Development of Micro Pixel Chamber and
Systematic Study on the Electrode Structure
Tsutomu Nagayoshi
Department of Physics, Faculty of Science, Kyoto University
Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan
nagayosi@cr.scphys.kyoto-u.ac.jp

This thesis was submitted to the Department of Physics,
Graduate School of Science, Kyoto University
on January 6, 2004
in partial fulfillment of the requirements
for the degree of Doctor of Science in physics.

Abstract
We have developed a novel gaseous two-dimensional imaging detector “Micro Pixel Chamber (µ-PIC)” based on Printed Circuit Board (PCB) technology. The problem of gas gain
non-uniformity was overcome by a new manufacturing technique. The uniformity of gas
gain within 7% was achieved in the whole detection area of 10 × 10cm2 . The maximum
gas gain of more then 104 was obtained without any other intermediate gas multipliers.
Long term stability of over 1000 hours was also attained with the gas gain of 6000. Further
improvement of the electrode structure was investigated based on the three-dimensional
simulation using GARFIELD and MAXWELL. Consequently, we found that the gas gainof more than 105 would be expected for a µ-PIC having a 150µm thick substrate. A fast
data acquisition system was also developed for the time-resolved readout. The signals of
512 channels from the µ-PIC are encoded to the position information using Field Programmable Gate Arrays (FPGAs) with a clock of 20 or 40MHz. The data taking rateof 7MHz was attained for real X-ray events. The two-dimensional spatial resolution of
120µm was achieved using this DAQ system. The proton tracks were successfully detected with the three-dimensional sampling interval of 2.6mm. The tracking resolutionof 260µm was obtained. For the future applications, several foundamental experiments
were performed: X-ray polarimetry, X-ray crystallography, MeV gamma-ray imaging, andneutron detection.

Contents
1 Introduction

12 Physical process of gas multiplication in µ-PIC

6

2.1

Drift and diﬀusion of primary electrons in gas . . . . . . . . . . . . . . . .

6

2.2

Gas multiplication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8

2.3

Gases for detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3 Micro-pattern gas detectors

18

3.1

MicroStrip Gas Chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.2

Intermediate gas multipliers . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.3

Pixel electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.4

New manufacturing technique . . . . . . . . . . . . . . . . . . . . . . . . . 24

4 Development of the Micro Pixel Chamber

26

4.1

Electrode structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.2

Two-dimensional readout . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.3

Fabrication of µ-PIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.4

Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.4.1

Streamer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.4.2

Field emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4.4.3

Discharges in MicroPattern detectors . . . . . . . . . . . . . . . . . 39

5 Data Acquisition System
5.1

42Overview of the DAQ system . . . . . . . . . . . . . . . . . . . . . . . . . 42i

CONTENTS

ii
5.2

Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

6 Performance of the µ-PIC

50

6.1

Performance test of the µ-PIC . . . . . . . . . . . . . . . . . . . . . . . . . 50

6.2

Long term stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

6.3

Operation in carbon tetraﬂuoride gas . . . . . . . . . . . . . . . . . . . . . 55

7 Improvement of µ-PIC

59

7.1

Problem of former µ-PICs . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

7.2

Simulation study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

7.3

7.2.1

Simulation method . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

7.2.2

Simulation Result . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Detector optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
7.3.1µ-PIC with a thick substrate . . . . . . . . . . . . . . . . . . . . . . 76

7.3.2

Substrate-ablated µ-PIC . . . . . . . . . . . . . . . . . . . . . . . . 80

7.4

Development of the improved µ-PIC . . . . . . . . . . . . . . . . . . . . . 85

7.5

Performance test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
7.5.1

Basic performance . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

7.5.2

Long term operation . . . . . . . . . . . . . . . . . . . . . . . . . . 91

8 Imaging and 3D tracking

94

8.1

X-ray imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

8.2

Performance of µ-TPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
8.2.1µ-PIC readout for the µ-TPC . . . . . . . . . . . . . . . . . . . . . 98

8.2.2

Heavy particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

8.2.3

Minimum ionizing particles

9 Application of µ-PIC. . . . . . . . . . . . . . . . . . . . . . 103
107

9.1

X-ray polarimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

9.2

X-ray crystallography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

9.3

Application of µ-TPC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

9.3.1

Gamma-ray imaging . . . . . . . . . . . . . . . . . . . . . . . . . . 116

9.3.2

Neutron imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

CONTENTS
10 Summary and conclusion

iii
128

List of Figures
2.1

Energy loss of various particles in MeV/(g · cm2 ). . . . . . . . . . . . . . .

7

2.2

Schematic drawing of a proportional chamber. . . . . . . . . . . . . . . . .

9

2.3

Properties of gases for X-ray. . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.4

Schematic drawing of the multiple scattering. . . . . . . . . . . . . . . . . 13

2.5

Drift and diﬀusion of electron in gases. . . . . . . . . . . . . . . . . . . . . 15

2.6

The ﬁrst Townsend coeﬃcients in each gases. . . . . . . . . . . . . . . . . . 16

3.1

Structure of a MWPC and its electric ﬁeld. . . . . . . . . . . . . . . . . . . 19

3.2

Schematic structure of MSGC. . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.3

Microscopic photograph of the GEM. . . . . . . . . . . . . . . . . . . . . . 21

3.4

Schematic view of a capillary plate. . . . . . . . . . . . . . . . . . . . . . . 22

3.5

Capillary-MSGC combined system. . . . . . . . . . . . . . . . . . . . . . . 23

3.6Structure of MDOT detector. . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.7

Schematic structures of several types of MPGDs.

4.1

Schematic structure of µ-PIC. . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.2

Photograph of the printed mother-board of the µ-PIC. . . . . . . . . . . . 28

4.3

Diagram of the circuit to divide the applied voltage to each anode electrode. 29

4.4

Signal shapes from both the anode and cathode electrodes. . . . . . . . . . 30

4.5

Manufacturing process of a polyimide-based µ-PIC. . . . . . . . . . . . . . 31

4.6

Blocking of the anode formation. . . . . . . . . . . . . . . . . . . . . . . . 32

4.7

Manufacturing process of a ceramic-based µ-PIC. . . . . . . . . . . . . . . 33

4.8

Microscopic photograph of a ceramic-based µ-PIC. . . . . . . . . . . . . . . 33

4.9

Microscopic photograph of a damaged electrode of µ-PIC1. . . . . . . . . . 34
iv

. . . . . . . . . . . . . . 25

LIST OF FIGURESv

4.10 The abnormally grown anode. . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.11 Microscopic photograph of a polyimide-based µ-PIC. . . . . . . . . . . . . 36
4.12 Growth of an avalanche into a streamer. . . . . . . . . . . . . . . . . . . . 38
5.1

Block diagram of the DAQ system. . . . . . . . . . . . . . . . . . . . . . . 43

5.2

Photograph of an ASD board. . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.3

Block diagram of the ASD chip. . . . . . . . . . . . . . . . . . . . . . . . . 44

5.4

Signals of 5.9keV X-ray obtained from the µ-PIC . . . . . . . . . . . . . . 45

5.5Photograph of the position encoding module. . . . . . . . . . . . . . . . . . 46

5.6

Block diagram of the position encoding module. . . . . . . . . . . . . . . . 47

5.7

Circuit diagram and a timing chart of the synchronizing circuit. . . . . . . 47

5.8

Photograph of the memory module. . . . . . . . . . . . . . . . . . . . . . . 48

5.9

Result of the high rate test. . . . . . . . . . . . . . . . . . . . . . . . . . . 49

6.1

Gain curve of the µ-PIC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

6.2

Obtained energy spectrum of 55 Fe source. . . . . . . . . . . . . . . . . . . . 52

6.3

Map of the measured gas gains. . . . . . . . . . . . . . . . . . . . . . . . . 52

6.4

Anode and feedback current. . . . . . . . . . . . . . . . . . . . . . . . . . . 53

6.5

Gas gain variation (1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

6.6

Gas gain variation (2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

6.7

Experimental setup for CF4 and high pressure test. . . . . . . . . . . . . . 57

6.8

Gas gain curve obtained by CF4 test. . . . . . . . . . . . . . . . . . . . . . 58

7.1

Mesh structure of the µ-PIC.

7.2

Basic µ-PIC structure generated by MAXWELL. . . . . . . . . . . . . . . 62

7.3

Description of the geometrical parameters of the µ-PIC. . . . . . . . . . . . 63

7.4

Electron drift of the µ-PIC with lower anode. . . . . . . . . . . . . . . . . 65

7.5

Relation between electron collection eﬃciency and anode height. . . . . . . 66

7.6

Electron drift of the µ-PIC with higher anodes. . . . . . . . . . . . . . . . 67

7.7

Terminal points of drift electrons for a MDOT detector. . . . . . . . . . . . 68

7.8

Simulated drift lines for thin and thick substrates. . . . . . . . . . . . . . . 69

7.9

Terminal points of electrons on the electrode plane of the µ-PIC. . . . . . . 70

. . . . . . . . . . . . . . . . . . . . . . . . . 61

7.10 Electron collection eﬃciency dependence on the substrate thickness. . . . . 71

LIST OF FIGURES

vi

7.11 Strength of the electric ﬁeld against the distance from the anode center. . . 72
7.12 Strength of the electric ﬁeld as a function of the substrate thickness. . . . . 73
7.13 Calculated eﬀective multiplication Meﬀ as a faction of the substrate thickness. 74
7.14 The electron collection eﬃciency as a function of the scale factor. . . . . . 76
7.15 Dependence of the electron collection eﬃciency on TDR. . . . . . . . . . . 77
7.16 Electric ﬁeld at the anode pixel against the anode diameter. . . . . . . . . 78
7.17 Electron collection eﬃciency as a function of the anode diameter. . . . . . 78
7.18 Dependence of the eﬀective multiplication factor against the anode diameter. 79
7.19 The limit voltage and the corresponding achievable gas gain. . . . . . . . . 81
7.20 Substrate-ablated µ-PIC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
7.21 Distribution of drift terminal points of electrons for the well-type µ-PIC. . 82
7.22 Simulated absolute gas multiplication as a function of the anode height for
the well-type µ-PIC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
7.23 Microscopic photograph of the well-type µ-PIC. . . . . . . . . . . . . . . . 83
7.24 Gas gain curve of the well-type µ-PIC. . . . . . . . . . . . . . . . . . . . . 84
7.25 Manufacture process of the latest µ-PIC . . . . . . . . . . . . . . . . . . . 86
7.26 Microscopic photograph of the latest µ-PIC. . . . . . . . . . . . . . . . . . 87
7.27 Photograph of a substrate board formed nine µ-PIC patterns. . . . . . . . 88
7.28 Microscopic photograph of shifted anodes observed in edge region. . . . . . 88
7.29 Gas gain curve of µ-PICs. . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
7.30 Obtained energy spectrum of 55 Fe source. . . . . . . . . . . . . . . . . . . . 90
7.31 Map of the measured gas gains. . . . . . . . . . . . . . . . . . . . . . . . . 90
7.32 Distribution of the gas gains. . . . . . . . . . . . . . . . . . . . . . . . . . . 90
7.33 Anode and drift current as a function of the counting rate of X-ray. . . . . 91
7.34 Fraction of the feedback current to the anode current. . . . . . . . . . . . . 91
7.35 Variation of the gas gain in 150 hours. . . . . . . . . . . . . . . . . . . . . 92
7.36 Variation of the gas gain in two months. . . . . . . . . . . . . . . . . . . . 93
8.1

X-ray image of a test chart. . . . . . . . . . . . . . . . . . . . . . . . . . . 95

8.2

Projected image of the test chart edge. . . . . . . . . . . . . . . . . . . . . 96

8.3

Transparency image of a universal printed board. . . . . . . . . . . . . . . 97

8.4

Two-dimensional image of uniformly irradiated X-ray. . . . . . . . . . . . . 98

LIST OF FIGURES

vii

8.5

Schematic structure of a TPC and the detection principle. . . . . . . . . . 99

8.6

Structure of the µ-TPC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

8.7

Photograph of the ﬁeld cage of the µ-TPC. . . . . . . . . . . . . . . . . . . 101

8.8

The photograph of the developed µ-TPC. . . . . . . . . . . . . . . . . . . . 101

8.9

Setup photograph of the beam test. . . . . . . . . . . . . . . . . . . . . . . 102

8.10 A three-dimensional track image of a proton. . . . . . . . . . . . . . . . . . 104
8.11 Residual distributions of the proton tracks. . . . . . . . . . . . . . . . . . . 105
8.12 A three-dimensional image of an electron track. . . . . . . . . . . . . . . . 105
8.13 Three-dimensional track of cosmic-ray muons. . . . . . . . . . . . . . . . . 106
9.1

The angular distribution of the photoelectrons. . . . . . . . . . . . . . . . . 108

9.2

Measurement of the “hit width” of electrodes. . . . . . . . . . . . . . . . . 109

9.3

Experimental setup of X-ray polarization detection. . . . . . . . . . . . . . 109

9.4

A typical distribution pattern of the hit number of anode and cathodeelectrodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

9.5

Variation of the diﬀerence factor as a function of the rotation angle ofpolyethylene scatterer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

9.6

Dependence of the modulation factor as a function of the X-ray energy. . . 111

9.7

Schematic drawing of the pixel readout µ-PIC. . . . . . . . . . . . . . . . . 113

9.8

Rotational photograph image of an ABT crystal. . . . . . . . . . . . . . . . 114

9.9

Directly reconstructed reciprocal lattice. . . . . . . . . . . . . . . . . . . . 114

9.10 Setup of the X-ray diﬀraction experiment. . . . . . . . . . . . . . . . . . . 115
9.11 Rotational image of a ylide crystal observed by the µ-PIC. . . . . . . . . . 115
9.12 Description of axes used for the event reconstruction. . . . . . . . . . . . . 116
9.13 Principles of gamma-ray detector using the Compton scattering. . . . . . . 117
9.14 Gamma-ray imaging detector. . . . . . . . . . . . . . . . . . . . . . . . . . 119
9.15 Typical reconstructed event. . . . . . . . . . . . . . . . . . . . . . . . . . . 120
9.16 Reconstructed image of two 137 Cs sources. . . . . . . . . . . . . . . . . . . 121
9.17 Schematic structure of the MeV gamma-ray detector. . . . . . . . . . . . . 122
9.18 Several three-dimensional proton tracks. . . . . . . . . . . . . . . . . . . . 124
9.19 Energy loss of the protons. . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
9.20 χ2 /d.o.f vs. dE/dx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

List of Tables
1.1

Summary of the Micro-Pattern Gas Detectors. . . . . . . . . . . . . . . . .

2.1

Physical properties of gases. . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.1

Summary of the µ-PICs developed by 2002. . . . . . . . . . . . . . . . . . 30

7.1

Geometrical parameters of the new µ-PIC. . . . . . . . . . . . . . . . . . . 85

7.2

Performance comparison of the previous and the new µ-PIC. . . . . . . . . 92

8.1

Properties of the gas mixtures of Ar-C2 H6 . . . . . . . . . . . . . . . . . . . 102

viii

3

Chapter 1
Introduction
Gas detectors are one of the radiation detectors most widely used since the beginningof particle physics. The proportional chamber which consists of a thin anode wire anda cathode tube is a well-known gas detector. It became the basis of all gas detectors
such as MultiWire Proportional Chambers (MWPCs), Drift Chambers, or Time Projection Chambers (TPCs). In particular, MWPCs are the ﬁrst gas detector having twodimensional position sensitivity and energy spectrometry. Because of the repulsive force
between anode wires, the electrode pitch of the MWPCs is restricted to about 1mm. This
limit of the wire pitch determines the spatial resolution. The Micro-Pattern Gas Detectors(MPGDs) were proposed in order to exceed the limit of MWPCs.
The history of MPGDs is traced back to the invention of the Micro-Strip Gas Chamber(MSGC) by A. Oed in 1988 [1]. A principle of the operation of an MSGC is same as aconventional proportional gas chamber. The main feature of an MSGC is the structureof the electrodes. Instead of wires in MWPCs, an MSGC has very narrowly spaced
electrodes made on a thin substrate. Hence, an MSGC has several excellent features:ﬁne position resolution (∼ 100µm) and high rate capacity (∼ 106 cps) [2]. Since 1992, we
had developed a two-dimensional MSGC and used for X-ray crystallography and solution
X-ray scattering experiments [3]. However, there still remained a serious problem ofdischarges, which caused a critical damages on the electrodes of MSGCs. Because of thisproblem, the gas gain of an MSGC was restricted to several hundreds. Although MSGC
was a quite promising detector for many application, such as high energy experiments,
1

CHAPTER 1. INTRODUCTION

2

X-ray crystallography, and X-ray astrophysics, there were only a few examples except
neutron imaging experiments [4] that achieved the the practical use of MSGCs.
Until now, many eﬀorts have been made to avoid discharges and to obtain a high gas
gain. One solution is a use of an intermediate gas multiplier. The capillary plate [5, 6],
the Gas Electron Multiplier (GEM) [7], and the Micromesh Gaseous Structure (MICROMEGAS) [8] are classiﬁed to this category. In these detectors, a ﬂoating structure is set above the readout electrodes such as an MWPC or an MSGC. Drift electrons
are multiplied by the strong electric ﬁeld applied between the electrodes. Now, GEM
and MICROMEGAS are widely studied for various applications, e.g. a Cherenkov lightimager [9], a tracking device for a high energy experiment [10], and a dark matter detector [11]. Although the attainable gas gain of single GEM or capillary plate was less than
1000, a high gas gain was obtained by multiple GEMs or capillary plates mounted in thesame gas volume. Our group had developed a hybrid MSGC combined with two-step capillary plates [12]. The gas gain of 5 × 103 was achieved, which was much higher than thatof a stand-alone MSGC. A gas gain more than 106 was obtained with the quadruple-GEM
detector, which was intended to be applied to a gaseous photomultiplier (gas PMT) [13].
However, these ﬂoating electrode structures make detectors too complicated to develop
large area detectors.
The other solution to obtain a high gas gain is a use of a pixel type electrode. A verystrong electric ﬁeld is made around the anode electrode, which is rapidly weakened withthe distance from the anode. Then the gas gain becomes higher than that of the strip
electrode for the same electrode pitch and the same applied voltage. The ﬁrst gas detector
which has a pixel electrode was the Gaseous Pixel Chamber [14]. However, usually thisdetector is not categorized into micro-pattern detectors, because of its wide electrodepitch (∼1cm). The Micro Dot detector (MDOT) [15] is the ﬁrst micro-pattern detector
which has a pixel electrode structure. Since MDOTs were manufactured using an integral
circuit (IC) technology on a silicon wafer, it was diﬃcult to make a large area detector.
The manufacturing technologies of MSGCs and MDOTs were based on IC technology.
Therefore, the detection area was restricted within a 10cm square. In order to extend thedetection area, an alternative manufacturing technique was required. That is the Printed
Circuit Board (PCB) technology, which provides us mechanical robustness as well as the3large detection area. Several types of MPGDs have been developed based on the PCB
technology, e.g. the Micro-Groove detector [16], the Micro-Well detector [17, 18], theMicro-Wire detector [19] etc. The manufacturing technologies of these detectors are thesame as GEM. The signal was read out by strip electrodes, then the achieved gas gain
was several thousands. The structure and manufacturing technologies for these detectors
are summarized in Table 1.1.
Name

ElectrodeTechnology

Electrode pitch

thickness

MSGC

Strip

IC

200µm

20µm

GEM

Mesh

PCB

140µm∗

50µm∗

MICROMEGASMeshElectroforming

25µm

3µm

MDOT

Pixel

IC

200µm

5µm

Micro-Groove

Strip

PCB

200µm

50µm

Micro-Well

Strip

PCB

120µm

50µm

Micro-Wire

Strip

PCB

100µm

50µm

µ-PIC

Pixel

PCB

400µm

100µm

* Pitch of the mesh.

Table 1.1: Summary of the Micro-Pattern Gas Detectors.

The Micro Pixel Chamber (µ-PIC) is one of the Micro-Pattern gas detectors which
has a pixel electrode structure. We have developed the µ-PIC using PCB technology.
The electrode structure of the µ-PIC is similar to the MDOT detector; pixel anodes are
surrounded by the cathode rings. However, the manufacturing process is quite diﬀerent.
At ﬁrst, a µ-PIC has a thick substrate of 100µm, which is an essential point to ensure
the higher gas gain. In addition, the PCB technology allows us to have a large detection
area up to 30 × 30cm2 . Our µ-PIC has following features:
1. pixel electrodes,
2. large detection area,
3. thicker substrate than MDOT detectors,

CHAPTER 1. INTRODUCTION

4
4. robustness against discharges,
5. mass productivity.

At ﬁrst, we made a prototype of the µ-PIC having the detection area of 3 × 3cm2 [20,21, 22] in 1999. Now, large area µ-PICs with the detection area of 10 × 10cm2 become
available through several improvements both of the manufacturing process and the electrode structure. The µ-PIC with a detection area of 30 × 30cm2 is now being developed.
In general, MPGDs have a capability of high counting rate up to 106 cps. In orderto obtain the performance eﬀectively, a multi-channel digital readout electronics with
fast operation speed is preferred. The former data acquisition (DAQ) system was developed based on this concept for the two-dimensional MSGC [2] which has 1024 readout
electrodes. This DAQ system consisted of 16 ampliﬁer-discriminator boards, 4 position
encoding modules, and a memory module. Event-by-event position encoding became possible by a pipeline processing method. The basic concept of the new DAQ system forµ-PICs is similar to that for MSGCs. Hence, it consists of ampliﬁer-discriminator boards,
position encoding module, and memory module. In contrast to the DAQ of MSGCs, theposition encoding was processed in one board with a system clock of several ten MHz.
A simple DAQ system was realized by Field Programmable Gate Arrays (FPGAs). The
data acquisition logic is suitably rewritable for various purposes.
There are many applications of the µ-PIC. In X-ray imaging, X-ray crystallography,
solution scattering, and X-ray polarimetry are major applications of µ-PIC. A Time Projection Chamber for charged particle tracking with ﬁne position resolution based on µ-PIC,
which is named “µ-TPC”, is another application. The µ-TPC is a promising detector for
gamma-ray imaging using tracks of recoil electrons due to Compton scattering, thermal
neutron imaging, and dark matter search experiments.
Especially for gamma-ray imaging, the µ-TPC needs to detect minimum ionizing particles such as electrons of sub-MeV energy region. Hence, the required gas gain for aµ-TPC is more than 104 . In order to realize a high gas gain and stability simultaneously,
the electrode structure have to be improved. The optimization of the electrode structureof µ-PIC have been carried out via a computer simulation.
In this thesis, I describe the development of µ-PICs, the optimization of the electrodestructure, the data acquisition system, and several applications of the µ-PIC. In chap-5
ter 2 and 3, the principle of operation and basic concept of MPGDs are described. The
manufacture of µ-PICs and the electrode structure is presented in chapter 4. The DAQ
system of µ-PIC and its performance is described in chapter 5. In chapter 6, I present thebasic performances of the µ-PIC. Chapter 7 is devoted to the attempt for the optimizationof the electrode structure. In chapter 8 X-ray and charged particle imaging propertiesare discussed. Several preliminary experiments for applications of µ-PIC are presented inchapter 9. Finally, I present the summary and conclusion in chapter 10.

Chapter 2
Physical process of gas
multiplication in µ-PIC
In principle, the operation process of the µ-PIC is same as that of a proportional wirechamber. In this chapter, the physical process of gas multiplication is described.

2.1

Drift and diﬀusion of primary electrons in gas

When a charged particle passes through the gas volume, it loses the kinetic energy by the
Coulomb interaction with the electrons of the gas atoms. Then electron and ion pairs are
released along the path of the incident charged particle. The cluster of an electron-ion
pair is called a “electron cloud”. The energy loss per unit length (dE/dx) is an intrinsic
value for a gas and also a function of the energy of the incident particle. When the
kinetic energy of the incident particle is similar to or over the mass of the particle, dE/dx
becomes minimum of typically several keV/cm in atmospheric pressure. The length x isoften replaced with the column density which is a product of length and density in orderto deal the energy loss independently of the medium. In this term, the energy loss isexpressed in the unit of MeV/(g · cm2 ). Fig. 2.1 shows the energy loss of various particles
in the unit of MeV/(g · cm2 ). The minimum energy loss is roughly the equal value formany particles with the unit charge. Then the charged particles of which energy exceeds
the mass of the particle are called “Minimum Ionizing Particles (MIPs)”.
6

2.1. DRIFT AND DIFFUSION OF PRIMARY ELECTRONS IN GAS

7

Figure 2.1: Energy loss of various particles in MeV/(g · cm2 ) [23].

The total number of electron-ion pairs nT released by a charged particle is
nT =

∆E
,
W

(2.1)

where ∆E is the total energy loss in the gas volume, and W is the average energy to
produce one electron-ion pair, or “W-value”. The W-value is typically 20-30eV. On
the other hand, photons such as X-rays or gamma-rays interact with a gas atom via
a photoelectric absorption, a Compton scattering, or a pair production, and emit the
primary electrons, which also ionize gas atoms and release electron cloud as the following
equation (2.1).
A generated electron cloud is drifted toward the anode electrode in an electric ﬁeld
applied in the gas volume while a cluster of positive ions moves to the drift electrode. The
drift velocity of ions w is proportional to the strength of the electric ﬁeld E and inversely
proportional to the gas pressure p, i.e. w = µE(p0 /p), where p0 means the atmosphericpressure (760Torr). The constant parameter µ is called “mobility” of ions, which is
intrinsic to each gas. The value of µ for argon gas is 1.7cm2 /(s · V). The dependences of
w on E and p can be represented by the reduced parameter E/p which is called “reduced
electric ﬁeld”.

CHAPTER 2. PHYSICAL PROCESS OF GAS MULTIPLICATION IN µ-PIC

8

On the other hand, the drift velocity of electrons can not be expressed as a simple
function like that of ions. Therefore experimental data (e.g. Ref. [24]) or results of asimulation is used. The drift velocity of electron is experimentally known to be order of
1cm/µs, which is 1000 times faster than that of ions of ∼ 1cm/ms. In general, the drift
velocity of electrons increases in proportional to the reduced electric ﬁeld E/p while theelectric ﬁeld is not so strong (∼<500V/cm). In a strong electric ﬁeld, the drift velocity isalmost saturated (see Fig. 2.5).
During the drift in the gas, the cluster of electrons or ions is gradually diﬀused due tothe multiple scattering. The diﬀusion of electrons along a given direction x is describedby Gaussian distribution. Usually, the scale of the diﬀusion is described as the standard
deviation at 1cm drift length. A typical size of the diﬀusion becomes the order of 100µm.
In an X-ray imaging detector having a drift length of a few mm, the diﬀusion is several
tens µm.
The examples of electron drift and diﬀusion in individual gas are described in the later
section (Sec. 2.3).

2.2

Gas multiplication

The main process of gas detectors is a gas multiplication of an electron in the strong
electric ﬁeld around the anode. This process is theoretically explained based on Townsend
theory. Here, the gas multiplication in a proportional wire chamber (Fig. 2.2) is explained,
because the physical process is the same as the µ-PIC.
At ﬁrst, primary electrons produced by an incident particle in the gas volume areforced by the applied electric ﬁeld in the chamber. In the moderate electric ﬁeld of∼1kV/cm in the drift region, the primary energy of the drifting electrons in the cloud isimmediately lost by collisions with the gas atoms, and drifted toward the anode electrodes
without any additional ionization. A small amount of electrons are lost by recombination
or attachment. When a low energy electrons collides with a positive ion, the electron
is sometimes combined with the ion. This is called “recombination”, which appears in
extremely weak electric ﬁeld ( 100V/cm). If an electron is captured by a neutral atom,
the electron and atom form a negative ion. This phenomena is called “attachment”. In2.2. GAS MULTIPLICATION

9V

cathode tuberc

anode wire

2r a

Figure 2.2: Schematic drawing of a proportional chamber.

the noble gas this eﬀect is almost negligible.
The electric ﬁeld near the anode electrode increases in proportional to 1/r, where r
is the distance from the center of the anode. Then as an electron goes near the anodeelectrode, the energy of an electron increases. When the energy exceeds the ﬁrst ionization
potential of the gas, the secondary ionization are occurred. These secondary electrons areaccelerated again, and they ionize gas atoms one after another. Hence, the number of thesecondary electron exponentially increases in the proximity of the anode electrode, whichis called an electron avalanche. If λ is the mean free path of an electron for the secondary
ionization, the number of ionization through the unit path of an electron is expressed
as α = 1/λ. This α is the ﬁrst ionization coeﬃcient or the ﬁrst Townsend coeﬃcient.
Consequently, the number of the total electron n after the path length of x becomesn = n0 exp(αx),(2.2)

where n0 is the initial number of the primary electrons. The number of the total electron
rapidly increases. Thus the α is an intrinsic value to the gas, and depends on the electricﬁeld strength. Since the electric ﬁeld depends on the position, α is also a function of
the path length of an electron. Therefore, the multiplication factor M of a detector is
expected as an exponentiated integral of the ﬁrst Townsend coeﬃcient along the drift

10

CHAPTER 2. PHYSICAL PROCESS OF GAS MULTIPLICATION IN µ-PIC

path,





n
M=
= exp α dx .
(2.3)
n0
M depends on the strength of the electric ﬁeld through the dependence of α. Therefore, M
is a function of the applied voltage through the dependence of α on the electric ﬁeld. Here,this multiplication factor does not include the losses of the electrons during the electron
drift via attachment or recombination. On the other hand, the measured gas gain isexperimentally calculated using the charge collected by the anode electrode. Suppose
the energy deposit ∆E of the incident particle is known, the measured gas gain Meﬀ isexpressed as:collected charge
Qcol
=
,
(2.4)
released charge(∆E/W ) · e
where Qcol , W , and e are the collected charge, W-value of gas, and the elementary charge
Meﬀ =(charge of an electron), respectively. Sometimes, some fraction of the electrons are lost
due to the recombination or other factors, and hence the measured multiplication factor
is smaller than the absolute multiplication factor deﬁned as the equation (2.3). Hence,
we deﬁne the electron collection eﬃciency as follows:
Reﬀ = Ncol /Ntot ,(2.5)

where Ncol is the number of primary electrons collected by the anode, Ntot is the total
number of primary electrons. The measured multiplication factor Meﬀ can be also written
using the electron collection eﬃciency Reﬀ as follows:
Meﬀ = Reﬀ M.

(2.6)

Note that the measured multiplication factor is not the absolute multiplication factor
deﬁned by the equation (2.3), but the measured multiplication factor expressed by theequation (2.4). Hereafter, we express the absolute multiplication factor as Mabs insteadof M. Actually, in classical wire chambers, most primary electrons are collected by theanode electrodes, then the measured and the absolute multiplication factors become thesame value. However, a wireless chamber such as an MSGC has a complicated structure,and some electrons attach on the detector structure other than the anode electrodes. In
this case, the degradation of multiplication factor due to this electron loss can not be
neglected.

2.3. GASES FOR DETECTORS

11

In the multiplication process, large number of positive ions are generated. When theseions return to the neutral state via recombination, a photon of UV energy region (∼10eV)
is emitted. Once the UV photon irradiates the electrode material, secondary electron(s)are emitted via the photoelectric eﬀect. This secondary electrons are also multiplied near
the anode electrodes. If the average number of the secondary electrons due to a positiveion exceed 1, this process occurs endlessly and causes discharges. In order to suppress thiseﬀect, some additional gases which absorb the UV photons are mixed with the base gas.
Organic gases such as methane or ethane is normally used as the additional gas, which iscalled “quenching gas” or “quencher”. These organic gases absorb the UV photons andchange the energy into the rotation or vibration of the molecule.

2.3

Gases for detectors

The behaviors of incident photons and incident charged particles in a detector depends on
the physical properties of the gas. The gas in a detector has to be suitably selected taking
into account of various purposes such as X-ray detection or charged particle tracking.
X-ray imaging
Noble gases such as argon and xenon are frequently used in gas detectors, because they
have no electronegativity. In an electronegative gas such as oxygen, the electrons releasedby the incident particle are immediately captured by the electronegative gas. For X-ray
detectors, a gas having a short absorption length of photon is required. As is well known,
the cross section of photoelectric absorption is approximately proportional to Z 5 andE 7/2 , where Z is an atomic number of the material, and E is the energy of an incident
photon, when photon energies are greater than the energy level of the K-absorption edge
and less than the electron mass. Absorption lengths of photon in typical gases are shownin Fig. 2.3 [25]. Argon is a useful gas for easy handling and its low cost. Because of its
high Z, xenon gas is the most suitable gas for this purpose. As well as its high eﬃciency,
xenon gas has another advantage of the short range for photoelectrons emitted from theXe atoms. Therefore, a ﬁne spatial resolution is obtained. Although krypton is a oneof noble gases, it is rarely used for detectors, because it contains the radioactive isotope,

1285

CHAPTER 2. PHYSICAL PROCESS OF GAS MULTIPLICATION IN µ-PIC

Kr which emits beta-ray of the maximum energy of 687keV.

Figure 2.3: Cross sections of photoelectric eﬀect (left) and mean free path (right) [25].

Charged particle tracking
When we design a tracking detector for charged particles, deﬂection of the tracks due tomultiple scattering have to be minimized. While electrons or other charged particles passthrough matter, the incident charged particles are repeatedly scattered by nuclei of thematter. The physical process is expressed as Molière theory [26]. The deﬂection angleof the track is associated with the each atom which composite a molecule. In concrete,
the root mean square of the deﬂection angle is expressed using a radiation length X0 as
follows [27] (see also Fig. 2.4):
13.6 MeV
z
θ=
βcp







x
x
1 + 0.038 ln
X0
X0



,

(2.7)

2.3. GASES FOR DETECTORS

13

where p, βc, and z are momentum, velocity, and charge number of the incident particle,
and x means the thickness of the scattering medium. The gases which consist of heavy
vacuummaterial

vacuumzx

Figure 2.4: Schematic drawing of the multiple scattering.

atoms have small radiation length. The probability distribution of the scattering angle
is approximated by a Gaussian function. Then the average angle is proportional to Z ofthe medium. Since the incident particle undergoes large scattering in heavy gas such as
xenon, light gases such as methane or ethane is more suitable than noble gases for particle
tracking.
Gamma-ray detection
In the detection of the gamma-rays in the MeV energy region, the Compton scatteringis the dominant interaction with a matter. Compton scattering is an elastic scattering
between a photon and an free electron. The cross section of Compton scattering is proportional to the atomic number Z which indicate the number of electrons bound in an
atom. Therefore, large cross section is obtained for heavy gas such as xenon. However,for low energy gamma-ray (∼< 100keV), the energy of scattered gamma-ray is broadened
by Doppler shift in heavy gas. Additionally, the tracks of the recoil electron tend to be
straggled in heavy gas such as xenon. Thus it is diﬃcult to measure the direction of therecoil electron, although this is a key technique for the gamma-ray imaging as describedin chapter 9. On the other hand, since the Compton scattering is an interaction of afree electron, the cross section is a function of only the number of electrons. That is

14

CHAPTER 2. PHYSICAL PROCESS OF GAS MULTIPLICATION IN µ-PIC

independent to whether the electron is bound in an atom or in a molecule. Therefore,
following expression is also possible; the cross section is proportional to the number ofelectrons bound in an atom or a molecule. In the carbon tetraﬂuoride (CF4 ), which has
42 electrons in a molecule, a large cross section is expected for Compton scattering. The
cross section of photoelectric absorption depends on the atomic number of each atomwhich composites a molecule. Therefore, the boundary energy region that the dominantinteraction changes from photoelectric absorption to Compton scattering is dependent on
the gas. The boundary energy for Ar and Xe are about 100keV and 300keV, respectively.
In contrast, since the atomic numbers of carbon and ﬂuorine are less than that of argon or
xenon, the quantum eﬃciency for photoelectric absorption is quite low. Thus, for CF4 gas,
Compton scattering is a dominant interaction even in low energy (< 100keV) gamma-ray.
Furthermore, small deﬂection of the track is expected, because multiple scattering is small
for the gas composed by light atoms. In addition, CF4 has an advantage of the larger
energy loss per unit length (dE/dx). The dE/dx for minimum ionizing particles (MIPs)
of CF4 is 7keV/cm which is about three times as high as that of argon. These physicalproperties are summarized in Table. 2.1 [29]. In this table, the numbers of electron inmolecules are also expressed as Z which is the same symbol as the atomic number.
Gas

Z

Density−3

W [eV]
2[10 g/cm ](dE/dx)MIP

Radiation[keV/cm]

length [m]Ar

18 1.78

26

2.44

110

Xe

54 5.86

22

6.76

15

C2 H6

18 1.34

27

1.15

340

CO2

22 1.98

33

3.01

183

CF4

42 3.93

54

7

92.4

Table 2.1: Physical Properties of gases at 20◦ C and 760 Torr [29].

Electrons in the gases
The drift velocity of electrons, the diﬀusion of an electron cluster, and the ﬁrst Townsend
coeﬃcient are also important parameters which determine the performance of the gas

2.3. GASES FOR DETECTORS

Ar / C2H6 (80 / 20)

CF4 / C2H6 (80 / 20)

14pure CF4

σtrans [µm] for 1cm drift

Drift velocity [cm/µsec]

Xe / C2H6 (70 / 30)

15

103

12
10
8
6
4

102

2
0
0

500

1000

1500
2000
E field [V/cm]

0

500

1000

1500
2000
E field [V/cm]

Figure 2.5: Drift velocities (left) and transverse diﬀusions (right) of electrons in each gas.
Diﬀusion is deﬁned as a standard deviation [µm] for 1cm drift.

CHAPTER 2. PHYSICAL PROCESS OF GAS MULTIPLICATION IN µ-PIC

Townsend coefficient [1/cm]

16

Xe / C2H6 (70 / 30)
Ar / C2H6 (80 / 20)

104

CF4 / C2H6 (80 / 20)103

pure CF4

102

10

1
10

2

10

3

10
E field [kV/cm]

Figure 2.6: The ﬁrst Townsend coeﬃcients in each gases.

detector. In particular, the drift velocity and the diﬀusion are key factors for the performances of Time Projection Chambers (TPCs). These parameters are intrinsic to the gases,
and are expressed as functions of the reduced electric ﬁeld E/p. However, it is not easy
to express these parameters as a simple function. Usually, experimental data or results ofsimulation are used to describe these parameters. As a simulation tool, the MAGBOLTZ
computer program [28] is widely used. The MAGBOLTZ solves the Boltzmann transport equations for electrons in gas mixture under the inﬂuence of a electromagnetic ﬁeld.
Figs. 2.5 and 2.6 show the calculated drift velocity of electrons, diﬀusion of electrons, andthe Townsend coeﬃcients of some gases which are often used in gas detectors. In theseﬁgures, properties of pure CF4 and CF4 /C2 H6 (80/20), Ar/C2 H6 (80/20), and Xe/C2 H6(70/30) gas mixtures are shown.
Short summary
As the results, these gases has following properties:
Ar-based gas High gas gain at the low voltage is expected. The spatial resolution and2.3. GASES FOR DETECTORS

17

detection eﬃciency is not good for high energy X-rays. This gas is suitable for basic
operation test of detectors because of its low cost. Also argon is a useful gas forparticle tracking detectors.
Xe-based gas High detection eﬃciency for X-ray is obtained. The ﬁne spatial resolution
is expected. This gas is suitable for X-ray area detectors, disregarding its high cost.
CF4 -based gas Small multiple scattering and small diﬀusion is expected. Because of
these properties and its large dE/dx, this gas is suitable for particle tracking detectors.

Chapter 3
Micro-pattern gas detectors
The Micro Pixel Chamber is one of the Micro-pattern gas detectors (MPGDs). Before
the discussion of our detector, we review several Micro-pattern detectors developed prior
to the Micro Pixel Chamber.

3.1

MicroStrip Gas Chamber

MultiWire Proportional Chambers (MWPCs) developed in 1968 [30] are commonly usedas large area imaging detectors (Fig. 3.1). In principle, the spatial resolution of MWPCs
is determined by the pitch of the anode wires. However a wire spacing is limited to about
0.5mm due to the repulsive force among the wires.
In the avalanche process, the same number of positive ions as that of the electrons are
generated around the anodes. These ions are drifted toward the drift electrodes. If anelectric ﬁeld of 1kV/cm is applied in a 1cm thick drift space, the drift time of the positive
ions from the avalanche to the drift electrodes is about 0.6msec in argon gas. During thistime, the electric ﬁeld in the chamber is distorted and hence the gas gain become lower,
because the electric ﬁeld is distorted by the space charge due to the positive ions, whichis called space charge eﬀect. Thus the rate capability is determined by the drift time ofthe positive ions; it becomes about 103−4 counts/sec · mm2 .
The Micro-Pattern Gas Detectors (MPGDs) such as MicroStrip Gas Chambers (MSGCs) were developed to overcome these limits of MWPCs. The MSGC was introduced
18

3.1. MICROSTRIP GAS CHAMBER

19

Figure 3.1: Structure of a MWPC and its electric ﬁeld [25].

by Oed in 1988 [1]. The schematic structure of the MSGC developed by our group is
shown in Fig. 3.2. In MSGCs, a narrow spacing of the electrode (< 500µm) was achieved
using alternately arranged anode and cathode strips printed on a insulating substrate instead of wires of MWPCs. This micro structure was realized using a modern lithography
technology. Because of the ﬁne electrode pitch, the spatial resolution of below 100µm
was easily achieved. Furthermore, less positive ions from an avalanche return to the drift
electrode than that of MWPCs, because they are immediately (∼< 1µsec) collected by
the cathode strips placed near the anode strips. Thus the achieved capability for highcounting rate was up to 106−7 counts/s · mm2 , which is more than 1000 times higher thanthat of MWPCs.
Because of these advantages, MSGCs were expected to be a good candidate for highintensity beam such as synchrotron radiation facilities or future high energy experiment.
Our group had developed two-dimensional MSGCs (2D-MSGCs) with a thin (20µm)
polyimide substrate [2] (see Fig. 3.2). The 2D-MSGC has following features: (1) ﬁne
position resolution of < 100µm, (2) rate capability for incident X-ray of 107 cps/mm2 , and(3) ﬁne timing resolution of < 100nsec. These features were utilized for rapid X-ray crystal
analysis and small angle X-ray scattering (SAXS) experiments of protein solutions [3].

20

CHAPTER 3. MICRO-PATTERN GAS DETECTORSDrift planesubstrate(20µm
polyimide)

20

0µ
m

200µm

Back strip(180µm width)
Anode strip(10µm width)

Cathode strip(100µm width)

Base(Ceramic)

Figure 3.2: Schematic structure of MSGC.

However, MSGCs have some serious problems. For two-dimensional readout, 2D-MSGC
has additional electrodes on the rear side of the thin substrate. These back electrodeaﬀect the electric ﬁeld of the drift region. More speciﬁcally, positive ions produced inan avalanche are attracted by the back electrode, and attached on the substrate. The
attached ions weaken the electric ﬁeld near the anode strips. Consequently the gas gain
decrease within a few minutes. In order to avoid this problem, the surface of the substrate
was covered by a thin layer of organic titanium. By this process, the substrate had slight
conductivity of 1012 Ω. The charging-up of positive ions was removed by the conductivity.
Another and more serious problem was the breakdown of electrodes by the discharges,
which caused critical damage to the detectors. Although the mechanism of the discharge
will be discussed in Sec. 4.4, they are attributed to the strong electrode near the cathodeedge. The discharge probability become critical when the gas gain is higher than 1000.
This is the limit of the gas gain for MSGCs.
These problem prevented the practical use of MSGCs. In the practical use, it is necessary to operate detectors in harsh environment. In order to operate stably, the MPGDs of
the next generation need to possess robustness against discharges and suitable electrodestructure to avoid charging-up of positive ions. Additionally, the mass production of a

3.2. INTERMEDIATE GAS MULTIPLIERS

21large area detector is an important factor for a practical use. The Micro Pixel Chamber
have been developed to satisfy these demands.

3.2

Intermediate gas multipliers

The gas gain of the MSGC was restricted to 1000 due to discharges. This gain is not highenough to obtain signiﬁcant S/N for various experiments. One solution to overcome thisproblem is the use of a intermediate gas multiplier. If primary electrons were multiplied
before reaching the MSGC, signiﬁcant amount of charge can be obtained by MSGC witha low gas gain.

Figure 3.3: Microscopic photograph of the GEM [31].

Gas Electron Multiplier A representative detector based on this concept is the Gas
Electron Multiplier (GEM) developed by F. Sauli [7]. The GEM consists of a 50µm
thick copper-clad polyimide foil with high density (typically 140µm pitch) holes. The
microscopic photograph is shown in Fig. 3.3. Since the GEM is manufactured using
Printed Circuit Board technology, mass production is realized at low cost. High voltage
is applied between both sides of the GEM. Primary electrons are multiplied in the strong

CHAPTER 3. MICRO-PATTERN GAS DETECTORS

22electric ﬁeld formed in each hole. When a GEM is operated at a gas gain of about 100,signiﬁcant signal is obtained by an MSGC with low gas gain (∼ 100). Then total gas gain
can exceed 10000 [32].
Although the attainable gas gain of single GEM is limited to several hundred, a higher
gas gain over 106 can be obtained by combination of multiple GEMs. Even one electron
is detectable by the multiple-GEM detector, which is used as a position sensitive gaseous
PMT for UV and visible photon imaging [9, 13].
Capillary plate The gas multiplication in the capillary plate was conﬁrmed by Sakurai [33]. It consists of a bundle of ﬁne glass capillaries with electrodes on both the inlet
and the outlet as shown in Fig. 3.4. Electrons are multiplied in the electric ﬁeld appliedin each hole. In our group, the capillary plate having the detection area of 95 × 95mm2
was used as a intermediate gas multiplier for the MSGC. Fig. 3.5 shows the schematic
structure of the hybrid system of an MSGC and a capillary plate [5, 6]. The high gas gains
more than 103 is achieved by the combination of the MSGC and the capillary plate, where
gas multiplication in each step is less than 100. The multiple capillary plates combinedwith an MSGC was also developed. The gas gain of 5 × 103 was achieved [12].
100µminconellead glass

1mm

30µm

Figure 3.4: Schematic view of a capillary plate.

A common feature of intermediate gas multiplier such as GEM or capillary plate is thatthey suppress discharges by the reduction of gas gains of each multiplier. There remains
some risk of discharges for the intermediate gas multiplier. Additionally, these ﬂoating3.3. PIXEL ELECTRODES

23

X-ray
Drift plane
HV

Ar:C2H6

Volume_A

4mm
R1

∆VD_U

100µm
Upper plane

∆VU_L

1mm
Lower plane

R2

4mm

30µm

Volume_B

∆VL_M

MSGC
Figure 3.5: Capillary-MSGC combined system.

structures make the detector complicated to develop a large area detector. Therefore, adetector with simple structure and high attainable gas gain is required for a large area
gaseous detector.

3.3

Pixel electrodes

One solution to obtain high gas gain is to use a pixel shaped electrode. The concentrationof electric ﬁeld lines on a pixel electrode is denser than that of a strip electrode for acertain voltage. Then a high gas gain is expected for a pixel detector.
The MicroDot (MDOT) detector is the ﬁrst micro-pattern detector having the pixel
type anodes [15]. A schematic structure and a cross-sectional view of the MDOT is shownin Fig. 3.6. The MDOT detector is manufactured on a silicon wafer based on the integral
circuit (IC) technology. Anode readout buses are buried in a 5µm thick SiO2 layer, andpixel anode is formed through the insulator layer. The gas gain of more than 104 isreported for stand-alone MDOT detector [34].
Since the depth of anode readout buses is only 2µm, the electric ﬁeld of the avalanche
region is strongly distorted by the readout buses. This may cause a degradation of gas

24

CHAPTER 3. MICRO-PATTERN GAS DETECTORS

Figure 3.6: Structure of MDOT detector [15].

gain. The relation between the gas gain and the substrate thickness is discussed in later
chapter. Additionally, the manufacturing technology for MDOT is diﬃcult to apply for alarge area detector. Large area MDOT detectors have not yet reported.

3.4

New manufacturing technique

In order to make a large area detector, the Printed Circuit Board (PCB) technology ispreferred rather than the IC technology. A detector manufactured by the PCB technology
have following features of expandability for a large area detector and mechanical robustness. As well as GEM, various type of detectors have been developed in the late 1990s,
for example, Micro-Groove Detector (MGD), WELL detector, and Micro-Wire detector.
Basically, above new detectors have the structure similar to a GEM. That is a 50µm thickmetal-clad polyimide foil and regularly formed electrodes. These structures are schematically shown in Figs 3.7. An electric ﬁeld is applied between the upper metal (cathode)and the lower metal (anode). Primary electrons are multiplied in the gap between thetwo metal layer similar to a GEM. The multiplied electrons are collected by planar anode3.4. NEW MANUFACTURING TECHNIQUE

25

electrodes.

(b)

(a)

(c)
Figure 3.7: Schematic structures of several types of MPGDs. (a) Micro-Groove Detector [16], (b) WELL detector [17], and (c) Micro-Wire detector [19].

Although the gas gains of more than 104 are obtained for these detectors, they arelikely to be used as a readout device for a combined detector system with the GEM.
For this use, a gas gain of the readout detector is not so high (∼< 103 ), then discharge is
avoided.
The Micro Pixel Chamber is a new gaseous imaging detector which has the featuresof the pixel type detectors and PCB-based detectors.

Chapter 4
Development of the Micro Pixel
Chamber
4.1

Electrode structure

Fig. 4.1 shows a schematic structure of the µ-PIC. This detector is made using a doublesided printed circuit board (PCB) technology. Anode and cathode strips are orthogonallyarranged on both sides of the insulating substrate with a thickness of 100µm, respectively.
This substrate contains a glass cloth (prepreg structure) as a reinforcement in order tosuppress deformation in the fabrication process. Each cathode strip has 400µmφ circularholes with a pitch of 400µm. 50µmφ cylindrical anode electrodes are formed on the anodestrips and pierced at the center of each cathode hole.
Using both signals from the orthogonal anodes and cathodes, two-dimensional readout
is easily realized. In 2D-MSGCs which have back electrodes on the rear side of thesubstrate, the induced charge on the back electrode due to cloud of ions around theanode is used for two-dimensional readout (see Fig. 3.2).
In the case of MSGCs with a thin substrate, the back electrode which is grounded isplaced on the opposite side of the anode electrodes having a positive electric potential forthe cathode. Then the vectors of the electric ﬁeld around the anode direct the substrate
due to the back electrodes. In the other words, as described in Sec. 3.1, the positive ionsgenerated in an avalanche is immediately attached on the substrate. This charging-up
26

4.1. ELECTRODE STRUCTURE

27

Drift plane

400µm

Cathode
Anode

400µm

100µm
50µm

Figure 4.1: Schematic structure of µ-PIC.

of positive ions causes a decrease of the gas gain [35]. In contrast, the back electrodesof µ-PIC is connected to the pixel anodes. Hence, all electric ﬁelds on the substrate run
upward. Then there are no charging-up of positive ions on the surface. The thickness ofthe substrate is relatively thick and similar to the distance between the anode and thecathode. This is the one of the outstanding features of our µ-PIC compared with theother pixel detectors made of silicon wafer using IC technology. The electric ﬁeld due toback electrodes can be weakened at the surface of the substrate. Thus the electric ﬁeld is
not distorted near the substrate by the back electrodes. These design parameters such as
the substrate thickness and the anode diameter will be discussed in detail in chapter 7.

High-voltage supply
Now, 10 × 10cm2 µ-PICs are available for performance tests. The µ-PIC is mounted on
the printed mother-board, on which ampliﬁers are connected to all readout electrodes(Fig. 4.2). In order to avoid the whole voltage down at discharges, the high voltage isindividually applied to every 32 anode strips as shown in Fig. 4.3. At ﬁrst, the highvoltage is divided into eight branch. Each branch has a 1GΩ resistor, and divided into 32
anodes through 1MΩ resistors. All cathode strips are grounded through 1MΩ resistors on

28

CHAPTER 4. DEVELOPMENT OF THE MICRO PIXEL CHAMBER

the mother-board. Once a discharge occurs at a certain electrode, the current due to thedischarge may aﬀect other electrodes and cause sequential discharges. The 1GΩ resistors
protect the electrodes from the discharge current.

Figure 4.2: Photograph of the printed mother-board of the µ-PIC.

4.2

Two-dimensional readout

Since MSGCs have parallel anode and cathode electrodes, two-dimensional readout was
not so easy. For two-dimensional readout, it has the backstrip electrodes which are arearranged orthogonally to the anode and the cathode electrodes for our thin substrate
MSGC. When an avalanche occurred around an anode, the charge is induced in thebackstrip electrode. The amount of the induced charge is a diﬀerence between the charges
collected by the anode and the cathode. Since major part of the total charge is collectedby the cathode, the pulse height from backstrips are reduced to 20∼30% of those fromanode electrodes [36]. In contrast, the µ-PIC has the orthogonal anodes and cathodes
which collect the charge directly. Therefore, two-dimensional readout is easily realized
without any other additional electrodes. Furthermore, as described above, the signal
charges from the anode and the cathode is almost same. Fig. 4.4 shows a signal from

4.3. FABRICATION OF µ-PIC

29

Anode HV

10MΩ
1M Ω
1G Ω
0.01 µ F

1M Ωfrom detector
to pre-amplifier

100pF

from detector
to pre-amplifier

100pF
1M Ω

from detector
to pre-amplifier

1M Ω
1G Ω

1M Ω

100pF
from detector
to pre-amplifier

100pF

from detector
to pre-amplifier

100pF

Figure 4.3: Diagram of the circuit to divide the applied voltage to each anode electrode.

both the anode and cathode electrodes for one event.

4.3

Fabrication of µ-PIC

We have developed several types of µ-PICs in order to achieve a high gas gain of ∼ 104
and stable operation simultaneously. We have examined several manufacturing processes
and the electrode structures, which are brieﬂy summarized in Table 4.1.

Prototype µ-PIC
The manufacturing process of the µ-PIC with a polyimide substrate is shown in Fig. 4.5.
The key technique of the manufacturing is an anode forming. In the case of the prototypeµ-PIC [20, 21] which has a detection area of 3 × 3cm2 on the polyimide substrate, electric
plating technique is used for the electrode formation. Copper was deposited in CuSO4
solution from the bottom of the via-hole of 50µm diameter drilled by laser. After theformation of the electrode, the surface of the electrodes was coated with nickel and gold inCHAPTER 4. DEVELOPMENT OF THE MICRO PIXEL CHAMBER

30

Figure 4.4: Signal shapes from both the anode and cathode electrodes.

Detector

Substrate

Electrode

Anode heightb (r.m.s.)Cathode widthamaterialmaterials

prototype

PolyimideCu, Ni, Auµ-PIC1

Ceramics

W, Ni, Auµ-PIC2

PolyimideCu, Ni, Au−40µm (5µm)

336µmµ-PIC3

PolyimideCu, Ni, Au−15µm (3.6µm)

314µmµ-PIC4-7

PolyimideCu, Ni−20µm (∼ 3µm)

314µm

< 5µm (33µm)∼+10µm (∼
< 2µm)

359µm
340µm

a: Nickel and gold were used for plating of the electrode surface.
b: Distance between the surface of the substrate and the top of an anode pillar. Negative value means
anode electrodes are below the surface.

Table 4.1: Summary of the µ-PICs developed by 2002.

4.3. FABRICATION OF µ-PIC

31

order to protect against oxidation. During the plating process, hydrogen gas is generated
from the anode electrodes. The gas bubble blocks up the via-hole (Fig. 4.6), and thenanode forming is stopped. Sometimes, other anodes were continuously formed over the
via-hole without bubble blocking. Hence, the anode heights become quite random. The
number of the anodes which were higher than the surface of the substrate was about 50%of the 75 × 75 pixels. The deviation of the anode height from the average was about 30µm(rms), which caused also the large non-uniformity of the gas gain.

Figure 4.5: Manufacturing process of a polyimide-based µ-PIC.

Ceramic-based µ-PIC
The manufacturing process of the ﬁrst µ-PIC (µ-PIC1) having a large area of 10 × 10cm2
was quite diﬀerent from that used for the prototype detector in order to obtain theuniformity of the anode height. It is shown schematically in Fig. 4.7. Instead of theelectronic plating, the electrodes of µ-PIC1 were formed using conductive paste made frommainly tungsten. In this case, anode and cathode electrodes are “printed” all together.
The ceramic (Al2 O3 ; alumina) substrate is more suitable than polyimide for this process.

32

CHAPTER 4. DEVELOPMENT OF THE MICRO PIXEL CHAMBER

H2 gascathodeanode

Figure 4.6: Schematic drawing which indicates the blocking of anode formation due to ahydrogen bubble.

Then we manufactured a ceramic-based µ-PIC having the detection area of 10 × 10cm2 .
The size of a ceramic substrate is limited by a deformation of substrate in the sintering
process. Very uniform electrodes was realized by this method as shown in Fig. 4.8.
Although the µ-PIC1 has very uniform shape electrodes, we found a serious problem
of discharges. The anodes and cathodes had been broken down after a few discharges if
the µ-PIC1 operated in Ar-C2 H6 mixture gas. The longest operation time was about 1
hour at the gas gain of ∼ 1000. The electrodes of the ceramic-based µ-PIC often became
conductive after a few small discharges with the resistance of several MΩ whereas the prototype (polyimide-based) µ-PICs endured several tens discharges. The damaged electrodeis shown in Fig. 4.9. When the electrode became conductive, the substrate was severely
damaged and changed into black. The polyimide substrate looked to be dissociated dueto the high temperature of a spark. In contrast, ceramics is never carbonized. Therefore,
other reason of conductivity have to be considered. This conductivity seems due to thestuck of the carbon on the surface of the ceramic substrate that is produced from ethane
gas in a discharge process. Ceramics (alumina) has a porous surface. This surface act as
an adsorbent material. Then the carbon dissociated from the ethane molecule is stack on
the surface of the substrate. Although it was hardly observed for the prototype µ-PIC,
the ceramic-based µ-PIC was always suﬀered from discharges. Therefore, the polyimide
substrate is preferred for stable operation. This carbon accumulation was not observed
for the operation in inorganic gases such as Ar-CO2 mixture gas. In the case, however,the µ-PIC was not operated stably.

4.3. FABRICATION OF µ-PIC

Figure 4.7: Manufacturing process of a ceramic-based µ-PIC.

Figure 4.8: Microscopic photograph of a ceramic-based µ-PIC.

33

34

CHAPTER 4. DEVELOPMENT OF THE MICRO PIXEL CHAMBER

Figure 4.9: Microscopic photograph of a damaged electrode of µ-PIC1.

Polyimide-based µ-PIC
All µ-PICs (from µ-PIC2 to µ-PIC7) made later had a polyimide substrate. Although
the basic structure and the manufacturing process are the same as those of the prototypeµ-PIC, the uniformity of the electrode formation have to be dramatically improved forthe larger area detector. For this purpose, following improvements were carried out for
the fabrication of large area polyimide-based µ-PICs.
Careful cleaning The non-uniformity of the anode height was caused by the ﬂuctuationof the start timing of the anode growing due to the pollution in the via-hole. A carefulcleaning of the bottom of each via-hole using ultrasonic cleaning was carried out to remove
such a pollutants.
Shape of a via-hole The shape of the via-hole plays an important role to remove
a hydrogen bubble in the via-hole. The via-hole shape of µ-PIC2-7 is slightly tapered,and the surface was desmeared. The desmear process is used to remove tiny pollutants
attached on the surface of the via-hole. These processes help a bubble to be removed
smoothly from the via-hole.
By these improvements, the blocking up of the via-hole by hydrogen bubble was almost
avoided. The deviation of the anodes was decreased from 33µm (prototype µ-PIC) to
5µm. In µ-PIC2, the anode formation is stopped at the anode height of 40µm below
the substrate in order to suppress the abnormal deposition of copper which is called4.3. FABRICATION OF µ-PIC

35“dendrite”. The photograph of an abnormally grown anode is shown in Fig. 4.10. Thisﬁgure shows the dendrite observed in µ-PIC2. The dendrites should be inactivated bypotting silicone rubber on the cathode hole because the extremely high electric ﬁeld
around the dendrites causes non-uniformity of gas gain or discharges.

Figure 4.10: The abnormally grown anode which is called “dendrite” (observed in µPIC2). The right ﬁgure shows a cross-sectional image of the dendrite.

In order to remove the bubble completely and make uniform electrodes, further improvements were carried out.
Rocking the substrate In order to accelerate the removal of the hydrogen bubble, thesubstrate was rocked in the solution during plating. Furthermore, periodic shock is also
eﬀective for this purpose. In addition, this process help the circulation of the solution.
In order to rock the substrate eﬀectively, a large plating tank was specially prepared forµ-PIC6 and 7.
Rate of plating During the plating, cathode electrode (made of copper) is gradually
dissolved due to the sulfuric acid contained in the solution. Therefore, a ﬁne tuning of
plating rate and composition of the solution is necessary to form a high anode pillar and

36

CHAPTER 4. DEVELOPMENT OF THE MICRO PIXEL CHAMBER

wide cathode strips simultaneously. The period of plating was adjusted within 20 hours.
Consequently, the width of the cathode strips (∼ 314µm) become narrower than that ofprototype µ-PIC (∼ 340µm).
After these improvements, uniformity of the anode height was kept within 3µm (rms).
However, the formation of the anode electrodes were stopped when the top of the anodes
reach 20µm below the surface of the substrate. This depth was inevitably needed in order
to avoid the dendrites. Consequently, the number of remained dendrites were decreased
within 3% of the whole anode pixels. The microscopic photograph of the polyimidebased µ-PIC is shown in Fig. 4.11. Attachment of carbon dissociated from ethane due

Figure 4.11: Microscopic photograph of a polyimide-based µ-PIC.

to discharge was hardly observed for the polyimide-based µ-PICs. Therefore, damageof discharges is much smaller than that of ceramic-based µ-PIC. Thus the stability was
drastically improved for the polyimide-based µ-PICs. The long time continuous operationof more than 1000 hours was realized with the gas gain of 3000 as described in chapter 6.
Although we had succeeded to suppress the break-down of the electrode of the µ-PIC,
following problems still remained. The ﬁrst problem is loss of the cathode strips during
the plating process. If the cathode strips are narrow, the electric ﬁeld near the cathode4.4. DISCHARGE

37electrode is not uniform. Second, some ﬂuctuation of anode height still remained within∼ 3µm. As described in later section (Sec. 6.1), this problem quite aﬀects to the gas
gain uniformity. At last, the top of all anode electrodes are 20µm below the surface ofthe substrate. The drift of the primary electrons are partly prevented by the edge of thesubstrate. This problem is discussed in chapter 7.

4.4Discharge

4.4.1

StreamerElectrons in the avalanche are spherically diﬀused due to the thermal ﬂuctuation during
the drift process. On the other hand, the positive ions are regarded as stopping at theposition of ionization, because the drift velocity of positive ions is much slower than thatof electrons. Thus, the shape of the growing avalanche is like a water drop consisting ofa spherical electron cluster and positive ion tail [25]. When the strength of the electricﬁeld generated by the space charge of the avalanche is similar to the drift electric ﬁeld,
the avalanche does not grow any more. In this phase, the strong electric ﬁeld is localized
in front and behind the primary avalanche. The secondary ionization is occurred by theUV emission from the excited atoms in the avalanche cloud, and secondary avalanches
grow in front and behind the primary avalanche. Eventually a long ﬁlament-like forward
and backward charge propagation appears. This become a conductive channel called“streamer”, which leads a fast discharge. These processes are schematically shown in
Fig. 4.12.
Once the total charge in one avalanche exceeds 107−8 e, the streamer formation rapidly
proceeds [38]. This limit charge is known as Raether limit. For a heavily ionizing particle(such as an α particle), about 104 electron ion pairs are typically created, and the maximum gas gain to avoid the Raether limit is between 103 and 104 . Thus the gas gain of
detectors is restricted so as to control the total charge of the avalanche within the Raether
limit.

CHAPTER 4. DEVELOPMENT OF THE MICRO PIXEL CHAMBER

38

anode

+

-- -- -+ +
++
+
+
+

(1)

(2)

E

-- -+
- -- -++- -+ +++++--- +++ +---+++(3)- - - +-+ +-+- ++ - -+ - + +- ++ - - - ++- - + + -++ +- - +- ++- +- +(4) - + + - ++ +- + +cathodeFigure 4.12: Growth of an avalanche into a streamer. (1) The initial ionization occurs in (2) The electron avalanche grows in the strong electric ﬁeld. (3) The
the electric ﬁeld E.
secondary avalanches occur by localized electric ﬁeld near the primary avalanche. (4) The
avalanche grows up to a conductive plasma called a streamer.

4.4. DISCHARGE

4.4.2

39

Field emission

Discharges in a gas detector are usually triggered by an electron. In ordinary operation,
electrons are generated only by the incident particles. However, discharges may occur
without irradiation. Thus, in this case, the electron(s) generated somewhere in the detector triggers a discharge. In a high electric ﬁeld, electrons are emitted from the surface of
metal due to the tunnel eﬀect [39]. The current density j due to the electron emission isexpressed by Fowler-Nordheim’s equation as follows:




E2
φ3/2
j = 1.55 × 10−6,
exp −6.83 × 107 v
φ
E

(4.1)

where E is the electric ﬁeld, and φ is a work function of the metal, which is between 4
and 5 for many metals. v is a correction term of the work function in the electric ﬁeld.
Here, the value of v is close to 1 [40]. The threshold electric ﬁeld to avoid discharges canbe calculated using this equation by measuring the rate of discharges. When discharge
occurs with the rate of ∼ 1/min, the current density becomes the order of 10−21 A/cm2 .
Then the threshold electric ﬁeld is about 800kV/cm. While the electric ﬁeld near theelectrode exceed this value, ﬁeld emissions are initiated and trigger a discharge.

4.4.3

Discharges in MicroPattern detectors

Surface discharge
The electrodes of MPGDs are usually formed on the surface of the substrate. In otherwords, MPGDs have “triple junctions” which consists of the electrodes, the dielectric(substrate), and the gas. The electric ﬁeld near the triple junction becomes strong due tothe discontinuity of the dielectric constants among the materials. In this case, break-down
due to the “surface discharge” or the “surface breakdown” from the triple junction is an
important process of the discharge. In particular, the strong electric ﬁeld which exceeds
the threshold of ﬁeld emission may be localized near the triple junctions of the cathode.
In the strong ﬁeld, electrons can be emitted from the cathode by a ﬁeld emission process.
When this electron collides the surface of the dielectric, the surface is positively charged
up through the Secondary Emission Electron Avalanche (SEEA) mechanism [41]. It is

40

CHAPTER 4. DEVELOPMENT OF THE MICRO PIXEL CHAMBER

considered that this positive charging-up proceeds to the surface discharge, although thedetailed process from charging-up to discharge is not clear yet.

Malter eﬀect
If the organic gases such as methane or ethane are used as quenching gases, some molecules
are dissociated by the incident particles. Then free radicals are formed, which is polymerized and stuck on the electrodes due to electrostatic attraction [42]. Once a thin insulator
ﬁlm is stuck on the surface of the cathode the positive ions accumulate on the insulator
layer, which acts as a capacitor. The electric ﬁeld in the insulator may become strong
enough to occur the ﬁeld emission. This electron emission caused by a thin insulator iscalled “Malter eﬀect” [43]. This is a kind of dielectric breakdown. The Malter electrons
cause a self-sustained discharge [44]. Sometimes, the electrons can be emitted all at once
like a jet, which is considered to be a main break-down triggering mechanism [45].

Surface conductivity
When the gas used in the detector contains an organic gas as a quencher, the molecule ofthe organic gas is dissociated by the high temperature due to a discharge. Then a clusterof carbon is produced along the discharge path. In MPGDs, the radical carbon may
be accumulated on the surface of the substrate between an anode and a cathode, whichmakes the surface of MPGDs conductive. The tendency of the accumulation depends
on the material of the substrate. If the substrate is made of a porous material such asalumina, hydrocarbon such as ethane is easily dissociated and attached on the surface,because the porous material act as a adsorbent. On the other hand, the polyimide surface
has less adsorbent property. Thus the carbon accumulation rarely occurs for the polyimide
substrate. However, once the surface becomes rough due to discharge damages, the carbon
maybe accumulate on the surface, and causes a breakdown. Also, the polyimide substrate
may be carbonized by a high temperature due to a discharge, which becomes a bridge ofconductivity.

4.4. DISCHARGE

41

Discharges in MSGC and µ-PIC
In MSGCs, the electric ﬁeld near the cathode edge becomes more than 100kV/cm which
is high enough for a gas avalanche. Therefore, once an electron is ejected from the cathodedue to ﬁeld emission, the electron undergo the gas multiplication near the cathode edge.
The multiplied electrons directly drift toward the anode and multiplication occurs again.
As a result, gas multiplication factor increases up to 105 [46]. The maximum gas gain of
MSGCs is restricted to ∼ 1000 by this multiplication due to electron emission from thecathode edge. In order to suppress the electron emission, the MSGC of which cathodeedge was covered by polyimide was developed [47]. In this case, it is necessary to consider
the accumulation of positive ions on the polyimide.
In µ-PIC, electrons emitted from the triple junctions run on the surface of the substratewith avalanche multiplication. Then these electrons may cause the surface discharge. In
any case, the discharges looks triggered by the electron emission in a high electric ﬁeld
near the cathode edge. In order to suppress discharges, we have attempted weakening thestrength of electric ﬁeld near the triple junction of the cathode edge, which is discussed
in later chapter.

Chapter 5
Data Acquisition System
The data acquisition (DAQ) system is one of the most important parts in the developmentof µ-PIC to handle a huge number of signals from ﬁne electrodes of µ-PIC. In this section,
the overview of the DAQ system for the imaging and its performance are described.

5.1

Overview of the DAQ system

The narrow electrode spacing of µ-PIC allows us to operate in high counting rate up to
107 counts per second and obtain a very ﬁne position resolution. In order to obtain thesefeatures, the fast readout electronics is inevitable, which handle a large number of signals
from 256 anodes and 256 cathodes of 10 × 10cm2 µ-PIC. In order to apply the µ-PIC
as a real time imaging detector, a data acquisition rate of more than 20MHz is required.
Especially for charged particle tracking using a TPC, the spatial resolution along the drift
direction is determined by the processing speed. For example, in a gas with the electron
drift velocity of 4.5cm/µsec, the spatial resolution for drift direction would be less than
1mm for the DAQ system operated with 50MHz, whereas that is 2.3mm for the DAQ rateof 20MHz. The analog data processing is ill-suited for the high-speed and multi-channel
data acquisition which is required for the µ-PIC. Alternately, we have developed the DAQ
system which handle all signals digitally.
The DAQ system for two-dimensional imaging consists of following parts:• 16 ampliﬁer-shaper-discriminator (ASD) boards,
42

5.1. OVERVIEW OF THE DAQ SYSTEM

43• one position encoding module,• one memory module,• one CPU board.
The block diagram of this DAQ system is shown in Fig. 5.1.
Position
encoding
moduleAMP-Discri board

HV
Micro TPC

Anode
Analog256ch
PreAMP

CathodeLVDS
Digital256ch... Discri.

Anode-position
Cathode-position

16ch sum AMP...

Analog Sum

16ch sum AMP

LVDS
Clock
-Counter

Digital33ch

MemorymoduleEvent-Number

AMP-Discri board
LVDS

AnalogAnalog256ch

Digital
256ch
16ch sum AMP...

Analog Sum

Figure 5.1: Block diagram of the DAQ system.

Pulses from anodes and cathodes are individually ampliﬁed by ASD boards (Fig. 5.2).
An ASD board has 16 ASD chips which were originally developed by KEK for ATLAS
Thin Gap Chambers (TGC) [48]. The ASD chip consists of a fast preampliﬁer (gain≈ 0.8V/pC), shaping amp (gain = 7), and discriminator. One ASD chip has four inputs,
four discriminator outputs, and four analog outputs from each preampliﬁer. The block
diagram of one channel of the ASD chip is shown in Fig. 5.3. Each 16 analog signals of
32 channels are summed at the end of the board. The discriminator output of ampliﬁer
boards is LVDS diﬀerential signal, which are directly fed to the FPGA chips in the position
encoding module.
Because the ASD signal from TGC is originally used for the trigger decision, theintegration constant is very fast (τ = 16nsec). However, this integration constant is too

44

CHAPTER 5. DATA ACQUISITION SYSTEM

16cm

16cm
Figure 5.2: Photograph of an ASD board.

Figure 5.3: Block diagram of the ASD chip.

5.1. OVERVIEW OF THE DAQ SYSTEM

45

short to collect all pulse charges from the µ-PIC. Therefore, the measured pulse heightof the analog output was lower than that expected from the gas multiplication. By thesimulation, if the integration constant of the preampliﬁer were extended to 80nsec, thesignal from the µ-PIC would be twice higher than that of using 16nsec integration constant
ampliﬁer [49]. The output signal of the improved ASD was compared with the original
ASD for TGC. Fig. 5.4 shows the signals of the 5.9keV X-ray obtained by µ-PIC withthe original ASD (upper signal) and the improved ASD (lower signal) [49]. Thus we havedeveloped the new ASD with the integration constant of 80nsec.

Figure 5.4: Upper signal and lower signal were obtained by original and improved ASDs,
respectively.

The digital signals from ASD boards are fed to the position encoding module, where
the position of the hit electrode is encoded. Fig. 5.5 shows the position encoding module,
which consists of ﬁve Field Programmable Gate Arrays (FPGAs; Xilinx Virtex-E). These
FPGAs accept LVDS signals without any external terminators. The position encoding
logic of FPGAs used here can be rewritten for various purposes. This system operates
with the clock of 20MHz or 40MHz. In one event all signals are synchronized to the clock
and processed in one clock using a pipeline processing.
The hit position of X-ray is encoded as following (see also Fig. 5.6). 256 channel LVDS
signals from anode are fed to two FPGAs. At ﬁrst, these signals are synchronized to the

46

CHAPTER 5. DATA ACQUISITION SYSTEM

Figure 5.5: Photograph of the position encoding module.

clock in each FPGA (Fig. 5.7), then the minimum and maximum channel position of the
hit anode electrode are also encoded here. Simultaneously, the minimum and maximum
channel position of the cathode is also encoded in the other two FPGAs by the same
method. These information from four FPGAs are fed to the ﬁfth FPGA, which requires
the both hits in the anode and cathode in the same internal clock, and encode the numberof hit electrodes (corresponding to the hit width) and the both center positions for anodehits and cathode hits. Here, an event with the hit width of less than or equal to 8 isregarded as a valid event. An event having the width of more than 8 is rejected as a noisy
or an accidental event. Resultant encoding information of output signal is fed to the VME
memory module as a sequence of 33bit parallel signal, which consists of 32bit main signal(position, width, etc.) and auxiliary 1bit WRITE signal. The correctly encoded signals
should have the WRITE signal as well as the 32bit main signals. This encoding logic
obtains an arithmetic mean of the hit signals whereas weighted mean of charge is often
used for conventional wire chambers. Since the size of a primary electron cloud is similar
to the electrode pitch of the µ-PIC, the spatial resolution of about 100µm is expected by5.1. OVERVIEW OF THE DAQ SYSTEM

47

this simple method.

Figure 5.6: Block diagram of the position encoding module.

(b)(a)
Figure 5.7: Circuit diagram (a) and a timing chart (b) of the synchronizing circuit [50].

The information obtained by the position encoding module are sent in LVDS level
through a 33 channel cable to a VME6U-based 32 bits memory module. This module
consists of SRAM memories of 32MByte in total, because high-speed read and write
operation is possible for SRAM. Fig. 5.8 shows a photograph of the memory module.
The validity check is carried out hear using the WRITE signal, and the accepted signals

CHAPTER 5. DATA ACQUISITION SYSTEM

48

are accumulated in the SRAM memories. This memory module is controlled by the CPU
board, which also controls other modules concerning with the DAQ system, such as ADCs.

Figure 5.8: Photograph of the memory module.

5.2

Performance

The performance of the DAQ system for the high counting rate was studied using an
X-ray generator (Kevex X-ray CU028, tungsten target). The acceleration voltage of thegenerator was 20kV and the intensity was controlled by varying the tube current between
0.01mA and 0.6mA. The DAQ was operated with the clock of 40MHz. The result of thehigh rate test is shown in Fig. 5.9, where the data acquisition rate against the tube currentis plotted. We achieved the maximum data acquisition rate of 7.7MHz, which is twice
higher than that of the previous DAQ system developed for two-dimensional MSGCs [6].
This system has a capability of 40×106 event per second for synchronous events. However,
X-ray usually enter the detector at random, and the duplication hits within one clockdegrades the capability. The number of incident events in one clock follows the Poisson
distribution. Therefore, the expected data acquisition rate R is calculated by the following

5.2. PERFORMANCE

49

formula:R(r) = r · exp(−r/c),(5.1)

where, r is a real event rate, and c is the clock cycle of the DAQ system. The maximum
value of R(r) is c/e at r = c. When the clock is 40MHz, the theoretical limit of the dataacquisition limit is 14.7MHz. The obtained counting rate was about half of the expected
counting rate from the Poisson distribution. The saturation is thought to be due to thetime-lag between the signal lines in the FPGAs.

Data acquired rate[Mcps]

8
7
6
5
4
3
2
1
0
0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

X-ray generator current[mA]

Figure 5.9: Result of the high rate test of the µ-PIC and the DAQ system.

Chapter 6
Performance of the µ-PIC
The performances of the µ-PIC, about gas gains, energy resolution, and long time stability
are described here. This study becomes a basis for all applications of the µ-PIC.

6.1

Performance test of the µ-PIC

The detectors were operated in Ar/C2 H6 (80/20) gas mixture at the atmospheric pressure.
The depth of the drift space was 1.2cm and an electric ﬁeld of 500V/cm was applied. The
positive high voltage was applied to anode electrodes, and all cathode electrodes weregrounded. Here the polyimide-based µ-PIC with lower anode (µ-PIC4-7) is described forall performance tests in this section.
µ-PIC was irradiated by a non-collimated radioactive source of 55 Fe (5.9keV), and theoutput charge of the sum of 32 × 256 pixels was measured. The gas gains were calculated
from the obtained charge. Fig. 6.1 shows the obtained gas gains as a function of theanode voltage, which are plotted as ﬁlled squares. The maximum gas gain of 1.5 × 104
was achieved, which is about 10 times higher than that of stand-alone MSGCs. Stable
operation was attained at the gas gain of between 3000 and 5000.
The obtained spectrum of 55 Fe source (5.9keV) was shown in Fig. 6.2 In this experiment, 32 × 32 pixels (12.8 × 12.8mm2 ) were used for signal readout. The spectrum
obtained from this area is shown in Fig. 6.2 (a). An escape peak is seen at 2.7keV as
well as the main peak of 5.9keV. Achieved energy resolution of this area was about 30%
50

51

Gas gain

6.1. PERFORMANCE TEST OF THE µ-PIC

104

103

500 520 540 560 580 600 620 640 660
Anode voltage [V]Figure 6.1: Gas gain curve of the µ-PIC as a function of the applied voltage.

(FWHM). The spectrum from whole detection area is also shown in Fig. 6.2 (b). This
spectrum was obtained using a Flash ADC (FADC). In this case, the escape peak is notclear. The energy resolution of this area was about 50% (FWHM).
The position dependences of the gas gain are measured for all detection area of theµ-PIC4. The detection area was divided into 8 × 8 regions, and gas gain was measured
from the peak of the spectrum for each region which consists of 32 × 32 pixels. The resultis shown in Fig. 6.3. There observed a large ﬂuctuation of the gas gain. The maximum gas
gain was about 5 times higher than the minimum gas gain. The standard deviation of thegas gain was more than 40%. All polyimide-based µ-PICs with the lower anodes have asimilar tendency of the gas gain ﬂuctuation. This deviation can be attributed to the nonuniformity of the anode heights of which deviation was 3-5µm. This non-uniformity of thegas gain causes unstable operation. If there are some spot of extremely high gas gain, themaximum voltage for stable operation is governed by the high gain area. Consequently,
low gas gain will obtained by the remaining (large) area.
One of the important properties of the µ-PIC is the capability of high counting rate.
In the conventional MWPCs, positive ions produced in the avalanche drifts toward theCHAPTER 6. PERFORMANCE OF THE µ-PIC

4500

Countscounts

52

4000

1000

3500

800

3000
2500

600
2000

400

1500
1000

200
500
0

0

500

1000

1500

0
0

2000 2500 3000
ADC channel

100 200 300 400 500 600 700 800 900 1000
FADC channel

(a)

(b)

Figure 6.2: Energy spectra of 55 Fe source. (a) Obtained from the detection area of

Anode12.8 × 12.8mm2 . (b) Obtained from the whole detection area (100 × 100mm2 ).

3500

8
7

3000

6

2500

5

2000

4

1500

3

1000

2
500
1
1

2

3

4

5

6

7
8
Cathode0

Figure 6.3: Map of the measured gas gains.

6.1. PERFORMANCE TEST OF THE µ-PIC

53

cathode electrode which is also used as a drift electrode. High rate capability is restricted
to 103−4 cps/mm2 by the drift time of positive ions (∼1msec). On the other hand, positive
ions from the avalanche are expected to drift toward the cathode electrode formed on thesubstrate near the anode. The drift time of the positive ions from an anode to a cathode isabout 200ns. If some positive ions escape toward the drift electrode, the feedback ions will
be observed as a current Id between the drift plane and the ground. In contrast, a current
Ia between the anode electrodes and the ground is the summation of the drift current Id
and the cathode current Ic corresponding to the collected charge by the cathode. Since
the cathode current can not be measured directly in our system, we measured the drift
current Id and the anode current Ia . The fraction of the feedback ion is deﬁned as a ratioof Id to Ia . Incident X-rays were collimated through a 10mm thick lead block with a holeof 6mm diameter. Fig. 6.4 shows the measured anode and feedback current for µ-PIC6 asa function of the counting rate of X-rays. The observed drift current was about 25% of
the anode current which corresponds to the total collected charge, and this fraction was
independent of the X-ray intensity.

Detector current [nA/mm2]

6µ-PIC No.6, gas gain: 5200,

Anode
Drift

5
4
3
2
1
0
0

4000

8000

12000

16000

20000

Counting rate [cps/mm2]

Figure 6.4: Anode and feedback current as a function of the counting rate of X-ray.

CHAPTER 6. PERFORMANCE OF THE µ-PIC

54

6.2

Long term stability

Long term stability of the gas gain is most important for various applications of µ-PIC. In
MSGCs, the gas gain decreases in one hour. It is explained by charging-up of positive ionson the substrate. This degradation can be avoided by ﬁne tuning of the anode-cathodeand anode-backelectrode voltages [2]. In this case, the gas gain slowly increases in a few
hours or longer. It is considered that this increasing of gas gain is due to the dielectric
polarization of the substrate.
On the other hand, there is no charging-up of positive ions for µ-PIC because of
the electrode structure as mentioned in Sec. 4.1. However, the eﬀect of the dielectric
polarization in the µ-PIC was unknown, which should be observed because polyimide isalso used as an insulator similar to MSGC.
The variation of the gas gain was measured for the previous µ-PIC for 120 hours(about 5 days) with the gain of more than 3000. During this measurement, the µ-PIC
was irradiated by an 55 Fe source with counting rate of ∼ 150counts/sec. Fig. 6.5 shows
the gas gain variation as a function of the elapsed time since the anode voltage supply.
Slight increase of the gas gain was observed in the ﬁrst 20 hours, which we think is dueto the polarization eﬀect of the polyimide substrate. In the ﬁrst a few hours, there weresteep increase of the gas gain. It can be understood as the electron attachment on thesubstrate. In this case, the electric ﬁeld due to the attached electrons is superimposed on
the primary electric ﬁeld due to the electrodes. As described in chapter 7, the electron
attachment is attributed to the lower anode height of 20µm below the substrate surface.
If the anodes were higher than the substrate, no electron attachment would be expected.
Thus variation of the gas gain would be small. This will be described in next chapter.
Another variation of the gas gain in a few months was also observed. Fig. 6.6 shows
the gain curve observed in 15 August 2002 (ﬁlled circle) and 23 October 2002 (ﬁlled
square). During this interval, the gas gain became about 2 times higher. This eﬀect is
explained as water contamination. Polyimide substrate is thought to contain some water
just after the fabrication. This water gradually evaporates during the operation, then
the water vapor contaminate the detector gas (Ar/C2 H6 mixture). Because, the watermolecule has polarity, some primary electrons are captured by the molecule. Therefore,
the resultant gas gain is considered to be degraded in the detector during the early phase6.3. OPERATION IN CARBON TETRAFLUORIDE GAS

55

4000
3500
gas gain

3000
2500
2000
1500
1000
500
0
0

20

40

60 80 100 120 140
time[hour]

Figure 6.5: Gas gain variation as a function of the time.

of the operation. The gas gain increases gradually with the evaporation of the water in thesubstrate. After a months, the gas gain become constant for all µ-PICs. In order to obtaina stable gas gain from the beginning of operation, it is important to completely evaporate
the water contained in the polyimide substrate using vacuum dehydration, which has tobe carried out in near future.

6.3

Operation in carbon tetraﬂuoride gas

As described in Sec. 2.3, the CF4 gas is suitable for a tracking of MIPs. Furthermore,high detection eﬃciency is expected by pressurized CF4 gas. However, there are few
samples which reported the performances in CF4 gas for pixel detectors. The operation
test of the µ-PIC in CF4 /C2 H6 mixture gas was performed using a prototype µ-PIC withthe detection area of 3 × 3cm2 . The variation of the gas gain in CF4 by changing thegas pressure was measured. The experimental setup is shown in Fig. 6.7. The µ-PIC(prototype) was installed in the gas pressure vessel. We measured the analog summed
signal from 10 × 70 pixels using a charged pre-ampliﬁer with a gain of 1V/pC. As shownin Figs. 6.8, the gas gain of over 103 was obtained at a pressure of 2.5atm, although

CHAPTER 6. PERFORMANCE OF THE µ-PIC

Gas gain56

15 August 2002

10

10

4

23 October 2002

3

500 520 540 560 580 600 620 640 660
Anode voltage [V]Figure 6.6: Gas gain variation in two months.

somewhat higher operation voltage was required than argon-based gas.
In this experiment, signiﬁcant stability in high gas gain up to 104 was not obtained.
The best condition should be found to obtain high gas gain and stability. For this purpose,
we will try other combination of gases, such as Ar/CF4 /C2 H6 .

6.3. OPERATION IN CARBON TETRAFLUORIDE GAS

57

valve
vacuumpump

signalanode HV
drift HV

gas inlet
valvesignalfeed-throughboard

anode HV
55

Fe

drift HVµ-PIC

Figure 6.7: Experimental setup for CF4 and high pressure test.

CHAPTER 6. PERFORMANCE OF THE µ-PIC

58

1 atm

2.5 atm

4

4

10

Gas gainGas gain

10

2 atm

pure CF4

10

3

800

CF4 / C2H6 (80 / 20)

3

10

900

1000

1100

1200
1300
Anode voltage [V]

700

800

900

1000 1100 1200 1300
Anode voltage [V]Figure 6.8: Gas gain curve obtained by CF4 test.

Chapter 7
Improvement of µ-PIC
We found that the µ-PICs with lower anode have some problems. In this chapter, we investigated these problems quantitatively using a three-dimensional simulation, and founda new electrode structure to overcome problems.

7.1

Problem of former µ-PICs

The µ-PICs with lower anode achieved high gas gain (∼ 104 ) and stable operation withlow gas gain of 3000. However, we need to obtain the stable operation with high gas gainof 10000 for its applications. First, we summarize the defects of the µ-PICs.
Non-uniformity of gas gain If we focus on a narrow region of the detection area, theenergy resolution of about 30% was obtained. As shown in Fig. 6.3, however, the gas gainstrongly depends on the position. The factor 5 of diﬀerence was measured between theminimum gas gain and the maximum one.
Low gas gain The required gas gain for detection of a MIP track is derived from theW-value and dE/dx of each gas. Using the data described in table 2.1, the gas gain of
more than 104 is needed for µ-PICs. Furthermore, stable operation is also required at
this high gas gain. Although the maximum gas gain of 104 was achieved, stable operation
could be obtained only in a low (< 5000) gas gain.
59

CHAPTER 7. IMPROVEMENT OF µ-PIC

60

These problems were considered to be attributed to the lower anode height, which was
due to the manufacturing conditions of the µ-PIC. As mentioned before, the anode height
was limited to ∼ 20µm below the surface of the substrate to keep the uniformity of theanode height. Even in this condition, the uniformity of the anode height is also restrictedwithin 3µm. In order to estimate how the performance of the µ-PIC would be improved
due to the uniform and high anode, we performed a three-dimensional simulation varying
the geometrical parameters of the µ-PIC.
The feature of the µ-PIC which clearly diﬀerentiate from the MDOT detector is the
thickness of the substrate. As mentioned in Sec. 3.3, the electric ﬁeld near the substrate of
the MDOT is aﬀected by the anode readout buses. Therefore, the eﬀect of the substrate
thickness must be investigated using the simulation.

7.2

Simulation study

7.2.1

Simulation method

Simulation code
Here, simulation studies were carried out using MAXWELL 3-D Field Simulator (hereafter, MAXWELL) [53] and GARFIELD [54]. The electromagnetic ﬁeld in a threedimensional structure was calculated using a ﬁnite element method by the MAXWELL
program. In this method, the structure is divided into ﬁne tetrahedrons called “mesh”.
The electromagnetic ﬁeld is calculated in each mesh point based on Maxwell’s equations.
A unit cell of the µ-PIC was created in the MAXWELL program, which has a size of
400 × 400µm2 and consists of a polyimide substrate with a relative dielectric constant of
3.5 and a pair of anode and cathode electrodes. Fig. 7.1 shows the mesh structure of aunit cell of the µ-PIC. Maps of the electric ﬁeld, the voltage, and the dielectric constantwere generated at each unit cell, which are called the “ﬁeld maps”. The inﬁnite repetitive structure (Fig. 7.2) was created by combining the unit cell with a periodic boundary
condition.
These maps were fed to GARFIELD and then the dynamics of the drift electrons7.2. SIMULATION STUDY

61

Figure 7.1: Mesh structure of the µ-PIC. The upper panel shows the full mesh including
the drift space. The lower one shows only the structure of the detector.

CHAPTER 7. IMPROVEMENT OF µ-PIC

62

Figure 7.2: Basic µ-PIC structure generated by MAXWELL.

were simulated. GARFIELD is a program for the detailed simulation of drift chambers.
Originally GARFIELD does not deal with dielectrics and three-dimensional structures.
However, GARFIELD can read the ﬁeld maps generated by MAXWELL. Primary electrons were uniformly distributed in the unit area 400µm above the substrate and the drift
paths of electrons were calculated by the Monte Carlo method. The multiple scattering ofdrifting electrons in the gas is calculated by the MAGBOLTZ program [28] which works as
a subprogram of the GARFIELD. The absolute multiplication factor Mabs was calculated
by equation (2.3). Because the observed gas gain is determined by Meﬀ which includes adegradation of factor Reﬀ , we focus on Meﬀ in the following discussion.
Basic parameters
The geometrical parameters of the µ-PIC used for the simulation are listed below, where
h is the height from the surface of the substrate.
1. Height of the anode electrode (variable; default = 10µm),
2. Width of the cathode strip (ﬁxed; 314µm),
3. Diameter of the cathode openings (ﬁxed; 260µm),

7.2. SIMULATION STUDY63

4. Substrate thickness t (variable; default = 100µm),
5. Diameter of the anode electrode d (variable; default = 50µm).
At ﬁrst, we discuss the Reﬀ dependence on the anode height. Moreover, Meﬀ dependences
on t and d are examined, in which we assume the anode height is 10µm above the surfaceof the substrate. These geometrical parameters of the µ-PIC are described in Fig. 7.3. Dmeans the distance between the anode pillar and the cathode edge. The default value of
D is 105µm. The thickness of the cathode electrodes is ﬁxed to 10µm for all calculations.
The anode-cathode voltage was 600V and the drift electric ﬁeld was 1kV/cm. The gas
mixture was Ar/C2 H6 (80/20).

h

d D
O

10 µm

t

Figure 7.3: Description of the geometrical parameters of the µ-PIC.

7.2.2

Simulation Result

Reﬀ dependence on the anode heightFirstly, we simulate the dependence of the electron collection on the anode height in orderto clarify the problem of the µ-PIC. The calculated drift lines of primary electrons areshown in Fig. 7.4 (a). This ﬁgure shows the cross-sectional view of the electrode structure.
The left-half path of the ﬁgure are obtained by a numerical calculation using the Runge
Kutta Fehlberg (RKF) method. Each drift lines calculated by this method traces theelectric ﬂux lines. The drift lines of right-half in the ﬁgure are the results simulated by

CHAPTER 7. IMPROVEMENT OF µ-PIC

64

the Monte Carlo (MC) calculation. The deﬂection of the electron track due to the multiple
scattering is included in the MC calculation. We found that some drift electrons terminate
on the substrate. The distribution of the terminal points of drift electrons are also shownin Fig. 7.4 (b), where all drift electrons are calculated by the Monte Carlo method. The
terminal points of the electron are roughly distributed along the anode readout electrodearranged on the rear side of the substrate. The terminal points are sorted into following
three types:
1. collected by the anode electrodes and contributing to the signal (signal event),
2. terminated on the substrate between the anode and the cathode (near-anode event),
3. terminated on the substrate between two cathode strips (gap event).
The electron collection eﬃciency Reﬀ is deﬁned as a fraction of type 1 events against the
total event. In the µ-PIC, Reﬀ was only about 30%. The dominant event was type 2
which accounts 65% of the total events. It means that the collected charge is only one
third of a primary electron cloud. Therefore, observed pulse height is only 0.3 times ofthat expected by the Townsend coeﬃcient.
We simulated the eﬀect of the anode height in the range from 40µm below the substratesurface (the level of µ-PIC2) to 10µm above the substrate (the level of µ-PIC1). Fig. 7.5
presents the result of this simulation, where the fraction of type 1, 2, and 3 events areplotted as ﬁlled circles, triangles and squares, respectively. Obviously, the fraction of
signal event increases with the raise of the anode level. Then the collection eﬃciency
would become close to 90% if the top of the anode exceeds the surface of the substrate.
The electrode structure of the µ-PIC with higher anodes was created in the MAXWELL
and the drift paths of primary electrons were calculated by the GARFIELD. Because thegap between the cathode strips also attracts some of primary electrons [37], the gap was
reduced from 86µm to 40µm in order to obtain high collection eﬃciency. Fig. 7.6 (a)shows the resultant drift lines of the primary electrons. Obviously almost all electrons
are collected by the anode electrodes. This is also shown in Fig. 7.6 (b), which shows
the distribution of the terminal points of the primary electrons. The electron collection
eﬃciency was attained to 97%.

7.2. SIMULATION STUDY

65

0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0

(a)

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08(b)

Figure 7.4: Electron drift of the µ-PIC with lower anode. (a)Cross-sectional view ofelectron drift. The paths of left-half are calculated by the Runge Kutta Fehlberg method.
The drift lines of right-half are simulated by the Monte Carlo method. (b)Distribution of
the terminal points of electron drifts.

CHAPTER 7. IMPROVEMENT OF µ-PIC

66

Fraction1signal eventnear-anode event

0.9

gap event

0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

-40

-30

-20

-10
0
10
Anode height [µm]Figure 7.5: Relation between electron collection eﬃciency and anode height. The ﬁlled
circles mean the electron collection eﬃciency. The ﬁlled squares and the ﬁlled triangles
mean the ineﬃciency.

7.2. SIMULATION STUDY

67

0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0

(a)

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08(b)

Figure 7.6: Drift lines of primary electrons and their terminal points of the electron forthe µ-PIC with higher anodes. (a) Cross-sectional view of them. The paths of left-halfare calculated by the Runge Kutta Fehlberg method. The drift lines of right-half are
simulated by the Monte Carlo method. (b) Distribution of the terminal points of electron
drifts.

CHAPTER 7. IMPROVEMENT OF µ-PIC

68

Meﬀ dependence on the substrate thickness
Pixel-type gas detectors developed prior to µ-PIC were made by the IC technology, andhence had a very thin insulator layer (several µm). Therefore, the electric ﬁeld in the drift
region near the surface of the substrate is aﬀected by the readout strips buried in the insulator. Some of the drift electrons would be directed toward the readout buses, rather
than the anode electrodes as shown in Fig. 7.7. This eﬀect would be observed as a degra-

0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
-0.01
0

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

Figure 7.7: Terminal points of drift electrons for a MDOT detector.

dation of the gas gain, or small Reﬀ values. Furthermore, the accumulated charge on thesubstrate might cause serious discharges. There are two approaches to avoid this problem.
The ﬁrst is to make a narrow-pitch detectors [55]. Actually, a narrow pitch (< 100µm)
MDOT detector was reported to have overcome this problem. Because discharges could
be problematic with the small anode-cathode distance, we adopted the other approach;a thick substrate. This approach was only realized with the PCB technology, and henceµ-PIC has its outstanding features in its thick substrate.
As described above, eﬃcient electron collection is expected for the higher anode electrodes. In order to evaluate the eﬀect of the substrate thickness quantitatively, we use

7.2. SIMULATION STUDY69

the µ-PIC structure having the anode pillar of 10µm above the surface of the substrate.
Figs. 7.8 and 7.9 show the drift lines of primary electrons and the distribution of
their terminal points. where the results of the thin (t = 5µm) and thick (t = 200µm)
substrates are shown in (a)s and (b)s, respectively. For t = 5µm, some of the drift lines
end at the substrate (Fig. 7.8 (a)) and the resultant electrons are terminated on thesubstrate (Fig. 7.9 (a)). It was obvious that the buried anode readout buses distorted
the drift electric ﬁeld for t = 5µm, because the electron end points are distributed above
the anode strips which are placed orthogonally to the cathode strips. On the other hand,most of the electrons are collected to the anode electrodes for t = 200µm.

(a)Fig. 7.10(b)

Figure 7.8: Simulated drift lines of primary electrons for (a) t = 5µm, and (b) t = 200µm.
In both ﬁgures, the drift lines drawn in the left half were calculated by the RKF method;
those in the right half were simulated by the MC method.

shows the Reﬀ dependence on t. For a thin (t < 10µm) substrate, Reﬀ was found to beless than 0.5, while that with a thick (t > 100µm) substrate exceeds 0.8.
Another parameter which determines Meﬀ is Mabs . Because Mabs strongly depends
on the electric ﬁeld around the anode electrodes, Mabs also depends on t. Fig. 7.11

CHAPTER 7. IMPROVEMENT OF µ-PIC

70

0.08

0.08

0.07

0.07

0.06

0.06

0.05

0.05

0.04

0.04

0.03

0.03

0.02

0.02

0.01

0.01

0
0

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

000.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

(a)

(b)

Figure 7.9: Terminal points of electrons on the electrode plane of the µ-PIC of (a) t = 5µm,
and (b) t = 200µm. 1000 events are shown in both cases.

Fraction

7.2. SIMULATION STUDY

71

1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
10

anode event

102
Substrate thickness [µm]near-anode event

gap eventFigure 7.10: Electron collection eﬃciency dependence on t (ﬁlled circles). For reference,
fractions of electrons that are terminated on the substrate in the cathode openings andthe gaps between the cathodes are shown by the triangles and squares, respectively.

CHAPTER 7. IMPROVEMENT OF µ-PIC

72(a) shows the electric ﬁelds as a function of the distance r from the anode center at
h = 11µm. With a thin (t = 5µm) substrate, the electric ﬁeld averaged over the top
area of the anode electrodes (hereafter Ea ) reaches only 60kV/cm, while that with a thick(t > 100µm) substrate exceeds 120kV/cm. Because Mabs strongly depends on Ea , a thicksubstrate is important for large Mabs value. The electric ﬁelds near the surface of thesubstrate (h = 1µm) are also shown in Fig. 7.11 (b) as a function of r. The electric ﬁeld
averaged along the circumference of the cathode edge (hereafter Ec ) is found to be 10
times stronger for thin (t = 5µm) substrate than that for a thick (t > 100µm) substrate.
Hence, the Townsend coeﬃcient for a thin substrate becomes larger near the cathodeedge, which is likely to cause discharges due to the emission of electrons from the cathodeelectrode, which may be attributed to that the distance between the cathode edge andthe anode readout bus (t) is much shorter than the distance between the anode pillar and

350
300

substrate
5 µm

250

200 µm1200

E [kV/cm]

E [kV/cm]

the cathode edge (D). On the other hand, if t is larger than D, the eﬀect of the readout

substrate
5 µm

1000

100 µm

100 µ m200 µ m

800

200

600
150

400

100
50

200

0

0

-50 anode
0 20

cathode40

60

80 100 120 140 160 180 200
r [µ m]

anode

-200
0

(a)

20

cathode40

60 80 100 120 140 160 180 200
r [µ m](b)

Figure 7.11: Strength of the electric ﬁeld as a function of the distance from the anodecenter: (a) the r axis is located 1µm higher above the top of the anode electrodes. (b)the r axis is located 1µm higher above the substrate.

7.2. SIMULATION STUDY73

bus is eﬃciently shielded by the substrate, and the Ec was weaker than the Ea . Ea andEc for various t values are shown in Fig. 7.12. It is intuitively expected that Ec should
be at least weaker than Ea though the mechanism of discharges will be discussed in later
section. Because Ea is comparable to Ec with t ∼ 100µm, thickness of the substrate of
the present µ-PIC is thought to satisfy the minimum point, though it is not still suﬃcient.

E field [kV/cm]

Thicker substrate (t ∼> 150µm) would help to realize larger Mabs with the µ-PIC.

103

102

0

100

200

300
400
500
Substrate thickness [µm]Anode (average)

Cathode edge

Figure 7.12: Strength of the electric ﬁeld at the center of an anode pixel (triangle), andnear to the cathode edge (square) as a function of the substrate thickness.

As a consequence of these two studies, thicker substrate gives larger Meﬀ . Fig. 7.13
shows Meﬀ as a function of t. With a substrate thicker than 150µm, Meﬀ is expected to
be twice larger than the current µ-PIC.
Scaling parameter; TDR
When we discuss the the dependence of performances on the substrate thickness for detectors having various geometrical parameters, it is useful to introduce a general geo-

CHAPTER 7. IMPROVEMENT OF µ-PIC

Effective multiplication

74

16000
14000
12000
10000
8000
6000
4000
2000
0
0

100

200

300
400
500
Substrate thickness [µm]Figure 7.13: Calculated eﬀective multiplication Meﬀ as a faction of the substrate thickness.

7.2. SIMULATION STUDY

75

metrical parameter. We deﬁne a new parameter, the “thickness-distance ratio (TDR)”;
TDR = t/D, where t and D are the substrate thickness and the distance between theanode and the cathode, respectively as shown in Fig. 7.3. The default values of D and t
are 105µm and 100µm, respectively. This new parameter have to be checked its reliability.
For this purpose, the geometrical parameters d, t, and D were varied within a reasonable
range. The structures similar to the original µ-PIC (TDR ≈ 1) was emulated with thescale factors of 0.5, 1.5, and 2.0, as well as the default value. The voltage between theanode and the cathode was also varied with the scale factor so that the electric ﬁeld at
the anode center becomes about 100kV/cm for all scale factors. The electron collection
eﬃciencies were calculated for each structure, which is shown in Fig. 7.14. Roughly constant values were obtained with the variation of 10%. Also the same calculations werecarried out varying TDR. Fig. 7.15 shows the variation of Reﬀ as a function of TDR for
the scaling factor of 0.5, 1.0, 1.5, and 2.0. In the case of large TDR (∼> 1.0), Reﬀ wasregarded as constant for several scaling factors. However, while TDR becomes smaller,signiﬁcant diﬀerence of the calculated Reﬀ was found for larger scaling factors. Consequently, this results mean that the TDR parameter is useful for the evaluation of theelectrode structure of which size is sub-mm level.

Meﬀ dependence on the anode diameter

The dependences of the Reﬀ and Mabs on the anode diameter (d) were also studied.
Although the current technology can manufacture only anode electrodes with a diameter
larger than 50µm, it is worth to investigate the optimum diameter. Fig. 7.16 shows Ea asa function of d. Obviously, Ea decreases with increasing d. On the other hand, large Reﬀ is
expected for the large d, as shown in Fig. 7.17. Fig. 7.18 shows the eﬀective multiplication
factor Meﬀ as a function of d. As is well known, this result shows that a smaller d 20-30µm
has a larger gain, however, it also indicates that a suﬃcient multiplication is expected foran d ∼ 50µm.

CHAPTER 7. IMPROVEMENT OF µ-PIC

Fraction

76

1

0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

0.4 0.6 0.81

1.2 1.4 1.6 1.8 2 2.2
Scale factor

Figure 7.14: The electron collection eﬃciency as a function of the scale factor. The scale
factor = 1 is the default value of d, t, and D.

7.3

Detector optimization

7.3.1µ-PIC with a thick substrate

In the previous section, we showed that the thicker substrate and smaller diameter of theanode electrode would help to improve the performance of the µ-PIC. Because smaller
anode electrodes are technically unreachable in the PCB technology, we evaluated the
maximum Meﬀ of the µ-PIC with a thick substrate considering the maximum anodevoltage for stable operation.
As indicated in earlier works about MSGCs [46, 47, 56], the dominant mechanismof discharges in micro-pattern gas detectors is a consequence of the ejection of electrons
from the cathode edge by spontaneous ﬁeld emission. At a normal operation condition of
MSGCs, the electric ﬁeld near the cathode edge exceeds 100kV/cm, which is high enoughfor a gas avalanche [46]. Electrons released from the cathode electrodes undergoes the gas
multiplication near both the cathode and anode electrodes, while the primary electrons
have gas multiplication only near the anode electrode. We can assume that the electricCollection efficiency

7.3. DETECTOR OPTIMIZATION

77

1

0.9
0.8
0.7
0.6
0.5
0.4
Scale factor (electrode pitch)

0.3

0.5 (200µm)
1.0 (400µm)
1.5 (600µm)
2.0 (800µm)

0.2
0.1
0

-1

10

1
Thickness-Distance Ratio

Figure 7.15: Dependence of the electron collection eﬃciency on TDR for the scale factorsof 0.5 (square), 1.0 (ﬁlled circle), 1.5 (inverse triangle), and 2.0 (triangle). The corresponding electrode pitches are also shown.

CHAPTER 7. IMPROVEMENT OF µ-PIC

E field [kV/cm]

78

substrate thickness = 100 µm

300
250
200
150
100
50
0
0

20

40

60

80
100
120
140
Anode diameter [µm]Fraction

Figure 7.16: Electric ﬁeld at the anode pixel as the function of the anode diameter.

1

0.9
0.8
0.7
0.6
0.5

0.4
0.3

substrate

0.2

100 µm0.1

150 µm

0
0

20

40

60

80

100
120
140
Anode diameter [µm]Figure 7.17: Electron collection eﬃciency as a function of the anode diameter.

Effective multiplication

7.3. DETECTOR OPTIMIZATION

79

20000
18000

substrate

16000

100 µm

14000

150 µm

12000
10000
8000
6000
4000
2000
0
0

20

40

60

80

100 120 140
Anode diameter [µm]Figure 7.18: Dependence of the eﬀective multiplication factor as a function of the anode
diameter.

CHAPTER 7. IMPROVEMENT OF µ-PIC

80

ﬁeld near the cathode governs the discharge process and “stable operation” can be judged
by the cathode ﬁeld. It is natural to think this discharge mechanism can be applied to µPIC, and we can estimate the possibly maximum anode voltage and Meﬀ before discharges
starting by the cathode ﬁeld Ec .
From the experimental results (Sec. 6.1), we knew that the attainable anode voltage
before discharges was about 630V, which corresponds to the maximum Ec of 200kV/cm.
Here 200kV/cm is used as the limit of Ec . As mentioned in the previous section, Ec
becomes weaker for a large TDR with the same anode voltage. In other words, the corresponding voltage Vlim becomes higher for a larger t. In Fig. 7.19, Vlim and corresponding
Meﬀ are shown as a function of TDR. For a thicker substrate of TDR > 1.5, maximum
anode voltage exceeds 800V, of which the maximum gas multiplication for that voltage
is higher than 106 , as shown in Fig. 7.19. Taking into account other factors which cause
discharges such as roughness of the cathode surface, the gas gain might achieve 105 for a
150µm thick substrate. Thus, geometrical structures of µ-PIC or other pixel-type detector
are thought to be optimized by increasing TDR.

7.3.2

Substrate-ablated µ-PIC

As discussed above, a thick substrate is thought to drastically improve the performanceof the µ-PIC. However, it would be technically diﬃcult to form an anode pillar througha thick substrate especially in case t/d  1. Even though we make a µ-PIC with a thicksubstrate, if the anode pillar is lower the surface of the substrate, Reﬀ becomes small
and we don’t achieve large Meﬀ . One of the solutions to retain large Meﬀ with a low
anode pillar is to ablate the substrate by a physical or a chemical process, e.g. laser
machining. Fig. 7.20 shows the structure of the µ-PIC with an ablated substrate (“welltype µ-PIC”) created by the Maxwell program. We simulated the performance of thistype µ-PIC for several anode height between h = −40µm to h = 0µm. All parameters
except the cathode width (340µm) were set to the default values. A wider cathode isused to improve Reﬀ . Fig. 7.21 shows the simulated electron terminal points for anodeheight at h = −20µm. Calculated Reﬀ was more than 99% for all the anode height. The
absolute gas multiplication Mabs depends on the anode height, as shown in Fig. 7.22.
We manufactured a prototype of the well-type µ-PIC. The substrate around the anode

81

1000
10

7

900
800

10

700

10

600
10

500

10

400
300

Limit voltage (left)

200

Limit multiplication (right)

100
0
0.4

0.6

0.8

1

10

6

5

4

Limit multiplication

V lim [V]7.3. DETECTOR OPTIMIZATION

3

2

10

1.2 1.4 1.6 1.8
2
Thickness-Distance Ratio

1

Figure 7.19: Dependence of the limit voltage (ﬁlled circle) and the corresponding achievable gas gain (ﬁlled square) as a function of the Thickness-Distance Ratio.

Figure 7.20: Substrate-ablated µ-PIC.

CHAPTER 7. IMPROVEMENT OF µ-PIC

82

0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
0

0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

Multiplication

Figure 7.21: Distribution of drift terminal points of electrons for the well-type µ-PIC.

18000
16000
14000
12000
10000
8000
6000
4000
2000
0
-45 -40 -35 -30 -25 -20 -15 -10 -5 0
5Anode height [ µm]

Figure 7.22: Simulated absolute gas multiplication as a function of the anode height for
the well-type µ-PIC.

7.3. DETECTOR OPTIMIZATION83

was ablated by a laser treatment, as shown in Fig. 7.23. The anode electrodes remained

Figure 7.23: Microscopic photograph of the well-type µ-PIC.

on the readout strip electrodes like pillars. Fig. 7.24 shows the measured gas gains of thewell-type µ-PIC. Measured gas gains are smaller than those of previous µ-PIC because
we ablated the substrate of the µ-PIC whose anode height are lower than h = −20µm.
The detector became unstable by discharges when the gas gain exceeded 103 . Although
the reason for the discharges is not very clear, we have some hypotheses, e.g. the shapeof the ablated well, and a distortion of the top shape of the anode pillars that occurred
during the ablation process. For these reasons, the electric ﬁeld might become locallystrong, and then discharge paths could easily be formed between the cathode edge andthe anode strip. A simulation shows that a slightly smaller diameter of the well than thecathode ring is much better for discharges. However, at present, it looks quite diﬃcult tocontrol the ablation process with the required accuracy for a large area detector. We will
try this type detector when the new technology becomes available.

CHAPTER 7. IMPROVEMENT OF µ-PIC

Gas gain

84

Ar90% + C2H610%
Ar80% + C2H620%

10

10

3

2

380 400 420 440 460 480 500 520 540 560
Anode voltage [V]Figure 7.24: Gas gain curve of the well-type µ-PIC.

7.4. DEVELOPMENT OF THE IMPROVED µ-PIC

7.4

85

Development of the improved µ-PIC

The conventional electroplating technology looked very diﬃcult to control the growthof the anode in such a ﬁne level. Then we applied the new manufacturing technology,
which is a combination of the chemical (electroless) plating and electric plating. Using
this new technique, a µ-PIC was manufactured as follows (see also Fig. 7.25). First, athin copper layer is formed on the all surface of the laser-drilled 100µm thick substrateusing electroless plating. Via holes are ﬁlled over the surface, and a thick copper layer isformed by electric plating (via-ﬁll plating). A copper surface is etched until its thickness
is reduced to ∼ 10µm. Finally, both anode and cathode electrodes are formed by etching
simultaneously, and hence the anode height (10µm) is quite same as that of the cathodesurface. The gaps between cathode strips is also formed by etching together with theanode pixels. Cathode material is not lost in this process whereas cathode strips of theprevious µ-PIC were slightly dissolved in the plating process. Then a narrow gap of 60µm
was realized. Other geometrical parameters are summarized in Table 7.1. A microscopic
photograph and a schematic drawing are shown in Fig. 7.26 together with those of theprevious µ-PIC. This new µ-PIC having the detection area of 10 × 10cm2 was named“µ-PIC8”. Very precise processing is possible for the etching technique. It allows us tomake almost uniform electrodes. The deviation of the anode height was within 3µm in
whole detection area (10 × 10cm2 ). Furthermore, there was almost no abnormal anodeformation; the fraction was less than 0.1%. This technique can be applied for larger area
detectors because the uniform electrodes was achieved in 10 × 10cm2 .

AnodeCathodeSubstrate

Parametervalueheight10µm

diameter∼ 50µm

width

340µm

hole diameter

260µm

thickness

10µm

thickness

100µm

Table 7.1: Geometrical parameters of the new µ-PIC.

CHAPTER 7. IMPROVEMENT OF µ-PIC

86

Figure 7.25: Manufacture process of the latest µ-PIC.

We succeeded to form nine µ-PIC patterns on one large polyimide substrate in one
manufacturing process as shown in Fig. 7.27. Now systematic shift of the anode position
is observed as shown in Fig. 7.28. The oﬀset increases with the distance from the center.
This systematic shift is attributed to the diﬀerence of the thermal expansion between
polyimide and copper. If the position of via holes are corrected before the manufacture
process by estimation of this diﬀerence, the accurate electrodes will be realized and alarger area (30 × 30cm2 ) µ-PIC can be made.

7.5

Performance test

7.5.1

Basic performance

The basic performances of µ-PIC8 were tested. The conditions of the experiment were
all the same as those of the previous µ-PICs described in chapter 6. The obtained gas
gains were plotted in Fig. 7.29 by ﬁlled circles together with the gas gain curve of theprevious µ-PIC. Obviously, the gas gains of the new µ-PIC were three times higher thanthose of the previous µ-PIC. This diﬀerence can be explained by the eﬀect of the higher
anode of the new µ-PIC. This result shows that most of the primary electrons due tothe incident particle are collected by the anode electrodes. In other words, the electron7.5. PERFORMANCE TEST

87

Figure 7.26: Microscopic photograph and schematic cross section of the previous µ-PIC(left) and the new µ-PIC (right).

88

CHAPTER 7. IMPROVEMENT OF µ-PIC

Figure 7.27: Photograph of a substrate board formed nine µ-PIC patterns. Each square
is a µ-PIC pattern. The center one is plated by nickel, and the other are not coated.

Figure 7.28: Microscopic photograph of shifted anodes observed in edge region.

7.5. PERFORMANCE TEST89

Gas gaincollection eﬃciency was close to 100%.

Previous µ-PIC
New µ-PIC

104

103
500

550

600
650
Anode voltage [V]Figure 7.29: Gas gain curve of the new µ-PIC (square) and the previous µ-PIC (circle).

The energy spectrum was measured using 5.9keV X-ray from an 55 Fe source. Fig. 7.30(a) shows the spectrum obtained from the detection area of 12.8 × 12.8mm2 . An escapepeak is clearly seen at 2.7keV as well as the main peak of 5.9keV. Achieved energy
resolution of this area was about 23% (FWHM). Fig. 7.30 (b) also shows the spectrum
from whole detection area (100 × 100mm2 ). Although the spectrum from whole detection
area was somewhat broadened, the energy resolution of 30% (FWHM) was kept.
Dependence of the gas gain on the detection position was also measured using an 55 Fe
source. The detection area was divided into 8 × 8 segments and gas gains were measured
in each segment which has 32 × 32 pixels. Fig. 7.31 shows the map of the measured gas
gain of the full detection area. The standard deviation of the gas gain distribution was
only 7%. The uniformity of the gas gain was drastically improved compared to that ofthe previous µ-PIC as shown in Fig. 7.32.
The feedback of the positive ions generated in the avalanche is measured using an

CHAPTER 7. IMPROVEMENT OF µ-PIC

Counts

90

300

1800
1600

250

1400
1200
Counts

200
150

1000
800
600

100

400
50

200
0
0

1

2

3

4

5

6

7

0
0

8
9 10
Energy [keV]

2

4

6
8
Energy [keV]

10

12

(b)

(a)

Figure 7.30: Energy spectrum obtained from the detection area of (a) 12.8 × 12.8mm2

Anode

and (b) 100 × 100mm2 by irradiating the 55 Fe source.

4000
8

18

3500

16

3000

14

2500

12

7

new µ-PIC(σ = 7%)

6
5
2000
4

10
8

1500

6

3
1000
2
500
1
1

2

3

4

5

6

7
8
Cathode0

previous µ-PIC(σ = 40%)4201000 1500 2000 2500 3000 3500 4000 4500
Gas gain

Figure 7.31: Map of the measured gas

Figure 7.32: Gas gain distribution of thegains.

previous and the new µ-PIC.

7.5. PERFORMANCE TEST

91

X-ray generator. In order to compare the result to the previous µ-PIC, the setup of the
X-ray generator was same as those of the previous one. Fig. 7.33 shows the anode andthe drift current of the previous and the new µ-PICs as a function of X-ray counting rate
per unit area. As a result, the observed feedback current was about 1/3 of the anodecurrent which correspond to the total collected charge. It was almost similar to that ofthe previous µ-PIC. Furthermore, this current was independent on the X-ray counting

Detector current [nA/mm2]

6

New µ-PIC, gas gain: 5300,
Previous µ-PIC, gas gain: 5200,

5

Feed-back ion current / Anode current

rate (Fig. 7.34).
Anode
Drift
Anode
Drift

4
3
2
1
0
0

4000

8000

12000

16000

20000

2

Counting rate [cps/mm ]0.5

New µ-PIC, gas gain: 5300
Previous µ-PIC, gas gain: 5200

0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
0

2000

4000

6000

8000

10000

12000

Counting rate [cps/mm2]

Figure 7.33: Anode and drift current as a

Figure 7.34: Fraction of the feedback cur-function of the counting rate of X-ray.

rent to the anode current.

7.5.2

Long term operation

Long term operation more than 1000 hours was also examined for the new µ-PIC at
the gas gain of > 6000. Fig. 7.35 shows the variation of the gas gain during 150 hours(corresponding to 6.25 days). As described in Sec. 6.2, almost constant gas gain for longterm is expected, because the accumulation of electron on the substrate is smaller thanthat of the previous µ-PICs. Actually, a slight increase of the gas gain was observed also
for the µ-PIC8. The variation from the initial gas gain was within 15%, which is about
half of that of µ-PIC4. It is considered that this small increase expresses a variation of
the gas temperature.
The increase of gas gain in a few months was also observed in the new µ-PIC. Thegas gain became about 1.5 times higher during 2 months as shown in Fig. 7.36. The

CHAPTER 7. IMPROVEMENT OF µ-PIC

Gas gain

92

7000
6000
5000
4000
3000
2000
1000
0
0

20

40

60

80

100

120
140
Time [hour]

Figure 7.35: Variation of the gas gain in 150 hours.

resultant gas gain of 1.6 × 104 was achieved. This gain increase can be explained as watercontamination as well as the previous µ-PIC, because the new µ-PIC is made of the samematerial as the previous one.
The obtained basic performances are summarized and compared with those of theprevious µ-PIC in table 7.2. Most properties were improved for the new µ-PIC.
Previous µ-PIC
4

New µ-PIC> 1.6 × 104

Max gas gain

1.5 × 10

Stable gas gain

∼3000

∼6000

Energy resolution (FWHM)

30%

23%

Gain uniformity (σ)

40%

7%

Feedback fraction

1/3

1/3

Table 7.2: Performance comparison of the previous and the new µ-PIC.

Gas gain

7.5. PERFORMANCE TEST

93

03 September 2003
06 November 2003

4

10

103
480

500

520

540

560

580
600
620
Anode voltage [V]Figure 7.36: Variation of the gas gain in two months.

Chapter 8
Imaging and 3D tracking
All applications of the µ-PIC is based on the two-dimensional imaging. Imaging of X-rays
or charged particles is the most interesting feature of the µ-PIC. Here, we describe about
the capabilities of X-ray imaging, and three-dimensional tracking of charged particles in
the use of Time Projection Chamber.

8.1

X-ray imaging

The µ-PIC has narrowly spaced electrodes, which provide the spatial resolution of about
100µm by a digital readout DAQ system. The DAQ system operates with the clockof 20 or 40MHz, then real-time imaging is obtained in principle, which contains ∼ 106
data per second. Then a high-quality moving image can be taken like a CCD-based
detector. Furthermore, energy information is also available as well as the position andtiming information. That will establish a new technique of imaging.
We evaluated the performances of X-ray imaging using a standard test chart for X-ray
imaging. The test chart is made of 0.05mm thick lead with the slits of various width. We
used an X-ray generator with the acceleration voltage of 12kV, which was set at a distanceof 35cm from the drift electrode of the µ-PIC. Irradiation at the surface of the detector
is expected to be uniform at this distance. A 1mm thick aluminum plate was used forﬁltering of low energy X-rays (∼<5keV). In order to obtain a clear image, we used the gas
mixture of Xe/C2 H6 (70 / 30) at atmospheric pressure, because the practical range of94

8.1. X-RAY IMAGING

95

photoelectrons is shorter than the electrode pitch. Diﬀusion of the electron cloud (rms)is 210µm for 1cm drift. Hence, the diﬀusion is expected to be about 60µm for the drift
length of 3mm, which is much shorter than the electrode pitch (400µm). Fig. 8.1 showsan X-ray transparency image of the test chart. As shown in the ﬁgure, even 0.5mm wideslits are clearly separated by eyes.

0.5mm slits

1cm
Figure 8.1: X-ray image of a test chart.

From a simple statistics, the spatial resolution (r.m.s.) of the detectors with digital
readout is expressed by the following formula:√σ = d/ 12,(8.1)

where d is the electrode pitch. Since the electrode pitch of the µ-PIC is 400µm, theexpected spatial resolution is 115µm. The actual spatial resolution was measured by the
knife edge method using the test chart. In order to suppress the range of the electron in
the gas, the acceleration voltage of the X-ray generator was set to be 10kV. A 1mm thickaluminum plate was also used for this measurement. The counting rate was projectedalong one of the edge of the test chart for 2cm as shown in Fig. 8.2. This proﬁle was ﬁtted

CHAPTER 8. IMAGING AND 3D TRACKING

96
using an error function:x2
erf(x) = √ · exp(−t2 ) d t.
π 0

(8.2)

Then the ﬁtting function is

x − a3,
Count(x) = a1 + a2 · erf √
2a4

(8.3)

where x means the length along the edge and a1 , a2 , a3 , and a4 are the ﬁtting parameters.
The calculated spatial resolution, a4 , was 120µm. This result was very close to the limit√
from the statistics: σ = d/ 12 = 115µm.
1400
1200

Counts

1000
800
600
400
200
0
0

0.5

1

1.5

2

2.5

3

Length along the edge [mm]Figure 8.2: Projected image of the test chart edge and the best ﬁt function. The areaused for the projection is shown in the right ﬁgure.

The image distortion is an important property for large area detectors. In the caseof an X-ray image intensiﬁer [51] which is commonly used for material science, there issome image distortion due to the focusing of photoelectrons. Image distortion is also
observed for the detector with analog readout such as charge division and delay line
methods. Therefore, raw images obtained by these detectors must be calibrated andcorrected suitably. To the contrary, the µ-PIC with the fast digital readout causes noimage distortion in principle. In order to verify this, a universal printed circuit board

8.1. X-RAY IMAGING

97

with 0.1 inch pitch and 1mm φ holes was used as a calibration pattern. The detection
area of 76.8 × 76.8mm2 is used for this experiment, which area corresponds to about 56%of the whole detection area. Fig. 8.3 (a) shows the transparency image of a universal
printed circuit board irradiated by the X-ray from the generator with the acceleration
voltage of 12kV. The grid in the image are the border lines between pre-ampliﬁers which
are drawn in every 32 channels. The close-up image is shown in Fig. 8.3 (b). The observed
image distortion is within the pixel size (400µm).

250
103
150

200
10

3

140
Anode

Anode

150

130

100

10

2

102

120
110

50
100
70

0
0

50

100
150
Cathode

200

80

90
100
Cathode

110

120

250

(b)

(a)
Figure 8.3: Transparency image of a universal printed board with 0.1 inch pitch (a) and its
close-up image (b). The grids in the image mean the border lines between pre-ampliﬁers
which are drawn in every 32 channels. Obviously, the image distortion is within the pixelsize.

We also evaluated the image uniformity of the µ-PIC using the detection area of
76.8 × 64.0mm2 . The acceleration voltage of the X-ray generator was 20kV, and thelow energy X-ray (∼<3keV) was ﬁltered by a 200µm thick aluminum plate. A 5cm thickpolyethylene block was used as a scatterer for a uniform irradiation. The obtained image

CHAPTER 8. IMAGING AND 3D TRACKING

98

is shown in Fig. 8.4. The counting rate was slightly high in the center region. That isdue to the intrinsic intensity distribution of the X-ray generator. Taking into account thevariation of intensity , the non-uniformity of image was about 6% for all detection area.

Anode

300

250

CountsCathode profile

2D image

300
250
200
150

250
200

100
50

200

0
0

150

100
100

50

100

150

200
250
Cathode strip number

50

100

150

200
250
Anode strip number

Anode profile

Counts

150

300
250
200

50

50

150
100

0
0

50

100

150

200

250

0

Cathode50
00Figure 8.4: The left panel is a two-dimensional image of uniformly irradiated X-ray. In
the right panel, the projection on the cathode axis (upper) and on the anode axis (lower).

8.2

Performance of µ-TPC

8.2.1µ-PIC readout for the µ-TPC

In order to observe tracks of charged particles, the µ-PIC works as a readout device ofa Time Projection Chamber (TPC). The TPC is a three-dimensional tracking device forcharged particles, which uses a two-dimensional array of pickup electrodes together with ameasurement of the drift time. The TPC consists of multiplication region and drift region
as shown in Fig. 8.5. A uniform electric ﬁeld is applied in the drift region. Conventionally,
the multiplication region consists of a MWPC with the segment cathode. An electron
cloud along the track of incident charged particles drifts toward the multiplication region

8.2. PERFORMANCE OF µ-TPC

99

Figure 8.5: Schematic structure of a TPC and the detection principle [57].

and picked up by the anode and the cathode. A three-dimensional track is reconstructedusing two-dimensional projected track detected by pick-up electrodes and the hit timingof each electrode. In a conventional TPC consisting of a MWPC and a long drift cage,
an avalanche occurs around the sense wire, and signals are induced on the cathode pads.
However, a part of ions produced in the avalanche gradually accumulate on the wall of
the drift cage, and these accumulated ions distort the electric ﬁeld in the drift volume.
In the case of the conventional MWPC-based TPCs, this problem was overcome by using
the gating grids [58]. While gas avalanche is grown around the anode electrodes, thegating grids are closed in order to block the feedback of positive ions to the drift region.
In contrast, as described in chapter 3, the feedback of the positive ions to the drift regionis suppressed by using a MPGD without gating grids. GEM readout for the TPC isdeveloped in CERN [59].
We made the TPC with the µ-PIC readout which is named the Micro Time Projection
Chamber (µ-TPC). The µ-TPC consists of the µ-PIC and the drift cage shown in Fig. 8.6.
This drift cage has an area of 10 × 10cm2 and an 8cm long drift region. A negative highvoltage of −3.3kV is applied to the top of the drift cage made of a 1mm thick aluminum
plate. The 0.3mm thick inner aluminum plane is connected to it through 22MΩ. A 4mm
wide ﬁeld cage electrode is connected to the plane through 10MΩ. Another fourteen ﬁeld

CHAPTER 8. IMAGING AND 3D TRACKING

100
gas out

gas in22ΜΩ −3300V
Al window (~1mm)
10mm

10ΜΩ
10ΜΩ
10ΜΩ
10ΜΩ
10ΜΩ
10ΜΩ

Al (0.3mm)
2700V

~1mm
4mm

2400V

1mm

2100V
1800V

75mm

10ΜΩ
10ΜΩ
10ΜΩ
10ΜΩ
10ΜΩ
10ΜΩ
10ΜΩ
10ΜΩ
10ΜΩ

1500V
1200V
900V
600V
300V

µ−PIC

5mm

2200pF~110mm22ΜΩFigure 8.6: Structure of the µ-TPC with the calculated electric potential (thick solidlines).

cage electrodes are spaced by 1mm, and each is connected to the next through 10MΩ. The
wide ﬁeld cage electrode prevent from attachment of positive ions on the insulating gap
between the electrodes. The photograph of the ﬁeld cage is shown in Fig. 8.7. Gas ﬂows
through the chamber via connectors at both sides of the top plate. Fig. 8.6 also shows
the calculated electric potential in the drift region by the GARFIELD program [54]. An
electric ﬁeld of 0.4kV/cm was produced uniformly in the inner region (∼80% of the full
drift volume). The ﬁeld cage was set in a gas vessel and mounted on the mother board
as shown in Fig. 8.8.

8.2.2

Heavy particles

Tracking of heavy charged particles with sub-relativistic energy is not so diﬃcult, because
the energy loss of the particle is large. Therefore, a dense track is expected to be observedwith a moderate gas gain (∼ 3000). We used the gas mixture of argon and ethane with the
mixture ratio of 80:20 and 90:10. The basic properties of each mixture are summarized8.2. PERFORMANCE OF µ-TPC

Figure 8.7: Photograph of the ﬁeld cage of the µ-TPC.

Figure 8.8: The photograph of the developed µ-TPC.

101

CHAPTER 8. IMAGING AND 3D TRACKING

102
in Table 8.1.

Mixture ratio Drift velocity [cm/µs]

Diﬀusion (σ[µm])

80:20

4.7

350

90:10

3.6

440

Table 8.1: Properties of the gas mixtures of Ar-C2 H6 in the drift ﬁeld of 0.4kV/cm.
Diﬀusion means the standard deviation for 1cm drift.

The performance of the µ-TPC was tested using high energy protons of 0.3-2.0GeV/cwith Ar/C2 H6 (80/20) gas mixture. A photograph of the experimental setup is shown in
Fig. 8.9. The energy of incident particles from the right of this ﬁgure is varied by themagnet. The Cherenkov counter and the time of ﬂight (TOF) counters were not used inthis experiments. Two plastic scintillators were used as trigger counters. The incident
particles were identiﬁed using pulse height information from the trigger counters. Theµ-PIC used in this experiment was the previous type.

Figure 8.9: Setup photograph of the beam test.

In this experiment, three-dimensional spatial resolution was measured using tracks ofheavy charged particles such as protons. These particles are hardly deﬂected by multiple

8.2. PERFORMANCE OF µ-TPC

103

scatterings. Therefore the tracks of the heavy charged particles are regarded as straightlines. Fig. 8.10 shows a typical three-dimensional image of a proton of 0.8GeV/c. The average sampling interval was about 1.5mm for the projection on the anode-cathode planeand 2.6mm for three-dimensional tracks. Each obtained track was ﬁtted with a threedimensional linear function. The position resolution is deﬁned as the root mean squareof the residual distribution. The residual distribution of the proton tracks is shown in
Fig. 8.11 by a solid line. The obtained three-dimensional position resolution was 320µm
for Ar/C2 H6 (80/20) gas mixture. This resolution is limited by the DAQ system clock of
20MHz. Drift velocity of electrons in the gas is 4.7cm/µsec, then the primary electrons
drift about 2mm during 1 clock time (50nsec). Although, two-dimensional spatial resolution of 250µm was obtained, the three-dimensional resolution was somewhat degradedby the resolution of the drift direction.
A similar experiment was performed for argon and ethane gas mixture with the mixture
ratio of 90:10. The position resolution was measured using recoil protons (0.5-1.0MeV)
due to fast neutrons from a radioactive source of 252 Cf. The residual distribution is also
shown in Fig. 8.11 by a doted line. The obtained position resolution was 260µm. The
diﬀerence between these two results is proportional to the diﬀerence of the drift velocityof the gas mixtures. The three-dimensional position resolution is restricted by the DAQ
clock of 20MHz. We are now improving a rate of clocks in the DAQ system to 50MHz,
by which a position resolution of about 210µm for an Ar/C2 H6 (90/10) gas mixture is
expected.

8.2.3

Minimum ionizing particles

Electron tracks were detected using Compton scattering of 662keV gamma-ray from a137

Cs source. The energy of the recoil electrons are estimated to be several hundred keV.

In this experiment, the µ-PIC worked as a µ-TPC for gamma-ray imaging (Ref [60, 61] andSec. 9.3.1). The µ-TPC was triggered by scatter gamma-rays detected by a NaI scintillator
behind the µ-TPC. One of the typical three-dimensional tracks of recoil electron is shownin Fig. 8.12. A winding track was clearly detected like a “electric cloud chamber”. In thisﬁgure, the direction of the electron was upward. The gas gain of the µ-PIC in this test
was about 3000.

CHAPTER 8. IMAGING AND 3D TRACKING

104

drift

FADC data(cathode-drift)
200

anode [cm]

2D track(cathode-anode)
10
9

180

8

160

7

140

6

120

5

100

4

80

3

60

2

40

1

20

0
0

1

2

3

4

5

6

7

200

400

600

800 1000 1200 1400 1600
cathode2D track(cathode-drift)
drift [cm]drift [cm]

3D track

10
9
8
7
6
5
4
3
2
1

0
10
an 9
od 8
e 7
[c 6
m 5
] 4

0
0

8
9 10
cathode [cm]

10
9
8
7
6
5
4
3
2

3

9 10
7 8 cm]5 6 thode [
2
4
a
c
1
2 3
0 0 1

1
0
0

1

2

3

4

5

6

7

8
9 10
cathode [cm]Figure 8.10: A three-dimensional track image of a proton (0.8GeV). The upper-left andlower-right images are two-dimensional projection onto the cathode-anode plane and thecathode-drift plane. The upper right graph is pulse heights obtained each cathode preampliﬁer.

8.2. PERFORMANCE OF µ-TPC

105

0.18
0.16

Ar : C2H6
90 : 100.14

80 : 20

0.12
0.1
0.08
0.06
0.04
0.02
0
0

0.2

0.4 0.6 0.8

1 1.2 1.4
residual [mm]Figure 8.11: Residual distributions of the proton tracks.

drift [cm]

10
9
8
7
6
5
4
3
2
1
0
10
an
od 8
e
[c 6
m
] 4
2
0 0

2

4

10
8
m]
c
[
6
ode
h
t
a
c

Figure 8.12: A three-dimensional image of an electron track originated in a Compton
scattering of a 662keV gamma-ray from a 137 Cs source.

CHAPTER 8. IMAGING AND 3D TRACKING

106

Tracks of cosmic-ray muons were also detected as shown in Fig. 8.13. The µ-TPC was
triggered by the coincidence of two plastic scintillators placed on both sides of the µ-TPC.
In this experiment, intervals of obtained plot was rather coarse. The tracking eﬃciency(> 3 points) was about 20% for the gas gain of 3000-5000. This result means that thesuﬃcient gas gain has not been obtained for the previous µ-PIC. Actually, the energy
deposition of MIPs is about 2MeV/(g/cm2 ). About 4 electron-ion pairs are produced in
the length of one electrode (400µm) by an MIP in an Ar/C2 H6 (80/20) gas mixture of
the atmospheric pressure. The required gas gain to detect dense tracks of MIP is ∼ 104 ,which has been obtained in the new µ-PIC.

anode [cm]

10
9
8
7
6
5
4
3
2
1
0
8
7
drift
[

6
cm]

5

4

3

2

1

4
5
6 cm]
7 [
8 e
9 thod0 10ca3

2

1

0

Figure 8.13: Three-dimensional track of cosmic-ray muons.

Chapter 9
Application of µ-PIC
Because of the imaging performance and ﬁne timing resolution, the µ-PIC is a promisingdetector for many ﬁelds of science, i.e. X-ray and gamma-ray astrophysics, X-ray crystallography, high energy particle physic etc. The development of the µ-PIC for several
applications being done by our group are brieﬂy described in this chapter.

9.1

X-ray polarimetry

Principle
The narrow spacing of the electrodes of the µ-PIC allows us to detect the polarizationof the incident X-ray. Incident X-rays kick out an orbital electrons of an atom via thephotoelectric eﬀect. The range of the emitted electrons (photoelectrons) in argon gas is∼1mm for 8keV X-ray and ∼5mm for 20keV X-ray, which are suﬃciently longer than theelectrode pitch of the µ-PIC. Thus, the photoelectrons are expected to run over several
electrodes. The diﬀerential cross section of the scattering angle of a photoelectron is givenbydσ
sin2 θcos2 φ∝,dΩ(1 − βcosθ)4(9.1)

where θ and φ are polar and azimuth angle of the photoelectron, respectively. β = v/cmeans the velocity of the photoelectron, which corresponds to the energy of incident X-ray.
If the incident X-ray is fully polarized, the angular distribution of the emission become107CHAPTER 9. APPLICATION OF µ-PIC

108

maximum at the direction of the electric ﬁeld of the incident X-ray as shown in Fig. 9.1.
While the range of photoelectrons is long, large asymmetry of the vertical and horizontal
width is obtained. Therefore, light gases such as argon and neon are rather suitable thanheavy gases such as xenon, because a long range of photoelectrons is obtained for thelight gases as mentioned in Sec. 2.3.

Polarized direction

Figure 9.1: The angular distribution of the photoelectrons for fully polarized X-ray.

The direction of X-ray polarization is measured by counting the hit electrodes horizontally and vertically as shown in Fig. 9.2.

Experiment
The photograph of the experimental setup is shown in Fig. 9.3. The polarized X-ray wasproduced using perpendicular Thomson scattering on a polyethylene block. The scattered
X-ray is fully polarized theoretically. As the X-ray source, we used an X-ray generator
same as that described in chapter 6 again. The µ-PIC was ﬁlled with the Ar/C2 H6 (80/20)
or Ne/C2 H6 (80/20) gas mixture of atmospheric pressure, and the drift ﬁeld of 500V/cm
was applied in a 12mm thick gas volume.

9.1. X-RAY POLARIMETRY

109

photoelectron
cathode strip

electron cloud

2 strips hit

anode strip
3 strips hit

Figure 9.2: Measurement of the “hit width” of electrodes.

Figure 9.3: Experimental setup of X-ray polarization detection.

CHAPTER 9. APPLICATION OF µ-PIC

110

8
7

Cathode Hit

6
5
C
4
3
2

A

1
1

2

3

4

5

Anode Hit678

Figure 9.4: A typical distribution pattern of the hit number of anode and cathode electrodes for the X-ray energy of ∼8keV. The size of each square indicates the number of
events.

ResultFig. 9.4 shows an example of the distribution of the hit number of anode and cathodeelectrode for the X-ray energy of ∼8keV. In this ﬁgure, the size of each square indicatesthe number of events. We deﬁne A as the number of events whose hit number of anodestrips are larger than that of cathode strips, and C as vice versa. If A is larger than C,
we regarded the direction of polarization as the direction of the anode strips. Here, wedeﬁne a degree of bias, or “diﬀerence factor” D as
D=

A−C
.
A+C

(9.2)

The diﬀerence factor varies with the rotation angle of the polyethylene scatterer. Although the maximum diﬀerence factor of 1.0 is expected from fully polarized X-ray, obtained diﬀerence factor is less than 1.0 because the tracks of photoelectrons are intricately
straggled. The range of photoelectrons become longer with the energy of incident X-ray.
Therefore, high diﬀerence factor was obtained from high energy X-ray as shown in Fig. 9.6.
The consistent result was obtained from a Monte Carlo simulation [62] using the EGS4

9.1. X-RAY POLARIMETRY

Difference Factor

111

Angle(o )Figure 9.5: Variation of the diﬀerence factor as a function of the rotation angle of polyethylene scatterer.

Difference Factor: Neon: ArgonEnergy (keV)

Figure 9.6: Dependence of the modulation factor as a function of the X-ray energy.

CHAPTER 9. APPLICATION OF µ-PIC

112
code [63].
Future prospect

In the case of a strip-readout detector such as µ-PIC, the degree of polarization is derived
as the variation of the diﬀerence factor. Therefore, the observation have to be done
at least two rotating angle. In contrast, the image of a photoelectrons would obtainby a pixel-readout detector. In this case, the degree of polarization can be obtained
without a rotation of the detector. We are developing a new µ-PIC with pixel-readout.
The schematic drawing is shown in Figs. 9.7. The thin electrode is formed on a ceramic
substrate with the pitch of 600µm. The surface of the ceramic substrate is covered by thinpolyimide layer in order to avoid the attachment of materials on the ceramic substrate.
This µ-PIC with pixel-readout is developed as a focal plane detector of a X-ray telescope. Until now, the only source from which X-ray polarization was detected was the
Crab Nebula [64], although the polarization has important information in astrophysics
i.e. magnetic ﬁeld around Supernova remnants.

9.2

X-ray crystallography

X-ray diﬀraction study is one of the most expected ﬁelds for µ-PICs. In this ﬁeld, a
integrating area detector such as an Imaging Plate (IP) [65] or CCD had been commonly
used. However, timing and energy information are not obtained by the integrating detector. Today, the main device for the X-ray crystallography is a 4-axis diﬀractometer usinga zero-dimensional pulse counting detector such as a scintillator. In this case, the detector
observe all diﬀraction spots. Therefore, it takes very long time (more than 1 day). On theother hand, an MPGD is a pulse counting area detector, hence rapid crystal analysis isavailable. In particular, for an MSGC, the wide dynamic range of 106 was obtained [66].
Therefore, the MPGDs are suitable device for X-ray crystallography.
A new analyzing method, “Continuous Rotation Photograph (CRP) method” hadbeen developed for a MSGC [3], in which all diﬀraction peaks can be obtained within one
continuous rotation of a crystal. Using the CRP method, the measurement time of 2.1sec
was achieved for an ammonium bitartrate (ABT) crystal by using a commercially-available9.2. X-RAY CRYSTALLOGRAPHY

113

540

µm

(a)

240 µm
60 µm 10µm

polyimide
overcoating
Cu 3 µm+
Ni 2 µm
ceramics

Cu

100 µm(b)
Figure 9.7: Schematic drawing of the pixel readout µ-PIC. (a) is a top view of the detector,and (b) shows a cross-sectional view of the detector.

114

CHAPTER 9. APPLICATION OF µ-PIC

X-ray generator in the laboratory [67]. Rotational photograph image and the directly
reconstructed reciprocal lattice of an ABT crystal using the CRP method is shown in
Figs. 9.8 and 9.9, respectively [3]. Both results were taken using a two-dimensional MSGC.
Because long term sequential operation is possible for the µ-PIC, and then statistically

0.6
0.4
0.2
0
-0.2
-0.4
0.4 0.2

Figure 9.8: Rotational photograph imageof an ABT crystal [3].

0.20.40
0 -0.2 -0.4-0.4-0.2

Figure 9.9: Directly reconstructed reciprocal lattice using the CRP method. The
unit of length is 1/Å [3].

good data is expected, the quality will be comparable to the IP or the CCD.
A preliminary experiment of the X-ray crystallography was performed at OhashiUekusa Laboratory of Tokyo Institute of Technology. The experimental setup is shownin Fig. 9.10. The sample was a crystal of ylide, which was set at 10cm from the detector
surface. This sample was irradiated by a collimated X-ray from an X-ray generator witha molybdenum target. The acceleration voltage and tube current were set to be 50kV and
250mA, respectively. The continuous component was reduced by a monochromator, andthe sample was irradiated by the Mo K-α line (17.5keV). The µ-PIC was ﬁlled with theargon-based gas of atmospheric pressure, and the drift ﬁeld of 600V/cm in a 1mm thickgas volume. The gas gain of the µ-PIC was set to be 3200 in order to avoid discharges.
Fig. 9.11 shows a rotational image of a ylide crystal observed by the µ-PIC with the9.2. X-RAY CRYSTALLOGRAPHY

115

µ-PIC

X-ray generator& monochromator

10 cm

36.9

12.5 cm

beam
stoppersampleFigure 9.10: Setup of the X-ray diﬀraction experiment.

Anode[cm]uPIC8 20031219 per22
100

10

900
8

800
700

6

600
500

4
400
300

2

200
0
0

2

4

6

8
10
Cathode[cm]100

Figure 9.11: Rotational image of a ylide crystal observed by the µ-PIC.

CHAPTER 9. APPLICATION OF µ-PIC

116

irradiation time of 30min, where the rate of the crystal rotation was 0.1/min. Becauseof the long irradiation time, statistically good data was obtained. Although we used
the argon-based gas in this experiment due to its easy-handling, in order to obtain more
precise diﬀraction spots, we would perform this experiment using the xenon-based gas innear future.

9.3

Application of µ-TPC

9.3.1

Gamma-ray imaging

Principle
In astrophysics, gamma-ray observation have become a very promising ﬁeld. However, thedetection of gamma-rays in MeV region is still uncultivated, although this region would
surely show us new aspects of high energy phenomena in the universe. This is almost due
to the nonexistence of the practical detector for this energy region after COMPTEL [68]aboard Compton Gamma-Ray Observatory (CGRO).

Figure 9.12: Description of axes used for the event reconstruction.

In this energy region, the incident gamma-ray is detected via a Compton scattering.

9.3. APPLICATION OF µ-TPC

117

Fig. 9.12 shows the description of axes used for the gamma-ray imaging. The angle φ isdeﬁned as the angle between the scattered gamma-ray and the incident gamma-ray, which
is derived from the kinematics of the Compton scattering as follows:cos φ = me c

2



1
1
−
, E0 = Eγ  + Ke ,
Eγ 
E0

(9.3)

where E0 is the energy of the incident gamma-ray, and Eγ  and Ke are the energies ofthe scattered gamma-ray and the recoil electron, respectively. The conventional detectors
such as COMPTEL consist of two position sensitive gamma-ray detectors: the scatterer
and the absorber. If the scattered gamma-ray deposit the whole energy in the absorber,as shown in Figs. 9.13, an “event circle” of the incident gamma-ray is determined using
the positions of the absorption point and the scattering point, and the energies of thescattered gamma-ray and the recoil electron. The incident direction is obtained as anintersection of at least three event circles.

E0

φ
e
γ

Low-Z

E0

φ
Semiconductor

E1

High-Z

E2

φ2

E1
E2
E3

Multiple Compton
Single Compton
(a)

(b)

Figure 9.13: Principles of gamma-ray detector using the Compton scattering.
In the classical Compton type detectors, it is diﬃcult to reconstruct diﬀused gammaray sources, because all event circles do not cross at one point.
Recently several detectors are developed [69]. These detectors, which are called multiple Compton detector (Fig. 9.13 (b)), consist of a number of scintillator or semiconductor

118

CHAPTER 9. APPLICATION OF µ-PIC

detectors. Each detector works as both scatterer and absorber. In these detectors, however, at least three event circles to obtain the incident direction of gamma-ray. Therefore,
they have the same problem as the single Compton detectors. In order to completely
reconstruct the incident gamma-ray by one event, the track of recoil electrons have to be
detected. This is quite diﬃcult for Compton detectors using solid state detectors.

Advanced Compton method
In order to obtain the incident direction by one Compton event, one additional informationis required. If the recoil angle of the electron is measured, we can deﬁne the angle δ whichis the angle between projected vectors of the recoil electron and the incident gamma-rayon the normal plane to the scattered gamma-ray (See Fig. 9.12). The original directionof the incident gamma-ray can be reconstructed as a small segment of a Compton event
circle. In order to measure the recoil angle of an electron, the recoil electron have to be
tracked accurately. For this purpose, the gas is suitable material as the scatterer.
In order to obtain a ﬁne three-dimensional track of a recoil electron, we have developed
the µ-TPC, which consists of the detection volume of 10 × 10 × 8cm3 and the 10 × 10cm2µ-PIC readout. This µ-TPC works as both the scatterer for incident gamma-rays and thetracker for recoil electrons. The observed track of the recoil electron provide the recoil
angle, and the energy deposit provide the kinetic energy Ke of the recoil electron. As theabsorber, an Anger camera consisting of a 10 × 10 × 2.5cm3 NaI(Tl) scintillator is used.
Scattered gamma-rays are detected by the scintillator and the energy and the absorption
point are measured with 5 × 5 3/4-inch PMTs. This Anger camera is set in front of
the µ-TPC (Fig. 9.14). The timing information which is obtained from the scintillator isalso used as a trigger signal for the µ-TPC. A three-dimensional track of an electron isreconstructed using the position and the relative timing information of an electron and thetrigger timing obtained by the scintillator. This is the ﬁrst detector which can reconstruct
the incident gamma-ray by one event.
Furthermore, the opening angle α between the recoil electron and the scattered gammaray can be used for background rejection. The α angle is derived by two method: geometrical and kinematical method. First, α is geometrically measured using the detected
electron track and the absorption point of the scattered gamma-ray. On the other hand,9.3. APPLICATION OF µ-TPC

119

Figure 9.14: Gamma-ray imaging detector.

α is derived by the kinematics of Compton scattering as follows:cos α =Ke (Eγ  − me c2 )
Eγ  Ke2 + 2Ke me c2

.

(9.4)

If the scattered gamma-ray does not deposit all energy in the scintillation detector (Compton escape event), the gamma-ray is not reconstructed correctly. In this case, the α measured geometrically is inconsistent with that derived from the Compton kinematics. Thisbackground rejection method is named “kinematical cut”. As well as Compton escape
events, other background such as charged particles can be rejected using the kinematicalcut. Thus no veto detector is needed for this µ-TPC based gamma-ray detector.
Experimental result
The imaging characteristic of the detector was examined using the 137 Cs radioactive source
which emits 662keV gamma-ray. The source was placed behind the mother board of theµ-PIC. The distance between the source and the µ-PIC was 5cm. The scintillator was
set in front of the µ-TPC. Fig. 9.15 shows a typical Compton event. The direction of the

120

CHAPTER 9. APPLICATION OF µ-PIC

incident gamma-ray was correctly reconstructed from one event.

Figure 9.15: Typical reconstructed event.

It is diﬃcult to resolve two gamma-ray sources with a conventional Compton detectorin particular when these two sources emit same energy gamma-rays. However, clearly
separated image was obtained by the µ-TPC based detector as shown in Fig. 9.16. The
angular resolution were 11.5 degrees for φ direction and 22.3 degrees for δ direction. The
angular resolution is restricted by the spatial resolutions of the µ-TPC and the Angercamera. As mentioned in section 8.2, the spatial resolution of the µ-TPC will increase
when a DAQ system with a clock of 50MHz will be completed. In order to improve
the spatial resolution of the scintillation camera, we are developing a pixellated CsI(Tl)scintillator instead of an Anger camera, and a Si PIN photodiode array is being developed
as a readout device of the scintillation camera [70].
Future prospect
As the next step, we are planning to develop the larger (more than 30 × 30 × 30cm3 )µ-TPC which is surrounded by scintillation camera (Fig. 9.17). In the near future, thisdetector will be mounted on a balloon. This detector cover the energy region of 0.1-

9.3. APPLICATION OF µ-TPC

121

Cs137 x 2
130

15

120

10

110
5

100

0

90
80

-5

70
-10

-15
-15

60
-10

-5

0

5

10

15

50

Figure 9.16: Reconstructed image of two 137 Cs sources. The real position of the gammaray sources are indicated as two crosses.

CHAPTER 9. APPLICATION OF µ-PIC

122

30MeV. In the low energy region (0.1-2MeV), the incident gamma-ray is reconstructedusing Compton scattering as described above. In the high energy region (1-30MeV), thepair creation process is used for the reconstruction of the incident gamma-ray. According
to a simulation work, the position resolution of less than 1 degree is expected at the energy
of 20MeV [60].

~1MeV γ~10MeV γ
Drift plane

e-

e-

µ-PIC

e+

PMTs
Scintillator

Figure 9.17: Schematic structure of the MeV gamma-ray detector based on µ-TPC.

9.3.2

Neutron imaging

Principle
Now neutron time-resolved imaging becomes important technique. In the X-ray crystallography, the material containing heavy atoms can be well analyzed. However, X-ray havepoor sensitivity for light atoms such as hydrogen, carbon, oxygen, and nitrogen. In contrast, the cross section of a neutron become large for these light atoms. These elements
constitute biomolecule such as protein. Thus, neutron diﬀraction study is becoming a
powerful tool for bioscience. Neutron position sensitive detectors (PSDs) with a large

9.3. APPLICATION OF µ-TPC

123

detection area is necessary for this purpose.
Additionally, excellent timing resolution ( msec) is required for pulsed neutron
source, because energies of thermal neutrons is measured using time-of-ﬂight method.
In particular, a capacity for operation in high counting rate are indispensable for use at aneutron beams of the next generation [71]. Gas-ﬁlled neutron detectors have an advantageof the large detection area.
For gaseous neutron detectors, 3 He is commonly used, where a following interaction
occurs:n + 3 He → p (578keV) + 3 H (193keV).
This interaction has a large cross section for thermal neutrons of about 25meV, and theenergy loss of the emitted nuclei in gas is quite large. The range of the emitted nuclei areabout 10mm for the proton and about 3mm for the triton in argon gas of atmosphericpressure. These range are long enough for tracking by a µ-PIC. Furthermore, the large
energy losses provide a dense sampling of the tracks.
Neutron detection using µ-TPC
As a preliminary experiment, we veriﬁed the principle of the neutron detection using ourµ-TPC. The µ-TPC was ﬁlled with the Ar/C2 H6 (90/10) gas mixture of atmosphericpressure. In this experiment, the µ-TPC was irradiated with fast neutron from a 252 Cf
radioactive source, the neutron energy of which peaks between 500keV and 1MeV. The
neutrons kick the hydrogen nuclei in C2 H6 , and part of the kinetic energy of the neutron
is transferred to the recoil nucleus, i.e. the proton. The tracks and the energy loss ofthe proton in the gas is detected by the µ-TPC. One ﬁssion of a 252 Cf emits 3.8 neutrons
and 9.7 gamma-rays on average. These gamma-rays were detected by a YAP scintillator(1inch × 1inchφ) and used for the trigger of the µ-TPC.
The µ-PIC was operated with the gas gain of 3000. This gas gain is high enough todetect dense tracks of protons. Fig. 9.18 shows the detected tracks of protons. The spatial
resolution of the track was 260µm, which is discussed in section 8.2.2. The energy lossof the protons was measured by a FADC (REPIC RPV160). The measured energy losses
are shown in Fig. 9.19. The waveforms show that the µ-TPC has suﬃcient performance
to detect tracks and Bragg curves of protons with energies of around 1MeV.

CHAPTER 9. APPLICATION OF µ-PIC

124

drift [cm]

n
8
7
6
5
4
3
2
1
0
10
6

e

od

an

8

]

m

[c

4
2
0 0

2

4

6

10
8
]
m
[c
e
cathodFigure 9.18: Several three-dimensional proton tracks.

Pulse height [V]9.3. APPLICATION OF µ-TPC

0.6

125
time[ µ s]

1

0

2

track
direction

0.5
0.4

neutron

0.3
0.2
0.1
0
0

1

2

3

4

5

6

7
8
Drift [cm]Figure 9.19: Energy loss of the protons. The direction of the tracks are recognized fromthe shape of the Bragg curve.

Background rejection
Since the neutron generation process utilize nuclear reactions, large number of gammarays are emitted in the process, which become a serious background source. A gamma-ray
kicks a electron from an atom via Compton scattering. Therefore, when electron tracks aredetected, it can be regarded as gamma-ray events. In order to discriminate neutron eventsfrom gamma-ray events, energy losses and straggling of each particle are used. The energy
loss of proton is much larger than that of electrons. Furthermore, the straggling of the
protons is also smaller than that of electrons. The discrimination power was tested using a
gamma-ray source (22 Na). Annihilated back-to-back gamma-rays of 511keV were emitted
from the source. One gamma-ray is detected by the µ-TPC and another is detected by
YAP scintillator and used as a trigger. The energy of gamma-ray is close to the Q value(764keV) of the 3 He(n, p)3 H reaction. The straggling of a track is characterized using χ2
deﬁned by
χ2 =



∆i
,
i=1,2,n−1,n σ

(9.5)

126

CHAPTER 9. APPLICATION OF µ-PIC

where n is the number of detected points, δi is the distance between the ith detected
point and the best-ﬁt straight line, and σ = 260µm is the measured spatial resolutionof the µ-TPC. The degree of freedom (d.o.f) was three for all of the tracks, because weused the ﬁrst two and the last two points to calculate χ2 . Figs. 9.20 show the plots ofχ2 /d.o.f against dE/dx for the events from 252 Cf and the events from 22 Na. We deﬁned
thresholds of neutron events for dE/dx (> 50keV/cm) and χ2 /d.o.f (> 3). More than
90% of gamma-ray events are rejected by this threshold.

9.3. APPLICATION OF µ-TPC

127

30

25

χ 2 / d.o.f.

20

15

neutron region

10

5

0
0

100

200

300
dE/dx [keV/cm]

400

500

600

500

600

(a)

30

25

χ 2 / d.o.f.

20

15

neutron region

10

5

0
0

100

200

300
dE/dx [keV/cm]

400

(b)
Figure 9.20: χ2 /d.o.f vs. dE/dx of the (a): 252 Cf events, and (b): 22 Na events. The
neutron regions are superimposed.

Chapter 10
Summary and conclusion
A novel gaseous micro-pattern detector “Micro Pixel Chamber (µ-PIC)” has been developed, and its performances have been examined. Because the µ-PIC was manufactured
based on the Printed Circuit Board (PCB) technology, it has the features of a large
detection area (10 × 10cm2 ) and mechanical robustness.
From the early development using the small prototype detector having 3 × 3cm2 detection area, the µ-PIC indicated the promising abilities as a radiation detector. The gas
gain exceeded 5000 and it also had a robustness for discharges. In addition, the PCB
technology made it possible to produce a large two-dimensional µ-PIC with a detection
area of 10 × 10cm2 . Within one year we developed two type of µ-PIC.
However, there have been serious problem for µ-PICs to be overcome. That is thelower anode electrodes below the substrate. The eﬀect of the lower anode was intensively
investigated by the computer simulation study using MAXWELL and GARFIELD. This
simulation study shows that the drift of primary electrons are prevented by the substrateedge near the anode top and 2/3 of primary electrons did not reach the anode but thesurface of the substrate. It means that the gas gain would increase three times higher if theanode top is higher than the surface of the substrate. In Addition, we found that thicker
substrate improves the absolute gas multiplication factor as well as the electron collection
eﬃciency, because the thick substrate shield the electric ﬁeld due to the anode readoutbuses placed on the rear side of the substrate. The attainable gas gain was estimatedfrom the simulation and the previous experimental data. If the substrate thickness exceed
128

129
150µm, the attainable gas gain to avoid discharges would exceed 105 .
Based on this study, we have reproduced the higher anode electrodes using a new
manufacturing technology. The new µ-PIC have a 100µm thick substrate with higher anduniform anode electrodes. The gas gain of more than 1.5 × 104 was obtained for the newµ-PIC. This new method also provided us the uniformity of the gas gain within 7% in
the whole detection area of 10 × 10cm2 . Furthermore, a long term continuous operation
over 1000 hours was achieved with the gas gain of more than 6000, which looks close to
the required gas gain to detect Minimum Ionizing Particles. The attained gas gain wasconsistent with the simulation. It means that we can design the structure of µ-PIC basedon the quantitative prospect by the simulation. In near future, the µ-PIC having thedetection area of 30 × 30cm2 will be completed based on this improved method.
A compact data acquisition (DAQ) system was developed for multi-channel and highrate readout of µ-PIC. It consists of ﬁve Field Programmable Gate Arrays (FPGAs) andoperates with the clock of 20-40MHz. The maximum data acquisition rate of 7.1MHz was
obtained using an X-ray generator.
The imaging performances of the µ-PIC were examined using the DAQ system withthe clock of 20MHz. As a result of X-ray imaging test, the position resolution of 120µm(r.m.s.) was obtained, which value is very close to the limitation determined by theelectrode pitch. The uniformity of the image was within 6% for the whole detection area.
The µ-PIC was applied to the TPC, which is named “µ-TPC”. The track of protons
were successfully detected with the sampling pitch of 2.6mm at 20MHz. Recently the 1mm
sampling has been obtained by a DAQ system with 50MHz clock. The three-dimensional
tracking resolution was 260µm for Ar/C2 H6 (90/10) gas mixture for each sampling points
for 20MHz sampling.
In order to explore the capability of µ-PIC for various science ﬁelds, we have carriedout following preliminary experiments. At ﬁrst, we measured X-ray polarization using
Thomson scattering of a polyethylene block. The asymmetric parameter of 0.15 wasobserved for 10keV X-ray using neon-based gas [62]. X-ray crystallography using theContinuous Rotation Photograph method was also performed using a ylide crystal. Oneof the most important applications of the µ-TPC is the gamma-ray imaging of MeV
energy region. We have developed a prototype of the MeV gamma-ray imaging detector

130

CHAPTER 10. SUMMARY AND CONCLUSION

based on the advanced Compton method. The ﬁrst gamma-ray image was successfully
obtained using two 137 Cs (662keV) sources. The detail of MeV imaging was alreadypublished elsewhere [60, 61]. Another important application of the µ-TPC is thermal
neutron imaging. The detection principle was veriﬁed with Ar/C2 H6 gas mixture and a252

Cf source [72].
As mentioned before simulation, µ-PIC has a much potential to innovate gas gains bysmall improvements,and we will soon try to develop the new perfect radiation detector.
The µ-PIC will soon establish a new imaging method using X-ray, gamma-ray, charged
particles, and neutrons.

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135

Acknowledgments
I am deeply grateful to Professor Toru Tanimori for his guidance and encouragements inthis work. This thesis would not be completed without his support. I would like to thankmy collaborators: Professor Takeshi Tsuru, Dr. Hidetoshi Kubo, Dr. Kentaro Miuchi, Dr.
Atsushi Takeda, Masaru Ueno, Reiko Orito, Yoko Okada, Atsushi Takada (Kyoto Univ.),
Dr. Oleg Bouianov (Espoo-Vantaa Institute of technology) and Dr. Marina Bouianov(CSC-Scientiﬁc Computing Ltd.). I am grateful for all the member of the cosmic raylaboratory in Kyoto University for their continuous discussions and encouragements.
I sincerely thank Dr. Atsuhiko Ochi (Kobe Univ.) and Yasuro Nishi (Rigaku) forthe helpful advice of the development of the detector and the DAQ system. I also thankDr. Yuji Nishi (RIKEN) for his kind guidance of detector physics. I appreciate Tomohisa
Motomura (D.T. Circuit Technology Co., Ltd.), Dr. Akira Yunoki (TOSHIBA), and the
staﬀs of D.T. Circuit Technology and TOSHIBA who manufactured the detectors andgave me several technical informations about the manufacturing process.
I thank Professor Tomofumi Nagae (KEK) and members of his laboratory, who helped
the experiment at T1 beam line in the Proton Synchrotron at KEK. I also thank Dr.
Hidenori Toyokawa, Dr. Masayo Suzuki (JASRI), and Dr. Tetsuro Fujisawa (RIKEN),
who helped the MSGC experiment at SPring-8. I appreciate Professor Yuji Ohashi andProfessor Hidehiro Uekusa (Tokyo Inst. of Tech.) for providing the experimental space.
This work was supported by a Grand-in-Aid for the 21st Century COE “Center for
Diversity and Universality in Physics”, and the Japan Society for Promotion of Sciencefor Young Scientists.
Finally, I thank my family for their kind support and encouragement.

136