Research and Development of Large-Aperture
Hybrid Photo-Detectors for Hyper-Kamiokande
Yusuke Suda
Department of Physics, University of Tokyo
January 7, 2014

Abstract
We have developed the prototype of the 8-inch Hybrid Photo-Detector (HPD)
and the high quantum efficiency (QE) 20-inch photomultiplier tube (PMT) for
Hyper-Kamiokande (Hyper-K) which is proposed as a next generation Megaton
class water Cherenkov detector. Our goal is to develop the high-QE 20-inch
HPD which satisfies the requirements to photosensors in Hyper-K.
We evaluated the performance of the 8-inch HPD. The timing resolution for
single photoelectron, the single photocelectron charge resolution, and the dark
rate at 0.5 photoelectron threshold are measured to be 1.2 ns (1 σ), 12 %, and2 kHz, respectively.
To ensure the usability of HPD and high-QE PMT, we evaluate the performance in a 200 ton water tank as a sensor for a water Cherenkov detector. For
the test in the tank, we checked the durability and safety in water of the 8-inch
HPD. No trouble has been found. We calibrated the gain of the sensors before
installation. Then, we installed eight 8-inch HPDs, five high-QE PMTs and 227
normal-QE PMTs into the tank. The installation of the sensors is successfullycompleted and the cosmic muon events were observed in the tank. The single
photoelectron resolution of the 8-inch HPD in the tank is better than that of
PMT. Because of the low temperature in the tank and closing the tank for twomonths, the dark rate of the 8-inch HPD decreases. The dark rate of the highQE PMTs measured in the tank is comparable to that of the normal ones. Thisis the first time to operate HPD and sensor which has a high-QE photocathodeas a sensor for a water Cherenkov detecter.
Through the test in the tank and more precise measurement, we plan todecide the photosensors for Hyper-K in 2016.

Contents
1 Introduction

12 Physics motivation
2.1 Physics targets . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1 Neutrino oscillation . . . . . . . . . . . . . . . . . . . . .
2.1.2 Leptonic CP violation . . . . . . . . . . . . . . . . . . . .
2.1.3 Nucleon decay . . . . . . . . . . . . . . . . . . . . . . . .
2.1.4 Astrophysics using neutrinos . . . . . . . . . . . . . . . .
2.2 The principle of water Cherenkov detectors . . . . . . . . . . . .
2.3 Hyper-Kamiokande project . . . . . . . . . . . . . . . . . . . . .

3
3
3
4
5
5
6
7

3 Photosensor R&D for Hyper-Kamiokande
3.1 Requirements to photosensors in Hyper-K . . . . . . . . . . . . .
3.2 Motivation of photosensor improvement . . . . . . . . . . . . . .
3.3 R&D of the Hyper-K photosensors . . . . . . . . . . . . . . . . .
3.3.1 Candidate photosensors . . . . . . . . . . . . . . . . . . .
3.3.2 High-QE photocathode development . . . . . . . . . . . .
3.3.3 Schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4 Hybrid Photo-Detector . . . . . . . . . . . . . . . . . . . . . . . .
3.4.1 The principle of HPD . . . . . . . . . . . . . . . . . . . .
3.4.2 The 8-inch HPD system used in this thesis . . . . . . . .
3.5 Goals of this thesis . . . . . . . . . . . . . . . . . . . . . . . . . .

9
9
10
10
10
11
11
13
13
13
14

4 Performance evaluation of 8-inch HPD
19
4.1 Output signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.2 Total Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.3 Dark rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.4 Timing resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.5 Charge distribution . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.6 Rate tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.7 Stability of gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.8 Linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.9 Response uniformity . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.10 Summary and next prototype of HPD . . . . . . . . . . . . . . . 30

i

CONTENTS

5 Test in the water Cherenkov detector
33
5.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.2 Pre-test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5.2.1 Durability test of the 8-inch HPD . . . . . . . . . . . . . 34
5.2.2 Soak test of the 8-inch HPD . . . . . . . . . . . . . . . . . 34
5.2.3 Pre-calibration . . . . . . . . . . . . . . . . . . . . . . . . 35
5.3 Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
5.4 DAQ system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
5.5 The measurement in the water tank . . . . . . . . . . . . . . . . 45
5.6 Summary and future plan . . . . . . . . . . . . . . . . . . . . . . 45
6 Summary and future prospects
53
6.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
6.2 Future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

ii

Chapter 1

Introduction
In this thesis, we report the performance of 8-inch Hybrid Photo-Detector(HPD) and the test in a 200-ton water Cherenkov detector. The performanceof high quantum efficiency (QE) 20-inch photomultiplier tube (PMT) is alsopresented. These are developed for Hyper-Kamiokande (Hyper-K), which is
proposed as a next generation Megaton class water Cherenkov detector [1].
The road to new physics is opened by the discovery of neutrino oscillation in
the atmospheric neutrino by Super-Kamiokande in 1998 [2]. Since then, many
experiments to explore neutrino oscillation parameters have been conducted.
Recently, the last mixing angle θ13 has been measured by T2K [3], Daya Bay[4], RENO [5] and Double Chooz [6]. Non-zero θ13 opens the opportunity tomeasure the CP phase δ in the lepton sector which is the last neutrino oscillation
parameter. Other than that, there are many Grand Unified Theories (GUTs)
proposed as beyond the standard models. Most GUTs predict nucleon decays.
Super-Kamiokande has found no evidence of nucleon decays. Astrophysics using
a supernova burst neutrino and a solar neutrino is needed to study the life ofstar. For these study, Hyper-K is strongly desired.
We have been developing the high-QE 20-inch HPD as a photosensor for
Hyper-K detector. HPDs are much simpler in structure and have better performance than conventional PMTs. In order to detect as many photons as
possible, the sensor is desired to have as high quantum efficiency as possible.
As a first step, we have developed the prototype of 8-inch HPD and the highQE venetian blind dynode 20-inch PMT. Through the R&D of these sensors, weevaluate HPD and high-QE technique and give a feedback to the developmentof the high-QE 20-inch HPD. In this thesis, the status of photosensor R&D forHyper-Kamiokande is presented.
Our physics targets and overview of Hyper-K are presented in Chapter 2.
The requirements to photosensors, overview of our R&D and the principle andsystem of 8-inch HPD are shown in Chapter 3. In Chapter 4, we describe theperformance evaluation of 8-inch HPD. We conduct the test in a 200-ton water
Cherenkov detector to evaluate the performance of new photosensors as thesensor for a water Cherenkov detector. Overview of the test, some preparation,
1

CHAPTER 1. INTRODUCTION

the installation and the measurement in the tank are discussed in Chapter 5. We
summarize our study and explain the future prospect of our R&D in Chapter 6.

2Chapter 2

Physics motivation
In this chapter, we describe the physics motivation for the Hyper-Kamiokande
project and what we need to realize the Hyper-Kamiokande.

2.1

Physics targets

We briefly explain the physics targets of Hyper-Kamiokande.

2.1.1

Neutrino oscillation

Here we describe the neutrino oscillation phenomenon. Assuming that neutrinos
have finite masses and the flavor eigenstates of neutrinos are different from themass eigenstates of neutrinos, the flavor eigenstates are expressed as:Xαi|να i =
UM
(2.1)
N S |νi i ,iwhere |να i (α = e, µ, τ ) is the flavor eigenstate, |νi i (i = 1, 2, 3) is the mass
αieigenstate, and UM
N S is an element of the 3 × 3 unitary matrix called the MNS(Maki-Nakagawa-Sakata) matrix [7]. The MNS matrix is expressed with four
independent parameters:




1
0
0
c13
0 s13 e−iδ
c12 s12 0
0 c23 s23   0
1
0  −s12 c12 0
UM N S =0 −s23 c23
0
0 1−s13 eiδ 0
c13
(2.2)


−iδ
c12 c13s12 c13s13 e
s23 c13  ,= −s12 c23 − c12 s23 s13 eiδ c12 c23 − s12 s23 s13 eiδ
iδiδs12 s23 − c12 c23 s13 e−c12 s23 − s12 c23 s13 e
c23 c13(2.3)
where θ is a mixing angle, cij = cos θij , sij = sin θij and δ is a CP violatingphase. CP violation in the lepton sector occurs if δ 6= 0. For the simplicity, here3CHAPTER 2. PHYSICS MOTIVATION

we consider two neutrino flavors να and νβ , and two neutrino mass eigenstates
ν1 and ν2 . We can rewrite Eq. 2.3 with a mixing angle θ:



 


|να i
|ν1 i
cos θ
sin θ
|ν1 i
=U
=
.
(2.4)
|νβ i
|ν2 i
− sin θ cos θ|ν2 i
In vacuum, the time evolution of the mass eigenstate is given as follows:|νi (t)i = e−i(Ei t−pi x) |νi (0)i(2.5)

where Ei and pi are the energy and momentum of νi . Assuming that theneutrino are highly relativistic:t=

LL≈ = L, pi =vcq

Ei2 − m2i ≈ Ei −

m2i
,
2Ei

where L is the position of the neutrino. Eq. 2.4 is written as: m2 L





1
−i 2E
1
|να (L)ie0−1 |να (0)i


=U
.
U
m2
2L
|νβ (L)i|νβ (0)i0e−i 2E2

(2.6)

(2.7)

We can calculate the oscillation probability between two flavors as follows:
P (να → νβ )= | hνβ (0)|να (L)i |2∆m221 L2
2
= sin 2θ sin4E
1.27∆m221 (eV2 )L(km)= sin2 2θ sin2,
E(GeV)(2.8)(2.9)(2.10)

where ∆m221 = m22 − m21 and we use E = E1 ≈ E2 . If neutrino oscillation
occurs, at least one of neutrinos has finite mass.
The finite mass of neutrino is not included in the standard model of theelementary particle. In 1998, Super-Kamiokande (Super-K) experiment discovered the neutrino oscillation with the measurement of the atmospheric neutrinos.
This is the first evidence that the new physics exists.
To build up the beyond the standard model, we want to know all of theelements of the MNS matrix. These elements can be measured by the neutrino
oscillation.

2.1.2

Leptonic CP violation

Now, we know the approximate value of all mixing angles, θ23 , θ12 and θ13 . Only
one parameter in the MNS matrix, the CP phase δ is still unknown. We havean opportunity to measure the δ thanks to the large angle of θ13 . CP violationin the lepton sector may be a key to solve the problem of the matter-antimatterasymmetry in our universe.
4CHAPTER 2. PHYSICS MOTIVATIONThe effect of the CP phase appears in the difference between the transition
probabilities P (να → νβ ) and P (ν̄α → ν̄β ). The most promising channel todetect the difference in transition probabilities is νµ (ν̄µ ) → νe (ν̄e ). Acceleratorbased neutrino oscillation experiment can measure these two channels. T2K(Tokai to Kamioka) experiment and NOvA (NuMI Off-Axis νe Appearance)experiment [8] are the accelerator-based neutrino oscillation experiments. In
T2K, the νµ (ν̄µ ) beam is produced by the J-PARC accelerator located in Tokaimura, shooting them into the Super-K and detect them. We expect that thefull sensitivity of the CP phase combined with the result of reactor neutrino
experiments is around 2σ.
To determine the CP phase, we need more statistics of neutrino events thanthat of T2K expected. A larger volume water Cherenkov detector than Super-K
and higher intensity neutrino beam are strongly required.

2.1.3

Nucleon decay

Grand Unified Theories (GUTs) are proposed as a candidate of a framework
beyond the standard model. GUTs unify the strong and electroweak forces.
Most GUTs allow baryon number violating interactions and predict nucleon
decays.
Super-Kamiokande has been searching for the nucleon decays such as p →e+ + π 0 and p → ν̄K + since 1996. Super-K is the largest detector for theobservation of nucleon decay in the world. But no evidence has been foundin Super-K. The current limit of proton partial lifetime is 1.4 × 1034 years for
p → e+ + π 0 .
One of GUTs, SUSY SO(10), predicts that the limit of proton lifetime isorder 1035 years. To detect the nucleon decay and determine GUTs, we need alarger detector than Super-K.

2.1.4

Astrophysics using neutrinos

At the end of life of massive stars, they explode and emit many neutrinos called
the supernova burst neutrinos. The energy of the supernova burst neutrinos
is expected to be about 10∼30 MeV. The observation of the supernova burst
neutrinos gives us a clue to explore the mechanism of supernova explosion. On
the 23rd of February 1987, the Kamiokande II [9], IMB [10] and Baksan [11]experiments observed the supernova burst neutrinos from SN1987A located in
the Large Magellanic Cloud. Neutrino astronomy starts from this discovery.
Unfortunately, no supernova explosion has been observed since the neutrino
burst of SN1987A. But, recently, it is said that Betelgeuse might be explodeand emit the supernova burst neutrinos.
The neutrino from the sun produced by the process of the nuclear fusion iscalled the solar neutrino. The energy of the solar neutrinos is below 20 MeV.
By observing the solar neutrino, we can study the state of inside the sun inreal time. The standard solar model is tested by the observation of the solar
neutrinos.
5CHAPTER 2. PHYSICS MOTIVATION

To detect the supernova burst neutrinos and the solar neutrinos as many as
possible, we need a large detector which can be observe MeV order neutrino.

2.2

The principle of water Cherenkov detectors

Super-K is one of water Cherenkov detectors. Water Cherenkov detectors mainly
consist of a tank to store water and many photosensors on the wall of the tank.
When a charged particle travels faster than the speed of light in the water, itemits Cherenkov light. The Cherenkov light propagates at a constant angle θ
specific to the velocity of the charged particle, which is written as:cos θ =

1,βn(2.11)

where β is the speed of the charged particle in unit of c and n = 1.33 is therefractive index of water (Fig. 2.1). The number of Cherenkov photons dN
generated per unit distance the charged particle travels dx per unit wavelengthdλ is calculate as:


d2 N
2πα
1
= 2 1− 2 2 ,(2.12)
dxdλλn βwhere α is the fine structure constant. If we use Super-K PMT which is sensitive
to the wavelength range of 300 ∼ 600 nm in a water Cherenkov detector, thenumber of observable Cherenkov photons is about 340 in water per 1 cm track
length for the charged particle which has a unit charge and velocity of β ≈ 1.
The emission of Cherenkov photons keeps on until the energy of the charged
particle E becomes as:
nmm=√
Ethr = p,(2.13)
1 − ( vc )2n2 − 1
where m is the mass of the charged particle. For electron, Ethr should be
0.78 MeV. We can roughly calculate the number of Cherenkov photons induced
by a 10 MeV electron is about 1500 photons. In Super-K II which has photocoverage 20 %, the light yield is about 2.8 photoelectrons/MeV. The photo-coverageand the quantum efficiency of the photosensor are critical for the study of low
energy neutrino.
The photosensors detect a ring image of the Cherenkov light. From thetiming and the total amount of light, we can identify the particle and reconstruct
the energy momentum vector of the particle. When a neutrino comes fromoutside the tank, it might interact with H2 O. Then, a corresponding charged
lepton is produced via charged current interaction. Using the photosensors, we
can detect the neutrino. Electron- and muon-like event can be distinguishedby the shape of the Cherenkov ring. In case of electron-like event, it will besmeared by the cascade shower.
The major advantage of to use water Cherenkov detectors is that it is easierto prepare a target (water) with large effective volume than other detectors suchas liquid scintillators or liquid argon time projection chambers.
6

CHAPTER 2. PHYSICS MOTIVATION

Cherenkov light

Charged particle

Figure 2.1: Sketch of that the charged particle which travels faster than the
speed of light emits the Cherenkov light in water.

2.3

Hyper-Kamiokande projectA larger water Cherenkov detector is needed for more precise measurement of
neutrino oscillation parameters, the measurement of CP asymmetry, search fornucleon decay, the observation of the supernova burst neutrino and the solar
neutrino and so on. For that purpose, Hyper-Kamiokande (Hyper-K) detector
is proposed as a next generation underground water Cherenkov detector.
Hyper-K has 0.99 (0.56) Megaton total (fiducial) mass, which is about 20(25) times larger than that of Super-Kamiokande (Fig. 2.2) [12]. The candidate
site is Kamioka mine, Gifu, Japan. There are a lot of pure water and hard
bedrocks in Kamioka mine. It is suitable to construct a Megaton class water
Cherenkov detector. Figure 2.3 shows a cross section of Hyper-K tank. The
tank consists of an inner detector and an outer detector. The outer detector
is a veto part for cosmic muon. In the inner detector, we need 99,000 20-inch
photosensors. Also in the outer detector, 25,000 8-inch photosensors are needed.
The photo-coverage which indicates a photosensitive area in the inner tank is
20 %. This is half photo-coverage of that of Super-K.
Since we need huge number of photosensors, the R&D of new photosensors
is critical to the Hyper-Kamiokande project.

7

CHAPTER 2. PHYSICS MOTIVATION

Figure 2.2: Hyper-Kamiokande detector [1].

Figure 2.3: Cross section of Hyper-Kamiokande detector [1].

8Chapter 3

Photosensor R&D forHyper-Kamiokande
3.1

Requirements to photosensors in Hyper-K

Major requirements to photosensors in Hyper-K are as follows:
Good timing resolution
Timing resolution is essential to reconstruct the vertex of neutrino interaction. Considering the transit time spread (TTS) of the single photoelectron of PMTs used in Super-K is 2.2 ns, we require the TTS of the single
photoelectron of the photosensor for Hyper-K to be better than 2.2 ns.
High single photoelectron sensitivity
In order to observe a low energy neutrino event which is induced by a
MeV order neutrino, the photosensor needs high single photoelectron sensitivity. We require the single photoelectron resolution of the photosensor
for Hyper-K to be better than that of Super-K PMT 60 %.
Large photosensitive area
This is required to achieve good energy resolution.
High Quantum Efficiency (QE)
In order to detect as many photons as possible, the photosensor is desired
to have as high quantum efficiency as possible.
Low dark rate
From the point of view of the data acquisition (DAQ) system and low
energy physics capability, the dark rate of a photosensor must be lessthan ∼ 4 kHz.
Wide dynamic range
In order to study very high energy event, a dynamic range up to ∼1, 000 photoelectrons is required.
9CHAPTER 3. PHOTOSENSOR R&D FOR HYPER-KAMIOKANDEHigh rate tolerance
In order to record a supernova burst neutrino event without event loss,
the sensor needs to be capable of handling high rate signal up to 1 MHz.
Simple structureHigh assembling precision of photosensors and high productivity are expected with a simple structure. This is important for cost reduction inmass production because we need 0.1 million photosensors.
The sensitivity of the p → ν̄K + event will be improved if the timing resolution of new photosensors became better than Super-K PMT. In the p → ν̄K +event, a proton in 16 O decays as follows:
16

O

→

15

+

→

νµ + µ+

K

+

µ

3.2

→

N + γ + ν̄ + K +

(3.1)
(3.2)

+

ν̄µ + νe + e .

(3.3)

Motivation of photosensor improvement

The performance of photosensor is one of key parameters to determine the performance of a water Cherenkov detector. For example, a better timing resolution
will result in a better vertex resolution, which in turn will lead to better reconstruction of particle properties from the Cherenkov light pattern. More direct
improvement is expected for several physics topics. One of such examples is
the search for the p → ν̄K + decay. In a water Cherenkov detector, K + fromp → ν̄K + decay cannot be directly detected because its momentum is below the
Chrenkov threshold and decay products are used to search for this decay mode.
In addition to a 236 MeV/c µ+ and its subsequent decay positron, tagging a
6.3 MeV γ ray emitted by the deexcitation of resulting 15 N nucleus provides arather clean signal of this decay in an oxygen nucleus. In this technique, theCherenkov light from 6.3 MeV γ should be distinguished from that of 236 MeV/cµ+ , which is separated by the K + decay time (τK + =12 ns). Thus, with a bettertiming resolution of photosensor, the efficiency of this decay mode is expected
to improve.

3.3

R&D of the Hyper-K photosensors

In this section, we explain the three candidates of Hyper-K photosensor, thegoal, schedule and the status of our R&D.

3.3.1

Candidate photosensors

There are three photosensors as candidates for photosensors in Hyper-K as
follows.

10

CHAPTER 3. PHOTOSENSOR R&D FOR HYPER-KAMIOKANDE

Table 3.1: The comparison of performance of candidate photosensors. ∗ indicates the circulated value.

20-inch type
HV
Gain
TTS (FWHM)
Collection Efficiency∗

Venetian Blind PMT

Box&Line PMT

HPD

2kV
107
∼5.5 ns
80%

2kV
107
∼ 2.7 ns∗
93%

8kV
104−5
∼0.75 ns∗95%

Venetian Blind dynode type PMT (R3600)
This is employed in Super-K. The performance is well confirmed.
Box&Line dynode type PMTThis is a new type of PMT which has a better timing resolution thanR3600.
Large-Aperture Hybrid Photo-Detector (HPD)
An HPD uses avalanche diode instead of dynodes to multiply the photoelectrons. The development of HPD is the main topic of this thesis.
Figure 3.1 shows the drawings of three candidates. The major difference between HPDs and PMTs is that HPDs substitute avalanche diode for dynodes
in PMTs to multiply the photoelectrons. Because of the difference, HPDs aremuch simpler in structure, have a better timing resolution and better resolutionof single photoelectron than PMTs. Table 3.1 shows the comparison of eachphotosensor. Transit time spread (TTS) is an index of the timing resolution.

3.3.2

High-QE photocathode development

As a common option for three candidates, we have been developing a high-QE
photocathode. Hamamatsu Photonics K.K. has recently developed a high-QE
R3600 which has 30 % QE, 1.5 times larger than that of Super-K PMT (normalQE R3600). The QE spectra as a function of wavelength for eight high-QE
R3600s and a typical value of normal-QE PMTs are shown in Fig. 3.2. First,
we evaluate the high-QE R3600. Then, we apply this high-QE technique toother candidates of Hyper-K photosensor.

3.3.3

Schedule

Figure 3.3 shows a schedule of R&D of photosensors. To select the Hyper-K
photosensor, we have to verify the performance and usability of the box&line
PMTs, HPDs and high-QE R3600s. We evaluate the basic performance of eachphotosensor. Then, we will test the usability of photosensors for Hyper-K in

11

CHAPTER 3. PHOTOSENSOR R&D FOR HYPER-KAMIOKANDE

508mm

508mm

20-inch Venetian
Blind dynode type
PMT (R3600)

508mm

20-inch Box &20-inch HPDLine dynode type
PMT

QE [%]

Figure 3.1: Three candidates of Hyper-K photosensors.

40

High-QE R3600

35

ZP0007
ZP0012
ZP001430
25

ZP0015
ZP0021×1.5

ZP0022
ZP0024
ZP002520

Normal SK PMT

15

Normal-QE R3600

10
5
0

300

350

400

450

500

550

600 650 700
Wave Length [nm]Figure 3.2: QE vs. wavelength for high-QE and normal-QE R3600.

12

CHAPTER 3. PHOTOSENSOR R&D FOR HYPER-KAMIOKANDE

We are here

20-inch
HPD

Preparation

Test in the water
Cherenkov detector

8-inchHPD20-inchB&L PMT
High-QE
R3600
2012

2013

2014

2015

Selection of Hyper-K Photosensor

Performance evaluation
Prototype production

2016

Figure 3.3: Schedule of R&D of Hyper-K photosensors.

a 200-ton water Cherenkov detector for 1 ∼ 2 years. Before starting the test,
we have some preparation such as pre-calibration, safety check and installation.
The detailed performance evaluation is carried out in parallel with the test in
the water Cherenkov detector. We plan to decide the photosensors for Hyper-K
in 2016.

3.4

Hybrid Photo-Detector

In this section, we describe the principle of HPD and the design of 8-inch HPD
system which we are currently using.

3.4.1

The principle of HPD

The HPD consists of a phototube and an avalanche diode (AD). AD is a kind of
semiconductor device. Figure 3.4 shows the principle of our HPD. An electron
produced by photoelectric effect is accelerated by a high electric field (∼ 8kV)
from the photocathode to the AD. Many electron-ion pairs are created inside
the AD via the ionization loss of the bombardment electron. We obtain thebombardment gain ∼ 1, 600. Then, electrons and ions are amplified by a AD
bias voltage, giving the avalanche gain 30 ∼ 100. With the two multiplication
mechanisms, the HPD realizes a gain of order 105 .

3.4.2

The 8-inch HPD system used in this thesis

The HPD used for the study described in this thesis is equipped with a preamplifier and a high voltage (HV) module inside the housing as shown in Fig. 3.5.

13

CHAPTER 3. PHOTOSENSOR R&D FOR HYPER-KAMIOKANDE

Photon

8kV

e-

e-

Bombardment gain
× ~1600
200~300V

AD

Avalanche gain
× 30~100

Figure 3.4: Principle of HPDs.

Figure 3.6 shows pictures of an 8-inch HPD without any equipment. A pictureof the 5mmφ AD which is used in 8-inch HPD is shown in Fig. 3.7.
Figure 3.8 shows a schematic diagram of the circuit inside an HPD. The HV
module made by CAEN produces two voltages; One is the high voltage applied
between the photocathode and the AD. The other is the bias voltage of the
AD. Usually, we apply the HV 8kV and the AD bias voltage around 300 V.
The operation of the HPD in water is safe because we do not need a HV cablein water. The specification of the HV module is shown in Table 3.2. We put
filters between the AD and the HV module to eliminate high frequency noise
coming from the HV module. A protection device against electric discharge suchas a varistor is placed in next to the decoupling capacitor. We employ an I-V
inverted differential amplifier as a preamplifier to eliminate common mode noise
coming from the AD. A raw signal of AD is a positive polarity. For convenience
to use HPDs in the same condition as PMTs, it is better to invert the raw signal.
The specification of the preamplifier is shown in Table 3.3. After a decoupling
capacitor which is placed in the end of preamplifier, we get the signal of the
8-inch HPD.
To control the HV and the AD bias voltage, we use a control power supply
shown in Fig. 3.9. This power supply has four functions; One is to apply 10 V
as the power of the HV module and the preamplifier. Second is to control the
HV and the AD bias via changing the HV control voltage and the AD bias
control voltage. Monitoring the over current of the HV module, if it occurred,a LED lamp turns red. The other one is transmission of the signal from HPD
to the DAQ system. The specification of the control power supply is shown in
Table 3.4.

3.5

Goals of this thesis

The most desirable solution to Hyper-K photosensor is to develop the highQE 20-inch HPD which satisfy the requirements to photosensors in Hyper-K

14

CHAPTER 3. PHOTOSENSOR R&D FOR HYPER-KAMIOKANDE

HV module

Preampliﬁer

HPD
Figure 3.5: Picture of 8-inch HPD.

Avalanche Diode
Figure 3.6: Picture of 8-inch HPD. The front view (left) and the top view(right).

15

CHAPTER 3. PHOTOSENSOR R&D FOR HYPER-KAMIOKANDE

5mmφ

Figure 3.7: Picture of 5mmφ avalanche diode.

HPD

HV module  

I-V inverted amplifier



I-V

 

Signal out

× 3

 

I-V

  

55V/mA × 2Figure 3.8: Schematic view of the circuit inside HPD.

Table 3.2: Specification of the HV module produced by CAEN.

Item

Value

Output voltage
Supply voltage
Current
Control voltage for HV and AD bias voltage
Enable voltage

160∼10 kV for HV
0∼500 V for AD bias voltage
5V∼ 120 mA
0∼4
5 V (on) / 0 V (off)CHAPTER 3. PHOTOSENSOR R&D FOR HYPER-KAMIOKANDE

Table 3.3: Specification of the preamplifier.
Item

Value

Band width
Gain
Supply voltage
Current
Output impedance

25 MHz54 kV/A
5V∼ 15 mA50 ΩLED over current monitor
Enable switch
HV control (0~4 V)
Bias control (0~4 V)
Power (100 Vac)

Front Panel
Signal out (Lemo)
Signal in (SMA)
Control 

Back Panel
Figure 3.9: Pictures of the control power supply.

Table 3.4: Specification of the control power supply.
Item
Output voltage
Supply voltage

Value
10 V for HV module and preamplifier0∼4 V for HV and AD bias control voltage
100 Vac17

CHAPTER 3. PHOTOSENSOR R&D FOR HYPER-KAMIOKANDE

because it is expected to have better performance than the other options. But
there is no large-aperture HPD which can be used in water Cherenkov detector.
Thus, we need to develop a new sensor for Hyper-K. As a first step, we havedeveloped the prototype of 8-inch HPD with QE ∼20% and the high-QE R3600
in cooperation with Hamamatsu Photonics K.K. Through the R&D of thesesensors, we evaluate HPD and high-QE technique and give a feedback to thedevelopment of the high-QE 20-inch HPD. Moreover, the 8-inch HPD has apossibility to be the photosensor for the outer detector of Hyper-K. For thatreason, the R&D of the 8-inch HPD and the high-QE R3600 is an important
step for Hyper-K.
In this thesis, we present the measured performance of the 8-inch HPDs.
After checking the performance of the 8-inch HPD, we prepare for the test in thewater Cherenkov detector. The measurement including the high-QE R3600s in
the water tank will be presented and compared with the result measured before
installation.

18

Chapter 4

Performance evaluation of
8-inch HPD
The performance of 8-inch HPD with the preamplifier is described in this chapter.

4.1

Output signal

Figure 4.1 shows the typical output signal of 8-inch HPD with the applied HVof 8 kV and AD bias voltage of 300 V. We use Hamamatsu high-repetition
picosecond light pulsar (PLP, model PLP-10, λ = 405 nm, pulse width 70 ps(typ.)), NIM standard clock module and an oscilloscope (LeCroy, wave runner
104MXi). The HPD has excellent capability of single photon detection comparedwith PMTs. The rise time of the HPD with the preamplifier is around 10 ns
and the pulse width is around 60 ns. Due to the bandwidth of the preamplifier,the time response of the signal becomes worse. An HPD without preamplifier
has the rise time ∼1.7 ns and the fall time ∼2.7 ns.

4.2

Total Gain

Figure 4.2 schematically shows the setup for the gain measurement. We use PLP
and VME standard ADC module (CAEN mod. V265). The input light intensity
is set to multi-photoelectron level which is regarded as Gaussian distribution in
charge. The charge of the output was measured by the charge integrating ADC
with 200 ns gate width. The gain is defined by (sigma of Gaussian)2 /{(mean ofGaussian)×(electric charge)}. Figure 4.3 shows the gain curve with fixed HV of
8 kV. The total gain of the 8-inch HPD with the preamplifier is (2 ∼ 5) × 107 .

19

CHAPTER 4. PERFORMANCE EVALUATION OF 8-INCH HPD

1p.e.
2p.e.
3p.e.
5mV

4p.e.

20ns

Figure 4.1: Typical waveform of 8-inch HPD with the preamplifier.

Optical ﬁber

Pico pulse
Laser

8-inch HPD

NIM

Clock
NIM
Gate
Gate inLight shielding box

ControlPower Supply

VME
ADC

PC

Figure 4.2: Schematic view of the setup for the measurement of the output
charge.

20

CHAPTER 4. PERFORMANCE EVALUATION OF 8-INCH HPD

Gain curve

70 ×10Total gain

6

Fitting Function

60[0]/(1-(Bias/[1])^[2])p0

50-9.029e+05 ± 0.02656

p1

448.5 ± 6.346e-08

p2-0.08539 ± 2.462e-09

40
30
20
10
0
200

250

300

350
400
AD bias voltage (V)Figure 4.3: Total gain as a function of the AD bias voltage with fixed HV of
8 kV.

4.3

Dark rate

The dark rate is measured as a function of threshold. In this measurement, we
use two Analog Timing Module (ATM) boards (Fig. 4.4). ATM is a DAQ board
which was used in Super-K I, II and III. A detailed description for ATM canbe found elsewhere [13]. ATM can count up the hit and attenuate the input
pulse. The hit is defined by the signal which exceeds the threshold level. The
range of the threshold is from 0 mV to 12.5 mV. The pulse height of a single
photoelectron signal from an HPD is about 15 mV at this time. The first ATM
is used to attenuate the pulse height of HPD to 10 % of its original value andsend it to the second ATM. The second ATM counts the hit by varying thethreshold level. The temperature of measurement place is 24 ◦ C.
Figure 4.5 shows the measured dark rate of 8-inch HPD. The dark rate is
2kHz at 0.5 photoelectron. The typical dark rate of Super-K PMT is ∼4kHz
with 0.25 photoelectron threshold. When used in a water Cherenkov detector,
the dark rate of 8-inch HPD is expected to decrease with operation time in thedark environment and with lower temperature (13 ◦ C) in the water tank. It will
be checked by the long term test in the water tank described in Chapter 5.

21

CHAPTER 4. PERFORMANCE EVALUATION OF 8-INCH HPD

ControlPower Supply

8-inch HPD

ATM 1

ATM 2

Signal IN

Signal IN

PMTSUM

PC

Threshold scan
0~12.5mV
Attenuate 1/10

Light shielding box

Dark rate (kHz)

Figure 4.4: Schematic view of the setup of dark rate measurement.

Dark rate

104
103
102
10

0.5 p.e.

1 p.e.

1
10-10

0.5

1

1.5

2

2.5
3
3.5
Threshold voltage (mV)

Figure 4.5: Dark rate as a function of the threshold.

22CHAPTER 4. PERFORMANCE EVALUATION OF 8-INCH HPD

Transit time spread
450

h

400
350

Entries

4144

Mean

-0.006419

RMSχ2 / ndf

300
250
200

1.45
204 / 45Prob

4.048e-22

Constant

345.1 ± 7.6

Mean-0.1075 ± 0.0195

Sigma

1.185 ± 0.018

150
100
50
0
-6

-4

-2

0

2

4
6
Transit time (ns)Figure 4.6: Transit time distribution of single photoelectron.

4.4

Timing resolution

We evaluate the transit time spread (TTS) as a measure of timing resolution.
The definition of the transit time is the time from a trigger of a light source
to 25 % pulse height of signal. To measure TTS, we record the waveforms of
8-inch HPD using an oscilloscope and analyze them offline. We define TTS as a
sigma of the Gaussian function fitted to the histogram of the transit time. TTSof an HPD is around 1.2 ns in sigma as shown in Fig. 4.6. Note that long risetime of the preamplifier and bad signal to noise ratio make TTS of the HPD
worse. An HPD itself has 0.62 ns TTS. Thanks to the absence of dynodes, the
TTS of an HPD is better than that for Super-K PMT (2.2 ns in sigma) [14],although the sensor sizes are different. We expect the TTS of 8-inch HPD will
be improved with the optimization of the preamplifier. Figure 4.7 shows TTS
vs. number of input photoelectrons. TTS gets smaller by increasing the numberof photoelectrons.

4.5

Charge distribution

We measure the charge distribution with the same setup shown in Fig. 4.2.
The gate width of the ADC is 200 ns. The charge distribution with single
photoelectron level light input is shown in Fig. 4.8. The flat region between
the pedestal peak and the single photoelectron peak is presumed to be due toelectrons backscattered on the surface of AD. To evaluate the mean and sigma
23

CHAPTER 4. PERFORMANCE EVALUATION OF 8-INCH HPD

Transit time spread (ns)TTS vs. Number of photoelectrons
χ2 / ndf

1.2

Prob
p0
p1

1

82.1 / 2
1.484e-18
1.572 ± 0.02735-0.5058 ± 0.0163

Fitting Function[0]/sqrt(x)+[1]

0.8
0.6
0.4

1

1.5

2

2.5
3
3.5
4
Number of photoelectrons

Figure 4.7: TTS as a function of a function of the number of input photoelectrons.

of the pedestal and single photoelectron, we employ a composite function which
consists of two Gaussian and an error function. The error function is used totake account of the effect of the backscattering.
An 8-inch HPD has 12 % single photoelectron resolution defined by (sigmaof single photoelectron peak)/(mean of single photoelectron peak). We evaluate
the signal to noise (S/N) ratio which is an index of the significance of separationof the single photoelectron from the noise level of pedestal. The S/N ratio is
given by (mean of single photoelectron peak)/(sigma of pedestal peak). For this
HPD, the S/N ratio is 12. We also evaluate the peak-to-valley (P/V) ratio. It isdefined as a ratio of the charge of single photoelectron peak to the valley between
the pedestal peak and the single photoelectron peak. The P/V ratio is around5.6. On the other hand, Super-K PMT has around 60 % single photoelectron
resolution in sigma, the S/N ratio of about 2.2 and the P/V ratio of 1.9 [14]. An
HPD has better single photoelectron resolution, the S/N ratio and the P/V ratio
than Super-K PMT. The charge distribution for multi photoelectrons is shownin Figure 4.9. Clear photoelectron peak can be seen up to four photoelectrons.

4.6

Rate tolerance

We measure the response of an 8-inch HPD to high rate input by changing
the light emission frequency of the laser diode (Fig. 4.2). Figure 4.10 shows24

CHAPTER 4. PERFORMANCE EVALUATION OF 8-INCH HPD

Charge distribution
χ 2 / ndf
Prob
FactorPeak
σ0119.8 / 49
7.364e-08
1063 ± 16.3
0.08822 ± 0.00829
0.6188 ± 0.0062
Factor
169.1 ± 5.9
Peak8.003 ± 0.034
σ10.9923 ± 0.0300
p6
1.958e-30 ± 6.164e-31
p7
0 ± 1.4
p8
0 ± 1.4
1pe BS ratio
0.202 ± 0.013

1000
800
600

ADC count : 31.6581

400

P/V : 5.23989± 0.951651
Gain : 5.00172e+07± 2.13099e+08

200
0
-5

0

5

10

15
20
Charge (pC)

Figure 4.8: Single photoelectron distribution in charge.

25

CHAPTER 4. PERFORMANCE EVALUATION OF 8-INCH HPD

Multi-photoelectron distribution
χ2 / ndf
473.8 / 161
Prob
1.465e-32p0375.9 ± 6.4
p1
9.577 ± 0.057
p2
2.133 ± 0.064
p3
587.1 ± 7.0
p4
18.8 ± 0.1
p53.304 ± 0.122
p6
652.2 ± 14.7
p7
27.8 ± 0.1
p8
3.082 ± 0.149
p9510.5 ± 39.8
p10
36.37 ± 0.20
p11
3.413 ± 0.285
p12
389.9 ± 8.2
p13
46.07 ± 0.45
p14
5.003 ± 0.634

700
600
500
400
300
200
100
0

0

20

40

60

80
100
Charge (pC)

Figure 4.9: Multi-photoelectron distribution in charge.

the measured output charge as a function of input light frequency with threedifferent light intensity. HPD has rate tolerance up to 500 kHz within 5 %variation. The rate tolerance of the current HPD is limited by the response of
preamplifier. It will be improved in the next version of the preamplifier.

4.7

Stability of gain

We measure the stability of gain of 8-inch HPD by continuously measuring
the pulse height of the output using an oscilloscope. Figure 4.11 shows themeasured stability. The horizontal axis shows the time from enabling the HVand the vertical axis shows the pulse height normalized to its average. We cansee the gain is stable within ±0.5 % fluctuation in 50 hours.

4.8

Linearity

Linearity is measured by varying the input light intensity using PLP and VME
ADC. We use an APD module (Hamamatsu C5331-11) as a light intensity monitor. Figure 4.12 shows the output charge as a function of the number of the
input photoelectrons. The number of the input photoelectron is estimated fromthe ADC count of APD normalized to the value when the light intensity is single
photoelectron level for HPD. Linearity holds up to around 60 photoelectrons,
26

Charge (pC)

CHAPTER 4. PERFORMANCE EVALUATION OF 8-INCH HPD

Rate tolerance

600

120pC
350pC
500pC

500
400
300

Stability

200
100
0

500kHz-100 3
10

104

5

10

6

10
Frequency (Hz)Normalized pulse height

Figure 4.10: Output charge of 8-inch HPD as a function of input light frequency
with three different light intensity.

Stability

1.05
1.04
1.03
1.02
1.01
1
0.99
0.98
0.97
0.96
0.95

20

30

40

50

60
Time (hour)

Figure 4.11: Normalized pulse height vs. time.

27

CHAPTER 4. PERFORMANCE EVALUATION OF 8-INCH HPD

Charge (pC)

Linearity
400
350

χ2 / ndf
Prob
p0
p1

8.904 / 4
0.0635428.95 ± 0.9784
5.214 ± 0.03767

300
250
200
150
100
50

10

20

30

40

50
60
70
80
Number of photoelectrons

Figure 4.12: Charge vs. number of photoelectrons.

which is limited by the saturation of the preamplifier. This is worse than SuperK PMT, whose output is linear up to 200 photoelectrons [15]. This output limitof HPD comes from the rated voltage of the preamplifier. The dynamic range
will be improved in a next version of the preamplifier.

4.9

Response uniformity

The setup for response uniformity measurement is shown in Fig. 4.13. Uniformity is measured by changing the position of the optical fiber, attached onto thephotocathode. We set the input light intensity to larger than 10 photoelectrons
which is regarded as Gaussian distribution in charge and maintain the intensity
during this measurement. A 1-inch PMT (Hamamatsu H7415) is employed asa light intensity monitor and an HPD is placed in a µ-metal shielding box toprevent an effect of the terrestrial magnetism. The HV of each HPD and PMT
is turned off when we open the shielding box. After 1 min from switching on
the HV, each data taking is started. Figure 4.16 and 4.17 show the response
uniformity over the photocathode along the X and Y-axis respectively. Relative output charge is defined by (HPD mean charge (pC))/(monitor PMT mean
charge (pC)). Relative output charge is uniform from center to 560 mm within
10 %. We also measured the HV dependence of response uniformity along the
Y-axis shown in Fig. 4.18. The response uniformity is independent of the HV.

28

CHAPTER 4. PERFORMANCE EVALUATION OF 8-INCH HPD

Mu-metal shielding box
1” PMT
Optical ﬁber

8” HPD

Pico pulse
Laser
ControlPower Supply
HV

NIM
Clock
NIM
Gate
Gate in

VME
ADC

PC

Figure 4.13: Schematic view of the measurement setup of response uniformity.

203mm
0mm 28mm

Y-axis

56mm

Cathode Pin

84mm
112mm

X-axis

140mm
Y-axisCathode PinSide View

Front View

Figure 4.14: Definition of the X and Y-axis and position of the optical fiber.

29

CHAPTER 4. PERFORMANCE EVALUATION OF 8-INCH HPD

Figure 4.15: Picture of the response uniformity measurement setup.

4.10

Summary and next prototype of HPD

We have confirmed basic performance of 8-inch HPD. HPDs satisfy some of therequirements to Hyper-K photosensor. HPDs have enough low dark rate, bettertiming resolution and single photoelectron resolution. But the rate tolerance
and the linearity of the current version of the 8-inch HPD do not satisfy therequirements. These limitations come from the specification of the preamplifier.
We will improve the preamplifier to satisfy the requirements in the future versionof 8-inch and 20-inch HPD.

30

Relative output charge (%)

CHAPTER 4. PERFORMANCE EVALUATION OF 8-INCH HPD

Response uniformity (X-axis)

120
100
80
60
40
20
0

-10

-5

0

5

10
X-axis (cm)

Relative output charge (%)

Figure 4.16: Response uniformity along the X-axis.

Response uniformity (Y-axis)

120
100
80
60
40
20
0

-10

-5

0

5

10
Y-axis (cm)Figure 4.17: Response uniformity along the Y-axis.

31

Relative output charge (%)

CHAPTER 4. PERFORMANCE EVALUATION OF 8-INCH HPD

120

HV dependence of response uniformity
LegendHV=6kV100

HV=7kV
HV=8kV80

HV=9kV

60
40
20
0
0
20
40
60
80
100
120
Distance from the center along the photocathode (mm)Figure 4.18: HV dependence of response uniformity along the Y-axis.

32Chapter 5

Test in the water
Cherenkov detector
We have confirmed that the 8-inch HPD has good performance in Chapter 4. As
the next step of photosensor R&D, we measure the performance as the sensor
for a water Cherenkov detector and the stability using a newly constructed 200ton water Cherenkov detector. In this chapter, we report the purpose of thetest in the water tank, some preparation before installation, sensor installation,
the measurement in the tank and its calibration.

5.1

Overview

HPD and the high-QE R3600 have never been used in a water Cherenkov detector. To ensure the usability of these two, we evaluate the performance in awater tank as a sensor for a water Cherenkov detector. We also evaluate thelong term stability of HPD and the high-QE R3600. We plan to perform thesame test for 20-inch HPD and 20-inch box&line dynode type PMT when they
become available.
In Kamioka mine, there are a 200-ton water tank and a water circulation system prepared for another experiment, EGADS (Evaluating Gadolinium’s Action
on Detector System) experiment (Fig. 5.1) [16]. We use this 200-ton tank forthe test of our new photosensors in a water Cherenkov detector. The tank can
accommodate up to 240 photosensors in total. We install eight 8-inch HPDs,
five high-QE R3600s and 227 normal-QE R3600s into the tank. Then, we evaluate the performance of each photosensor and start the long term stability in thesame experimental environment and condition. The test in the water Cherenkov
detector will continue for 1 ∼ 2 years.
Before putting inside the water tank, we check the durability, short term
stability and operation capability in water of HPD at first. Then we calibrate
the gain of the 8-inch HPDs and high-QE R3600s. After that, we install 240
photosensors into the tank. We present the calibration and the first data after33

CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

200 ton tank
7m

Figure 5.1: Picture of 200-ton tank (EGADS tank).

installation into the tank.

5.2

Pre-test

Before installation, we checked that HPDs have enough durability and no problem to be used in water.

5.2.1

Durability test of the 8-inch HPD

To verify the durability of HPDs, a power cycling test is conducted. We installeight 8-inch HPDs into the tank. Assuming that we will switch on/off 10 times
for each HPD in one day during 1 year, HPDs are required to have a durability
which means HPD is not breakdown by 30,000 times of HV on/off. Figure 5.2
shows a setup for the durability test. We switch on/off the enable line of a HPD
30,000 times via a mechanical relay board. One cycle takes 20 seconds, 10 sec
for enable on and 10 sec for enable off. We operate HPDs over one month intotal. Through these tests, there was no breakdown of HPD. We conclude that
HPDs have enough durability to be used for the test in the EGDAS tank.

5.2.2

Soak test of the 8-inch HPD

Because HPDs use higher voltage than R3600, we need to check the safety ofuse of HPDs in water. Figure 5.3 shows the setup for the soak test. We operate34

CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

Control Power Supply
Relay Board
PC

Figure 5.2: Setup for the durability test.

HPDs in water drum for 10 hours one by one. We check the leak current whichflows from HPD to ground using a clamp meter (HIOKI 3276) as shown in
Fig. 5.4. During the soak test, we have no trouble and observe no leak current.

5.2.3

Pre-calibration

We measure the gain curve of HPDs by the same setup shown in Fig. 4.2.
Because the accuracy of the gain measurement is better with a higher output
charge, the input light intensity is set to many photoelectrons level. We measure
the output charge of HPD with varying the AD bias voltage with fixed HV of
8 kV. The output is scaled the single photoelectron equivalent charge using aseparately measured single photoelectron charge for one voltage setting. The
gain curves of HPDs are shown in Fig. 5.5.
Using the gain curves, the gain of HPDs is adjusted to 8.2 pC/photoelectron
which corresponds to ∼13 mV of pulse height within +1.2 %/ − 2.8 % (Fig. 5.6).
The gain of high-QE R3600s and normal ones are also calibrated as shown in
Fig. 5.7. The P/V ratio of each photosensor is measured with the calibrated AD
bias voltage as shown in Fig. 5.8. We can see the P/V ratio of HPDs are greaterthan that of PMTs. The P/V of high-QE R3600s are same level compared withthat of normal-QE R3600s. We also measure the dark rate of each photosensor
with the calibrated AD bias voltage as shown in Fig. 5.9. The dark rate ofeight HPDs are comparable with the normal-QE R3600s. The dark rate of thehigh-QE R3600 are a bit larger than that of the normal ones.
35

CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

Drum
ControlPower Supply

8-inch HPD

Oscilloscope

Clamp meter

Insulator

Figure 5.3: Setup for the soak test.

Clamp
meter

Figure 5.4: Pictures of the jig for HPD (left), HPD in water drum (center) and
Clamp meter (HIOKI 3276) (right).

36

Charge of single photoelectron (pC)

CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

14

EHD073
EHD074
EHD07812

EHD080
EHD083
EHD084

10

EHD092
EHD094
EHD0958
6
42
0200 220 240 260 280 300 320 340 360 380
Avalanche diode bias voltage (V)Number of photosensors

Figure 5.5: Variation of the gain curve of HPDs.

h2

Entries
Mean
RMS

1.8
1.6

10
8.112
0.1161

Photosensor

1.48-inch HPD

1.2
1
0.8
0.6
0.4
0.2
0
7.6 7.7 7.8 7.9 8 8.1 8.2 8.3 8.4 8.5 8.6
Charge of single photoelectron (pC)
Figure 5.6: Charge of single photoelectron of ten HPDs.

37

Number of photosensors

CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

Photosensor
Normal-QE R3600
High-QE R3600

25

20

15

10

5

0
1

1.5

2

2.5
3
3.5
4
Charge of single photoelectron (pC)

Number of photosensors

Figure 5.7: Charge of single photoelectron of PMTs.

Photosensor
Normal-QE R3600
High-QE R3600
8-inch HPD

10

1
0

1

2

3

4
5
6
Peak to valley ratio

Figure 5.8: P/V ratio of each photosensor.

38

Number of photosensors

CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

Photosensor
Normal-QE R3600
High-QE R3600
8-inch HPD

10

1
0

10

20

30

40
50
Dark rate (kHz)

Figure 5.9: Dark rate of each photosensor.

5.3

Installation

The installation of photosensors into the EGADS tank is conducted from July
16th to August 13th in 2013. Figure 5.10 shows the map where our photosensors
are placed. First we check the signal of the photosensor using an oscilloscope,and then clean up and attach the jig to fix the photosensor on the tank. Afterthat we install the photosensor into the tank and connect the cable of thephotosensor to the cable connected into the DAQ board. For DAQ system,
we use the ATM board which is used in previous Super-K. Finally we checkthe signals of each photosensor one by one using an oscilloscope (Fig. 5.11).
We successfully mounted photosensors to the 200-ton water Cherenkov tank in
August (Figs. 5.12 and 5.13).
A few days after filling water into the tank, one HPD (No. 74) became
unoperatable. We think that this trouble was caused by malfunctioning of the
HV module. The development of more robust HV module is an urgent issue ofthe HPD R&D. There is no further breakdown of photosensors. Seven HPDs
are available for the long term test.

5.4

DAQ system

In this section, we show the DAQ setup for the test in the water tank.
The control power supply for eight HPDs, power supply for 232 PMTs,
ATMs and some electronics are shown in Fig. 5.14. The power supply of thehigh-QE R3600s and that of the normal ones are common. One ATM has 12channels and we employ 22 ATM boards. When the signal which exceeds the39CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

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                   
   

8-inch HPD
High-QE R3600

Figure 5.10: Photosensor map.

threshold voltage is inputted into an ATM board, it integrates the charge of the
input signal with 400 ns window and records the timing of the signal. We cantrigger the DAQ by using some electronics which is placed under the control
power supply. Figure 5.15 shows the back plane of the control power supply.
The 70 m cables coming from inside the tank are connected to the control power
supply and the ATM boards.
To monitor the HPDs, we set two voltage logger, a temperature logger anda relay board as shown in Fig. 5.16. The voltage logger records the control
voltage of the HV and AD bias voltage to monitor the stability of the HV andAD bias voltage. Four HPDs have Pt100 as a temperature sensor inside themodule. The temperature logger records the temperature inside the HPDs to
study the temperature dependence of gain. To control remotely the enable of
HPDs, we use the relay board. In emergency situation, we can immediately
disable the HPDs. Two connectors relay the connection each monitor module
to the control power supply.
After establishment of DAQ setup, we conduct the noise reduction. We
found that the periodic noise in the signal of HPDs come from a VME module.
Attaching a ferrite core to an AC cable of a VME crate, the noise is reduced.
We also improved grounding to reduce the noise level.

40

CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

Cleaning

Assembling

Installation

Connection

Figure 5.11: Procedure of installation.

41

CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

Figure 5.12: Picture when the installation completed.

R3600

8-inch HPD

Figure 5.13: 8-inch HPD with R3600s in tank.

42

CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

TKO power supply

ATM board

TKO crate

Control power supply
for HPDs

HV power supply
for PMTs

Electronics

Figure 5.14: Picture of the front view of the electronics.

TKO crate
70m cables forhigh-QE R3600s
70m cables for
8-inch HPDs
200 ton tankControl power supply
for HPDs

Figure 5.15: Picture of the back view of the electronics.

43

CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

Connectors

Voltage logger

Relay board

Temperature logger

Figure 5.16: Picture of the side view of the electronics.

44

CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

5.5

The measurement in the water tankAfter the installation, we measured the single photoelectron signal and the dark
rate of photosensors and calibrated the gain of HPDs. The cosmic muon-likeevents observed in the tank are shown in Figs. 5.17. The EGADS detector works
as a water Cherenkov detector. Figure 5.18 shows a setup for the measurement
using a laser diode (OPG-1000-NIM, λ = 405 nm, pulse width (70∼80 ps)). The
light emitted by the laser diode is diffused uniformly inside the tank through adiffuser ball. By changing the light intensity of the laser diode, we measure thecharge of the single photoelectron of 8-inch HPDs. The threshold of all HPDs
is fixed at −1.5 mV which corresponds to about 1/5 photoelectron. Figure 5.19
shows a result for one HPD (No. 82). The single photoelectron resolution isabout 30 % in sigma, while 40∼50 % in case of PMTs. The difference between
the single photoelectron resolution 12 % as shown in section 4.5 and this value
30 % comes from the difference of the gate width and the existence of the 70 m
cable. The multi photoelectron distribution measured with an HPD is shown in
Fig. 5.20. We can see clear separation of photoelectrons up to four photoelectrons. Figure 5.21 shows the photoelectron resolution defined by sigma/mean ofnth photoelectron peak as a function of the number of photoelectrons. The photoelectron resolution becomes better with the larger number of photoelectrons.
We calibrate the gain of HPDs using the same system of Fig. 5.19. The target
value is 7.6 pC/photoelectron. The gain is adjusted to 7.6 pC/photoelectron
within 4.7 % (Fig. 5.22).
After the calibration, we measure the dark rate of all HPDs using the VME
scaler. The result is shown in Fig. 5.23. The single photoelectron is around
8 mV. Channel 0 is for the dead channel as described in previous section.
Channel 1 and 2 have relatively high dark rate. We suspect that these highdark rate are caused by some HV module issues. The other five HPDs have thedark rate less than 2 kHz at 0.5 photoelectron threshold. Figure 5.24 shows thecomparison of the dark rate of an HPD (No. 150) between before and after the
installation into the water tank. At 0.5 photoelectron level, the dark rate taken
in May 22th, October 12th and November 21th are 2 kHz, 0.6 kHz and 0.4 kHz,
respectively. Because of the low temperature in the tank and closing the tankfor two months, the dark rate decreases. Figure 5.25 and Fig. 5.26 show thedark rate of the high-QE R3600s and the normal ones. At this time, we use anamplifier with a gain of 4.4. The dark rate of the high-QE R3600s at around
10 mV which corresponds to 0.25 photoelectron is comparable to that of thenormal ones.

5.6

Summary and future plan

The installation of photosensors is successfully completed in August. The measurement in the water tank shows that HPDs have better single photoelectron
resolution, clear separation of photoelectrons and enough low dark rate in the
tank.

45

CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

Charge (pC)
800

Normal-QE PMT
High-QE PMT
600

600

8-inch HPD
500

400

400

200

0

300
-200

200
-400

100

-600

-800
-800

-600

-400

-200

0

200

400

600

800

0

Charge (pC)
800

Normal-QE PMT
High-QE PMT
600

8-inch HPD

600

500

400

400
200

0

300

-200

200
-400

100

-600

-800
-800

-600

-400

-200

0

200

400

600

800

0

Figure 5.17: Cosmic muon-like event. The color shows the charge of the hit.
The warm color is for large charge. The cold color is for small charge.
46

CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

ATM (Analog TimingGate
Module)

Delay

Clock
500Hz

Laser
Diode

200ton tank 12#channel#input#
 2TAC+2QAC/channel,##1ADC/board# ADC#process#speed#:#5.5sec/hit#
Signal line 70m
 Dynamic#range#:#~450#pC#
 Read#the#data#via#TKO#bus#and#VME#
Threshold< 1p.e.
LV line HITSUM:#200ns###O15mV#

70m

PC

threshold#

Diffuser ball

Ultra pure water
Control Power Supply

ATM board

Figure 5.18: Setup for the single photoelectron measurement in 200-ton tank.

Single photoelectron charge distribution
charge_sig

160

EntriesMean
RMS
χ 2 / ndf
Prob
p0
p1
p2
p3
p4
p5

140
120
100

4400
3.699
4.102
138.5 / 61
5.896e-08
134.3 ± 3.4
0.8772 ± 0.0663
1.848 ± 0.044
78.38 ± 2.65
7.574 ± 0.103
2.196 ± 0.096

80
60
40
20
0
-10

-5

0

5

10

15

20

25

30 35 40
Charge (pC)

Figure 5.19: Single photoelectron distribution of an 8-inch HPD measured in
EGADS tank.

47

CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

Multi-photoelectron charge distributionhEntriesMean
RMS
χ2 / ndf
Prob
p0
p1
p2
p3
p4
p5
p6
p7
p8
p9
p10
p11

2000
1800
1600
1400
1200
1000
800
600
400

1098857.771
4.326
1808 / 100
01745 ± 13.6
3.799 ± 0.010
1.025 ± 0.011
1532 ± 11.9
7.536 ± 0.015
1.19 ± 0.02
873.1 ± 9.6
11.22 ± 0.03
1.204 ± 0.038
398.4 ± 6.2
14.98 ± 0.06
1.463 ± 0.066

200
0

0

5

10

15
20
Charge (pC)

Photoelectron Resolution

Figure 5.20: Multi photoelectron distribution of an 8-inch HPD.

Fitting Function[0]/sqrt(x)+[1]

0.28
0.26

χ2 / ndf

0.24
0.22

13.03 / 2p00.3653 ± 0.008992

p1-0.09764 ± 0.00676

0.2
0.18
0.16
0.14
0.12
0.1
0.08
1

1.5

2

2.5

3

3.5
4
Number of photoelectrons

Figure 5.21: Photoelectron resolution of an 8-inch HPD as a function of numberof photoelectrons.

48

Number of photosensors

CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTORh2

Entries
Mean
RMS

1.8

7
7.668
0.2113

1.6
1.4

+4.7%

-4.7%

1.2
1
0.8
0.6
0.4
0.2
0

6.6

6.8

7

7.2 7.4 7.6 7.8
8
8.2 8.4
Charge of single photoelectron (pC)

Dark rate (kHz)

Figure 5.22: Single photoelectron peak values of HPDs after single photoelectron
calibration.

103

Legend
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7

8 mV ~ 1 p.e.

102

10

4 kHz

1
10-1
10-2

0

10

20

30

40
50
Threshold voltage (mV)

Figure 5.23: Dark rate distribution of eight 8-inch HPDs measured in EGADS
tank. Ch 0 is for a broken HPD.

49

Dark Rate (kHz)CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

EHD0083
2013/03/22, Bias=302V

103

2013/10/12, Bias=330V
2013/11/21, Bias=331V

102

10

1

10-1

10-2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6
Threshold (p.e.)Dark Rate (kHz)

Figure 5.24: Comparison between before after the installation.

High-QE R3600
CH0

3

10

CH1
CH2
CH3
CH4

102

10

1

10-1
0

10

20

30

40

50

60
70
Threshold (mV)

Figure 5.25: Dark rate of five high-QE R3600 with an amplifier × 4.43.

50

Dark Rate (kHz)CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

Normal-QE R3600
CH0

3

10

CH1
CH2
CH3
CH4
CH5

2

10

10

1

10-1
0

10

20

30

40

50

60
70
Threshold (mV)

Figure 5.26: Dark rate of six normal-QE R3600 with an amplifier × 4.43.

These are the first measurement of the 8-inch HPD and the high-QE R3600
in the water Cherenkov detector. We calibrate the gain of HPDs. We will
start the long term testing in February 2014. Comparing with the normal-QE
R3600s, we evaluate the usability of HPDs and high-QE photocathode in thelong term testing. The first version of prototypes of 20-inch HPD and 20-inch
box&line dynode type PMT will be available in early 2014 (Fig. 5.27). We selectthe photosensor for Hyper-K during 2016.

51

CHAPTER 5. TEST IN THE WATER CHERENKOV DETECTOR

Hamamatsu Photonics K.K.

20-inch HPD

20-inch Box&Line dynode type PMT

Figure 5.27: Picture of 20-inch HPD and 20-inch box & line dynode type PMT.
Courtesy of Hamamatsu Photonics K.K.

52

Chapter 6

Summary and future
prospects
6.1

Summary

We have developed the prototype of 8-inch HPD. The 8-inch HPD satisfies
some requirements to photosensors in Hyper-K. It has a simple structure, large
photosensitive area, dark rate of 2 kHz at 0.5 photoelectron, timing resolution of
1.2 ns and single photoelectron resolution of 12 %. But some requirements suchas the linearity and the rate tolerance do not satisfy the requirements and need
improvement. In order to install the HPD into the water Cherenkov tank, we
confirmed the durability and safety operation in water. We installed eight 8-inch
HPDs, 5 high-QE R3600s and 227 normal-QE R3600s. The single photoelectron
resolution of the 8-inch HPD is better than R3600 in the tank. The dark rate of
the 8-inch HPD in the tank decreased compared with the measurement before
installation. One HPD was broken a few days after filling the water. We suspectthat the problem is caused by the HV module trouble. We also have developed
the high-QE R3600. The single photoelectron resolution and P/V ratio of thehigh-QE R3600 are the same that of the normal-QE R3600. The dark rate of
the high-QE R3600 is comparable to that of the normal-QE R3600.

6.2

Future prospects

The major topics of R&D of HPD are the R&D of 20-inch HPD, the test in thewater Cherenkov detector and the improvement of the preamplifier and the HV
module. The first version of prototype of 20-inch HPD will be available in early
2014. The test in the water Cherenkov detector will start in early 2014. We aredeveloping the next version of the preamplifier to make the best performance ofHPD. To ensure that HPD is useful to use in a water Cherenkov detector, thestable operation of the HV module is the key.

53

Acknowledgements
I would like to express my great appreciation to my supervisor Prof. Masashi
Yokoyama for his overall support. Through discussion with him, I could understand my work more deeply. Without him advices, this thesis would not exist.
Many thanks to Dr. Yasuhiro Nishimura and Seiko Hirota. They taught me
how to operate HPD, how to take and analyze data. Whenever I am preparing
a transparency, they gave me many comments even if it is midnight.
I must thank to Prof. Masato Shiozawa, Prof. Yoshinari Hayato, Dr. Shoei
Nakayama, Dr. Hidekazu Tanaka, Prof. Tsuyoshi Nakaya, Prof. Masahiro Kuzeand Dr. Akimichi Taketa for their guidance and supervision. I also want to thankDr. Tomonobu Tomura who graduated my laboratory, Kazuki Iyogi, Hiroyoshi
Uchida, Shota Takahashi, Kunxian Huang, Hiroki Takiya, Tadashi Hosokawa,
Osamu Takachio, Yuki Nakano, Naoya Oka, Katsuma Nakagawa, Yuto Haga,
Isao Kametani, Keiji Tateishi, Yosuke Ohnishi, Asato Orii, Tateo Nakajima,
Masatoshi Kobayashi, Yuji Okajima and Zhi Li for my happy life in Kamioka.
I am indebted to the members of Hamamatsu Photonics K.K. Dr. Yoshihiko
Kawai, Takayuki Ohmura and Masatoshi Suzuki, especially for the development of HPD and high-QE PMT.
I need to thank to the members of EGADS experiment Prof. Masayuki Nakahata, Prof. Mark Vagins, Prof. Yusuke Koshio, Dr. Motoyasu Ikeda, Dr. Astuko Kibayashi, Dr. Takatomi Yano and Takaaki Mori. They approved to install
our photosensors into the EGADS tank and helped us to take data in the tank.
My appreciation also goes to the members of Aihara/Yokoyama laboratory
Prof. Hiroaki Aihara, Dr. Yoshiyuki Onuki, Dr. Denis Epifanov, Sogo Mineo,
Clement Ng, Yasutaka Kanazawa, Nobuhiro Shimizu, Taichiro Koga, Junya
Sasaki and Yifan Jin for my fruitful life in Hongo. I would like to thank Kuniko
Kono who supports my laboratory for her fantastic assist. I would also like toappreciate Dr. Akito Kusaka, Dr. Hiroyuki Nakayama and Jiayin Wang for theirresearch and development of HPD.
Finally, I would like thank to my family and all my friends.
54

Bibliography[1] K. Abe, T. Abe, H. Aihara, Y. Fukuda, Y. Hayato, et al. Letter of Intent: The Hyper-Kamiokande Experiment — Detector Design and Physics
Potential —. 2011.
[2] Y. Fukuda et al. Evidence for oscillation of atmospheric neutrinos.
Phys.Rev.Lett., 81:1562–1567, 1998.
[3] K. Abe et al. Observation of Electron Neutrino Appearance in a Muon
Neutrino Beam. 2013.
[4] F.P. An et al. Observation of electron-antineutrino disappearance at Daya
Bay. Phys.Rev.Lett., 108:171803, 2012.
[5] J.K. Ahn et al. Observation of Reactor Electron Antineutrino Disappearance in the RENO Experiment. Phys.Rev.Lett., 108:191802, 2012.
[6] Y. Abe et al. First Measurement of θ13 from Delayed Neutron Capture on
Hydrogen in the Double Chooz Experiment. Phys.Lett., B723:66–70, 2013.
[7] Ziro Maki, Masami Nakagawa, and Shoichi Sakata. Remarks on the unified
model of elementary particles. Prog.Theor.Phys., 28:870–880, 1962.
[8] R.B. Patterson.
The NOvA Experiment:
Nucl.Phys.Proc.Suppl., 235-236:151–157, 2013.

Status and Outlook.

[9] K. Hirata et al. Observation of a Neutrino Burst from the Supernova SN
1987a. Phys.Rev.Lett., 58:1490–1493, 1987.
[10] R.M. Bionta, G. Blewitt, C.B. Bratton, D. Casper, A. Ciocio, et al. Observation of a Neutrino Burst in Coincidence with Supernova SN 1987a in
the Large Magellanic Cloud. Phys.Rev.Lett., 58:1494, 1987.
[11] E.N. Alekseev, L.N. Alekseeva, I.V. Krivosheina, and V.I. Volchenko. Detection of the Neutrino Signal From SN1987A in the LMC Using the Inr
Baksan Underground Scintillation Telescope. Phys.Lett., B205:209–214,
1988.
[12] Y. Fukuda et al. The Super-Kamiokande detector. Nucl.Instrum.Meth.,
A501:418–462, 2003.
55

BIBLIOGRAPHY[13] Hirokazu Ikeda, Masaki Mori, Tohru Tanimori, Keni-ichi Kihara, Yoichiro
Suzuki, et al.
Front end hybrid circuit for Super-Kamiokande.
Nucl.Instrum.Meth., A320:310–316, 1992.
[14] A. Suzuki, M. Mori, K. Kaneyuki, T. Tanimori, J. Takeuchi, et al. Improvement of 20-inch diameter photomultiplier tubes. Nucl.Instrum.Meth.,
A329:299–313, 1993.
[15] K. Abe, Y. Hayato, T. Iida, K. Iyogi, J. Kameda, et al. Calibration of theSuper-Kamiokande Detector. Nucl.Instrum.Meth., A737:253–272, 2014.
[16] Takatomi Yano. Status of the Gadolinium project for Super-Kamiokande.
PoS, ICHEP2012:382, 2013.

56