.6
40.2
1.1
2ch HPD (Single)
10.3
7.6
8.4
2ch HPD (Sum)
14.8
11.1
8.9
1ch HPD
17.7
13.8
7.3
B&L PMT [25]35
4.8
SK PMT [25]
53
1.9

4.5

Timing resolution

The timing resolution of HPD is evaluated by measuring the transit time spread(TTS), which is defined by the time from a trigger of a light source to 0.5
p.e. threshold as shown in Fig. 4.9.
Fig. 4.11 shows the setup for the measurement of the timing resolution.
The pico-pulse laser is triggered by the input/output (I/O) resistor and thelight intensity is controlled by an attenuator. The output signal of the HPD is
divided and measured by an ADC and a TDC. The gate triggers of the ADC
and the TDC are synchronized by I/O resistor and we can measure the charge
and transit time of the HPD simultaneously.
39

Fig. 4.10 is the result of the single photoelectron TTS measurement. The
1 p.e. TTS of each photodetector after applying time walk correction are overwritten. The evaluated TTS in FWHM is summarized in Table 4.3. The TTS
of 1ch HPD becomes 3.3 ns with bias voltage of 520 V and 2.7 ns with bias
voltage of 570 V after time walk correction. This is twice better than that of
SK PMT and better than that of B&L PMT.
We also measure the TTS for multiple p.e. signal of HPD. Fig. 4.12 shows
the measured TQ-map (the relation between charge and timing) of 1ch HPD
and Fig. 4.13 is the evaluated TTS in FWHM with time walk correction foreach number of photoelectrons. It is shown that the TTS of HPD is better thanthat of SK PMT and B&L PMT especially for multiple p.e. signal1 .

Figure 4.9: Definition of transit time.

Figure 4.10: Single photo-electron transit time spread (TTS) of each photodetectors. Left (right) figure contains TTS of HPD with the bias voltage ∼520 V(∼570V).
1 The 1 p.e. TTS of HPD in Fig. 4.13 becomes worse than that in Fig. 4.10 because theused preamplifier is diﬀerent (the circuit structure is the same but the components is diﬀerent)
and the time walk correction using TQ-map has less impact on small p.e. signals than largep.e. signals

40

Table 4.3: The summary of single photoelectron timing resolution for each photodetectors.
PD Type
Conditions
TTS(FWHM) [ns] TTS(FWHM, time walkcorrection) [ns]
SK PMT
7.0
B&L PMT 4.0
1ch HPD
Bias ∼520 V 4.0
3.3
1ch HPD
Bias ∼570 V 3.0
2.7

Figure 4.11: Setup of the timing resolution measurement.

41

Figure 4.12: The TQ-map (time charge map) of 1ch HPD.

Figure 4.13: Multi photo-electron transit time spread (TTS) of each photodetectors.

42

4.6

Dark rate

Dark count rate is the signal pulse rate of photodetectors without light input.
The dark pulse of HPD is caused by the thermal electron emissions from thephotocathode and mainly observed as 1 p.e. signal.
We measure the dark rate of HPD as a function of the threshold after keeping
it in a dark box for a few days with applying high voltage and bias voltage atroom temperature (25◦ C). We use an Analog Timing Module (ATM) board, a
DAQ board used in Super-K I, II and III. A detailed description for ATM canbe found in Ref. [26]. The threshold of ATM can be set from 0 mV to 12.5mV. Because the pulse height of the signal of HPD is ∼15 mV, we use divider
to make the height in half.
Fig. 4.14 shows the measured dark rate of 1ch HPD and 2ch HPD. The dark
rate of HPD is ∼10 kHz with a threshold of 0.5 p.e. Considering the temperaturedependence, the dark rate of HPD is expected to be the same level as that of
SK PMT (∼4 kHz) at water temperature (∼16◦ C) in HK operation from thedark rate measurement of a 50 cm HPD with 5mm diameter [24] or that of awaterproof 50 cm HPD in Section 4.10. It is still not stabilized completely andthere might be room to reduce the level further.

Figure 4.14: The dark rate of 1ch and 2ch HPD as a function of the threshold.
The doted red line shows the dark rate of each channel of 2ch HPD and thesolid red line shows the sum of tow doted red line.

43

4.7

Photon detection eﬃciency

We cannot measure the collection eﬃciency of photodetectors directly, so wemeasure the total detection eﬃciency, which is equal to collection eﬃciency (CE)× quantum eﬃciency (QE), relatively by comparing with reference PMTs.
Fig. 4.15 shows measured relative total detection eﬃciency, QE measured byHamamatsu photonics and calculated relative CE. The total detection eﬃciencyof HPD is the same level of that of B&L PMT, while the CE of HPD is lower
than B&L PMT.
This is because the trigger eﬃciency of HPD is low as shown in Fig. 4.16.
The noise level of HPD in pulse height is larger than that of B&L PMT andthe threshold level of HPD is set higher than that of B&L PMT. There are flat
regions between pedestal and 1p.e. or 1p.e. and 2p.e. peaks due to the contamination of photo-electron backscattering on the AD. Therefore, the backscattering
signals which pulse height are lower than threshold level become undetectable
and these reduce the trigger eﬃciency of HPD. It is necessary to reduce thenoise of preamplifier in order to solve this problem.

Figure 4.15: The measured relative total detection eﬃciency (= relative collection eﬃciency (CE) × quantum eﬃciency (QE)), QE measured by Hamamatsu
photonics and calculated relative CE.

44

Figure 4.16: The charge distrubition and trigger eﬃciency of an HPD and a
B&L PMT.

4.8

Afterpulse

Afterpulse of a large aperture HPD or PMT occurs O(1)∼O(10) µs after the
initial photoelectron signal and becomes the background especially for low rate
event detection like Michel electron. The cause of afterpulse is positive ion
generated by photo-electron and residual gases in glass. In HPD, There are no
multiplied electrons between dynodes like PMTs, therefore it is considered thatthe possibility of generating positive ion becomes much lower and the afterpulse
rate is less than that of PMTs.
We measured afterpulse rate by counting signals between -5 to 50 µs time
windows by TDC. Fig. 4.17 shows the count rate normalized by the main pulse
counts in each 500 ns window. The existence of negative rate point is due tosubtraction of dark count rate. Table 4.4 shows the sum of the normalized countrate between 0.5 to 50 µs. The afterpulse rate of HPD is the same level of SK
PMT and less than that of B&L PMTs 2 .

2 The data of B&L PMTs is measured at 2015. The recent developed B&L PMTs are more
improved and have lower afterpulse rate.

45

Figure 4.17: The afterpulse rate of each photodetector between -5 to 50 µs time
windows from main pulse.

Table 4.4: The rate of afterpulse to mainpulse for each photodetector.
PD Type
Rate [%]
1ch HPD< 2.3
SK PMT< 2.5
B&L PMT (old ver.)∼30
B&L PMT (reduced afterpulse ver.)
1∼13

4.9

Rate tolerance

It is necessary for the photodetectors of HK to have suﬃcient high rate tolerance
in order to detect the high rate event like a supernova explosion. If supernova
explosion occurs at 10 kpc from the earth, O(100) kHz event rate in O(10)MeV energy is expected. This yield O(107 ) photons per second at the whole
HK detector. Because the HK has forty thousand photodetectors per tank andmean number of detected photons per photodetectors is less than O(1) p.e. , itis considered that 1 MHz rate tolerance of photodetectors is suﬃcient for HK.
We measured the signal output stability of HPD by changing the frequency
of input light pulse. As shown in Figure 4.18, the output is constant within 5%
up to 1.2 MHz for 50 p.e. signal in charge. It is suﬃcient to detect the supernova
burst events which is assumed in HK.
The cutoﬀ frequency is limited by the pulse width of the preamplifier (∼100

46ns). In order to improve the rate tolerance of an HPD, it is necessary to makeits fall time shorter and develop a quick response preamplifier. Below the cutoﬀ
frequency, the output of HPD slightly increases as the signal rate increases inboth charge and pulse height. The reasons are considered as follows: First is
the pile up due to the ringing of output pulse. It may cause the base line shift
and we have to correct this eﬀect. Second is the instability of the light source.
In this measurement, we do not monitor the output of light source and there isa possibility to change the number of input photons. Third is the gain increaseof the AD. It is planned to do more detail measurement.

Figure 4.18: The output signal stability of HPD in charge and pulse height.

4.10

Viability test of an 50 cm HPD in water

Photodetectors for HK will be operated in water for 20 years. B&L PMTs and 20
cm HPDs are under the viability test [27] in EGADS (Evaluating Gadolinium’s
Action on Detector System), which is a 200-ton water tank in Kamioka mineas shown in Fig. 4.19 and 4.20. On the other hand, the long term performancestability of a 50 cm HPD has never been evaluated.
Because the basic performance of a prototype 50 cm HPD is confirmed, wemove to the viability test in water as the next step. We make a waterproof
50 cm HPD in July 2016 and do the performance test in water as shown in
Fig. 4.21. Fig. 4.22 shows 1 p.e. and multi p.e. signal of a waterproof 50 cm
HPD. The noise level of a waterproof HPD is the same as a prototype 50 cm
HPD and we can separate each photoelectron signal clearly (Fig. 4.23).
We conduct a continuous operation test in dark box filled with water for 20
days in the end of July and confirmed no trouble and stable operation. Fig. 4.24
shows the variation of dark rate and water temperature during continuous operation of a waterproof HPD. For the initial period of the operation, the dark ratechanges because of the stabilization of the photodetector and the temperaturechange. After the temperature is stabilized, the HPD works stably. Fig. 4.25
shows the relationship of a temperature and a dark rate of a waterproof HPD. It
was confirmed that the dark rate of an HPD at a water temperature (∼ 15◦ C)
in HK becomes half of that at a normal temperature (∼ 25◦ C).

47

Figure 4.19: The schematic view of Figure 4.20: The extend elevation of
EGADS tank (a 200-ton water tank). EGADS tank. Green circles show theplaces planning to install 50 cm HPDs.

Figure 4.21: The waterproof HPDs and the picture of operation in water.

Figure 4.22: 1 p.e. and multi p.e. signal of a waterproof 50 cm HPD.

48

Figure 4.23: The multi p.e. charge distribution of a waterproof 50 cm HPD andcharge of each number of photo-electron peaks.

Figure 4.24: The variation of dark rate Figure 4.25: The relationship of a temand water temperature during continu- perature and a dark rate of a waterproof
ous operation of a waterproof HPD.
HPD.

49

Chapter 5

Conclusion
5.1

Summary of performance evaluation

We have developed the prototype of the large aperture diameter HPD and confirmed its basic performance. The evaluated performance and comparison withother photodetectors are summarized in Table 5.1.
The charge resolution of an HPD is 17.7% which satisfy the requirement
and timing resolution is 2.7 ns in TTS (FWHM) which reaches the required
value. They are better than other candidate photodetectors. The dark rate ofan HPD is ∼10 kHz at the room temperature. It is expected to reach the HK
requirement (∼4 kHz) at the water temperature supposed for HK operation.
The afterpulse rate is less than 2.5% and it satisfy the requirement. It is better
than that of B&L PMT and the same level as SK PMT. The single photon
detection eﬃciency is measured relatively and confirmed that it is better thanthat of SK PMT and the same level as B&L PMT. It clears the requirement.
An HPD has the rate tolerance up to 1.2 MHz. It is less than the required value,
but the required value is conservative and 1 MHz rate tolerance is suﬃcient todetect any supernova burst events. In order to confirm the waterproof assemblyof an 50 cm HPD for future long operation in water, we do the waterproof HPD
test and it works stably for two weeks with applying high voltage.

50

Table 5.1: The summary of the evaluated performances of the 50 cm HPDs andcomparison with SK PMT and B&L PMT.
Required Value SK PMT B&L PMT HPDRise Time [ns]
10.6
7.6
13.4
Fall Time [ns]
13.2
16.0
59.2
FWHM [ns]
18.8
12.2
41.3
Charge resolution [%]
∼50
53
35
17.7
Timing resolution [ns]
∼2.4
7.0
4.0
2.7i
Dark rate [kHz]
∼4
4ii
8iii
10iv
Afterpulse [%]< 15%
1 ∼ 13< 2.3 < 2.5
Detection eﬃciency (rel1
1
1.85
1.76
ative)
Rate tolerance [MHz]
10
87
1.2

5.2

Remaining issues

For the requirement mentioned Section 2, dynamic range, magnetic field tolerance, lifetime and pressure rating is not evaluated in this thesis. The outputof the 50 cm HPD is linear up to 100 photoelectron signals. This output limitof an HPD comes from the operating voltage of the preamplifier. The dynamic
range will be improved in a next version of the preamplifier. The magnetic field
tolerance is evaluated by other measurement [28]. The lifetime will be evaluatedin the future long term validity test in water. To test the pressure rating, we
can use existing facilities which is used for the pressure rating test of B&L PMT
and the procedure is established.
Additionally, to make an 50 cm HPD practical for use in a water Cherenkov
detector, there are several remaining issues.
Cathode transit time diﬀerence
The cathode transit time diﬀerence (CTTD) of 50 cm HPD is not uniform.
There are about 2 ns fluctuations. To improve the CTTD of an HPD, it isnecessary to redesign the shape of the glass bulb. Current shape of the 50
cm HPD is the same as that of 50 cm B&L PMT and it is not optimized for
a 50 cm HPD. Because the structure of an HPD is simpler and the numberof components is less than that of a PMT, the short bulb shape as shown in
Fig. 5.1 is suitable for an HPD. By optimizing the shape of the glass bulb, we
can improve not only CTTD but also collection eﬃciency and magnetic field
tolerance.
i at the maximum AD bias voltage (∼570 V).

ii at water temperature (∼16C◦ ).

iii at water temperature (∼16C◦ ).
iv at room temperature (∼25C◦ ).

51

Flasher
An HPD emits photons and becomes a flasher. A flasher becomes backgroundin a water Cherenkov detector and has a bad influence on other photodetectors.
The mechanism of HPD flasher is considered such that electrons are emitted
from the aluminum vapor-deposited on an end surface, creeping the surfaceof the bulb, shot to the stem of the bulb to generate photons. To solve thisproblem, it is proposed that chromium is vapor deposited on an end surface andsuppress the electron emission.

Figure 5.1: The shape of the glass bulb.

5.3

Future prospect

An HPD can satisfy the basic requirement for HK except for dynamic range.
It is improved for the next version of the preamplifier. Additionally, there areroom to improve other performances of the preamplifier. By tuning parameters
and reducing the noise of preamplifier signal output, the timing resolution canbe improved and the lower threshold level can be set which result in increasing
the single photon collection eﬃciency. It enhances the sensitivity of short timeor low energy physics events like the p → ν̄K + mode because the lifetime of
the kaon is ∼12 ns and the energy of the nuclear de-excitation γ ray is ∼6.3
MeV. By making the fall time of the signal waveform shorter, the rate tolerance
will be improved. It makes possible to observe the supernova explosion near theearth and study the mechanism more presicely.
It is necessary to study which configuration is most suitable for HPD to
use in HK. 1ch HPD and single channel read-out is assumed to have higher
collection eﬃciency because of no insensible field on the AD due to no segment
meshes and lower amount of DAQ data than multi-channel HPD. On the other
hand, the multi-channel HPD, especially for 5ch HPD, and each channel readout is assumed to have the position sensitivity on the photocathode because theincident photon which hit the center (surrounding) of the photocathode is likely52to be collected to the center (surrounding) of the AD. By using this information,
it is considered that we can study the event near the tank wall more precisely
and expand the fiducial volume of HK. These studies are ongoing[28] and will
be continue.
The most influential factor to make a 50 cm HPD practicable is to eliminate
flasher. Therefor, we plan to perform R&D for suppressing flasher and this will
be finished in one year. After the elimination of flasher, we will do the validity
test for a long term in water to ensure that an HPD is suitable to be used in awater Cherenkov detector.

53

Acknowledgements
I would like to express the deepest appreciation to Prof. Masashi Yokoyama forgiving an opportunity to research in high energy physics and his support in mystudies.
I owe a very important debt to Dr. Yasuhiro Nishimura for teaching how to
operate HPD and take data, Dr. Akimichi Taketa for teaching how to design
and fabricate the preamplifier of HPD.
Special thanks to the members of Hamamatsu Photonics K.K., Dr. Yoshihiko
Kawai, Takayuki Ohmura and Masatoshi Suzuki, especially for the developmentof HPD.
I am deeply grateful to the staﬀ of Kamioka, Prof. Masato Shiozawa, Prof. Yoshinari Hayato, Dr. Hidekazu Tanaka, Dr. Shoei Nakayama, Dr. Takatomi Yanofor the help of the research in Kamioka and supervision.
Many thanks to the student at Kamioka, Seiko Hirota, Yuji Okajima, Daisuke
Fukuda, Miao Jiang, Ryosuke Akutsu, Fumimasa Muto, Yutaro Sonoda for theenjoyable life in Kamioka. Without their help, I could not perform my researchin Kamioka.
I would like to thank the members in Aihara-Yokoyama laboratory, Prof. Hiroaki Aihara, Dr. Yoshiyuki Onuki, Nobuhiro Shimizu, Yusuke Suda, Taichiro
Koga, Junya Sasaki, Yifan Jin, Fuminao Hosomi, Hiroko Niikura, Naruhiro
Chikuma, Alejandro Mora Grimaldo Johnny, Wan Kun, Tatsuki Ose, Riku
Tamura for my fruitful life in Hongo. I also would like to thank Kuniko Kono
who supports our laboratory for her fantastic assist.
Finally, I would like to express my gratitude to my parents for continuously
supporting and encouraging me.

54

Appendix A

Long term measurement ofdark rate of Hyper-K
photodetectors
As a candidate of photodetector for Hyper-K, the B&L PMT is well establishedand satisfies the performance requirements except for the dark rate. The dark
rate of photodetectors is one of the most important factor to determine the whole
HK detector performance, especially for the newly developed reconstruction
algorithm like neutron tagging [29].
The dark rate of a PMT depends on the applied high voltage, the trigger
threshold level, the temperature and the period kept in a dark environment.
In order to study whether the dark rate of B&L PMT decreases after a long
operation in a dark room and the eﬀects of threshold and temperature, we have
performed a long-term measurement of PMTs at an underground room in ICRR,
Kashiwa, from the end of 2015.

A.1

Setup

Fig. A.1 and A.2 show the setup in the dark room. The photodetectors are stored
in a cardboard case and located in a shelf. A light source is installed in front ofeach photodetector and we can measure the dark rate and charge simultaneously.
The temperature can be controlled by the air-conditioning system of the dark
room.
Table A.1 shows the summary of photodetectors used in this long measurement. The measurement can be divided into two terms by installed photodetectors. First term is from October 2015. We use two SK PMTs and two B&L
PMTs. Second term is from August 2016. We installed additional five newly
developed B&L PMTs.

55

Figure A.1: Image of setup in dark room.

Figure A.2: The setup of DAQ system for long measurement.

56

A.2

Result from October 2015 to September 2016

Fig. A.3 shows the time variation of gain and dark rate of photodetectors from
October 2015 to September 2016. The discontinuity around May 2016 is due tothe change of applyed high voltages to match the gain of photodetectors to thecriterion used in SK (1 p.e. ∼2.2 pC, gain∼ 1.4 × 107 ). After changing the HV,
the gain and dark rate of photodetectors are stabilized.
In table A.1, the averaged dark rate is summarized. After about half a year
operation in a dark room with applying high voltage, the dark rate of SK PMTs
(CD9092 and AB7836) become around 5 kHz, which are the same level of PMTs
in SK, while that of B&L PMTs (EA0047 and ZB8208) is more than 10 kHZ
and does not satisfy the requirements of HK.

Figure A.3: The time variation of gain (upper) and dark rate (lower). The left
column shows the whole history of gain and dark rate and the right column
shows the history after May 2016.

57

A.3

Result from August 2016 to November 2016

Fig. A.4 shows the variation of the dark rate after installing additional five B&L
PMTs. Until the end of August 2016, the dark rates had been changing due
to stabilization. The variation at the beginning of September is caused by thechange of the temperature in the dark room. After September 8, the dark rates
stabilized. The averaged dark rate of newly installed PMTs is 7.6 kHz andimproved compared with old versions, but does not reach the HK requirement(∼4 kHz).

Figure A.4: The time variation of the dark rate after installing newly developed
B&L PMTs

Table A.1: Summary of the type of measured photodetectors and averaged dark
rate after stabilization. WPASSY is short for waterproof assembly.
Serial No.
Type
Installed time Averaged dark rate [kHz]AB7836
SK PMT
October 2015
4.4
CD9092””
6.1
ZB8208
B&L PMT”
21.7
EA0047””
11.7
EA0225
B&LPMT WPASSY August 2016
6.5
EA0241””
8.8
EA0248””7.8
EA0250””
7.4
EA0270””7.6

58

We measure the temperature dependence of dark rate using the temperaturechanging period. The results are shown in Fig. A.5. We also measure the highvoltage and trigger threshold level dependence of the dark rate as shown in
Fig. A.6. By cooling a PMT, the dark rate of a PMT decreases in accordancewith Richardson’s law for thermionic emissionrR (T ) = AT 2 exp(−

W
)
kT

(A.1)

while when the temperature goes below zero degree celsius, non-thermal eﬀects
becomes non-negligible and the dark rate follows
rnt (T ) = GAc exp(−

T
)
Tr

(A.2)

where A is a normalization factor, W is work function [eV], k is the Boltzmann
constant (= 8.617 [eV/K]), G is an empirical constant (= 5 ± 1 [kHz/cm2]), Ac
is the cathode area [cm2] and Tr is the adjusted slope of the data (= 100 [K])[30].
To reduce the dark rate by half, we are improving B&L PMTs with Hamamatsu Photonics. The dark rate is evaluated roughly in Hamamatsu for a few
weeks and precisely in Kashiwa for the period of monthly basis. The environment of these two measurement is diﬀerent. Therefore, we are planning to fill
the gap by using the data of dark rate dependence on the temperature, highvoltage and threshold level.

Figure A.5: The temperature dependency of PMT dark rate.

59

Figure A.6: The high voltage and trigger threshold level dependency of the dark
rate.

60

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