Development of Advanced Compton Imaging Camera
with Gaseous Electron Tracker
andFirst Flight of Sub-MeV Gamma-Ray Imaging
Loaded-on-Balloon ExperimentAtsushi Takada
Department of Physics, Faculty of Science, Kyoto University
Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan
takada@cr.scphys.kyoto-u.ac.jp

This thesis was submitted to the Department of Physics,
Graduate School of Science, Kyoto University
on May 8, 2007in partial fulfillment of the requirements
for the degree of Doctor of Philosophy in physics.

Abstract
For a MeV gamma-ray telescope in the next generation , we developed an Electron Tracking
Compton Camera, which consists of a tracker and an absorber. The camera obtains the energy
and the direction of both the scattered gamma-ray and the recoil electron, and determines boththe energy and the direction of the incident gamma-ray, photon by photon. Also this camera
rejects the backgrounds using the kinematics of Compton scattering. We use a µ-TPC as the
tracker and the GSO:Ce pixel scintillator arrays as the absorbers. A µ-TPC is a gaseous time
projection chamber, which has a high gas gain of 2 × 104 and a good position resolution of ∼ 500µm. Our Compton camera is expected to have a good signal to noise ratio and a good angularresolution due to a gaseous high resolution tracker.
In this thesis, I reported things as below. First, in order to verify the principle of the gammaray reconstruction and the background rejection due to the Compton scattering kinematics, weconstructed a prototype camera using µ-TPC ﬁlling with an Ar gas. By this prototype, we obtained the images and the spectra of the gamma-rays from the radioactive sources, and measured
the performance of the prototype camera. Next, in order to measure diﬀuse cosmic gamma-rays
and atmospheric gamma-rays, we designed and constructed a ﬂight model detector with an
improvement of the detection eﬃciency using a Xe gas as a Compton scattering target. By
this improvement, the detection eﬃciency was 10 times larger than that of the prototype andthe FOV spread to 3 str. Finally, we loaded the ﬂight model on a balloon, and launched on
September 1 2006 as the 1st ﬂight of Sub-MeV gamma-ray Imaging Loaded-on-balloon Experiment (SMILE). The balloon reached to 35 km at altitude, and the level ﬂight continued during
4 hours. We succeeded in the detection of ∼ 200 downward gamma-rays during the 3.5 hourslevel ﬂight (live time 3.0 hours). The detected photon number was consistent to the simulated
one, which was ∼ 200 photons in 3 hours. By this balloon experiment, we succeeded in theobservation of the ﬂuxes of diﬀuse gamma-ray and atmospheric gamma-ray between 100 keV
and 1 MeV.

Contents
1 MeV Gamma-Ray Astronomy
1.1 Production Mechanism of Gamma-Ray . . . . . . . . . . . . . . . . . . . . . . . .
1.1.1 Synchrotron Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.2 Bremsstrahlung . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.3 Inverse Compton Scattering . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.4 Nuclear Transitions and Decay . . . . . . . . . . . . . . . . . . . . . . . .
1.1.5 Annihilation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Sky in Gamma-Ray . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.1 All sky map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.2 Galactic diﬀuse gamma-ray . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.3 Extragalactic diﬀuse gamma-ray . . . . . . . . . . . . . . . . . . . . . . .
1.3 Source of Gamma-Rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.1 Supernova Remnant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.2 Spin-Down Pulsars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.3 Black Hole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.4 Galactic Center . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.5 Active Galactic Nuclei (AGN) . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.6 Solar Flare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.7 Gamma-Ray Burst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1
1
3
3
4
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6
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6
6
9
9
10
13
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16

2 Detection of Gamma-Rays
2.1 Interaction of Gamma-Rays and Matter . . . . . . . . . . . . . . . . . . . . . . .
2.1.1 Photoelectric Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2 Compton Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.3 Pair Creation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Sub-MeV/MeV Gamma-Ray Imaging . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Active and Passive Collimators . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2 Coded Aperture Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3 Gamma-Ray Lenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.4 Compton Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.5 Pair Tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.6 Summary of MeV Gamma-Ray Imaging . . . . . . . . . . . . . . . . . . .
2.3 Low Energy Gamma-Ray Observatories . . . . . . . . . . . . . . . . . . . . . . .
2.3.1 CGRO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.2 INTEGRAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3 Advanced Compton Imaging
3.1 Imaging by Electron Tracking Compton Method . . . . . . . . . . . . . . . . . .
3.2 Background Rejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Doppler Broadening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4 Multiple Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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i

CONTENTS
3.5

Composition of Advanced Compton Camera . . . . . . . . . . . . . . . . . . . . .
3.5.1 Goal for Next Generation Telescopes . . . . . . . . . . . . . . . . . . . . .
3.5.2 Requirements for Tracker and Absorber . . . . . . . . . . . . . . . . . . .
3.5.3 Advanced Compton Camera with Gaseous Electron Tracker . . . . . . . .

38
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41

4 µ-PIC, µ-TPC & Scintillator
4.1 µ-PIC (Micro Pixel Chamber) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.1 Introduction of µ-PIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.2 Morphology of Pixels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.3 Readout Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.4 Performance of µ-PIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1.5 µ-PIC as an X-ray Imaging Detector . . . . . . . . . . . . . . . . . . . . .
4.2 Time Projection Chamber with µ-PIC (µ-TPC) . . . . . . . . . . . . . . . . . . .
4.2.1 Structure and Principle of µ-TPC . . . . . . . . . . . . . . . . . . . . . .
4.2.2 Gas Electron Multiplier . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.3 Drift and Diﬀusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.4 Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 Scintillation Camera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.1 Scintillator & PMT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.2 HV Operation/Readout System . . . . . . . . . . . . . . . . . . . . . . . .
4.3.3 Image and Energy Spectrum . . . . . . . . . . . . . . . . . . . . . . . . .

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55
57
58

5 Tracking of Charged Particles
5.1 Tracking of Cosmic Muon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.1 DAQ System for Muon Tracking . . . . . . . . . . . . . . . . . . . . . . .
5.1.2 Tracks of Muon & Track Eﬃciency . . . . . . . . . . . . . . . . . . . . . .
5.1.3 Clock Counter Histogram . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.4 dE/dX Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.5 Position Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.6 Threshold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Tracking of Recoil Electron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1 Fixing of the Track Points . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.2 Connection of the Points . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.3 Selection of the Stopping Electron . . . . . . . . . . . . . . . . . . . . . .
5.2.4 Judgement of the Initial/End Point of the Track . . . . . . . . . . . . . .
5.2.5 Determination of the Recoil Direction . . . . . . . . . . . . . . . . . . . .

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6 Prototype Camera
6.1 Prototype Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.1 Absorber & Tracker of Prototype . . . . . . . . . . . . . . . . . . . . . . .
6.1.2 Alignment of Prototype Camera . . . . . . . . . . . . . . . . . . . . . . .
6.1.3 Data Acquisition System of Prototype Camera . . . . . . . . . . . . . . .
6.2 Reconstruction of Gamma-Ray . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.1 Event Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.2 Reconstructed Gamma-Rays . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.3 Eﬀect of α Cut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.4 Comparison with Classical Compton Imaging . . . . . . . . . . . . . . . .
6.3 Performance of Prototype Camera . . . . . . . . . . . . . . . . . . . . . . . . . .

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ii

CONTENTS
7 Flight Model Detector
7.1 Design of Flight Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1.1 Cosmic-Ray Fluxes at Balloon Altitude . . . . . . . . . . . . . . . . . . .
7.1.2 Design of Flight Model for First Flight . . . . . . . . . . . . . . . . . . . .
7.1.3 Particle Incoming to Detector . . . . . . . . . . . . . . . . . . . . . . . . .
7.1.4 Expectation of Gamma-Ray Detection . . . . . . . . . . . . . . . . . . . .
7.2 Data Acquisition of Flight Model Detector . . . . . . . . . . . . . . . . . . . . . .
7.2.1 Compton Camera Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2.2 Charged Particle Tracking Mode . . . . . . . . . . . . . . . . . . . . . . .
7.2.3 Dead Time Measurement & Condition Monitoring System . . . . . . . . .
7.3 Flight Model µ-TPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.1 Properties of TPC Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.2 Gain Uniformity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.3 Clock Counter Histogram . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.4 Energy Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.5 Position Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3.6 Summary of Flight Model µ-TPC . . . . . . . . . . . . . . . . . . . . . . .
7.4 Performance of Flight Model Detector . . . . . . . . . . . . . . . . . . . . . . . .

79
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86
86
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88
89
90
90

8 Balloon Flight95
8.1 First Flight of SMILE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
8.1.1 Speciﬁcation of Physical Instrument . . . . . . . . . . . . . . . . . . . . . 95
8.1.2 Flight Path and Operation Mode . . . . . . . . . . . . . . . . . . . . . . . 97
8.2 Status of Detector in Balloon Flight . . . . . . . . . . . . . . . . . . . . . . . . . 98
8.2.1 House Keeping Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
8.2.2 Count Rate of Scintillators . . . . . . . . . . . . . . . . . . . . . . . . . . 98
8.2.3 Current of µ-TPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
8.2.4 Dead Time & Event Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
8.3 Results of Experiment & Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 103
8.3.1 Charged Particle Tracking Mode . . . . . . . . . . . . . . . . . . . . . . . 103
8.3.2 Compton Camera Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
9 Summary & Future Work

115

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Chapter 1

MeV Gamma-Ray Astronomy
Once, the astronomy meant the studies of the stars’ and planets’ movements with the visible lightobservation, and hence the image of the universe was static and silent. However, astronomers
now observe not only the electromagnetic waves from radio to gamma-ray but also cosmic-rays,
neutrinos and so on. By these new windows, we have got the new various information that theuniverse is active and violent.
Gamma-ray is one of these new windows. Figure 1.1 shows the history of the gamma-ray
observatories in space. The gamma-ray observations started in 1961, which was almost the sametime as the discovery of Sco X-1. After the prediction by Hayakawa et al. in early 1950s about
the existence of the gamma-ray radiation due to the decay of π 0 created by the interaction ofcosmic-rays and interstellar matter, several gamma-ray sources were discovered by the satellites,
OSO-7 (≥ 500 MeV) in 1967, SAS-2 ( > 30 MeV) in 1972 and COS-B (2 keV - 5 GeV) in 1975.
Besides, low-energy gamma-rays were observed by Granat which was launched in 1989 by Russia
and France, and CGRO which was launched in 1991 by United States. Recently, INTEGRAL
was launched in 2004 by ESA, and GLAST will be launched soon in 2007. On the ground,very high-energy gamma-rays of energy about 1012 eV were observed by Cherenkov telescopes,
Whipple, HEGRA and CANGAROO, in 1990s.
Although gamma-rays mean all electromagnetic waves with the energy above a few hundred
keV, the gamma-rays of each MeV - TeV energy band provide us the diﬀerent information even
if they were emitted from same object. A gamma-ray observation in the MeV region provides
the information of the nucleosynthesis, the particle acceleration and the interaction of cosmicrays and interstellar matter. MeV gamma-rays are hardly attenuated from source objects tothe earth, but can’t pass through the atmosphere (Figure 1.2). Thus celestial MeV gamma-rays
must be obtained outside the atmosphere. In this energy band, the number of photons is lessthan X-ray, and the complete absorption of photon is diﬃcult because of the dominated processof Compton scattering. Besides this energy band has the backgrounds of photons produced by
the hadron process of cosmic-ray and satellite body, although the band above 100 MeV has nobackground. Consequently the observation is very diﬃcult, and the MeV gamma-rays astronomy
has not be advanced than other energy bands.

1.1

Production Mechanism of Gamma-Ray

Thermal radiation is an emission from a large population of electromagnetically interacting
particles in equilibrium, with their mean energy characterized by temperature T . The spectrumof radiation intensity follows the ‘black body’ distribution, and the energy density of radiation
at frequency ν isIν =8πhν 31,
c3 ehν/kB T − 1

(1.1)
1

Chapter 1. MeV Gamma-Ray Astronomy

Figure 1.1: The history of Gamma-Ray Observatories [1]2Production Mechanism of Gamma-Ray

Figure 1.2: The attenuation by atmosphere in multi-wavelength [2]where h is Planck’s constant, and kB is Boltzmann’s constant. For the thermal gamma-raysof 1 MeV, the corresponding temperature would be above 109 K. Thus, thermal process is notreasonable, and nonthermal processes are the more reasonable source of gamma-rays. In the
MeV region, several nonthermal processes exist as follows.

1.1.1

Synchrotron Radiation

When a relativistic electron moves via the magnetic ﬁeld B, an electron trajectory will bechanged by the Lorentz force. Because any acceleration of the charged particle can be viewed
as the modiﬁcation of the ﬁeld, the kinetic energy of the electron is transformed into the energyof the electromagnetic ﬁeld. For the relativistic electron, such an energy lose as an emission ofphoton, which is called Synchrotron Radiation.
Synchrotron radiation spectrum has a peak at the characteristic as()
3h eB⊥
Ee 2
hνc =
,
(1.2)
4π me c me c2
where, e, me , Ee are charge, mass, energy of the electron, respectively, and B⊥ = B sin θ withthe pitch angle θ which is the angle between the particle trajectory and the direction of themagnetic ﬁeld. The total energy loss rate by synchrotron radiation of an electron moving in themagnetic ﬁeld B is
(
)
dEe
4
B2
−
= σT c γ 2 β 2 ,(1.3)
dt syn 38π
48πewhere σT = 3m
2 c4 is Thomson cross section.
e

1.1.2

Bremsstrahlung

Another important process of gamma-ray production is ‘Bremsstrahlung’. When an electron
passes very near the nucleus, the electron’s trajectory is substantially changed by the strong
electric ﬁeld of the nucleus, and photons are emitted.
3Chapter 1. MeV Gamma-Ray Astronomy
The spectrum of bremsstrahlung radiation remains ﬂat up to roughly the electron kinetic
energy, and it drops sharply toward zero above. For relativistic electrons in a fully ionizedplasma (no screening), the bremsstrahlung loss is
(
(
)
)
Ee
dEe
2 2
−
= 4na Z r0 αc ln+ 0.36 Ee .
(1.4)
dt B
me c2
In the case of total screening, above formula is modiﬁed to
)
)
(
(
dEe
183
1
2 2
−
= 4na Z r0 αc ln −1/3 −
Ee ,
dt B
18
Z

(1.5)

2

where na is the target atom density, r0 = mee c2 is the classical radius of the electron, and α isthe ﬁne structure constant.

1.1.3

Inverse Compton Scattering

When an energetic photon collides with a low energy electron, the photon is scattered andsome of the photon energy transfers to the electron. This process is called Compton scattering.
The inverse process also surely exists, and generates a gamma-ray: when the low energy photons
collide with energetic electrons, these photons gain the some amount of the energy of the electron
via the collision. This ‘Inverse Compton Scattering’ is important in the regions of the high photon
density.
The energy loss for an electron in the photon ﬁeld energy density wph is written as
)
(
4
dEe
= σT cwph γ 2 β 2 .
(1.6)
−
dt IC
3

1.1.4

Nuclear Transitions and Decay

A nucleus has several speciﬁc, quantized states of the energy to bind the nucleons. These nuclear
states have typical energy bands of the MeV scale, and hence the transition between those statesof a nuclear absorbs or emits the MeV gamma-ray photons. Thus, the gamma-rays are emitted
from ‘de-excitation’ of a nucleus:X ∗ −→ X + γ,

(1.7)

and ‘radioactive decay’:
X −→ Y ∗ + e+ −→ Y + γ.

(1.8)

De-excitation occurs by the energetic collisions of cosmic-rays with the interstellar gas nuclei,
in the space. On the other hand, a radioactive decay occurs by nuclearsynthesis in a supernova
and a core of heavy stars. The examples of line gamma-ray emissions by nuclear transitions
expected in the universe are listed up in Table 1.1.
Pion is a boson involved in the strong nuclear interaction, and is created during the stronginteraction events such as collisions of cosmic-ray protons with ambient-gas nuclei. The dominant
π-producing channels in hadronic interactions are as follows
p + p −→ p + p + aπ 0 + b(π + + π − ),

(1.9)
−

p + p −→ p + n + π + aπ + b(π + π ),+0+(1.10)

where a and b are positive integers. A π 0 decays rapidly into two gamma-rays with a proper
time of 9 × 10−17 sec, and the distribution of the photon energy has a peak at about 70 MeV atthe center of gravity system, which is the half of the rest mass of pion. Because pions createdby energetic protons has a momentum distribution, the observed gamma-ray spectrum of thepion decay is broadened by Doppler shift.
4Production Mechanism of Gamma-Ray

Table 1.1: The gamma-rays by nuclear transitions [1, 3, 4]
process
Energy [MeV]12∗
De-Excitation
C
4.438
14 N∗
2.313, 5.105
16 O∗
2.741, 6.129, 6.917, 7.117
26 Mg∗
1.809
56 Fe∗
0.847, 1.238, 1.811
radioactive decay

56 Ni (6.10 d)

60 Fe (1.5 × 106 y)

0.158, 0.270, 0.480, 0.759, 0.812
0.847, 1.238, 2.598
0.122, 0.136
1.157
1.809
1.173, 1.333

n + 1 H →2 D + γ

2.223

56 Co (77.2 d)
57 Co (271.7 d)
44 Ti (63 y)
26 Al (7.4 × 105 y)

capture1.1.5

Annihilation

An electron-positron annihilation is an important source of gamma-rays. In the annihilation,
two or more photons are created, and the total energy of electron and positron is distributed tothese photons. Positron and electron may form a bound system, which is called positronium,
and two diﬀerent states exist. One is the ground state, and a positronium decays to two gammarays. In this case, each photon has an energy of 511 keV, which is equal to the rest energy of anelectron. The other state is the parallel-spin state, and it decays to three gamma-rays having
the continuum spectrum.
Actually, the 511 keV line was observed at near the Galactic Center by SMM and OSSE, andimplied the annihilation rate of ∼ 2 × 1043 sec−1 . Annihilation photons suggests the existence ofthe electron-positron plasma. Positrons are produced by β + -decay, decay of π + and the collisionof hadronic antiparticles and normal matter. The examples of the radioactive isotopes causing
β + -decay are 26 Al, 44 Ti, 56 Co, and the distinct nova products 13 N and 18 F. On the other hand,
π + s would be produced at the vicinity of the compact stars.
However, positrons would be also produced by the decay of the intrinsic radioactive isotopes
or the interactions of the cosmic-rays with the satellite platform. Therefore, spectral decomposition of annihilation photons from the inner Galaxy is considered to be quite diﬃcult.

5Chapter 1. MeV Gamma-Ray Astronomy

1.2

Sky in Gamma-Ray

1.2.1

All sky map

For the all sky survey in gamma-ray, Compton Gamma-Ray Observatory (CGRO) satellite had
4 detectors: COMPTEL, EGRET, OSSE and BATSE. COMPTEL found about 30 gamma-ray
sources in the MeV region [5], and EGRET found about 270 sources in the sub GeV region[6]. Figures 1.3 and 1.4 show the all sky maps of the gamma-ray obtained by EGRET and
COMPTEL, respectively. These images show that the gamma-rays are radiated from not only
the compact objects but also the solar ﬂare. Besides, in EGRET observations, Figure 1.3 says
there are about 170 undeﬁned objects of which counter part are not identiﬁed by other energy
bands.

Figure 1.3: The distribution of the gamma-ray objects by EGRET [6]1.2.2

Galactic diﬀuse gamma-ray

The gamma-rays from the Galactic Plane were also observed by those detectors. Figures 1.4,
1.5 and 1.6 show the galactic diﬀuse gamma-ray radiation in both energy regions.. Figure 1.7
shows the multi-wave energy spectrum of the galactic diﬀuse gamma-rays and X-rays. In theenergy region from sub MeV to MeV, photons are mainly produced by the bremsstrahlung andthe inverse Compton scattering of an electron [7].
On the other hand, the galactic diﬀuse gamma-rays in the MeV region consist of not only thecontinuum component but also the line emissions. Figure 1.5 the all sky map of 1.8 MeV from thedecay of 26 Al. The source of the gamma-ray of 1.8 MeV is the decay of 26 Al (T1/2 =∼ 106 year),which is considered to be synthesized in the cores of massive stars.
However, in the observation of the point-sources near the galactic plane, the galactic diﬀuse
gamma-rays make diﬃcult to observe them as the background.

1.2.3

Extragalactic diﬀuse gamma-ray

Extragalactic diﬀuse gamma-rays come from the outside of the Galaxy, of which the distribution
is uniform in the all sky. Figure 1.8 is the spectum of the extragalactic diﬀuse gamma-ray.
The spectrum is explained by the combination of the emissions from AGNs and the type Ia
supernovae. Particularly, in the energy region of MeV gamma-rays, the mojority is considered
to be gamma-rays by type Ia supernovae in galaxies.
6

Sky in Gamma-Ray

Figure 1.4: The all sky map by COMPTEL (1 - 30 MeV) [1]Figure 1.5: The all sky map in 1.8 MeV by COMPTEL [1]Figure 1.6: The all sky map by EGRET (≥100 MeV) [1]7

Chapter 1. MeV Gamma-Ray Astronomy

Figure 1.7: The spectrum of galactic diﬀuse gamma-ray [7]. Also shown is the bremsstrahlung(dot-dashed line), inverse Compton scattering (short-dashed line), and π 0 (triple-dotdashedline) model, positronium continuum (long-dashed line) and the thermal Raymond-Smith plasmacomponent (dotted line). The total of the model components is also presented (solid line).

Figure 1.8: The spectrum of extragalactic diﬀuse gamma-ray [8]. The dot-dashed line and dashedline are estimated contributions from Seyfert I and Seyfert II, respectively. The tiple dot-dashedline is steep-spectrum quasar contribution, and the dotted line is Type Ia supernovae. Also thelong dashed line is a possible blazar contribution assuming an average power law index of -1.7
below 4 MeV and -2.15 at higher energies. The solid line indicates the sum of all components.

8

Source of Gamma-Rays

Figure 1.9: The light curve of SN1987A (infrared - ultraviolet) [9]1.3

Source of Gamma-Rays

1.3.1

Supernova Remnant

The supernova is an important site of the nucleosynthesis, and is considered to create the heavier
nuclei than Fe. In the nucleosynthesis process in the supernova explosion, the radio isotopes
are also produced, and hence the gamma-ray emission by the decay of those isotopes is a goodprobe to investigate the nucleosynthesis.
If a binary includes a white dwarf, the material from a companion star gradually accretes onto
the white dwarf. Then it makes the pressure in the core of the white dwarf higher. Eventually
the white dwarf explodes with the thermonuclear supernova explosion (type Ia supernova). In
the type Ia supernova, 56 Ni is expected to be produced of which amount is predicted about 0.6
M⊙ , and after the explosion 56 Ni decays such as:
56

Ni −→56 Co −→56 Fe.

(1.11)The light curve of the type Ia supernova is well explained by the combination of the decay timeof those nuclear transitions. In the type II supernova, which is the gravitational collapse of theiron core as the ﬁnal stage of the stellar evolution of the stars more massive than about 8 - 10
M⊙ , 56 Ni is also produced, although the amount is less than that of a type Ia supernova. Figure
1.9 shows the light curve of SN1987A, which is explained by the decay time of 56 Co practically.
Also the gamma-ray emission due to the 56 Co decay was observed from SN1987A.
In the supernova explosion, a lot of neutrons are ejected from the core, the nuclei of theouter shell rapidly capture the neutrons before the transition to a stable states, and changes
to neutron-rich isotopes. Since such nuclei are so unstable, they transmit to the stable nuclei
through β-decays. This process rapidly proceeds, which are called r-process. The existence of
the isotopes in Table 1.1 has been observed in the supernova remnants so far.

9

Chapter 1. MeV Gamma-Ray Astronomy

Figure 1.10: The schematic view of pulsar [10]1.3.2

Spin-Down Pulsars

Gamma-Ray Pulsars
Pulsars radiate a short periodic pulse, and hence are considered to be neutron stars spinningwith a high speed. A neutron star also has a strong magnetic ﬁeld of about 1012 G. Because themagnetic axis inlines to the rotation axis generally, as shown in Figure 1.10, the emission near themagnetic poles sweeps around the rotation axis. Since the emission region of the pulsar rotatingaround the axis is observed from the earth, we see it like a light house. Within thousand pulsars
observed so far, the emission of gamma-rays are only observed from several of them (Table 1.2).
The multiwave spectra of the gamma-ray pulsars are presented in Figure 1.11.
Table 1.2: Gamma-ray Pulsars [5]PulserPeriod P [msec] Ṗ [10−15 ] distance d [kpc]
Crab
33.34
421.2
2.0
PSR B1509-58
150.65
1537
4.4
Vela
89.29
124.3
0.5
PSR B1706-44
102.4
93.0
1.8
PSR B1951+32
39.53
5.849
2.5
Geminga
237.1
10.98
0.16
PSR B1055-32
197.1
5.8
1.5

Especially, the Crab pulsar was observed from the radio to the GeV gamma-ray, and it is
one of the most famous pulsars. Figure 1.12 are light curves in each energy region. These ﬁgures
say that the sub MeV to the MeV energy bands have 2 features:• Although the ﬁrst peak is dominant in other energy bands, the dominant in this bandvaries to the second peak.
• The pulse proﬁle has the ‘Bridge’ structure between the ﬁrst and the second peak.
The study of the emission from the pulsar is still underway, and these features in the MeV band
have not been explained.
10Source of Gamma-Rays

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Figure 1.11: The spectra of gamma-ray pulsars [11]11Chapter 1. MeV Gamma-Ray Astronomy

Figure 1.12: The pulse proﬁle of crab pulsar [12]Figure 1.13: The spectrum of crab nebula (solid line: synchrotron radiation, dashed line: inverse
Compton scattering) [13]12Source of Gamma-Rays

Soft state

-2.0
-2.5

Hard state-3.0

BATSE Spectral Index (20-100 keV)

-3.5
-4.0

0.5

BATSE Flux (20-100 keV)

0.2
-1

ASM Count Rate (cts s )

0.1
100

-1

0.3

-2

0.4

Flux (cm s in 20-100 keV)

Spectral Index

-1.5

80

RXTE ASM Count Rate
(2-10 keV)

60
40
20
8500

9000

9500
10000
10500
Truncated Julian Day (TJD)

11000

11500

Figure 1.14: The light curve of Cyg X-1[14]

Figure 1.15: The spectra in Soft state and Hardstate (Cyg X-1) [14]Pulsar Nebula
Crab pulsar has a nebula around it. The nebula is observed from the radio to the TeV gamma-ray
as a non-pulsed emission. Figure 1.13 shows the multi-wave spectrum of the Crab nebula. The
spectrum is explained by the combination of synchrotron radiation and inverse Compton scattering. Also in the MeV band, the emission from the Crab nebula was observed by COMPTELand OSSE, and it is suggested that the emission is due to synchrotron radiation.

1.3.3

Black HoleIn the ﬁnal evolution of a heavy star with a mass above 30M⊙ , the core is considered to collapse
to a black hole after a type II supernova. A black hole has a boundary line, which is called ‘event
horizon’, from where the electromagnetic wave can’t escape. However if a black hole makes abinary system with a star, an accretion disk is created similar to the white dwarf because thematter from the companion star ﬂows onto the black hole. This accretion disk radiates photons
in wide band including X-rays and gamma-rays. Until now, there is no solid evidence for the
existence of a black hole.
Although the existence of the black hole is not established, there are several ‘black holecandidates’, which are infered with its mass and the size. Cygnus X-1 is the most prominent
black hole candidate. Those black hole candidates commonly indicate a violent time proﬁle,as shown in Figure 1.14. Another feature of the time variability is that the energy spectrum
changes suddenly between Soft state and Hard State at time. Figure 1.15 shows the spectra of
the Soft/Hard state of Cyg X-1.
The emission model of the Soft state is depicted in Figure 1.16, and the model of the Hardstate in Figure 1.17. In the soft state, the disk is bright in the X-ray region, in which theblackbody radiation from the accretion disk and the Compton scattering by the non-thermal
electron are dominant in this state. On the other hand, the hard state has a peak at a few
hundred keV, and the dominant component is the Compton scattering by the thermal electron.
In addition, both states look to have the component of Compton reﬂection with the cold disk
[14].
Near the event horizon, ions accelerate up to 100 MeV by the strong gravitational potential,and hence surely π 0 must be created. Therefore, a thermal radiation of π 0 , which makes a peakat 70 MeV, is expected to be observed. Such radiation may be an evidence for the existence of
black hole.

13

Chapter 1. MeV Gamma-Ray Astronomy

Figure 1.16: The emission model of Soft(High)State [14]Figure 1.18: The spectrum of Galactic Center by OSSE [15]1.3.4

Figure 1.17: The emission model of Hard(Low) State [14]Figure 1.19: The map of Galactic Center at 511 keV by
SPI/INTEGRAL [16]Galactic Center

When an electron meets a positron, two or three gamma-rays are produced by the annihilation.
When the annihilation occurs at rest, the photon energy is 511 keV equal to the rest energy ofan electron (section 1.1.5). Actually, OSSE observed the 511 keV from the Galactic Center inFigure 1.18, which says that the annihilation in the Galactic Center exists certainly.
The distribution of 511 keV was broad in the Galactic Plane. The 511 keV emission means
the existence of the positron, and the positrons are produced by the β + decay of radioactive
isotopes, which are provided by the nucleosynthesis in the supernovae and heavy stars.
At the Galactic Center, the existence of a black hole with the mass of ∼ 3×106 M⊙ is strongly
considered [17]. Therefore, if there is a mass accretion, the MeV gamma-ray radiation due toπ 0 decay is also expected.

14Source of Gamma-Rays

Figure 1.20: The schematic view of AGN [18]1.3.5

Active Galactic Nuclei (AGN)

The Active Galaxy is a galaxy with the strong emission from its nuclei, and the nuclei of such agalaxy is called Active Galactic Nuclei. At the Active Galactic Nuclei, the existence of a blackhole with the mass of 106 − 109 M⊙ is considered. Figure 1.20 shows the schematic view of AGN.
Radio wave observation indicates that many AGN have jets, which is emitted with a narrow
beam from the center. If the radiation from the jets is stronger than the radiation from theaccretion disk or the thermal radiation of the near disk, the obtained multiwave spectrum is
explained by the combination of synchrotron radiation and inverse Compton scattering (Figure
1.21, 1.22).
There are 10 AGNs detected by COMPTEL and 94 AGNs by EGRET, respectively. Thus,
AGNs are also the important observable sources in the sub-MeV/MeV gamma-ray bands.

Figure 1.21: The multiwave spectrum of Centaurus A [19]Figure 1.22: The Spectrum of Blazar Mkn501
[20]

15

Chapter 1. MeV Gamma-Ray Astronomy

Figure 1.23: The spectrum of Solar Flare (June 4, 1991) [21]1.3.6

Solar Flare

The Sun, the most familiar celestial object, also radiates gamma-rays. From 1970’s, a few line
gamma-rays have been known to be emitted in the solar ﬂare sometimes.
Figure 1.23 shows the spectrum of the solar ﬂare. By these measurements, many of theneutron and positron are expected to be produced in the region of ﬂare. Moreover, by the acceleration and the heating of the particles in the corona at the time of solar ﬂare, the accelerated
electrons emit X/gamma-ray with bremsstrahlung. Because the Sun is the nearest star, it is themost detailed observable star and is a very important gamma-ray source.

1.3.7

Gamma-Ray Burst

The Gamma-Ray Burst (GRB) was discovered by the Vela satellite in early 1970’s. It is atransient phenomenon that gamma-rays arrival from speciﬁed direction suddenly. GRB has aviolent time proﬁle, and the bursting time is very short, but spread in several orders (Figure1.24).
Until now, many observations such as HETE-2 and SWIFT have been carried out, and ourunderstanding for GRB is dramatically recovered: GRB is a very giant burst at the distanceof cosmology, and GRB has a mother galaxy. The obtained spectrum can be explained withsynchrotron radiation, in MeV gamma-ray (Figure 1.25). BATSE on CGRO discovered 2700
GRBs, and the distribution is ﬂat in the all sky (Figure 1.26). But GRB is a puzzle, that is notunderstood of even the central engine until now.

16

Source of Gamma-Rays

2.5
x10

2

8–13 keV
BATSE-SD0

2

4

x10

2

x10

24–120 keV
BATSE-LAD0

3
2
1
1.5
1
0.5
0
4

320–1090 keV
BATSE-LAD0

1–2 MeVCOMPTEL
Burst Mode2
2–4 MeVCOMPTEL
Burst Mode

1.5
1
3

4–8 MeVBATSE-SD4

x10

1

x10

2

2

2

Ep (MeV)

1

0

50

100

0

50

100

1.5
1
0.5
0

α

Rate (counts/s)

x10

4

1.5

-1
-2
Time Since BATSE Trigger (s)Figure 1.24: light curve of GRB by BATSE and COMPTEL [22]17Chapter 1. MeV Gamma-Ray Astronomy

3

1

10

–2

0

10

-1

10

BATSE SD0
BATSE SD1
BATSE LAD0BATSE SD4OSSE
COMPTEL Telescope
COMPTEL Burst Mode
EGRET TASC

-2

10

-3

10

-4

10

–2

–1

E2 NE (erg · cm · s )GRB 990123

2

10

–1

–1

Flux (photons · cm · s · MeV )

10

-6

10

-7

10

-8

10

0.01

0.1

1

10

Photon Energy (MeV)

Figure 1.25: The spectrum of GRB [22]Figure 1.26: The all sky map of GRB by BATSE [23]

18

100

Chapter 2

Detection of Gamma-Rays
2.1

Interaction of Gamma-Rays and Matter

Because photons are electrically neutral, they can’t ionize the material. However, once highenergy photons interact with material, an electron is emitted via several processes dependingon their energy. The ionization of the surrounding material by such an electron enables us todetect X-rays and gamma-ray.
There are three processes in the interaction between X/gamma-rays and the matter: photoelectric absorption, Compton scattering and pair creation. The cross sections of these interactions depend on the energy of X/gamma-rays and the atomic number Z of the matter.
Figure 2.1 shows each cross-section of the interaction with Argon, and Figure 2.2 shows whichinteraction is dominant.
cross section [cm2/g]

104
3

10

102

1

ic
ctr
ele
oto ionPh sorpt
ab

10In
sc coheatt reerin ntg

10-1

To
Att tal
en
ua
tio
n

Coher
en
scatte t
ring

10-2

on
Pair creati ld)
fie(Nuclear

-3

10

n
atio
cre field)
Pair ctron
(Ele

-4

10

-5

10 -3
10

10-2

10-1

1

3

102
10
10
Photon Energy [MeV]

Figure 2.1: The cross-section of theinteraction of gamma-rays and matter(Argon) [24]2.1.1

Figure 2.2: The signiﬁcant interaction of gammarays and matter [25]

Photoelectric Absorption

The photoelectric absorption is a dominant interaction of photons with the energy less than
100 keV. In this process, a photon provides all energy to a electron in the atom, and then thiselectron, called photoelectron, is ejected from the atom. When a photon of the energy E0 makesa photoelectron, the kinetic energy of the photoelectron is as follows,
Ke = E0 − Ebind ,

(2.1)
19

Chapter 2. Detection of Gamma-Rays
where Ebind is the binding energy of the photoelectron in the atom. If E0 is higher than thebinding energy of the K shell, the probability of the interaction with an electron in K shell ishighest. Then the cross section of the photoelectric absorption of a K shell electron is:
(
)(
)4 (
)7/2
√ 5 8π 2e2
me c2
σK = 4 2Z
r
,
(2.2)
3 e
4πε0 ~cE0−7/22where σK is proportional to Z 5 and E0 , and re is the classical electron radius: re = 4πϵ0eme c2 .
If E0 is less than the binding energy of K shell, the photon interacts with an L shell electron.
Therefore, near the K shell binding energy, the energy dependence of the cross section has alarge discontinuity edge (Figure 2.1). Simultaneously a binding electron in the higher energy
level trends to transit to the empty level by an emitting X-ray. The energy of the emitted X-ray
is equal to the energy diﬀerence of those two levels. Also sometimes an electron of the nearlysame energy is ejected instead of the X-ray emission, which is called Auger electron.

2.1.2

Compton Scattering

In the energy band from a few hundred keV to 10 MeV, the dominant interaction is the Compton
scattering, which is an elastic scattering of a gamma-ray and an electron. When a gamma-ray
makes Compton scattering, the gamma-ray provides a part of the initial energy to an electron,
then the recoil electron runs away, and simultaneously the gamma-ray is scattered as its energy
becomes lower. When the gamma-ray in the energy of E0 makes Compton scattering with a freeelectron, the energy of the scattered gamma-ray is
E′ =

E0
,
E0
1 + me c2 (1 − cos ϕ)(2.3)

and the energy of the recoil electron is
Ke = E0 − E ′ =E0(1 − cos ϕ)
me c2
E0 ,
E0
1 + me c2 (1 − cos ϕ)(2.4)

where ϕ is a scattering angle. The angular distribution of the scattered gamma-rays is represented by the diﬀerential scattering cross section as follows,
(
)2 (
)(
)
dσ
1
1 + cos2 ϕ
k 2 (1 − cos ϕ)2
2
= Zre
1+
, (2.5)dΩ
1 + k(1 − cos ϕ)
2(1 + cos2 ϕ) [1 + k(1 − cos ϕ)]where σ is the cross section, Ω is the solid angle and k = mEe0c2 . Then the cross section is
proportional to Z. The angular distribution is shown in Figure 2.3, where you note that theforward scattering is dominant when E0 is higher.
In a real detector, electrons do not rest and they have the ﬁnite momentum of the orbit.
Therefore, the gamma-ray scattered at the ﬁxed angle from a monoenergetic source have some
ﬂuctuation in their energy (the “Doppler broadening”, see section 3.3).

2.1.3

Pair Creation

If the gamma-ray energy exceeds twice of the rest mass of the electron, a pair of an electron anda positron are created with the interaction between the photon and an electric ﬁeld around thenucleus. This process is called a pair creation, which is a dominant interaction of the photons
above 10 MeV. The energies of an electron and a positron emitted from the photon of the energy
E0 must be satisﬁed with the following equation,
E0 = E− + E+ + 2me c2 .

(2.6)

The cross section is proportional to Z 2 .
20

Sub-MeV/MeV Gamma-Ray Imaging

Figure 2.3: The angular distribution of Compton scattering [25]2.2

Sub-MeV/MeV Gamma-Ray Imaging

In general astronomy, it requires both measurements of the energy and the incident direction ofphoton for imaging. For the X-ray imaging, the photons can be focused by reﬂection. Figure
2.4 is a schematic view of an X-ray focusing. Only when an X-ray comes to the mirror at a very
shallow angle, the X-ray makes reﬂection. The mirror is Al or glass coated by Au generally.
Because the focusing is a mapping from the direction to the position, we can get the energy andthe incident direction of each photon at the same time using a 2-dimensional position sensitive
detector on the focal plane. However the limit of the energy in focusing by reﬂection is order of

Figure 2.4: The schematic view of an X-ray focusing [26]Figure 2.5: The eﬀective area of X-ray
telescopes [26]

10 keV, and the reﬂection ratio falls suddenly in the higher energy (Figure 2.5). For this reason,
the focusing of sub-MeV/MeV gamma-rays by reﬂection is impossible, and the other techniques
are necessary.
On the other hand, around a satellite, lots of MeV gamma-rays are generated by the excitation of the nuclei in the material of the satellite by the cosmic-rays, and furthermore such a
MeV gamma-ray is scattered several times by Compton process. Therefore, the observations of
the MeV gamma-rays in space severely suﬀer from huge background gamma-rays.

21

Chapter 2. Detection of Gamma-Rays

2.2.1

Active and Passive Collimators

For getting the incident direction of a photon, the simplest method is the collimation of theField Of View (FOV). The camera consists of a position sensitive detector and collimators, as
shown in Figure 2.6. By collimators, the detected photons are expected to come from the insideof FOV of the collimator. However, for higher energy photons, some photons are scattered in
the collimator or come through the collimator. Thus, gamma-rays sources outside of FOV makemany background, and obstruct the detection of a faint source in FOV.

Figure 2.6: The schematic view of passive collimator

For surely rejecting gamma-rays from the outside of FOV, there are two possible improvements. One is the simple method: using a thicker collimator. The collimator becomes thicker,
the higher stopping power the collimator has. But, in this way, FOV is narrower, eﬀective area issmaller and the collimator is heavier. Because the observations of MeV gamma-ray in astronomy
require the measurements on the satellite-borne or balloon-borne, these depravations make large
demerits for the actual detector-construction. The other way is using an active collimator. For
the rejection of the reaction in the collimator, a veto-counter having sensitivity for gamma-rays,
like a high-Z scintillator, is useful. The background events, scattering in the collimator, canbe rejected by the anti-coincidence of the veto-counter. On the other hand, the camera may
become more sensitive for intrinsic background and the dead time of the measurement may
increase. Therefore, the observation with collimator is not so sophisticated but quite simple andconventional in astronomy.
Until now, OSO-3, SMM and OSSE were based on this method, and HXD loaded on Suzaku
is also a telescope of this kind.

22Sub-MeV/MeV Gamma-Ray Imaging

2.2.2

Coded Aperture Imaging

Now, the practical imaging method is the Coded Aperture Imaging, which was adopted in
SIGMA, INTEGRAL and SWIFT. The detector of coded aperture imaging consists of a position
sensitive detector and a coded mask which carries out a mapping from the incident direction to
the image of the mask’s shadow. The coded mask consists of the optical thin material and theoptical thick material, and the mask can be described with a matrix:{
1 : the optical thin part,
Mij =
(2.7)
0 : the optical thick part.

Figure 2.7: Coded mask [27]When the gamma-rays come from the direction of j with the intensity of sj , the direction is
projected to the image of mask’s shadow, and the photon number of Mij sj arrive to the detectorof the position i, as shown in Figure 2.7. Therefore, in the detector of position i, the detected
photon number is:∑di =Mij sj + bi ,(2.8)
jwhere bi is the number of the background photons detected in the position i, because the photons
from the outside of FOV is the background. Although the incident direction sj is obtained byresolving this formula, there are two unknown parameters of sj and bi , so that the estimationof background is very important.
The angular resolution and the FOV depend on the distance between the coded mask andthe detector. For the same mask and the same detector, the camera has a larger FOV in the
shorter distance of them, while it has a better angular resolution in the longer distance. IBIS
loaded on INTEGRAL used the full coded FOV of 9◦ × 9◦ and obtained the angular resolutionof 0.2◦ in FWHM [1].
The coded mask is a kind of a collimator, and it is expected that the mask ideally absorbs
the gamma-ray completely. But, because the cross section of the photoelectric absorption isdecreased in proportion to E −7/2 , the Coded Aperture Imaging is not so good for the imagingof the high energy photons basically. The imaging of high energy photons is required of the
thicker mask. Actually, the mask of IBIS (20 keV - 10 MeV; INTEGRAL) is made of a 1.6 cm
thick tungsten plate, and SPI (3 keV - 8 MeV; INTEGRAL) has a 3 cm thick tungsten mask.
Still, there remains another problem, that gamma-rays scattered in the mask are detected as
background.
23

Chapter 2. Detection of Gamma-Rays

2.2.3

Gamma-Ray Lenses

Although gamma-rays can’t be focused by reﬂection, there are two focusing methods for MeV
gamma-rays. These focusing methods are called Gamma-Ray Lenses. One makes use of Lauediﬀraction, and the other makes use of a phase Fresnel lens. Generally, the focusing has thehigh angular resolution, but the FOV is narrow. Besides, the focusing can reduce the intrinsic
background, because the detection volume is quite smaller than other imaging methods.
Laue Lenses
The focusing by Laue diﬀraction is called Laue lens. Although the Laue lenses give us a very
good angular resolution, they focus only the gamma-ray of speciﬁc energy due to the principleof the Blagg scattering as follows,
2d sin θ = nλ.

(2.9)

Then it is impossible to get the wide band energy spectrum using this lens. Here d is the crystal
plane spacing, θ is the diﬀraction angle, n is the reﬂection order and λ is the wavelength. The′′
lens of CLAIRE, which was launched in the balloon experiment in 2001, has the FOV of 45 ,the energy band of 169 - 171 keV and the focal length 3 m [28].

Figure 2.8: The schematic view of Laue Lens [29]Fresnel Lenses
The other method is a phase Fresnel lens. Because the refraction index for gamma-ray in matteris slightly lower than 1, it is possible to focus gamma-ray. The Fresnel lenses have the angularresolution limit in the µarcsec, but they require a very long focal length of 109 m. Therefore itis diﬃcult to realize this method currently.

24

Sub-MeV/MeV Gamma-Ray Imaging

2.2.4

Compton Imaging

When the gamma-ray makes Compton scattering in a detector, it is hard to detect the total
photon energy because the scattered gamma-ray brings out some part of the incident photon
energy. Then, the Compton scattering is a hard process in an usual detector, although it is themost dominant interaction in the sub-MeV and MeV energy regions.
If the information of both the scattered gamma-ray and the recoil electron are detected, theenergy and the direction of the incident gamma-ray can be reconstructed. This imaging method
is called “Compton Imaging”. The detector based on Compton imaging has a large FOV because
no collimator is required. In actual, COMPTEL loaded on CGRO had a FOV of 1 steradian[30].
Compton imaging is an unique imaging method to detect both the energy and the direction
for photon by photon, in the sub-MeV/MeV band. Therefore this imaging is an attractive
method, and many detectors are in development: MEGA, NCT, LXeGRIT, TIGRE and so on.
Classical Compton Imaging
The Classical Compton Imaging used in COMPTEL makes use of the ﬁrst Compton scatteringin the material. A classical Compton imaging camera consists of two Z detectors of whichmaterials are light (low-Z) and heavy (high-Z), respectively. A low-Z position sensitive detector
is placed forward, and a high-Z position sensitive detector is placed backward. Figure 2.9 isa schematic view of a classical Compton imaging camera. An incident gamma-ray makes aCompton scattering in the low-Z detector, and the scattered gamma-rays are absorbed in thehigh-Z detector. From these detectors, the following information are measured,• the low-Z detector: the recoil electron energy E1 , and the Compton Point (scattering
point)• the high-Z detector: the scattered gamma-ray energy E2 , and the absorption point of thescattered gamma-ray
E0

ϕ

E1
E2
Figure 2.9: The schematic view of Classical Compton Imaging

Then, using E1 and E2 , the incident gamma-ray energy E0 and the scattering angle ϕ arecalculated
E0 = E1 + E2 ,cos ϕ = 1 − me c2

(

1
1
−
E2 E1 + E2

(2.10)).

(2.11)

25

Chapter 2. Detection of Gamma-Rays
The direction of the scattering gamma-ray is also obtained by connecting the Compton point
and the absorption point, although the two parameters of the direction of the recoil electron is
not measured. Therefore the reconstructed direction of the incident gamma-ray is limited in acircle (called an event circle as shown in Figure 2.9).
For getting the direction of the gamma-rays source, the detector requires at least 3 photons,
as shown in Figure 2.10, where the source position is determined fully by piling up circles (theright ﬁgure of Figure 2.10). Although COMPTEL rejected background by using the time of
ﬂight between the up and down detectors [30], the signal to noise ratio was not high, and FOV
is limited.

Figure 2.10: The Imaging of Classical Compton and the detection of GRB with COMPTEL(GRB910505 [31])Multiple Compton Imaging [32]For the classical Compton imaging, a good process to obtain the correct direction of an incident
gamma-ray is one Compton scattering in the forward detector and a perfect photoelectric absorption in the backward detector. However, sometimes the gamma-ray scattered in the forward
detector makes also a Compton scattering in the backward detector, and the second scatteredgamma-ray escapes. Then such as multiple Compton scattering event becomes background forthe classical Compton imaging.
The Multiple Compton Imaging is an advanced method to resolve this problem. Figure 2.11
is a schematic view of a detector for the multiple Compton imaging, and the detector consistsof layers of several thin position-sensitive detectors. The semiconductor detectors, such as CdTeor Si/Ge strip detector, are used as a thin detector. When a gamma-ray makes more times
Compton scattering in the detector, the incident gamma-ray energy E0 , the ﬁrst scattering
angle ϕ1 and the second scattering angle ϕ2 are absolutely obtained√
4me c2 E2E2 + E2 2 + 1−cos2ϕ2,(2.12)
E0 = E1 +2()
1
1
cos ϕ1 = 1 − me c2−,(2.13)
E2 + E3 E1 + E2 + E3()
1
1
cos ϕ2 = 1 − me c2−.
(2.14)
E3 E2 + E3
where E1 , E2 , E3 are the deposit energies of the ﬁrst, the second and the third interaction,
respectively. These equations say that it can detect the incident gamma-ray energy even if the
ﬁnal scattered gamma-ray escapes from the detector. Also this method may reduce backgrounddue to the improvement of the eﬃciency. On the other hand, the reconstructed direction remains
an event circle same as the classical Compton imaging. Besides, in the reconstruction of an event,there is a problem of sorting the detected points in the interaction order.
26

Sub-MeV/MeV Gamma-Ray Imaging

E0

ϕ1
E1
E2
E3

ϕ2

Figure 2.11: The schematic view of a Multiple Compton camera

Nuclear Compton Telescope (NCT) is an example of the multiple Compton imaging. NCT
consists of some 3-dimensional position sensitive germanium detectors, and it measured thebackground of 200 - 800 keV on balloon experiment in 2005 [33].
Electron Tracking Compton Imaging
In comparison to the multiple Compton imaging, which is an improvement method on the energy
information, the Electron Tracking Compton Imaging aims to improve the measurement of thedirection of the incident gamma-ray. In the classical/multiple Compton imaging, only the energy
and the position of the recoil electron is measured, which makes it hard to specify which partof the event circle the gamma-ray comes from.
The camera based on the electron tracking Compton imaging consists of a tracker, which
detects the track and energy of the recoil electron, and an absorber, which detects the absorption
point and the scattered gamma-ray energy (Figure 2.12). By the detection of the direction of
the recoil electron, we obtain the fully ray-traced gamma-ray image.

E0

Ke
α
Eγ
Figure 2.12: The schematic view of an Electron Figure 2.13: The schematic view of MEGA [34]Tracking Compton camera
The angle α between the recoil electron and the scattering gamma-ray is measured for eachgamma-ray geometrically, and also this angle is obtained by the calculation using the energy of27

Chapter 2. Detection of Gamma-Rays

Figure 2.14: The schematic view of pair tracking camera

recoil electron and scattered gamma-ray kinematically. Therefore we can select the good eventsof which the kinematic calculated angle is consistent with the measured one. Because of the
background rejection by the angle α, the electron tracking Compton imaging ﬁts for the MeV
gamma-ray astronomy, whose serious problem is the obstruction by background.
Medium Energy Gamma-ray Astronomy (MEGA) is an example of the advanced Compton
camera. Figure 2.13 is the schematic view of MEGA. The tracker consists of the stack of
the silicon strip detector and the absorber is CsI scintillator. MEGA detects gamma-ray withclassical Compton imaging below 2 MeV, with electron tracking Compton imaging in 2 - 8 MeV[34]. But the accuracy of α is low because MEGA uses the solid tracker.

2.2.5

Pair Tracking

Above 10MeV, a pair creation becomes a dominant interaction, and the pair tracking camera
consists of a tracker, a converter and a calorimeter, as shown in Figure 2.14. When the gammaray comes in the camera, it makes a pair creation in the converter made of the tungsten sheets.
The created electron and positron run through the tracker, and stop in the calorimeter. By
measuring the tracks in the tracker and deposit energies in the calorimeter, respectively, themomenta of the electron and the positron are measured. Then the momentum of the incident
gamma-ray is obtained by the sum of them. SAS-2, COS-B and EGRET were pair tracking
telescopes using a spark chamber, and LAT of GLAST is also one of similar detectors using thesilicon tracker.
The pair tracking camera is very similar to the advanced Compton camera. In actual, MEGA
makes use of pair tracking for the detection of gamma-ray above 8 MeV.

2.2.6

Summary of MeV Gamma-Ray Imaging

I summarize the characters of each sub-MeV/MeV gamma-rays imaging method in Table 2.1.
This table says that there is no ‘all-rounder’. Therefore, the selection of suited method for theobservation target is required, for example, Laue Lenses ﬁts for observation of the radioactive
isotopes distribution in the supernova remnant, Advanced Compton Imaging is suited to the allsky survey.

28

29

e

1
×
×

e

1
×
×

e

e

e

△
3
×
×

△
3
△
detectrejecte△1Compton Imaging
Classical Multiple Advanced
100 keV - 10 MeVe××
circlecircle
point
e
△
△
e
△
△
e
△
△

1
△
×

e
e
e
e
e

point

e

≥ 100 MeV

Pair Tracking

Table 2.2: The properties of some gamma-ray telescopes [1]
COMPTEL
EGRET
SPI
IBIS
Compton Gamma-Ray Observatory
INTEGRAL
0.1 - 10
1 - 3020 - 3 × 104
0.02 - 8
0.015 - 10
Passive collimatorClassical Compton
Pair Tracking
Coded Mask
6.8 % @ 1 MeV
6.5 % @ 3 MeV∼ 20 %
2 keV @ 1 MeV
10 % @ 1 MeV
3.8◦ × 11.4◦
1 str
9◦ × 9◦ (19◦ × 19◦ )
1.25◦
2.5◦
12 arc min−5−5−6
2 - 5 × 10
0.3 - 3 × 10
5 × 10 @ 1 MeV 4 × 10−4 @ 1 MeV
2 × 10−4 @ 1 MeV 1.7 × 10−4 @ 1 - 3 MeV 5 × 10−6 @ > 100 MeV∼ 10−45 × 10−4◦
10 arc min
8.5 arc min
5 − 10 arc min∼1
30 arc sec

OSSE

×

×

e
e
e

point

point
e
e
e

e

e

Focusing
Laue
Fresnelﬁxed energy ﬁxed energy¶: in unit of MeV, †: 3σ for 106 sec and the unit is γ/cm2 /sec, ‡: 3σ for 106 sec and the unit is γ/MeV/cm2 /sec.

Parameter
Satellite
Energy range¶
Imaging method
Energy resolution
Field of View
Angular resolution
Narrow-line sensitivity†
Continuum sensitivity‡
Location accuracy¶: Full coded, †: Partial coded.

energy bandtrue imaging
limitation for 1 photon
specify incident direction
detect for point source
detect for spread source
ﬁeld of view
Angular resolution
minimum photon
background rejection
escape eventwith Collimator
Collimator
Coded Mask
≤ 100 keV
≤ MeVe×
opening angle
can’t limit
e
×
¶
e
(△† )
△
×
×
e
×
e
×
1
0.5× pixel number
××××

Table 2.1: The summary of MeV gamma-ray imaging

Sub-MeV/MeV Gamma-Ray Imaging

Chapter 2. Detection of Gamma-Rays

2.3

Low Energy Gamma-Ray Observatories

In this section, I brieﬂy summarize several detectors loaded on the satellites for the low energy
gamma-ray observation so far.

2.3.1

CGRO

NASA’s Compton Gamma-Ray Observatory (CGRO) was operated from April 1991 to June
2000. Its mission was the ﬁrst all sky survey in the wide energy bands between Sub-MeV and
GeV. CGRO had 4 telescopes: OSSE, COMPTEL, EGRET and BATSE. In these instruments,
BATSE was a gamma-ray camera for GRB observations. The main properties of OSSE, COMPTEL and EGRET are listed up in Table 2.2, and the detected sources with COMPTEL and
EGRET are listed in Table 2.3.
OSSE
The Oriented Scintillation-Spectrometer Experiment (OSSE) used the active and passive collimators. Figure 2.15 shows the schematic view of the one detector of OSSE, in which there
were 4 identical detectors on CGRO. The main detector is a phosphor-sandwich (phoswich) detector consisting of the NaI(Tl) crystal (diameter: 33 cm, thickness: 10.2 cm) and the CsI(Na)crystal (thickness of 7.6 cm). In front of the Nal(Tl) crystal, a passive tungsten collimator was
mounted. Around the main detector and the tungsten collimator, there was the annular shieldof 8.5 cm thick NaI(Tl) crystal for the anticoincidence. For the rejection of charged particles, athin plastic scintillator (0.6 cm thick) was placed on the collimator. OSSE had an energy range
from 0.1 MeV to 10 MeV and had a FOV of 3.8◦ × 11.4◦ .

Figure 2.15: The schematic view of OSSE [1]Figure 2.16: The schematic view of
COMPTEL [30]

COMPTELThe COMPTEL was the ﬁrst Compton telescope launched on a satellite and was based onclassical Compton imaging. The schematic view of COMPTEL is shown in Figure 2.16. The
low-Z detector of it was a liquid organic scintillator NE213A (geometrical area: 4188 cm2 ), andthe high-Z detector was a NaI(Tl) crystal (geometrical area: 8744 cm2 ). For the rejection of
charged particles, each detector was surrounded by an anticoincidence counter dome. For the
background rejection, COMPTEL used the time of ﬂight between the low-Z detector and thehigh-Z detector. COMPTEL had a angular resolution of ∼ 1.25◦ for 10 MeV gamma-ray.
30

Low Energy Gamma-Ray Observatories

Table 2.3: The detected sources with COMPTEL and EGRET [1, 5]
Type of source
COMPTEL
EGRET
Spin-Down Pulsars
3
Crab, Vela,
6
Crab, Vela
PSR 1509-58
Geminga,
PSR 1786-44,
PSR 1055-52,
PSR 1951+32
Other Galactic sources|b| < 10◦

7

Cyg X-1,
Nova Persei 1992,
GRO J1823-12,
GRO J2228+61,
GRO J0241+6119,
Crab Nebula,
Carina/Vela region

Normal Galaxies
Active Galactic Nuclei

10

Cen A, etc.

Gamma-Ray Line Source

7

SN191T (56 Co),SNR RX J0852-4642 (44 Ti),
Cas A (44 Ti), Vela (26 Al),
Carina (26 Al),Cyg region (26 Al),RE J0317-853 (2.223 MeV)

Unidentiﬁed Sources
Total Number
Gamma-Ray Burst

5
32
31

2

Cen X-3,
Crab Nebula

1

LMC

77

Cen A, etc.

186
273
4

As shown in Table 2.3, EGRET detected about 270 gamma-ray sources. On the other hand,
COMPTEL detected only about 30 steady sources. It seems the reason is that COMPTEL
could not reject backgrounds completely by the TOF of both detectors. Therefore, the actual
sensitivity of COMPTEL was lower than the design sensitivity.
EGRET
The Energetic Gamma-Ray-Experiment Telescope (EGRET) was based on pair tracking imaging
detector. The schematic view of EGRET is shown in Figure 2.17. The central unit of EGRET
was a multilevel wire-grid spark chamber with interleaved tantalum conversion layers. The
trigger counter consists of the plastic scintillator sheets inserted into the lower part of the sparkchamber, and the calorimeter consists of NaI(Tl) crystal with the thickness of 20 cm. For
rejection of charged particles, the anticoincidence hood with the 2 cm thick plastic scintillator
surrounded the spark chamber.

2.3.2

INTEGRALThe INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) mission of the ESA
was launched in 2002. INTEGRAL has two gamma-ray telescopes: SPI and IBIS, and also has
31Chapter 2. Detection of Gamma-Rays

Figure 2.17: The schematic view of EGRET [1]two additional monitor instruments: JEM-X, an X-ray instrument between 3 keV and 35 keV,and OMC, an optical telescope observing at 500-850 nm. The main properties of SPI and IBIS
are listed up in Table 2.2.
SPI
The SPectrometer on Integral (SPI) is based on coded aperture imaging. The detector of SPI
consists of the array of 19 Ge crystals cooled to 85 K, and thus, SPI has a good energy resolutionof 2 keV for 1 MeV. The mask of SPI is made by the 3 cm thick tungsten, and the distance
between the mask and the detector is 1.71 m. The whole detector is surrounded by the active
BGO shield of mass of 511 kg viewed by 181 PhotoMultiplier Tubes (PMT). Due to the highenergy resolution, the SPI’s sensitivity for narrow-line gamma-rays is improved at least by afactor of 10 better than other gamma-ray telescopes.
IBIS
The Imager on Board of the Integral Satellite (IBIS) also uses a coded aperture imaging. The
coded mask of IBIS is the 16 mm thick tungsten mask. The detector consists of 2 layers. The
lower layer is the array of 4096 CsI scintillators viewed by the silicon PIN photodiodes (each CsI
size: 9 × 9 × 30 cm3 , total area: 3318 cm2 ), named PICsIT. The upper layer is the array of 16384
CdTe pixels (pixel size: 4 × 4 × 2 mm3 , total area: 2621 cm2 ), named ISGRI. ISGRI detectsfrom 15 keV to 400 keV, and PICsIT covers from 200 keV to 10 MeV. The detector layers are
surrounded by the BGO active shield, and a passive tungsten collimator is placed between themask and BGO shield. The distance between the tungsten mask and the upper layer detector
is 3.2 m, so that IBIS has a good angular resolution of 12 arc sec.

32

Chapter 3

Advanced Compton Imaging
3.1

Imaging by Electron Tracking Compton Method

The electron tracking Compton imaging measures both the energy and the direction of theincident gamma-ray by obtaining with the energy and direction of the scattered gamma-ray andthe recoil electron. In Figure 3.1, the energy and direction of the scattered gamma-ray are Eγ
and ⃗g , the energy and direction of the recoil electron are Ke and ⃗e, the scattering angle is ϕ,
the recoil angle is ψ and the diﬀerential angle between ⃗g and ⃗e is α, where ⃗g and ⃗e are the unit
vectors. Then, the incident energy E0 and the incident direction ⃗s of the initial gamma-ray aredescribed, respectively:
E0 = Eγ + Ke ,
)
(
sin ϕ
sin ϕ
⃗g +
⃗e,
⃗srcs = cos ϕ −
tan α
sin α
√
Ke (Ke + 2me c2 )
Eγ
⃗g +
⃗e.
=
Eγ + KeEγ + Ke

(3.1)
(3.2)
(3.3)

where the scattering angle is ϕ, which is written as follows,cos ϕ = 1 −me c2 Ke,Eγ + Ke Eγ(3.4)

and the recoil angle ψ is
(
)√
me c2
Ke
cos ψ = 1 +.
Eγ + Ke
Ke + 2me c2(3.5)⃗s, E0ψ

ϕ⃗e, Ke⃗g , EγαFigure 3.1: The parameters in Electron Tracking
Compton Imaging
33

Figure 3.2: The deﬁnition of ARM & SPD

180

600

500

400

300

200

Ke [keV]1000180αlim [degree]

angle [degree]

Chapter 3. Advanced Compton Imaging

160

160

140

140

120

ϕ

100

120

α

100

80
80

60
60

40
40

ψ

20

20

0
0

100

200

300

400

500

600
Eg [keV]00

Figure 3.3: The Eγ dependence of ϕ, ψ, α (E0 =662 keV)

0.5

1

1.5

2

2.5

3

3.5

4

4.5 5
E0 [MeV]Figure 3.4: The E0 dependence of αlim

The diﬀerential angle α between ⃗g and ⃗e is described by the deﬁnition:cos αgeo = ⃗g · ⃗e.

(3.6)

On the other hand, α is described with Eγ and Ke by the Compton kinematics:
(
)√
me c2
Ke
cos αkin = 1 −.
EγKe + 2me c2(3.7)

The angle α has a minimum value depending on the energy of the incident gamma-ray, and theminimum is described:{0(k < 1)E0k=cos αlim = k−1 √ k2 −1.
(3.8)
me c 2(k ≥ 1)
k+2
k(k+2)
Figure 3.3 shows the dependence of ϕ, ψ and α on Eγ , and Figure 3.4 shows the dependence of
αlim on E0 , respectively.
The angle α is a characteristic parameter of the electron tracking Compton imaging. The
α makes it possible to select the Compton scattering events from the backgrounds. The α
is obtained from the equation (3.6) and (3.7), where (3.6) depends on only the geometricalinformation, and (3.7) depends on only the kinematic information. Thus, the αgeo is independent
from the αkin . For this reason, we can select good events, in which Compton scattering occurs
in the tracker and scattered photon absorbs perfectly in the absorber, by requiring the follows
αgeo = αkin .

(3.9)

In the Electron Tracking Compton Imaging, the accuracy of the event reconstruction isestimated using two parameters. One is the Angular Resolution Measure (ARM), which is theaccuracy of the scattering angle:()
me c2 Ke∆ϕARM = arccos (⃗s · ⃗g ) − arccos 1 −.
(3.10)
Eγ + Ke Eγ
The other is Scatter Plane Deviation (SPD), which is the accuracy of the determination of thescattering plane:
( (
))
(
)
⃗s × ⃗g⃗srcs × ⃗g⃗s × ⃗g ⃗srcs × ⃗g
∆νSPD = sign ⃗g ·×arccos·.
(3.11)|⃗s × ⃗g | |⃗srcs × ⃗g ||⃗s × ⃗g | |⃗srcs × ⃗g |where ⃗s is the real direction, and ⃗srcs is the reconstructed direction of the incident gamma-ray.
Thus, the error region of one event is a sector shape, as shown in Figure 3.2.
34

pa

C

sti
c

A

rti

cle

Background Rejection

Backward Peak

re

lat

ivi

D1
B
D

CountsF

E

Forward Peak

D2

F
Signal

EGRET

E

COMPTEL

OSSE

D

A&C

Continuum

CGRO Platform

E+F

−20

Figure 3.5: The various background of COMPTEL
[35]

3.2

−15

−10

−5

0
ToF [nsec]

5

10

15

20

Figure 3.6: The TOF distribution [35]Background Rejection

MeV gamma-rays are produced by various processes such as a de-excitation of nuclei, hadroninteractions and a decay of radioactive nuclei. Actually, COMPTEL had various backgrounds
as shown in Figure 3.5, of which detail was described in [35]. Each process is:
A Internal Single Photon: Gamma-rays, which are produced at the inside of the detector byneutron capture or by radioactive isotopes (like 40 K), may make Compton scattering in
the forward detector, and its scattered gamma-rays are absorbed in the backward detector.
B External Single Photon: Gamma-rays are produced at another detectors or the satellite
platform by neutron capture, decay of radioactive isotopes and scattering, and then they
may make Compton scattering in the forward detector and its scattered gamma-rays areabsorbed in the backward detector.
C Internal Multi-photon: Multi-photons are produced by the interactions, like 27 Al(n,α)24 Na
or 27 Al(n;n′ , · · · )27 Al. Then some photons may hit the forward detector and also thebackward detector simultaneously.
D External Multi Photon: In another detectors or the satellite platform, some gamma-rays
may be created by the same processes of C, and some photons may hit the forward detector
and also the backward detector simultaneously.
E Random Coincidence: Sometimes, the diﬀerent photons produced by the independentinteractions hit the forward detector and also the backward detector accidentally.
F Cosmic-Ray Interaction: When cosmic-ray comes into the satellite, it interacts with material at the diﬀerent positions, generates some photons simultaneously. Then those photons
hit both the forward and backward detectors.
• Other Process: As the other backgrounds, neutron, electron and atmospheric gamma-rays
were measured.
COMPTEL rejected the backgrounds using the time of ﬂight (TOF) between both detectors.
Figure 3.6 shows the distribution of TOF by simulation. Although the most events are backward
events in which gamma-ray interacts in the backward detector and then in the forward detector,
these events can be easily rejected using the timing above 1 MeV. But D, E and F events
make continuum component, and the forward peak includes the one third of them. Since theinteraction of A and B events are the real Compton interaction, they are the intrinsic backgroundand can’t be rejected. Moreover, if there is a decay in the forward detector, C events have nodiﬀerence in TOF distribution. Therefore, even if we pick up the forward peak, the most of theselected events are still background events as shown in Figure 3.6.
35

Chapter 3. Advanced Compton Imaging
In comparison with classical Compton imaging, electron tracking Compton camera can reduce the background using the angle α. Because of this kinematic ﬁt, we can enhance the correct
events, and C, D, E and F events can be rejected. Therefore, signal to noise ratio becomes better
than classical Compton imaging.

3.3

Doppler Broadening

In the previous sections, we suppose that Compton scattering occurs with a free electron. That
process is called unbound Compton scatter. However, in actual detectors, electrons are boundedin the potential energy of the nuclear or molecular orbits. Thus, electrons have a kinematicenergy before the Compton scattering, and the actual scattering (bound Compton scattering)is aﬀected as follows,• scattering cross section:
Especially, for the low energy photon under 100 keV, the cross section is slightly increased.
• distribution of scattering angle:
In comparison with (2.5), the forward/backward scattering is reduced slightly.
• energy of scattered gamma-ray and recoil electron:
Because the electron before the scattering has a ﬁnite energy, the energy of scatteredgamma-ray that are scattered at a ﬁxed angle from a monoenergetic source has a narrow
distribution around the estimated energy from an unbound Compton scattering. Thiseﬀect is called Doppler broadening.
In Compton imaging, the scattering angle is obtained by the energy of scattered gamma-ray
and the energy of recoil electron. Therefore, the accuracy of the scattering angle (ARM) is
certainly aﬀected by Doppler broadening, and Doppler broadening makes the intrinsic limit for
the angular resolution of the ARM.

Figure 3.7: The ARM distribution of each elec- Figure 3.8: The Z dependence of the ARM [36]tron orbit (E0 = 200keV, Si) [36]Since Doppler broadening is caused by the motion of the electron before the scattering, theenergy distribution of scattered gamma-ray is changed at which orbit the electron recoils. Figure
3.7 shows the uncertainty of the ARM of each orbit. Because the electron at the outer orbit
has the lower kinematic energy, the eﬀect of Doppler broadening is smaller, and the uncertaintyof the ARM is the smaller. The dependence of the uncertainty of the ARM on Z is shown in
Figure 3.8. When Z of target material is larger, Doppler broadening makes a larger uncertaintyof the ARM. Figure 3.9 shows the energy dependence of Doppler broadening. The energy ofan incident gamma-ray is higher, the eﬀect of Doppler broadening is smaller because the initialenergy of an electron is smaller enough.
36

Multiple Scattering

Figure 3.9: The energy dependence of Doppler broadening [36]3.4

Multiple Scattering

The electron tracking Compton camera can determine the scatter plane using the direction of
the recoil electron, and the incident gamma-rays are reconstructed speciﬁcally. The scatter plane
is determined from the scattered gamma-ray direction ⃗g and the recoil electron direction ⃗e. The
accuracy of ⃗g depends on the distance between two detectors and the position resolutions ofthem. On the other hand, the recoil electron loses the directional information ⃗e because of themultiple scattering in the material of the tracker. Thus, multiple scattering is an intrinsic limitof SPD.
The uncertainty of the scattered angle is described by Moliere theory, which approximatesto a Gaussian for the small angle scattering [37]. At the range x, the scattered angle is:
[
( )]
√
x
13.6MeVx
1 + 0.038 ln,(3.12)
θrms =
βcp
X0
X0
where βc, p and X0 are the velocity of an electron, the momentum and the radiation length,
respectively. Figure 3.10 shows the scattered angle depending on the electron energy, and Figure
3.11 shows the dependence on the range.

70

90

100keV

80
70

Si

60

60

50

50

θrms [degree]

90

50keV

80

θrms [degree]θrms [degree]

90

200keV

80
70
60

Si
50

Ar 2atm

Ar 2atm

40

40

30

30

30

20

20

20

Xe 1atm
Ar 1atm

10
0
-2
10

-1

10

1
10
track length [mm]Xe 1atm

10
0
-2
10

Ar 1atm
-1

10

1
10
track length [mm]40Si

Xe 1atm

10
0
-2
10

Ar 2atm
Ar 1atm

-1

10

1
10
track length [mm]Figure 3.10: The scattered angle depending on electron energy (left: 50keV, center: 100keV,right: 200keV; gas temperature: 20◦ C)

If the tracker consists of the stack of silicon strip detectors, the recoil electron should passvia a few silicon layers for the determination of the recoil direction. Generally, the thickness ofthe silicon strip detector is 300 - 500 µm. When the recoil electron has an energy of 500 keV, thescattered angle is about 60◦ , so that it is hard to determine the recoil direction. On the other37

Chapter 3. Advanced Compton Imaging

70

Si

90

1mm

80
70

θrms [degree]

500µm

80

θrms [degree]90

θrms [degree]

90

70

60

60

60

50

50

50

40

40

Xe 1atm

30

30

20

20

10

10

Ar 2atm
Ar 1atm

00

50 100 150 200 250 300 350 400 450 500
energy [keV]

0
0

5mm

80

40

Xe 1atm

30
20

Ar 2atm
Ar 1atm
50 100 150 200 250 300 350 400 450 500
energy [keV]1000

Xe 1atm
Ar 2atm
Ar 1atm
50 100 150 200 250 300 350 400 450 500
energy [keV]

Figure 3.11: The scattered angle depending on range (left: 500µm, center: 1mm, right: 5mm;
gas temperature: 20◦ C)

hand, with a tracker using argon gas at 1 atm, if the recoil direction is obtained determinable at
1 mm, the scattered angle is ∼ 15◦ and the SPD accuracy becomes well than that of the silicon
tracker. Therefore, a gaseous detector is selected as a tracker.

3.5

Composition of Advanced Compton Camera

3.5.1

Goal for Next Generation Telescopes

After COMPTEL, there is only INTEGRAL launched as the satellite of gamma-ray observatories, and LXeGRIT, NCT and CLAIRE are balloon experiment with short observation time.
Since SPI and IBIS on INTEGRAL uses a coded aperture imaging, the sensitivity for the continuum spectrum is worse than that of COMPTEL, as shown in Figure 3.12. Therefore, thedevelopments of the detector with a higher sensitivity and observations using such a camera arewished. Then, we set the goal of our detector to ‘ten times better sensitivity than COMPTEL’.

Figure 3.12: The continuum sensitivities of Figure 3.13: The eﬀective area of X/gammaX/Gamma-ray observatories [38]ray observatories [39]Figure 3.13 shows the eﬀective area of various observatories. This ﬁgure says that COMPTEL
had been expected to achieve 1 mCrab source detection in 106 sec observation time if there wereno background. However, actually, the background rejection power of COMPTEL was limited,as shown in Figure 3.6, and the signal to noise ratio was quite worse than expected. Therefore,
the electron tracking Compton camera, even if its eﬀective area is as same as COMPTEL, isexpected to reach the goal by the improving the signal to noise ratio.
38Composition of Advanced Compton Camera

3.5.2

Requirements for Tracker and Absorber

A camera based on electron tracking Compton imaging consists of a tracker and an absorber,so I explain the requirements on each detector.
A tracker, which is the target of Compton scattering and detects the momentum of the recoil
electron, requires the accuracy of the recoil direction. Hence a gaseous detector looks better asa tracker than a solid state detector. On the other hand, since the density of solid is higher
by 3 order than that of gas, a solid detector has a larger cross section per unit volume than agaseous detector. However, a gaseous detector with a large volume is easily made. Figure 3.14
is the energy dependence of Compton scattering probability. This ﬁgure says that Xe gas of
50 × 50 × 50 cm3 at 1 atm which has an eﬀective area of ∼ 30 cm2 , which is nearly equal to thatof COMPTEL (∼ 40 cm2 ) for 1 MeV gamma-ray. Therefore, a gaseous detector can become agood Compton scattering target. As a Compton scattering target, lower Z material is better,because lower Z material has a wider energy band of Compton scattering dominance, and hasless eﬀect of Doppler broadening (see Figure 2.2 and 3.8). Besides, for the detection of the recoil
direction with an enough accuracy, the determination of the direction within a few mm lengthfrom the Compton point is required. Since the energy of the recoil electron is in the range from∼ 50 keV to several hundred keV, the energy loss of the recoil electrons are minimum ionizing
during a few mm from the Compton point. For this reason, the tracker must trace the recoil
electron with several points within a few mm. In summary, a tracker requires the following
features.
• Large volume gaseous detector : Gas has little impact of multiple scattering, and large
volume makes up for a small cross section.
• Compton scattering dominant gas : A gas, which consists of the low Z atoms but havea lot of electron per a molecule, is good as a scattering target of Compton process, forexample CF4 and CH4 .

Compton probability [%]• Fine Minimum Ionizing Particle (MIP) tracing : The determination of the recoil direction
with a good accuracy requires a ﬁne position resolution and ﬁne sampling with a sub-mm
pitch.

10

Xe 1atm

1
Ar 2atm

10-1

10-2
-1
10

Ar 1atm

1

10

2

10
E0 [MeV]Figure 3.14: The probability of Compton scattering (depth of 50cm)
Absorbers for the scattered gamma-rays are placed around the tracker. In the tracker,
gamma-rays are scattered in various directions. Then an absorber is required to cover the large
39

Chapter 3. Advanced Compton Imaging

10

FWHM of Position Resolution [cm]

FWHM of ARM [degree]area. Besides, the scattered gamma-rays often have most energy of the incident gamma-rays in
the Compton process, because the probability of the forward scattering is higher than that of
the backward scattering. Therefore a high Z absorber is better for the detection of the scatteredgamma-rays, of which energy is from hundred keV to a few MeV. The energy and position
resolutions of the absorber are strongly related with the resolution of the ARM. In principle, theARM has a limit due to Doppler broadening. Then the uncertainty by each resolution is wishedless than that of Doppler broadening. If an Ar gas is used in a tracker, the uncertainty of theE0 = 100 keV
E0 = 200

keV

°

0

1

∆νA

RM

∆ ν AR

M

M

∆νA

RM

∆νA

RM

10-10

20

40

60

10-3

80 100 120 140 160 180
Scattered angle φ [degree]

∆νA

RM

1

°

=5
=2
°

=1

°

.5

=0

°

.2

M
∆ ν AR

10-2

=1

°

∆ ν AR

10-1

0 keV
E0 = 50
MeV
1
=
E0

1

10

=0

°

.1

=0

10
Distance from Compton point [cm]Figure 3.15: The dependence of ARM un- Figure 3.16: The ARM uncertainty (∆νARM )certainty by Doppler broadening on scatter- depending on position resolution and distanceing angle (E0 : energy of incident gamma-ray, from Compton point
GEANT4 simulation)

102

0.1 MeV

10%

φ=
@6

φ=

15
0 °

90 °

45 °

eV

φ=

15 °

0k

φ=

@6
62k

M

eV

eV
1

1%

5°

50

10

φ=

Figure 3.17: The requirement of energy resolution for absorber. The
solid line means the ARM uncertainty
by energy resolution, equal to theuncertainty by Doppler broadening
with constant scattering angle, andthe dashed line is such condition withconstant energy of incident gammaray. The dotted line is typical energy resolution of real detectors (en√ergy resolution [%] ∝ 1/ energy).

62k
eV

0.2 MeV

FWHM of Energy Resolution [%]ARM by Doppler broadening behaves as shown in Figure 3.15. From this ﬁgure, the uncertaintyof the ARM for 500 keV gamma-rays with the scattering angle of 20◦ is ∼ 1◦ at FWHM. On theother hand, Figure 3.16 shows the variations of the ARM uncertainty depending on the position
resolution and the distance from the Compton point, respectively. Figure 3.16 shows that theabsorber should have the spacial resolution under 1 mm for the ARM uncertainty of 1◦ . The
requirement of the energy resolution is determined from Figure 3.17. In Figure 3.17, the solidlines means that the uncertainty due to Doppler broadening is equal to the uncertainty due tothe energy resolution when the scattering angle is constant. This ﬁgure says that the absorber

3

102
10
Energy of Scattered Gamma-Ray [keV]40Composition of Advanced Compton Camera
should have an energy resolution of a few percents at least. For summarize, the absorber requires
the following features.
• Large eﬀective area: An absorber is required to surround the tracker.
• High stopping power: Absorber must have a mass enough to stop the scattered gammarays.
• Fine Position resolution: Absorber should have a position resolution of ∼ 1 mm so that theuncertainty of scattered direction must be smaller than that due to Doppler broadening.
• Good Energy resolution: Absorber should have an energy resolution of several percent for
hundreds keV.

3.5.3

Advanced Compton Camera with Gaseous Electron Tracker

From above-mentioned requirements, we constructed an advanced Compton camera with theability of the electron tracking. As the tracker, an usual multi-wire proportional counter does
not ﬁt, because the required ﬁne pitch is not realized in a wire chamber. As an absorber,semiconductor detectors are diﬃcult to use, because its use is not reasonable for a large eﬀective
area, although they has a ﬁne energy resolution.
Therefore, we have developed an advanced Compton camera consisting of µ-TPC and ascintillation camera. µ-TPC is a Time Projection Chamber (TPC) based on µ-PIC, which is
our original gaseous micro-pattern detector having both a ﬁne position resolution and a large
detection area. The scintillation camera is realized at the reasonable cost, although it has aworse energy resolution than that of the semiconductor detectors. Then we got a good absorberto make large and thick detection area. An electron tracking Compton camera is comprised fromµ-TPC and the position-sensitive scintillation camera, as shown in Figure 3.18. As a ﬁnal goal,
we aim at the ten times better sensitivity than COMPTEL for MeV gamma-rays by developing
this detector.
~1MeV γ~10MeV γ
Drift plane

e-

e-

µ-PIC

e+

PMTs
Scintillator

Figure 3.18: The schematic view of Advanced Compton Camera with Gaseous Electron Tracker

41

Chapter 4µ-PIC, µ-TPC & Scintillator
4.1µ-PIC (Micro Pixel Chamber)

The key of the electron tracking Compton camera is the development of the tracker having an
enough position resolution for the recoil electrons. Since an electron is easy to lose the recoil
direction information by the multiple scattering, a high position resolution of a few hundred µm
is required for the tracker. As such a tracker, we have developed µ-TPC consisting of a micro
pattern detector, µ-PIC.

4.1.1

Introduction of µ-PIC

Micro Pixel Chamber (µ-PIC), which has been developed from 2000, is a kind of Micro Pattern
Gaseous Detector (MPGD) [40]. Figure 4.1 is the schematic view of µ-PIC. The µ-PIC has apixel electrode like a sliced proportional counter. The substrate is made by polyimide, and theelectrodes are Cu coated with Ni. µ-PIC is manufactured by the print circuit board technology,
and thus the device with a very large area can be easily developed. Actually, the large µ-PIC of
30 × 30 cm2 is now working. Each pixel is aligned with the pitch of 400 µm. µ-PIC has ∼ 65000
pixels in an area of 10 × 10 cm2 and ∼ 6 × 105 pixels in an area of 30 × 30 cm2 , respectively. The
anode and cathode electrodes are connected on the strip, respectively, and these electrodes run
perpendicularly. Therefore, µ-PIC has a 2 dimensional position sensitivity with a ﬁne position
resolution.
The small pixel structure causes a high gas gain (20000 at maximum, and 6000 in stable
operation) and a long stability (1000 hours with gain of 6000), because the pixel electrode ishardly damaged by discharge, which was a serious problem for MicroStrip() Gaseous Chamber.
Also µ-PIC can work under the high intensity of ∼ 107 count/ sec mm2 by the smallness ofthe pixel.

Figure 4.1: The structure of µ-PIC
43

Chapter 4. µ-PIC, µ-TPC & Scintillator

Figure 4.2: 10 × 10 cm2 µ-PIC

Figure 4.3: 30 × 30 cm2 µ-PIC

Figure 4.4: 3 types of pixels: good pixel (left), dead pixel (middle), bad pixel (right)

4.1.2

Morphology of Pixels

The pixel electrode of µ-PIC is manufactured by the following process. The ﬁrst process is theelectroless plating for creating the seeds of via-ﬁll plating. Next is the via-ﬁll plating, in which
the anode-holes are ﬁlled up, and the polyimide substrate are coated with copper. Then, copper
is etched to obtain a ﬂat surface. Finally, the pixel electrodes are created by etching.
The created pixels are classiﬁed into the three groups by eye scan.
1. Good pixels: The structure of the pixel is ideal, as shown in the left of Figure 4.4 andthese pixels make the stable operation. The majority of the pixels is classiﬁed into thistype.
2. Dead pixels: The dead pixels are due to the miss-plating. The anode hole of the dead pixel
is not ﬁlled up by copper, as shown in center of Figure 4.4, and the electric ﬁeld does notconcentrate to the center of the pixel. Then there is no electron avalanche and no signals.
3. Bad pixels: The bad pixels are due to miss-etching. The bad pixel has a protrusion on theedge of the cathode electrode, as shown in the right of Figure 4.4. On the surrounding of
the protrusion, the electric ﬁeld is too strong to suppress the discharge.
A dead pixel has no signal, even if a high voltage is supplied. Thus, a dead pixel is no problem
for the stable operation. However, if there are many dead pixels in a small area, the image isbroken naturally. A 10 × 10 cm2 µ-PIC has few dead pixels at present (< 10−3 %). But a 30 × 30
44

µ-PIC (Micro Pixel Chamber)

Figure 4.5: Data Acquisition System for µ-PIC [41]cm2 µ-PIC at the ﬁrst production had 0.1% dead pixels in the whole area. Especially in theworst area, the pixels of 40 % in 3 × 5 cm2 were dead pixels.
A bad pixel causes discharges, which obstructs the stable operation. The bad pixels distributeinto the whole area at random. The existence ratio is less than 0.01 % in 10 × 10 cm2 µ-PIC andthat is less than 0.1 % in 30 × 30 cm2 µ-PIC. The reduction of dead pixels is the ﬁrst priorityof µ-PIC improvement.

4.1.3

Readout Circuit

Since a µ-PIC has a lot of readout strips (a 10 × 10 cm2 µ-PIC has 512 ch, and a 30 × 30 cm2µ-PIC has 1536ch), we have developed a readout system for µ-PIC. The schematic view of thissystem is shown in Figure 4.5.
Each of all strips connects to a preampliﬁer chip one by one, and the ampliﬁer feeds bothan analog signal and a discriminated digital signal. The analog signal is summed with every
16 channels on the board, and the wave form is digitized by Flash ADC (FADC). The digital
signal is fed to the position encoder one by one. The position encoder takes the coincidence of
the anode signals and the cathode signals, and then it calculates the position of the hit strips.
The number of the hit electrode is sent from the position encoder to a memory module on a
VME system. This hit pattern readout system is rapid, because a CPU on the VME reads only
the hit strip numbers and the clock counter of the coincidence.
High Voltage Supply Board
In a µ-PIC, a positive High Voltage (HV) is supplied on the anodes via the high voltage supply
board, as shown in Figure 4.6, and the right of Figure 4.6 shows the circuit diagram. The board
supplies a high voltage to 256 anode strips. For a 30 × 30cm2 µ-PIC readout, three boards areneeded for x or y coordinates readout. In this board, 256 strips are gathered into 16 groups,
and each group is connected to the HV supply via a register of 1 GΩ, then one group is almost
insulated from other groups. Thus, even if the leak current ﬂows in one group, the pixels ofother groups can be operated safely. In one group, a high voltage is supplied to every 4 strips,
so that eﬀective area is not reduced with the current ﬂow of one group.
This HV supply board also works as a part of the feedthrough, as shown in Figure 4.7. The
readout of the signals from all strips with the usual vacuum feedthrough connectors is very hard.
45

Chapter 4. µ-PIC, µ-TPC & Scintillator

Figure 4.6: The photograph and the circuit diagram of the high voltage supply board

Figure 4.7: The schematic view of the connection of a µ-PIC and a HV supply board

The raw signals before a preampliﬁer is too small to be transfered using a long cable. By thisfeedthrough system, we can easily get signals from the vacuum vessel without the deteriorationof the signal to noise ratio.

ASD Bread Board & Rack
As a preampliﬁer for µ-PIC, we use ASD (Ampliﬁer-Shaper-Discriminator) chips, which weredeveloped by KEK for Thin Gap Chamber of ATLAS experiment in LHC at CERN [42]. The
ASD chip has 4 input lines per 1 chip. Each line has a preampliﬁer, main-ampliﬁer, discriminator, analog output and digital outputs. The preampliﬁer has a time constant of 16 nsec, andthe analog signal is an output of this preampliﬁer output signal. The digital outputs are thediscriminator outputs in LVDS.
We use this ASD chip on the bread board shown in Figure 4.8. This board has 64 inputs
and 16 ASD chips per 1 board. All analog signals are summed once in every 16 channels, andalso 32 channels are summed using the 16 channels summing signals. Thus, we get 4 analog
outputs of summing 16 channels or get 2 analog outputs summing 32 channels. Also, all of thedigital output of ASD chips are fed one by one.
The ASD bread board is inserted in the rack, as shown in Figure 4.9. The rack includes 4
bread boards and a common threshold level supply for the discriminator of 4 × 16 ASD chips.
We can select the internal generation or external-input for the supply of threshold level. At theback plane of the rack, the high voltage supply board is inserted. For the short timing gate of 10
nsec in the position encoder, the diﬀerence of the pattern length from the µ-PIC strip readout
to the digital outputs is less than 3 nsec.
46

µ-PIC (Micro Pixel Chamber)

Figure 4.8: ASD bread board

Figure 4.9: ASD Rack

Position Encoder
The digital outputs of the preampliﬁer bread board are inputted into the position encoder(Figure 4.10). The position encoder consists of 8 FPGAs (Field Programmable Gate Array),and has 1576 LVDS digital inputs, 5 LVDS I/O ports and the memory-writing port. In 8 FPGAs,
6 FPGAs are the same type FPGA for the position calculation, one for the encoding and theother is for the DAQ.

Figure 4.10: Photograph of position encoder

Figure 4.11 shows the block diagram of the position encoder. The position encoder is synchronously operated with the 100 MHz clock generated by FPGA 7. Each FPGA for the position
calculation (FPGA 1-6) has 256 LVDS inputs. The Anode hit signals provided by ASD chips
are fed into three FPGAs 1-3, and the cathode ones are into FPGAs 4-6. Each FPGA forthe position calculation computes the maximum and minimum number among the hit lines in
one clock of 100 MHz, and then send the minimum and maximum number to FPGA 7. WhenFPGA 7 receives the minimum and maximum number of each FPGA for the position encoding,it takes a coincidence of the x-coordinate FPGA 1-3 and the y-coordinate FPGA 4-6 within one
clock. When there is a coincidence, FPGA 7 calculates the minimum and maximum ID for bothx and y coordinates. After then, FPGA 7 sends these data to the memory module with thecount of the clock from the trigger input asynchronously. DAQ-FPGA (FPGA 8) is used forthe communication with other modules, like ADCs or CPU. Besides, FPGA 7 also has an I/O
port for the communication with other module. Using the I/O ports of FPGA 7 and 8, we will
construct even a complicated system compactly.

47

Chapter 4. µ-PIC, µ-TPC & Scintillator

Figure 4.11: The hardware mechanism of the position encoder48

µ-PIC (Micro Pixel Chamber)

4.1.4

Performance of µ-PIC

Gas Gaineffective gas gainµ-PIC gets a signal ampliﬁed by gas multiplication like a proportional counter. A ﬁne structure
electrode of Micro Pattern Gaseous Detectors (MPGD) suﬀers from a discharge, since a highvoltage at narrow gap of a few hundred µm between an anode and a cathode. In principle,
any MPGD has a triplet junction, at which gas, electrode and insulator connect. Such a triplet
junction is considered to cause a discharge easily. Therefore the gas gain of MPGD is limited
by discharge around the triplet junction. For example, Micro Strip Gas Chamber was able tobe used at a maximum stable gain of ∼ 1000.
The gas gain of µ-PIC is shown in Figure 4.12. This ﬁgure says that µ-PIC has a maximum
gain of ∼ 20000, which is very high gain for MPGD. Also µ-PIC have realized a good stability
with the gain of ∼ 6000 during more 1000 hours. Thus, µ-PIC is a stable detector having a highgas gain.

Ar 80% + C2H6 20%
Ar 90% + C2H6 10%
104

103

400

420

440

460

480

500

520

540

560

580 600 620
anode HV [V]Figure 4.12: The eﬀective gas gain as a function of the anode voltage (SN040223-1)

Uniformity
Since a 10×10 cm2 µ-PIC has 65536 pixels (= 256 ch × 256 ch) and a 30×30 cm2 one has 589824
pixels (= 768 ch × 768 ch). The non-uniformity of the pixel structure causes a non-uniformityof the gain on µ-PIC.
Figure 4.13 shows the gain map of the 10 × 10 cm2 µ-PIC, which surely indicates that µ-PIC
has a good uniformity. The Root Mean Square (RMS) of the gain variation on the whole areais ∼ 5 %. On the other hand, Figure 4.14 shows the gain map for the 30 × 30 cm2 µ-PIC. The
ratio of the gain between the minimum gain area and the maximum gain area is 2.2, and thegas gain uniformity is 16.7 % at the RMS.

49

effective gas gain

Chapter 4. µ-PIC, µ-TPC & Scintillator

3500
3000
2500
2000
1500

4000
3500
3000
2500
2000
1500

1000

1000

500

500

0
10
An 8
od
e[ 6
cm
]

0
700
an 600od 500
e 400
300
200
100

4
2
0 0

2

4

10
8
m]
c
[
6
e
od
Cath

0

500 400
700 600decatho

300 200

100 0

Figure 4.13: The eﬀective gain map of 10 × 10 Figure 4.14: The eﬀective gain map of 30 × 30
cm2 (SN040426-1)
cm2 (SN041129-1)

4.1.5µ-PIC as an X-ray Imaging Detectorµ-PIC is used as an X-ray imaging detector. Figure 4.15 shows a spectrum of 55 Fe measured
by 10 × 10 cm2 µ-PIC. Because of the good uniformity of a gas gain, the energy resolution
for X-rays of 5.89 keV is 30 % at FWHM for the whole area. Figure 4.16 is an X-ray imageof a test chart irradiated by the X-ray generator where the shadow of 2 slits per 1mm can be
distinguished clearly. Using the image of the edge of this test chart, we obtained the position
resolution is ∼ 120 µm at σ. This value is quite equal to the predicted one from statistics, which√≃ 115 µm.
is 400µm
12MnKαpedestal

Ar-escFigure 4.15: The spectrum of 55 Fe (whole area
Figure 4.16: X-ray imaging of a test chart withof 10 × 10 cm2 ; Ar 90 % + C2 H6 10 %, 1 atm)
10 × 10 cm2 µ-PIC [43]50Time Projection Chamber with µ-PIC (µ-TPC)

Figure 4.17: The schematic view of µ-TPC

4.2

Time Projection Chamber with µ-PIC (µ-TPC)

4.2.1

Structure and Principle of µ-TPC

When a charged particle passes through a gas, the gas around the particle track is ionized, inwhich a group of ionized electrons are called ‘electron cloud’. If there is an adequate electric ﬁeld
in the gas, the electron cloud drifts toward the electric ﬁeld with the constant velocity. At thistime, the drift time from the ionization point to the termination on the anode can be transferedto the distance between the ionization point and the termination point. Therefore, if we know
the start timing of the drift, the distance between the ionization point and the anode is measured
by the drift time. Such a system is called a Time Projection Chamber (TPC). A 2-dimensional
detector, for example MWPC, is usually used, and the TPC can measure 3-dimensional positionsof a track of a charged particle.
µ-PIC is a 2-dimensional gaseous detector, and is surely used as a readout detector of TPC.
A TPC with µ-PIC is called µ-TPC, and the schematic view is shown in Figure 4.17. In ouradvanced Compton camera, this µ-TPC is used as a tracker, which detects a ﬁne recoil electron
track.

4.2.2

Gas Electron Multiplier

The energy deposit of MIP in the argon gas is 2.54 keV/cm at the normal pressure, in which a
MIP makes about 3.9 electrons per 400 µm. For the detection of MIP tracks, we must detectsuch few electrons, which is very diﬃcult. The MIP tracking needs a high gas multiplication of
2 × 104 at least with µ-PIC system, but the stable gas gain of µ-PIC is only about 6 × 103 . Then
we use a Gas Electron Multiplier (GEM) [44] for the compensating this gain gap.
A GEM was developed by Sauli et al., and it is a polyimide foil with Cu-plated electrodes atboth sides, as shown in left of the Figure 4.18. A GEM foil has a lot of small holes as shown inright of the ﬁgure. If the diﬀerent voltage are supplied to Cu electrodes on both sides, a strong
electric ﬁeld is generated inside of this hole. When the seed electron drifts into this hole, the gas
multiplication is caused in such a strong electric ﬁeld. The foil is very thin with the thickness of
50 µm and consists of the low-Z material. But the distance of both electrodes is too short fora GEM foil to get a high gain alone, and hence the typical gain of single GEM is about 100.
We use this GEM foil as a pre-multiplier for µ-TPC (Figure 4.19). A seed electron drifts into
the GEM, and the ﬁrst multiplication is caused by the GEM with the gain of about 10. After
that, the multiplied electrons drift to µ-PIC, and second multiplication is caused by µ-PIC with51

Chapter 4. µ-PIC, µ-TPC & Scintillator

Figure 4.18: The photograph and microphotograph GEM. The hole diameter is 70 µm, and thepitch of holes is 140 µm.

effective gas gaina gain of thousands. Therefore, we can take an enough signal with a gas gain of above 2 × 104 .
Figure 4.20 shows the gas gain of the GEM + µ-PIC system. In comparison with the gain of asingle µ-PIC, the gain of the hybrid system is about 10 times larger than that of a single µ-PIC.

105

EM

104

ith

G

W

e

IC

on
al

P
µ3

10

350

Figure 4.19: The schematic view of µ-PIC+ GEM system

4.2.3

400

450

500
550
600
µ -PIC Anode Voltage [V]Figure 4.20: The comparison of the gain curveof GEM + µ-PIC and that of µ-PIC alone(SN040922-1, Ar 90 % + C2 H6 10 %, ∆VGEM =250 V)

Drift and Diﬀusion

Electron Drift
In an adequate electric ﬁeld, the pairs of an electron and an ion, which are ionized by a charged
particle, drift with the constant velocity. The ion drift velocity v+ is described with the electricP
ﬁeld intensity E and the gas pressure P : v+ = µ+ E
. Here, the constant µ+ is the mobility,
which depends on the gas.
On the other hand, an electron does not have a constant mobility, and the electron mobility
complicatedly depends on both E and P . The electron drift velocity v− is described such as:
v− =

eE
τ,
me

(4.1)
52

Time Projection Chamber with µ-PIC (µ-TPC)

Drift velocity [cm/ µsec]

where, τ is the mean free time. Figure 4.21 shows the electron drift velocity as a function of theelectric ﬁeld. Because τ also depends on E/P , v− depends on E/P . This ﬁgure says that the
behavior of v− strongly depends on the type of the gas even at the same E/P .
14
12
CF 4 100%

108
6
Ar 80% + C 2H 6 20%

4Ar 90% + C 2H 6 10%

200

Xe 70% + C 2H 6 30%

200

400

600

800

1000

1200

1400

1600 1800 2000
E field [V/cm]

Figure 4.21: The electron drift velocity as a function of the electric ﬁeld

Diﬀusion
Drifting electrons, which at ﬁrst localize in the ionization point at time t = 0, gradually diﬀuse
by the collisions with a gas molecule. At the time t and position x, the transverse diﬀusion ofelectrons in the width of dx is described by
(
)
x2
N
exp −
dx,
(4.2)
dN = √
4Dt
4πDt

σlong [µm/ cm]

σtrans [µm/ cm]

where N is the number of electrons, D is a diﬀusion constant. The diﬀusion constant depends
on the gas condition, and also it depends on the electric ﬁeld in the case of an electron. Figure
4.22 shows the standard deviation of the transverse diﬀusion, and Figure 4.23 shows that of thelongitudinal diﬀusion. Since the diﬀusion of electrons limits the position resolution for the TPC,
the a gas having a small diﬀusion, like CF4 , is better as a TPC gas.

3

10

3

10

Ar 90% + C 2H 6 10%
Ar 80% + C 2H 6 20%

Ar 80% + C 2H 6 20%

Xe 70% + C 2H 6 30%

Ar 90% + C 2H 6 10%102

Xe 70% + C 2H 6 30%

102
CF 4 100%
CF 4 100%

0

200

400

600

800

1000

1200

1400 1600 1800 2000
E field [V/cm]

0

200

400

600

800

1000

1200

1400 1600 1800 2000
E field [V/cm]

Figure 4.22: The standard deviation of trans- Figure 4.23: The standard deviation of longiverse diﬀusion
tudinal diﬀusion

53

Chapter 4. µ-PIC, µ-TPC & Scintillator

4.2.4

Spectroscopy

The X-ray spectra were measured by the hybrid TPC with GEM and µ-PIC as shown in Figure
4.24. The signal to noise ratio is better than that with a single µ-PIC due to the higher gain.
Since the TPC system has a large gas volume, TPC can detect comparatively high energy Xrays. Also the X-ray spectrum of 60 keV from 241 Am is shown in Figure 4.24. The CuKα
ﬂuorescence line is seen in this spectrum due to the irradiation of X-rays to µ-PIC/GEM copper
electrodes.
Figure 4.25 shows the dependence of energy resolution on the energy, where the dotted line
is got by ﬁtting:¯()
E −0.30∆E ¯¯[%] = 5.4 × 10.
(4.3)
E ¯FWHMkeV
This energy resolution of TPC reaches to about 30 % at 6 keV.

14000

18000
16000

12000

CuKα

14000

10000

AgKα

CuKα

12000

8000

10000

6000

8000
6000

4000
4000
2000
00

2000
10

20

30

40

50

60

00

70
80
Energy [keV]

59.5 keV
10

20

30

40

50

60

70
80
Energy [keV]

Figure 4.24: The X-ray spectra with µ-TPC (left: 109 Cd, right: 241 Am; Ar 90 % + C2 H6 10 %,1 atm, gas ﬂow)

FWHM of resolution [%]

102

10
102
Energy [keV]

10

Figure 4.25: The dependence of energy resolution of the µ-TPC on incident photon energy

54

Scintillation Camera

Figure 4.26: Hamamatsu multi-anode PMT H8500 (left) & GSO:Ce pixel array (right)

4.3

Scintillation Camera

Since a scintillator as an absorber of an MeV gamma-ray camera detects both the position andthe energy, the stopping power of the absorber limits the detectable energy range of the MeV
gamma-ray camera.
As a position sensitive detector, an Anger camera using a monolithic scintillator is an conventional but a powerful method. But if we use a larger and thicker scintillator, the scintillation
camera would be a quite large and have huge background, which makes it diﬃcult to coincidewith a tracker. Furthermore, for the balloon/satellite loading, a large Anger camera has a highrisk that a large monolithic scintillator might be broken by shock.
Another method is using an array of pixel scintillators. For a Pixel Scintillator Array (PSA),
the position resolution is determined by the pixel size. PSA separates a large area of the gammaray camera to some units. A scintillator has a dead time of a few µsec for each hit. Since thedetection area of an Anger camera is the monolithic scintillator, the pileup of the several signals
easily happens for a strong irradiation. On the other hand, such a pileup hardly happens using
the PSA, and thus the coinciding with a tracker is easy than the use of an Anger camera.
Besides, PSA has a low risk for the break of the scintillator due to its divided structure. On theother hand, PSA needs a lot of readout electronics.
From above reasons, we selected a PSA method for our MeV gamma-ray camera, although
the low power and compact readout system is necessary.

4.3.1

Scintillator & PMT

Table 4.1 lists the properties of various scintillators. As the scintillator for an absorber, therequired properties are a good energy resolution and a high stopping power. For the use of the
pixel structure, the scintillator can be easily handled. Also, for the balloon/satellite loading, thescintillator is required a radiation hardness. For these purposes, we selected GSO:Ce scintillator.
Table 4.1 says GSO:Ce scintillator has high Z, a high density, a high stopping power, reasonableenergy resolution, fast decay time, strong radiation hardness and no hydroscopic. Therefore
GSO:Ce matches to PSA as an absorber of our MeV gamma-ray camera.
For a photon sensor of the scintillator, we selected a multi-anode Flat Panel PMT H8500
made by Hamamatsu Photonics (left of Figure 4.26). This PMT has 8 × 8 pixels with each sizeof 6 × 6 mm2 and geometrical area of 52 × 52 mm2 , and its eﬀective area thus is 89 %. The gain
uniformity of H8500 is not so good, and the ratio of the maximum gain to the minimum one isabout 3.
For matching the PMT pixel size with the scintillator pixel size, we selected a 6 × 6 mm2
GSO:Ce pixel, where 13 mm height is due to the radiation length of GSO:Ce. A GSO arrayconsists of 8 × 8 pixels, and a reﬂector is inserted between each pixel (right of Figure 4.26).
55

NaI(Tl)
CsI(Tl)CsI(Na)BGO
LSO:CeYSO:Ce
YAP:CeYAG:Yb
GSO:CeLGSO
LaBr3 :Ce
LaCl3 :Ce
CWO
PWO
BaF2

scintillator

Density[g/cm3 ]
3.67
4.53
4.53
7.13
7.4
4.45
5.35
4.57
6.71
7.2
5.3
3.79
7.9
8.28
4.89

Table 4.1: The properties of various scintillators [45]Abs. coeﬃcient 511keV Decay time wave length light yield hydroscopic[1/cm][ns][nm](relative)
0.34
230
415
1
strong
0.44
1050
550
0.85
weak
0.44
630
420
0.90
strong0.92
300
4800.07-0.12
no
0.87
40
420
0.4-0.75
no
0.38
40
420
0.3-0.45
no
0.46
28
370
0.4
no
3
330/500
0.02
no
0.70
30-60
440
0.18
no
43
420
0.4
no
0.45
28
385
1.30
strong0.34
26385
0.7-0.9strong
0.92
5000
475
0.3-0.4no1.15< 3 < 20
430
0.04
no
0.48
0.6/620220/310
5/16
no

56

10
105−6
104−5

> 106

102−3
105
104
102−3

radiation hardness[Gray]
10
10

1.9
1.9
2.3
2.2
2.2

1.85
1.8
1.8
2.15
1.82
1.79
1.94
1.82
1.85

refraction index

Chapter 4. µ-PIC, µ-TPC & Scintillator

Scintillation Camera

4.3.2

HV Operation/Readout System

For the reduction of the number of the readout channel, we use the resistor matrix board asshown in Figure 4.27, and obtain the position of a hit pixel using the Center Of Gravity (COG).
On this board, the anode of 3 PMTs in horizontal rows and the interval of the edge of the column
are connected with 100 Ω register chips respectively. Then, we get a hit position by reading 4
readouts from the corners. These readouts are connected to the preampliﬁer board, which has
4 preampliﬁers having a time constant of 5.4 µsec, and the output of the preampliﬁer is fed into
the shaper having a time constant of 0.5 µsec. Then the shaping signal is fed to the peak hold
ADC. Also the summing signal of 4 preampliﬁer-outs is used for the generation of the trigger.

Figure 4.27: The resistor board

For the balloon/satellite loading, a compact high voltage power system for PMTs is inevitable. We selected EMCO Q12N-5 as a DC-HV convertor (Figure 4.28), and its speciﬁcation
is listed up in Table 4.2. We control this DC-HV convertor by supplying DC level from DAC,
which is operated by a VME CPU. We connect this convertor to PMT one by one, therefore wecan control HV of each PMT individually.

Figure 4.28: DC to HV convertor (EMCO
Q12N-5)

Table 4.2: The properties of EMCO Q12N-5
parameter
value
Input Voltage
0.7 - 5 V
Input Current< 50 mA (No load)< 175 mA (Full load)
Output Voltage
0 - −1.2 kV
Output Current< 0.4 mA
Conversion Factor ∼ 60 %Ripple Noise< 0.25 %Weight4g
Size
1.27 × 1.27 × 1.27 cm3
Operating Temp.
−25 - +70 ◦ C

Finally, we composite a compact GSO-PSA unit shown in Figure 4.29. One unit has 4 ch
analog signals, 1 signal output line for the trigger, and 1 D-sub 9 pin power line (3 pairs from
DAC and preamp power). By putting several GSO-PSA units around the tracker, a large areaabsorber are easily constructed.

57

Y COG

Chapter 4. µ-PIC, µ-TPC & Scintillator

1
0.9
0.8

102

0.7
0.6
0.5

10

0.4
0.3
0.2
1

0.1
0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1
X COG

2000
1800
1600
1400
1200
1000
800
600
400
200
0 0.2

0.4

0.5

0.6

0.7

0.8

Figure 4.30: The COG image of GSO (upper)
and the projection of the box-enclosed area
(lower)Figure 4.29: The photograph of one PSA unit

4.3.3

0.3

Image and Energy Spectrum

By illuminating the whole area of GSO-PSA by an radioactive source, we measure the image of
the COG as shown in Figure 4.30, where we can distinguish each pixel clearly.
Figure 4.31 shows the 137 Cs spectrum obtained from the whole area of one unit, and thetypical energy resolution is 11 % for 662 keV at FWHM. The energy dependence of the energy
resolution is shown in Figure 4.32, In this Figure, the dotted line is ﬁtted as
¯
)
(
∆E ¯¯
E −0.522[%] = 3.3 × 10.
(4.4)
E ¯FWHMkeV

FWHM of energy resolution [%]Figure 4.33 shows the distribution of the gain of each pixel, and the RMS is ∼ 0.23. We foundthat the GSO absorber has a dynamic rage of 80 keV - 1 MeV over the whole area.

40000
35000
30000
25000
20000
15000
10000

102

10

5000
0

200

400

600

800

1000

1200

1400
Eγ [keV]

102

3

10
Eγ [keV]

Pixel

Figure 4.31: The spectrum of 137 Cs measured Figure 4.32: The energy resolution depending
with the GSO-PSA
of the GSO-PSA on energy

90
80
70
60
50
40
30
20
10
0

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8
2
gain ratio to the avrage

Figure 4.33: The gain distribution of each pixel of the GSO-PSA

58

Chapter 5

Tracking of Charged Particles
5.1

Tracking of Cosmic Muon

5.1.1

DAQ System for Muon Tracking

For a performance test of the µ-TPC, we measured the tracks of cosmic muons. We put plastic
scintillators above and under the µ-TPC as shown in Figure 5.1, and took a coincidence of theupper and lower plastic scintillators as a µ-TPC trigger. This trigger was fed into the position
encoder, and the encoder started to count 100 MHz clock from the trigger generating during 4µsec. When a µ-TPC signal came in the position encoder, the encoder stored the clock counterand the maximum/minimum number of the hit anode/cathode strip in FIFO of FPGA 7. If there
is a coincidence of the anodes and the cathodes during clock counting, the encoder generated aninterrupt signal to the VME CPU after the encoder outputs the hit informations to the memory
module. After the CPU ﬁnished the data taking, CPU sent an acquisition end signal to theencoder, and the encoder cancels the veto signal for the next trigger.

Figure 5.1: The setup for cosmic muon detection

As a gas of the TPC, Ar 90 % + C2 H6 10% was used in ﬂow. The hybrid µ-TPC with GEM
was operated with the gains of 10 for GEM (∆VGEM = 250 V) and 2500 for µ-PIC (VAnode = 500
V), respectively, and hence the total gain of 2.5 × 104 was achieved. The maximum drift length
was 7.6 cm, and the electric ﬁeld in the drift region was ∼ 400 V/cm. Also the electric ﬁeld of
the induction from GEM was ∼ 2 kV/cm.
59

Efficiency [%]#Chapter 5. Tracking of Charged Particles

104

3

10

100

80

60
102
40
10
20
1
10

20

30

40

50

60

70

80

90

0

100
Nhit

1

10

102

NMin

Figure 5.2: The histogram of hit points number Figure 5.3: The tracking eﬃciency, which is the
Nhit
ratio of Nhit ≥ NMin events to the trigger number

5.1.2

Tracks of Muon & Track Eﬃciency

Cathode [cm]Figure 5.2 is a histogram of the number of hit points (Nhit ) for one muon track. The average of
Nhit is 14.8 for a 10 cm muon track.
By considering the geometrical trigger eﬃciency (0.902), the detection eﬃciency, which is
the ratio of the event number to the trigger number, is measured to be 98.7 %. Figure 5.3 shows
the tracking eﬃciency, which is the ratio of number of Nhit ≥ NMin events to the trigger of thecoincidence of the plastic scintillators. The tracking eﬃciency of Nhit ≥ 3 is 97.0 %, and that of
Nhit ≥ 10 is 69.0 %. Its reason is as follows. µ-TPC has a poor eﬃciency for the tracks running
vertically or parallel in µ-PIC, and the major cosmic muon passing through the µ-PIC are nearin parallel to the µ-PIC. Therefore, the tracking eﬃciency is not so good in this setup.
Figure 5.4 shows the examples of several tracks of cosmic muons in the µ-TPC. MIPs likecosmic muons create only 4 electrons by ionization during 400 µm, which is the pitch of µ-PIC
pixels. Although it is thus diﬃcult to detect MIPs by MPGD, µ-TPC can measure ﬁne tracksof MIPs as shown in Figure 5.4.

4
2
0

-2
-4
10
Dr 9
ift 8
[c 7
m 6
] 5

4

3

2

1

0

-2

-4

0

4 ]
m
2
de [c
o
n
A

Figure 5.4: The track images of cosmic muons with µ-TPC

60

#Tracking of Cosmic Muon

104

1400
1200

3

10

1000
800

2

10

600
10

400
200

1
50

100

150

200

250

Figure 5.5: The histogram of clock counter

5.1.3

0
0

300
Clock

1

2

3

4

5

6

7

8
9
10
dE/dX [keV/cm]

Figure 5.6: The dE/dX spectrum of cosmic
muons. The solid curve is the best ﬁt with a
Landau distribution

Clock Counter Histogram

Figure 5.5 shows the distribution of the drift time of the µ-PIC. This distribution of the drift
time looks to be divided into 4 regions: the clock of 0-8, 9-24, 25-225 and 226-400.
The events in the ﬁrst region are pre-trigger events. Because the position encoder calculates
the hit position in a pipeline processing and several clocks are required for the calculation, there
is a few time latency before generating the trigger. The second region is due to the gap events.
The seed electrons, which are ionized in the gap between µ-PIC and GEM, do not pass throughGEM. Therefore, the gain is lower by a factor ∼ 10, and the signals of these electrons are so smallthat they hardly exceed the threshold. Then, the amount of gap events are quite less than those
events in the drift region. In the third region, the events are generated in the drift region. Good
events are ionized in the drift region, and multiplied by both GEM and µ-PIC. The ﬁnal region
consists of the drift-over events. These events are due to the random coincidence completely.
Because the time width of one clock is 10 nsec, the time width of the third region is 2 µsec.
The maximum drift length is 7.6 cm, and we thus get the drift velocity of 3.8 cm/µsec at ∼ 400
V/cm. This value is consistent to the nominal one for the gas mixture in Figure 4.21.

5.1.4

dE/dX Spectrum

Figure 5.6 is the spectrum of the energy deposit per unit length dE/dX, and the solid curveis the best ﬁt with the Landau distribution. This ﬁtted Landau distribution has a peak of 2.6
keV/cm. On the other hand, the dE/dX of MIP in Ar gas is 2.5 keV/cm. Therefore thisspectrum is well consistent to the energy deposit of MIP.

5.1.5

Position Resolution

For the measurement of the position resolution on tracking method, we obtained the distance
between the hit points and the track line obtained by ﬁtting. Figure 5.7 shows the distribution of
the residuals, where data are ﬁtted with a 2 dimensional Gaussian in circular polar coordinates:
√
(
)
2π
r2
P (r)dr = 2 r exp − 2
dr.
(5.1)
σres
2σres
Obtained σres is 486 µm. Although the diﬀusion constant σ is ∼ 1 mm at the drift length of ∼ 5
cm, the obtained σres is smaller than the diﬀusion σ because we use the center of the minimum
hit strip and the maximum hit strip as a hit point.
61

Chapter 5. Tracking of Charged Particles

60000

2500

50000

2000

40000
1500

30000
1000

20000
500

10000
0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6
1.8
2
Residual [mm]

00

2

4

6

8

10

12

14

16

18

20
Ae

Figure 5.7: The histogram of residual between
the obtained points and the ﬁtted straight line. Figure 5.8: The histogram of the electron number for exceeding threshold Ae
The solid line is obtained by ﬁtting

5.1.6

ThresholdWe estimated how much electrons are required to exceed the threshold in ASD chips. Assumingthat the hit width, which is the distance between minimum and maximum hit strip, is due to thetransverse diﬀusion, the number of electrons for exceeding threshold Ae is described as follows,
Ae =

ϵ∆l
w

∫ max − min +0.5strip2max − min−0.5strip
2

√

)
(
x2
1
dx,
exp − 22σtrans
2πσtrans(5.2)

where ϵ is the energy deposit per unit length, ∆l is the track length per clock and w is theaverage energy required to ionize one electron. Figure 5.8 is the histogram of Ae , and about 2electrons are required to exceed the threshold.

5.2

Tracking of Recoil Electron

When µ-TPC is used as a tracker of the MeV gamma-ray camera, µ-TPC must track a low
energy recoil electrons which is easily scattered in the gas. The electron’s track looks winding
even in the gas. Therefore it is very diﬃcult to reconstruct a low energy electron track evenwith a gaseous detector. In this section, we will estimate the eﬃciency of tracking electron
by a GEANT4 simulation. We simulate the electron track in the µ-TPC using the following√
parameters: the transverse diﬀusion of σtrans = 470 µm/ cm, the longitudinal diﬀusion of√
σlong = 235 µm/ cm, the time constant of preampliﬁer of 16 nsec and the 100 MHz encodingof the hit positions. Here the electron energy is distributed in the range between 0 - 300 keV.
Also the gas is Ar 90 % + C2 H6 10 % at 1 atm.
In order to reconstruct the recoil electron track in the Compton camera, following 5 steps
are proceeded in the analysis (Figure 5.9).
1. Fixing of the Track Points: In general, a drifted electron cloud extends over several strips.
Then we record the ﬁve encoded values of a µ-TPC hit data, which consist of the clock
counter, the maximum ID and the minimum ID of the anode strip, the maximum ID andthe minimum ID of the cathode strip. Therefore we must determine the hit position on
the extended hit strips.
2. Connecting: As shown in the left-lower of Figure 5.9, there are several connections between
the hit points, and we have to ﬁnd the correct one (see 5.2.2).
62

Tracking of Recoil Electron

Figure 5.9: The schematic view of the process to reconstruct the recoil electron track. We obtaina minimum and a maximum of the hit strip number (Step 0). The ﬁrst step is the determinationof the track points. Next, we reconstruct the track by connecting the track points. Finally, we
determine the initial/end point of the track and the initial direction.

3. Selection of the Stopping Electron: For the reconstruction of the incident gamma-ray, theenergy of the recoil electron is needed. However, the recoil electron may escape from theﬁducial volume of the TPC. Thus we must select the electron stopping in the µ-TPC.
4. Judgement of the initial/end point of the electron track
5. Determination of the recoil direction: We determine the direction of the recoil electron byﬁtting the initial part of the track with a straight line.
The analysis of the recoil electron track aﬀects seriously to the eﬃciency and the angular resolution of the MeV Compton camera. Therefore, the study of the recoil electron tracking is very
important for the gamma-ray reconstruction.

5.2.1

Fixing of the Track Points

When the width of the hit strips extends signiﬁcantly, the extended hit point would include two
or more track points. Then, for the electron tracking, we select the real track points under theconditions as follows.
√•(xmax − xmin )2 + (ymax − ymin )2 < 5σtrans : As a track point, we obtain the center of hit()
min ymax +ymin
area, xmax +x
,
.
2
2
(
)
(
)
min
min
• xmax − xmin < 3σtrans : We obtain two points, xmax +x, ymin and xmax +x, ymax .
2
2
(
)
(
)
min
min
• ymax − ymin < 3σtrans : We obtain two points, xmin , ymax +yand xmax , ymax +y
.
2
2
63

Chapter 5. Tracking of Charged Particles[%]

Here, xmin and xmax are the minimum ID and the maximum ID of the hit anode strip, respectively, and ymin and ymax are the minimum ID and the maximum ID of the hit cathode strip.
The discrepancy between the obtained track points and the simulated electron track is shown in
Figure 5.10. From this ﬁgure, the position resolution of the electron tracks is ∼ 500 µm, which
is nearly equal to the position resolution of the muon tracks (Figure 5.7). Also the ratio of thenumber of the used hit points (Nuse ) to the number of the all hit points (Nhit ) is shown in Figure
5.11. This ﬁgure says that the events using all hit points are 70 % and the events using ≥ 80 %points are 90 %, by this method.
102

9000
8000

10

7000
6000
1
5000
4000
10-1

3000
2000

10-2

1000
00

1

2

3

4
5
residual [mm]

0

0.2

0.4

0.6

0.8

1
Nuse/Nhit

Figure 5.10: The position resolution of the elec- Figure 5.11: The ratio of the number of usedtron tracks
hit data (Nuse ) to the number of all hit data
(Nhit )

5.2.2

Connection of the Points

For the search of the Compton point, the roughly sorting of the track points is needed. However,the sorting of the track points in the correct order is a very diﬃcult problem. Then we calculate
the nearly minimum length path as the approximate path by the method as follows.
1. Combine the Nearest Point: As the ﬁrst step of the sorting, we reconstruct the track of(n + 1) points by the combination of the nearest point to the track of reconstructed using
n points. We obtain the initial track by the repeat of such point adding.
2. 2-opt and Or-opt: For the improvement of the track, we use the 2-opt method and the
Or-opt method, with using the random number.
• 2-opt: We select two points at random, and reverse the order between the two points,
as shown in Figure 5.12.
• Or-opt: We select two points at random and cut the part track between the twopoints, and we insert the part track at the random selected point of the residual
track, as shown in Figure 5.13.
We select two methods at random, and we compare the length of the new track with thatof the previous track. If the length of the new track is shorter than the previous one,
we adopt the new track order. We repeat the search of the shorter track until ﬁnding no
shorter length.
Figure 5.14 shows the absolute value of the correlation coeﬃcient (Ccor ) between the correct
order of the hit points and the obtained one. In this ﬁgure, the events of Ccor > 0.8 is 75 %.
Thus this sorting method looks eﬀectively functional.
64

Tracking of Recoil Electron

Figure 5.13: The schematic view of Or-opt[%]

Figure 5.12: The schematic view of 2-opt

10

1

10-1
0

0.2

0.4

0.6

0.8

1
Ccor

Figure 5.14: The correlation coeﬃcient between the correct rank and the obtained rank

5.2.3

Selection of the Stopping Electron

For the selection of the stopping electron events, we study the correlation between the energy
and the track length. Figure 5.15 shows this correlation in an ideal detector. In this ﬁgure,the events on the solid line correspond to the dE/dX of MIPs. If the electrons deposit all of
the energy in the µ-TPC, such events concentrate along the dashed line. The dashed line is
described as()
Ke 1.82−3
Le [cm] = 3.60 × 10
,
(5.3)
keV
where Le is the electron track length, and Ke is the electron energy.
On the other hand, taking into account of the position resolution and the eﬃciency of thesorting in the simulation, the correlation between the energy and the track length is changed asshown in Figure 5.16. The solid line and the dashed line are the same as Figure 5.15. Figure
5.16 says that the correlation of the energy and the track length is kept even if the position
resolution and the eﬃciency of sorting are considered. Therefore, the events of the stopping
electrons are selected by collecting the events near the dashed line.

5.2.4

Judgement of the Initial/End Point of the Track

For the judgement of the initial/end point of the electron tracks, we use the ratio of the track
length to the distance of the edge point and the middle point. Because the eﬀect of multiple
scattering increases in the end of the track, this ratio of the end part is expected larger thanthat of the initial part. The judgement eﬃciency by this ratio is ∼ 60 %. Figure 5.17 shows the
65

20

300

Track Length [mm]Track Length [cm]Chapter 5. Tracking of Charged Particles

18
250

16
14

200

12
10

50
140

45

120

40
35

100

30
80

150

25

60

8

20

100

6

15

40

4

10

50

20

2
00

20

40

60

80

100

120
140
Energy [keV]

0

5

00

Figure 5.15: The relation of the electron energy
and track length in the ideal. The tilt of thesolid line means the dE/dX of MIPs. If theelectron deposits all of the energy, the relationof the energy and the track length is on thedashed-line.

20

100

80

80

60

60

40

40

20

20

1

2

3

4

5

60

80

100

120
140
Energy [keV]0

Figure 5.16: The relation of the energy andthe track length with the position resolution ofthe µ-TPC. The tilt of the solid line means thedE/dX of MIPs, and the dashed line means the
stopping electrons.

100

00

40

00

6
7
8
9
10
residual of initial point [mm]after α cut

20

40

60

80

100

120

140

160
180
∆ [degree]

Figure 5.17: The position resolution of the ini- Figure 5.18: The diﬀerence angle between thetial point
obtained angle and the direction of the electron
ejecting.

residuals of the initial point, which has been judged to be the front one by this ratio, and theresolution of the initial point is ∼ 1 mm.

5.2.5

Determination of the Recoil Direction

The ﬁnal step is the determination of the direction of the recoil electron. For obtaining of thedirection, we use the initial part of the track consisting of several track points, of which length isless than 1.5 cm, and then we determine the direction with ﬁtting by a straight line. Figure 5.18
shows the diﬀerence angle between the obtained direction and the direction of electron ejecting.
In the gamma-ray reconstruction, scince we require the Compton scattering kinematics (α cut),the events of the correctly obtained direction are selected. The ejected direction of a part of theevents is obtained within the angular resolution of ∼ 30◦ in simulation, and hence the resolutionof the SPD is expected to be about 60◦ at FWHM.

66

Chapter 6

Prototype Camera
6.1

Prototype Setup

In order to the investigate a fundamental feature of the electron tracking Compton camera, weconstructed a prototype Compton camera consisting of a 10 cm µ-TPC and a GSO-PSA. In thischapter, I describe the system and the performance of the prototype camera.

6.1.1

Absorber & Tracker of Prototype

The speciﬁcations of the absorber and the tracker in the prototype camera are summarizedin Table 6.1 and Table 6.2, respectively. Also Figures 6.1 and 6.2 are the photographs of thebottom absorber and the side absorber, respectively. Using this absorber and this tracker, weconstructed an MeV gamma-ray camera.
Table 6.1: The speciﬁcation of prototype absorber
Scintillator
GSO:CePMT
Hamamatsu Photonics H8500
Pixel Size
6 × 6 × 13 mm3Bottom Camera
158 × 158 mm2 (3 × 3 PMTs)
Side Camera
158 × 52 mm2 ×4 surfaces(3 × 1 PMTs ×4)
Energy Range
80 - 900 keVEnergy Resolution FWHM[%] = 3.3 × 102 (Eγ /keV)−0.52

Table 6.2: The speciﬁcation of prototype tracker
Gas
Ar 90 % + C2 H6 10 %, gas ﬂow 1 atm
Volume10 × 10 × 7.6 cm3
Fiducial Volume
8 × 8 × 7.6 cm3
Gas Gain∼ 25000
Drift Velocity
3.8 cm/µsec
Encoding Clock
100 MHz
Position Resolution∼ 485 µm
Threshold2 electron( K )−0.30
eEnergy Resolution
FWHM[%] = 5.4 × 10 keV
(
)1.82
K
e
Length of stopped electron
Le [cm] = 3.60 × 10− 3 keV

67

Chapter 6. Prototype Camera

Figure 6.1: The photograph of the bottom ab- Figure 6.2: The photograph of the side absorbersorber

6.1.2

Alignment of Prototype Camera

When we observe the gamma-rays coming from a quite far point source such as a celestial object,
the gamma-ray comes into a detector in parallel. But, in the laboratory, it is very diﬃcult to
get the parallel light using a radioactive source. For simulated parallel light (< 1◦ ), we have to
put a radioactive isotope at ∼ 5 m from the detector, and the intensity of gamma-rays is too
weak. Then, we put a radioactive isotope at the appropriate distance from the camera, and get
an image by taking the cross point of the source plane and the reconstructed line, as shown in
Figure 6.3, where we use this distance as a known parameter.

Figure 6.3: The side view of alignment
Figure 6.3 shows the alignment of the prototype camera, and Figure 6.4 is the photograph ofthe prototype camera. The distance between the camera window and the source plane is ∼ 40
cm. Around this camera, there is no shield and no veto counter.

6.1.3

Data Acquisition System of Prototype Camera

Figure 6.5 is the diagram of the data acquisition system for the prototype camera. As a triggerof the data acquisition, we use a sum of the signals from 4 corners of a GSO unit. However,68Prototype Setup

Figure 6.4: The photograph of prototype cam- Figure 6.5: The diagram of data acquisitionerasystemFigure 6.6: The timing chart of various signals in the data acquisition cycle

Figure 6.7: The timing chart of various signals in the clear cycle

69

Chapter 6. Prototype Camera
since the tracker is a gaseous TPC, most of the incident gamma-rays would pass through the
TPC and interact with the GSO scintillators. Therefore, the most of the triggered events do
not have the TPC data. Then we clear the GSO data when the triggered event does not havea TPC data. The timing charts in the data acquisition cycle and the clear cycle are shown in
Figures 6.6 and 6.7, respectively.
A generated trigger is fed to the position encoder. When this trigger comes into the position
encoder, the encoder generates a gate signal for the peak hold ADCs, and also starts to counta 100 MHz clock. In counting the clock, the encoder waits the signal from the µ-TPC during 4µsec. In the case of no TPC signal, the encoder sends a clear signal to the peak hold ADC forcanceling the held data, and then clears the veto signal for a next trigger. On the other hand, inthe case of a signal coming, the encoder accumulates the hit information during 4 µsec from thetrigger and sends it to the memory module. Then it sends a stop signal to the Flash ADC at
4.5 µsec. After the data transfer to the memory module, the encoder makes an interrupt to the
VME CPU. Then the VME CPU takes the data of the memory module, peak hold ADCs andﬂash ADCs. Finally, the CPU sends the acquisition end signal to the encoder. If the encoder
catches the end signal, the encoder clears the veto signal for a next trigger. The position encoder
also makes the signal about the ID of the hit GSO unit, which let us know which GSO unit
made the trigger.

6.2

Reconstruction of Gamma-Ray

6.2.1

Event Selection

When we reconstruct the incident gamma-rays with (3.1) and (3.2), we select the real Compton
events by applying the following criteria.
• Number of the Triggered GSO Units: A real Compton scattering event must have one
electron track in TPC and one hit GSO unit. Thus, the number of the triggered GSO
units is one in each real Compton event. If that number is more than two, the event isdue to the background such as a charged particle event, a multiple scattering event or arandom coincidence.
• Rejection of Low Energy Electron: Because the electron with the energy less than 15 keV
is aﬀected by multiple scattering, the accuracy of the determination of the direction of
the recoil electron is very poor. On the other hand, for the determination of the recoil
direction, we require three points of the electron track at least. Therefore, we require
Ke > 15 keV and Nhit > 3.
• Fully Electron Contained Events: For the reconstruction of the incident gamma-ray, theenergy of the recoil electron is required. However, the radiation length of a gas is so longthat the recoil electron may escape from the drift volume of the TPC. Therefore, we must
require the containment of the full track of the recoil electron in the µ-TPC. Then, wedeﬁne the ﬁducial volume of the µ-TPC and the initial point of the electron track must
be contained in it. Also, for the selection of the stopped electron events, we require (5.3).
• Compton Scattering Kinematics: With the reconstruction of the incident gamma-ray, we
obtain the scatter angle by (3.4). Because the cos ϕ must be between −1 and 1, following
relation is required,0≤

me c2 Ke≤ 2.
Eγ + Ke Eγ

(6.1)
70

Reconstruction of Gamma-Ray

source: 137Cs 662 keV°

αkin: 88.9°
αgeo: 79.8°
φ: 27.7°
ψ : 61.2

E0: 629.9 keV
Eγ : 551.9 keV
Ke: 78.1 keV

XY: source position: reconst. point: absorbed point: Compton point

15

Y [cm]

20050818/per7 track_01-0444

10

5

0

-5

Y [cm]

-10

15

-15
-15

10

-10

-5

0

5

10

15
X [cm]

-10

-5

0

5

10

15
X [cm]XZ
Y [cm]

5
0

20

15

-5

10

-10

5

-15
20
-X 15
[cm 10
]
5

0

0

-5
-10

-15

-20 -40

-30

-20

-10

0

10

20
]
Z [cm

-5

-10
-15

Figure 6.8: The typical event of the reconstructed gamma-ray

For matching to the kinematics of Compton scattering, we select events by α. The incident
direction is described by the summing of the momenta,√
Ke (Ke + 2me c2 )
Eγ
⃗g +
⃗e.
(6.2)
⃗skin =
Eγ + KeEγ + KeFor a real Compton scattering event, ⃗skin is a unit vector within the measurement errors.
Then, as the α cut, we require the following condition,|⃗skin | − 1 ≤ ∆.

6.2.2

(6.3)

Reconstructed Gamma-Rays

Figure 6.8 is an event display for the typical Compton event measured by this prototype detector,
where we ﬁnd the energy and track of the recoil electron, the energy and absorbed point of
scattered gamma-ray. We can check that the Compton scattering is reconstructed correctly.
Moreover, the source position is also reconstructed accurately.
With changing the source position, we measured the movement of the images as shown in
Figure 6.9. All images have a peak near the real source position. On the other hand, Figure
6.10 shows the obtained images of the diﬀerent energy gamma-rays. It is clearly seen that thepeak of the obtained image spreads for the lower energy of the incident gamma-rays.
Two diﬀerent sources of a 137 Cs (662 keV) and a 54 Mn (835 keV) were put at (4cm, 4.5cm)
and (−4cm, −4.5cm), respectively, and we obtained the spectrum as shown in the left of Figure
6.11. The peaks of 662 keV and 835 keV are clearly seen in this spectrum, and the continuum
component of the gamma-rays scattered at the outside of the camera is also seen under 600
keV. On the other hand, the Compton edge is not seen in this histogram, because this camera
measures both the recoil electron and the scattered gamma-ray.
71

Chapter 6. Prototype Camera(12, 0)Y [cm]

Y [cm](0, 0)

40

40

20

20

0

0

-20

-20

-40

-40

0

20

X [cm]-40

40
X [cm](20, 0)(32, 0)

40

-20

0

20

Y [cm]-20

Y [cm]

-40

40

40

20

20

0

0

-20

-20

-40

-40

-40

-20

0

20

40
X [cm]

-40

-20

0

20

40
X [cm]

Figure 6.9: The obtained images of the diﬀerent source positions. The gamma-rays sources of
all images are 137 Cs, and the energy range is 613 - 711 keV.

72

Reconstruction of Gamma-Ray

133 Ba 313 - 399 keV

Y [cm]

Y [cm]

57 Co 104 - 140 keV

40

40

20

20

0

0

-20

-20

-40

-40

-40

-20

0

20

-40

40

-20

0

20

X [cm]

54 Mn 786 - 884 keV
Y [cm]

Y [cm]

133 Cs 613 - 711 keV

40
X [cm]

40

40

20

20

0

0

-20

-20

-40

-40

-40

-20

0

20

40

-40
X [cm]

-20

0

20

40
X [cm]

Figure 6.10: The obtained images of several diﬀerent sources. The source position of all imageare (0cm, 0cm).

73

Chapter 6. Prototype Camera

Y [cm]

20

240
220

15

200
10

180
160

5

140
0

120
100

-5

80
-10

60
40

-15

20
00

200

400

600

800

-20
-20

1000
E0 [keV]

-15

-10

-5

0

5

10

15
20
X [cm]

Figure 6.11: The obtained spectrum and the image by the prototype camera. We put a 137 Cs(662 keV) and a 54 Mn (835 keV) at (4cm, 4.5cm) and (−4cm, −4.5cm), respectively.

Y [cm]

20

Y [cm]

20
15

15

10

10

5

5

0

0

-5

-5

-10

-10

-15

-15

-20
-20

-15

-10

-5

0

5

10

-20
-20

15
20
X [cm]

-15

-10

-5

0

5

10

15
20
X [cm]

Figure 6.12: The image in the energy range of Figure 6.13: The image in the energy range of613 - 711 keV
785 - 885 keV

3

40

3.5

Y [cm]

Y [cm]

3.5

3

40

2.5

2.5

20

20
2

2

0

0
1.5

1.5

-20

-20
1

-40

1
-40

0.5

-40

-20

0

20

40
X [cm]

0

0.5

-40

-20

0

20

40
X [cm]

0

Figure 6.14: The normalized image of 137 Cs Figure 6.15: The normalized image of 137 Cs
without α cut (613 - 711 keV)with α cut (613 - 711 keV)74

Performance of Prototype Camera
The right of Figure 6.11 is the obtained image by this measurement. This image has peaks
near the 137 Cs and 54 Mn. On the other hand, by applying the energy cut of 613 - 711 keV and
785 - 885 keV, the obtained images are Figure 6.12 and 6.13, respectively. Both images havean unique peak near the gamma-ray source corresponding to the energy range. Therefore, theprototype camera can separate multi-energy sources using the energy information.

6.2.3

Eﬀect of α Cut

Figures 6.14 and 6.15 are the normalized images before and after α cut, respectively, where thesource position is (0cm, 0cm). In both images, the white region means 0 photons. Comparing
these images, although both images have a peak near the source position, the spread of theimage becomes narrow and the signal to noise ratio is quite improved, after applying the α cut.

6.2.4

Comparison with Classical Compton Imaging

50

1

40

0.9

30

Y [cm]

Y [cm]For the comparison with the classical Compton imaging, which has no information of the direction of the recoil electron, we analyzed the same data by the classical method and our method.
The left of Figure 6.16 is obtained by the classical method, and the right ﬁgure is obtained byour method. From these images, it is clearly seen that the image obtained by our method hasless background than that obtained by classical method, which indicates the usefulness of theelectron tracking.

0.8

20

0.7

10

0.6

0

50

1

40

0.9

30

0.8

20

0.7

10

0.6

0

0.5

-10

0.4

-20

0.3

-30

0.2

0.2

-40

0.1

0.1

-50
-50 -40 -30 -20 -10

0.5
-10
0.4

-20

0.3

-30
-40
-50
-50 -40 -30 -20 -10

0

10

20

30

40 50
X [cm]

0

10

20

30

40 50
X [cm]

0

Figure 6.16: The comparison of the image by the classical Compton imaging (left) and theimage by the electron tracking Compton imaging (right). The gamma-ray source is 137 Cs at(12cm, 0cm).

6.3

Performance of Prototype Camera

Figure 6.17 shows the energy spectrum under each event selection, and Figure 6.18 shows theenergy resolutions of the µ-TPC, the GSO-PSA, and the combined one, where the energy resolution of our Compton camera was found to be limited by the that of GSO-PSA.
Figure 6.19 shows the resolution of the ARM ∆ϕARM , and Figure 6.20 shows that of the SPD∆νSPD . We obtained the resolution of the ARM by ﬁtting with a lorentzian, and obtained theresolution of the SPD by ﬁtting with the combination of a gaussian and constant component.
Figure 6.21 shows the energy dependence of the resolution of ARM and SPD. The resolution of
the ARM becomes better as the energy of the incident gamma-ray increases, because it depends
75

FWHM [%]

count

Chapter 6. Prototype Camera

102

104

raw
3

10

GSO

fully contained e

MeV camera
µ-TPC

102

10
10

α cut

1
0

200

400

600

800

1000

1200

1400
E0 [keV]

1
3

102

10

10
E [keV]

Figure 6.17: The spectra of the sum energy ofµ-TPC and GSO-PSA under each event selecFigure 6.18: The energy resolution of the µtion (137 Cs).
TPC, the GSO-PSA and the Compton camera.

90

300

80
250
70
60

200

50
150
40
30

100

20
50
10
0

-150

-100

-50

0

50

0

100
150
∆ φARM [degree]

-150

-100

-50

0

50

100
150
∆ ν SPD [degree]

Figure 6.19: The histogram of the uncertainty Figure 6.20: The histogram of the uncertaintyof ARM (133 Ba, 313 - 399 keV). The solid line of SPD (133 Ba, 313 - 399 keV). The solid line
is obtained by ﬁtting with a Lorentzian.
is obtained by ﬁtting with the combination ofa gaussian and constant.

on the energy resolution of the absorber and the tracker. On the other hand, FWHM of the
SPD is nearly constant. This reason is that the detectable energy range of the recoil electron
has little change with the energy of the incident gamma-rays, because that range is limited by
the drift volume.
The detection eﬃciency of on-axis source is shown in Figure 6.22. The prototype camera has
the maximum eﬃciency of 3 × 10−5 at about 350 keV, and has the energy range from 100 keV
to 850 keV. Figure 6.23 shows the dependence of the eﬀective area on the zenith angle. Thisﬁgure says that the FWHM of the eﬀective area is ∼ 70◦ , therefore the FOV of the prototype isabout 1 str.

76

FWHM [degree]

Performance of Prototype Camera

SPD

102

MEGA’s SPD

ARM

MEGA’s ARM

10Doppler limit+ Energy res.
Doppler limit

1
102

3

104
E [keV]

10

Figure 6.21: The energy dependence of the resolution of ARM and SPD. For the comparison,
the resolutions of MEGA detector in the electron tracking mode are also plotted. The circles
and the boxes are the FWHM of the ARM and the SPD of our camera respectively, and thetriangles are the FWHM of the ARM of MEGA in the electron tracking mode. The cross is thetwo times of the HWHM of the SPD of MEGA. The dashed-line is the intrinsic limit of ARM
by Doppler broadening in an Ar gas. Also, the solid line is the calculated ARM resolution withconsidering of Doppler broadening and the energy resolution of µ-TPC and GSO.

effective area [cm2]efficiency

1

10-1

10-2
probability of Compton scattering

356 keV

-3

10

-3

10

10-4
observed value-5

10662 keV

-6

10

10-7

102

10-40

3

10
E [keV]

5

10

15

20

25

30

35 40 45 50
Angle [degree]

Figure 6.22: The dependence of the detection Figure 6.23: The dependence of the eﬀective
eﬃciency on the energy of the incident gamma- are on the zenith angle
rays. The solid line is the Compton scattering
probability in an Ar gas of 8 cm depth.

77Chapter 7

Flight Model Detector
7.1

Design of Flight Model

For the MeV gamma-ray observation in astronomy, we must launch this detector in space. Then,for the future observation in spacecraft loading, we have a plan of the balloon experiments, SubMeV gamma-ray Imaging Loaded-on-balloon Experiment (SMILE). The SMILE plan consists of
3 steps. At the ﬁrst step, we conﬁrm the gamma-ray detection using this camera for observing
diﬀuse cosmic gamma-rays and secondary ones generated by the collision between high energy
cosmic-rays and nuclei in the air. The second step will be the observation of bright sources, like
Crab or Cyg X-1, in the short time of about 6 hours using a larger detector (∼ 30 × 30 × 30
cm3 ). The ﬁnal step will be the observation of several sources in the long time more than a few
days.
This work is the ﬁrst step of the SMILE project, and concretely we aimed the following
items;
1. Operation of µ-TPC & Gamma-Ray Detection at High Altitude: The µ-TPC is our original
detector and has no ﬂight experience. At the very high altitude, the cosmic-rays including
the heavy ion come and deposit large amount of their energy in the detector. Therefore
we must study the performance of µ-PIC and GEM for such charged particles. Although
we have already measured the gamma-ray detection on the ground using the prototype
camera. We have to check the performance of our camera for detecting celestial gammarays in such condition.
2. Observation of Diﬀuse Cosmic Gamma-Rays and Atmospheric Gamma-Rays: For the future observation, it is necessary to know how much background gamma-rays from theair aﬀects. As the background gamma-rays, there are atmospheric gamma-rays and the
gamma-rays produced by the interaction of cosmic-ray with the gondola. Then, by measuring the variation of gamma-ray ﬂux depending on the altitude, we aim to measure both
diﬀuse cosmic gamma-rays and atmospheric gamma-rays separately.
We construct a ﬂight model detector to match with these purposes.

7.1.1

Cosmic-Ray Fluxes at Balloon Altitude

Figure 7.1 shows the spectra of diﬀuse cosmic X/gamma-rays reported by various balloon experiments [46], where the ﬁtting for those data is described as
(

dNγ
dE

)

= 30 × E −2.0±0.2

photons cm−2 sec−1 str−1 keV−1 .

cosmic79(7.1)

Chapter 7. Flight Model Detector
When cosmic gamma-rays pass through the air, a part of them are attenuated by the atmosphericdepth or scattered in the air. Therefore, the ﬂux of diﬀuse cosmic gamma-ray is changed as:
(
)
(
)
dNγ ′
dNγ
= (1 + p(z, E)) exp (−z/τ ),(7.2)
dE cosmic
dE cosmic
where z and τ is the atmospheric depth and the mean free pass of the total attenuation in theair, respectively. p(z, E) is the ratio of the scattered component to the gamma-rays reaching
the detector without any interactions. For example, p(100keV) ∼ 1 and p(1MeV) ∼ 0.25 atz ∼ 10g/cm2 [46, 47]. Also this ratio has the dependence of p(z, E) ∝ log(1 + z/τabs ) on theatmospheric depth [48], where τabs is the mean free pass of the absorption in the air.
On the other hand, the spectra of atmospheric gamma-rays, secondary proton, secondary
electron and secondary positron in the air shower depend on the geomagnetic latitude [49]. At
the Sanriku Balloon Center (geomagnetic latitude: 30.4◦ ) and z = 3.8g/cm2 , those spectra is
described as
)
(
)−1.7
(
)−2.5
(
−1.5
E
dNγ
E
E
5
= 250
+ 1.14 × 10
e−( 120MeV ) ,(7.3)
dE atmos
MeV
MeV
(
)
(
)−1.18dNp
E= 0.1
100MeV ≤ E ≤ 400GeV,(7.4)
dE atmos
100MeV
(
)
(
)−2.7
dNe−
E
= 0.3
100MeV ≤ E,
(7.5)
dE atmos
100MeV
(
)
(
)
dNe+
dNe−
= 1.66,(7.6)
dE atmosdE atmosin unit of photons sec−1 m−2 str−1 MeV−1 , respectively. Below 100 MeV, the spectra of protonand electron/positron were assumed by the extrapolation from the ﬂux in the higher energy
region as
)
(
)−1
(
dNp
E
= 0.1
,
(7.7)
dE atmos
100MeV
)
(
)−1
(
E
dNe−
= 0.3
.
(7.8)
dE atmos
100MeV
These spectra are shown in Figure 7.2. In the following simulation, we use these spectra.

7.1.2

Design of Flight Model for First Flight

Our advanced Compton camera, however, has a low detection eﬃciency because of the gaseousCompton target. In order to increase the eﬃciency, we used a Xe TPC gas in the ﬂight model
tracker instead of Ar, which has a higher cross section of Compton scattering. Another improvement is the increase of the drift volume from 10 × 10 × 8 cm3 to 10 × 10 × 14 cm3 , which increases
twice the detection eﬃciency. However, the increase of the drift length needs the increase of thearea of GSO units. Finally, we determined the design of MeV gamma-ray camera in Figure 7.3.

7.1.3

Particle Incoming to Detector

Since it is very diﬃcult to operate the detector and the electronics in a vacuum, a pressure
vessel to seal them in is necessary. Then we construct an Al vessel with the diameter of 1 m,
the height of 1.4 m and the thickness of 3 mm, which has an attenuation factor of 13 % for 100
keV and 5 % for 1 MeV. In order to estimate how much particle will come into the detector by80

Design of Flight Model5

count sec-1 m-2 str-1 MeV-1

10

104
3

10

γ

102
10

e+

1

10-1

p

10-2
-3

10

e-

10-4
-5

10

-6

10

10-7-1 -0.5

0

0.5

1

1.5

2

2.5

3 3.5 4Log (E/MeV)

Figure 7.1: The diﬀuse cosmic X/gamma-ray Figure 7.2: The spectrum of atmosphericcharged particles and atmospheric gamma-rays
spectra reported by various experiment [46]Figure 7.3: The schematic view of ﬂight model detector

simulation, the source particle were generated from the zenith direction with the spectra shownin Figure 7.2. As the sensitive area, we assumed a 30 cm cube at upper half of the inside of thevessel, and count the particles coming into this sensitive cube as shown in Figure 7.4.
Then the obtained spectra for the various particles are shown in Figure 7.5. From Figure 7.5,
the dominant component is found gamma-rays. But, because our detector has a low eﬃciency
for gamma-rays, the detected rate for gamma-rays must be ∼ 1 Hz. On the other hand, since theeﬃciency for the charged particles is nearly 1, the trigger rate of the charged particles is expected
to be a few kHz. Therefore, if we use a simple trigger system, almost all triggered events would
be occupied by charged particles, and the live time would be dramatically decreased. For thisreason, we must adopt a sophisticated trigger system to reject charged particles.
Since a minimum ionizing particle typically deposits an energy of 890 keV/mm in the GSO
scintillator, the energy deposit in our GSO unit by charged particles reaches 12 MeV. On theother hand, the ﬂight model detector is designed to detect the gamma-rays below 1 MeV.
Therefore, with the upper threshold of about 1 MeV, the charged particles would be easily
separated by the gamma-rays detected in GSO, because the quenching factor of the general
scintillator, which is the light yield of photon to proton, is about 0.1. Then, we adopt two
thresholds for GSO units: the lower one is for the trigger generation, and another one is for theveto of charged particles. Also we placed a thin plastic scintillator over the tracker as a veto for81count / (sec MeV)

Chapter 7. Flight Model Detector

5

10

104
3

10

e−

γ

102
10

e+

1

p

10-1

n
π+

10-2
-3

10

π−

-4

10

-5

10 -1 -0.5

0

0.5

1

1.5

2

2.5 3 3.5 4
Log (E/[MeV])

Figure 7.5: The spectra of incoming particles.
Figure 7.4: The schematic view of the pressure The dashed lines are count rates of source ﬂux
vessel. we deal the 30 cm cube in this vessel as into 30 × 30 cm2 .
a sensitive area.

downward charged particles.

7.1.4

Expectation of Gamma-Ray Detection

To simulate the performance of our camera, we constructed an MeV gamma-ray camera witha TPC with a Xe gas and GSO units in the simulation, of which properties are summarizedin Table 7.4 and Table 4.1. In the GEANT4 simulation, we considered Doppler broadening,
multiple scattering in the TPC gas, the alignment, the position encoding of µ-TPC and theenergy resolution of both µ-TPC and GSO. However, we did not consider the diﬀusion of theelectron cloud and the response of preampliﬁer because of the reasonable calculation time.
Figures 7.6 and 7.7 show the dependences of the detection eﬃciency on the incident gammaray energy and the dependence of the eﬀective area on the incident angle, respectively. As shownin Figure 7.6, a detection eﬃciency of the ﬂight model is estimated ∼ 10−4 , and the energy range
from 100 keV to 1 MeV is expected to be measured. Moreover, from Figure 7.7, the eﬀective
area is decreased a half at the zenith angle of ∼ 60◦ , and the FOV of this detector is estimatedabout 3 str.
Using above results, the expected photon ﬂux of the cosmic gamma-ray is presented in Table
7.1, and about 100 photons would be detected during 3 hours observation at the altitude of 35
km. On the other hand, as the altitude decreases, the less number of cosmic diﬀuse gamma-rays
would come and the more number of atmospheric gamma-ray would come into the detector.
Table 7.2 summarized the expectation of the photon ﬂux at various altitude during 3 hours
observation. Consequently, we note that the total detected photon ﬂux would not be changed
so much, but only the ratio of cosmic and atmospheric gamma-ray would be changed.

7.2

Data Acquisition of Flight Model Detector

Figure 7.8 is the block diagram of the data acquisition system of the SMILE. This system isbased on the system of the prototype camera, and the veto system for the charged particles wasadded. In addition, this system has a GPS module, a scaler for the status monitoring, and alsoa discriminator for the remote control of the threshold. The CPU of the system is a VME bus
82

Data Acquisition of Flight Model Detector

102

effective area [cm2]

Efficiency

1

10-1

10-2
-3

Compton scatteringin TPC gas

1Comp. in TPC + Abs. in GSO

Comp. in TPC+ Abs. in GSO

10-1

10

10-4

10

Comp. + Abs.
e stops in TPC

10-2

after analysis

after analysis

10-5
10-6 2
10

10-3

3

10

10-40

104
Energy [keV]

20

40

60

80

100 120 140 160 180
Incident Angle [degree]

Figure 7.6: The simulated detection eﬃciency Figure 7.7: The simulated eﬀective area as a
as a function of the incident gamma-ray energy. function of the incident angle. The incident
The incident angle is 0◦
gamma-ray energy is 300 keV.

Table 7.1: The expectation of the detected photon number of cosmic gamma-ray for 3 hourslevel ﬂight at the altitude of 35 km
EnergyFluxp†Eﬃciency
Vessel attenuate Number
2[keV][γ / (cm sec str)]
100 - 200
1.50 × 10−1
0.686 2.11 × 10−4
0.894
57.1
−2
200 - 400
7.50 × 10
0.560 2.50 × 10−4
0.919
37.9
−2
−4
400 - 600
2.50 × 10
0.424 1.70 × 10
0.930
8.46
600 - 750
1.00 × 10−20.338 6.13 × 10−5
0.939
1.26
750 - 900
6.67 × 10−30.264 3.85 × 10−5
0.946
.532
†: p is the ratio of the scattered component to the gamma-rays reaching the detector without any interactions.

Table 7.2: The expectation of detected photon number at various altitude during 3 hours
Altitude [km] Pressure [g/cm2 ] Cosmic Atmospheric by charged particle All
30
12.2
69.5
185
21.0
275
32
9.07
86.5
137
15.6
239
33
7.83
93.7
119
13.5
226
34
6.77
99.9
103
11.6
214
35
5.86
105
88.7
10.1
204
37
4.42
113
66.9
7.60
188

83

Chapter 7. Flight Model Detector

Figure 7.8: The diagram of SMILE data acquisition systemboard computer on Linux, and controls all system. Both monitoring data and the event dataof Nhit > 3 are acquired via VME bus, and are saved in the IDE HDDs. The system has two
HDDs, and saves the same data, respectively for the redundancy.
We designed two operation modes in the DAQ system. One is the Compton Camera Modefor the gamma-ray detection. The other is the Charged Particle Tracking Mode for checking
the detector performance. We change two modes by the remote operation via the CPU. The
working status of the data acquisition program is monitored by another program. As the failsafe, if the acquisition program would break down at fault, the script starts up the acquisition
program again automatically.

7.2.1

Compton Camera Mode

The Compton camera mode is the main data acquisition mode for the SMILE system. The
detection logic is the same as that of the prototype system basically. This mode consists ofthree diﬀerent type triggers. One is a GSO unit trigger for the gamma-ray detection. Second isa veto trigger for the rejection of charged particles. The last is a GPS trigger. For the generationof triggers, the position encoder waits during the 100 nsec after the ﬁrst trigger incoming due
to the time lag between the lower threshold hit and the upper threshold hit of the GSO unit.
GSO Unit Trigger
When the pulse height of the GSO signal is between the lower and upper thresholds, the discriminator generates a GSO unit trigger. The position encoder receives the GSO unit trigger,
and then the encoder starts the Compton camera logic, which is similar to the prototype procedure. Figures 7.9 and 7.10 are the time charts of this mode in case of TPC signal incoming and84

Data Acquisition of Flight Model Detector

Figure 7.9: The timing chart with a GSO unit Figure 7.10: The timing chart with a GSO unit
trigger and TPC signals
trigger and no TPC signal

Figure 7.11: The timing chart with a veto trig- Figure 7.12: The timing chart in the charged
particles tracking modeger

no coming, respectively. Because the maximum of the drift length of the ﬂight model TPC islonger than that of the prototype TPC, and the drift velocity in Xe gas is slower than Ar gas,
the waiting time for TPC signals is 8 µsec.
Veto Trigger
When the pulse height of the GSO signal is larger than the upper threshold or when the signal
comes from the anti-coincidence counter, the DAQ system generates the veto for the trigger, as
shown in Figure 7.11.
If a veto trigger was generated, charged particles would pass through the µ-TPC, and would
deposit a large energy in the GSO scintillator unit. Therefore, the system must wait for the
maximum drift time (∼ 8 µsec) or the recovering time of the GSO signal undershoot (∼ 30µsec). Thus, the position encoder must wait during 100 µsec after a veto trigger. Since the
DAQ system requires several msec for the acquisition data, the veto system is very useful todecrease the dead time.
GPS Trigger
Our system has a GPS module to measure the balloon position and the current time. The GPS
module also generates a pulse per 1 sec. We use this pulse as a trigger, in order to test thesystem condition. By this GPS trigger, the position encoder works as like the GSO unit trigger
85

Chapter 7. Flight Model Detector
and TPC signal incoming (Figure 7.12). In addition, we measure both the pedestal of the peakhold ADC and the noise level of µ-TPC in the ﬂight.

7.2.2

Charged Particle Tracking Mode

The Charged Particle Tracking Mode is used for the check of the µ-TPC’s performance at thehigh altitude. Figure 7.12 shows the timing chart in this mode. When the position encoder
receives any two triggers of GSO unit or plastic scintillator during the 100 nsec, the procedure
starts. This mode does not acquire the GSO unit data, since the purpose of this mode is thetest of the µ-TPC and the GSO data are not required. We can know the triggered scintillator
by the ‘Trigger hit ID’.

7.2.3

Dead Time Measurement & Condition Monitoring System

To monitor the dead time, the position encoder generates ‘10 MHz clock’ and takes ‘the coincidence of VETO and 10 MHz clock’. We count the number of the coincidence by the scaler inthis system, and obtain the dead time from the ratio of these counts. Also the scaler counts thehit number of each scintillator without VETO, for the check of the scintillator performance.
The system also has a DC-type ADC system for the measurements of the balloon systemstatus. This ADC measures the voltage of the power supplies, the current of µ-TPC HV, thegeomagnetic ﬁeld, the tilt angles of the vessel, the pressure, and the temperatures of several
points inside or outside the vessel. For the measurement of the position of the balloon and thecurrent time, the CPU often contact to the GPS module via a serial port.
Then the CPU saves the data of the scaler, the DC-type ADC and the GPS module into two
HDDs.

7.3

Flight Model µ-TPC

As a ﬂight model TPC, we developed a TPC with the drift length of 15 cm, as shown in Figure
7.13. Since the gas volume of the ﬂight model TPC is twice bigger than that of the prototype
due to the longer drift region, the Compton probability becomes twice higher than that of the
prototype even with the same gas. The gas vessel is made of a G10 frame and an Al window
with the thickness of 2 mm, and µ-PIC is put on an Al ﬂange with the thickness of 16.7 mm. A
GEM is placed at the distance of 4 mm above the µ-PIC. This vessel is ﬁlled with a gas and issealed by O-ring.

Figure 7.13: The photograph of 15 cm drift cage (left) and the schematic view of the gas vessel(right)86

Flight Model µ-TPC

Drift velocity [cm/ µsec]

5
4.5

Table 7.3: The gas properties of the ﬂight
4model µ-TPC
3.5
Ar 90% + C H 10%
3
Parametervalue
2.5
component
Xe + Ar + C2 H6
2mass ratio
80 : 18 : 2
Xe 80% Ar 18% + + C H 2%
1.5
pressure ratio
54 : 40 : 6
1
0.5
density
3.97 g/cm3
00
200 400 600 800 1000 1200 1400 1600 1800 2000
Radiation length
5.4 × 103 cm
E field [V/cm]
dE/dX of MIPs
5.27 keV/cm
average of Z
37.5
Figure 7.14: The electron drift velocity of 2
w value
24 eV
diﬀerent gas
2 6

σlong [µm/ cm]

σtrans [µm/ cm]

2 6

3

3

10

10

Xe 80% Ar 18% + + C2H6 2%

Xe 80% Ar 18% + + C2H6 2%

Ar 90% + C2H6 10%

Ar 90% + C2H6 10%

102

102
0

200

400

600

800

1000 1200 1400 1600 1800 2000
E field [V/cm]

0

200

400

600

800

1000 1200 1400 1600 1800 2000
E field [V/cm]

Figure 7.15: The transverse diﬀusion of 2 dif- Figure 7.16: The longitudinal diﬀusion of 2 different gas
ferent gas

7.3.1

Properties of TPC Gas

In order to measure the diﬀuse cosmic gamma-rays, we must increase the detection eﬃciencyof the ﬂight model Compton camera to ten times of that of the prototype camera. Therefore,
as a TPC gas, we used a Xe gas. Since Xe gas gives us a lower gas gain than Ar gas withthe same voltage, a higher voltage is required for enough gain to detect MIPs tracking, and thestable operation becomes diﬃcult. Then we mixed Xe with Ar in order to decrease the operation
voltage. Also ethane gas is added as the quenching gas.
The theoretical properties of the TPC gas are listed up in Table 7.3. Expected dE/dX for
MIPs in this gas is about 5.3 keV/cm, which is twice of that of Ar gas. Also the average of Z
is twice of Ar. Therefore, the Compton scattering probability is about twice higher than thatof Ar gas, and the ﬂight model TPC is expected to detect the higher energy electrons than the
prototype TPC. Figure 7.14 shows the electron drift velocity, and Figures 7.15 and 7.16 show
the transverse and longitudinal diﬀusion in the TPC gas and Ar gas, respectively. With the
TPC gas, the electron drift velocity is a half of the drift velocity in Ar gas, and the diﬀusion isnearly equal to that in Ar. Therefore the longitudinal position resolution of this gas is better
than Ar gas.

7.3.2

Gain Uniformity

We operated the ﬂight model µ-TPC with the anode voltage of 550 V for µ-PIC and the differential voltage of 400 V for the GEM. The obtained gain uniformity is shown in Figure 7.17.
87

Chapter 7. Flight Model Detector
Gain Map

45000
40000
35000
30000
25000
20000
15000
10000
5000
0
250
Ca 200
tho 150
de
100

3

10

50
0 0

50

100

150

250
200
de
Ano

102

0

100

200

300

400

500

600

700
Clock

Figure 7.17: The gain uniformity of the ﬂight Figure 7.18: The histogram of the clock countermodel µ-TPC
FWHM of resolution [%]

102

800
700

AgKα

600
500
400
300
200
100

10

0
0

10

20

30

40

50
60
Energy [keV]

10102
Energy [keV]

Figure 7.19: The spectrum of 109 Cd by the Figure 7.20: The energy resolution of the ﬂight
model TPC
ﬂight model TPC

The average of gas gain is about 3.5 × 104 , and the RMS of the gain uniformity is about 12 %.
In comparison with the prototype TPC, the ﬂight model TPC has a higher gas gain, but theuniformity is worse. This makes the energy resolution worse. However, the tracking eﬃciency
is expected to be not aﬀected because of the enough high gain.

7.3.3

Clock Counter Histogram

Figure 7.18 shows the distribution of the drift time obtained by the ﬂight model TPC, where thedrift region corresponds to the clock of 10 - 600. The length of the drift region is about 14 cm,
therefore the electron drift velocity is about 2.37 cm/µsec. Because the electric ﬁeld in the drift
region is 380 V/cm, the theoretical drift velocity is 2.5 cm/µsec from Figure 7.14. Therefore,
the measured velocity is well consistent to the Figure 7.14.

7.3.4

Energy Resolution

Figure 7.19 is an obtained energy spectrum of 109 Cd, and Figure 7.20 shows the energy resolution.
In Figure 7.20, the dashed line is obtained by ﬁtting, and is described as follows,
¯
(
)
δE ¯¯
E −0.22[%] = 9.7 × 10.
E ¯FWHM
keV

(7.9)
88

Drift [cm]Flight Model µ-TPC

14
12
10
8
6
4
2

0
10
Ca 9
th 8
od 7
e 6
[c 5
m 4
] 3

9 10
7 8 [cm]
6
5
de
2
Ano
3 4
1
2
0 0 1

σ [mm]

Figure 7.21: The 5 tracks by the ﬂight model TPC

350

2
1.8
1.6

300

1.4

250

1.2
200

1

150

0.8

100

0.6

50

0.4
0.2

0

0.5

1

1.5

2

2.5

3

3.5

4
4.5
5
Residual [mm]

00

2

4

6

8

10

12
14
Drift [cm]Figure 7.22: The histogram of residuals at the
Figure 7.23: The position resolution depending
drift length of 6 - 7 cm
on the drift length

In comparison with the prototype TPC, the energy resolution of the ﬂight model is worse, which
may be due to the non-uniformity of the gain. Another reason is that the TPC gas of the ﬂight
model is sealed, while the gas of the prototype TPC ﬂows. Therefore, the gaseous purity of theﬂight model is worse than the purity of the prototype TPC, by the outer gas. Because the outergas captures the seed electrons and the gain uniformity is not good, the ﬂight model has theworse energy resolution than the prototype.

7.3.5

Position Resolution

With the charged particle tracking mode, we observed the cosmic muons. In Figure 7.21, several
tracks of cosmic muons obtained by the ground experiment using the ﬂight model TPC aredepicted.
For the measurement of the position resolution of tracking, we obtained the residual between
the hit points and track obtained by ﬁtting. Figure 7.22 is a histogram of the residual at thedrift length of 6 - 7 cm. In this ﬁgure, the solid line is obtained by ﬁtting with a 2 dimensional
89

Chapter 7. Flight Model Detector

Table 7.4: The speciﬁcation of ﬂight model tracker
Gas
Xe + Ar + C2 H6 , sealed 1 atm
Volume10 × 10 × 14.5 cm3
Fiducial Volume
9 × 9 × 14 cm3
Gas Gain∼ 35000
Drift Velocity
2.4 cm/µsec
Encoding Clock
100 MHz√√
Position Resolution σFM [mm] = (4.2 × 10−2 )2 + (3.4 × 10−1 z)2Energy Range≤ 150 keV
( K )−0.30
eEnergy Resolution
FWHM[%] = 5.4 × 10 keV

Gaussian in circular polar coordinates (5.1). Also Figure 7.23 shows the position resolution
depending on the drift length. The solid line in this ﬁgure is√√σFM [mm] = (4.2 × 10−2 )2 + (3.4 × 10−1 z)2 .
(7.10)

7.3.6

Summary of Flight Model µ-TPC

I summarize the performance of the ﬂight model µ-TPC in Table 7.4 based on the ground test
results. In comparison with the prototype TPC, the ﬂight model µ-TPC has a larger volume
and a larger Compton scattering probability, but the energy resolution is worse than that of the
prototype. However, since the ﬁrst priority of the ﬂight model developing is the improvementof the detection eﬃciency, this TPC has the enough performance.

7.4

Performance of Flight Model Detector

For the ﬂight model of the MeV gamma-ray Compton camera, we constructed a bottom absorber(Figure 7.24) and 4 side absorbers (Figure 7.25). The performance of each GSO unit is similar
to that of the prototype. With these GSO arrays and the ﬂight model µ-TPC (Table 7.4), weconstructed the ﬂight model MeV gamma-ray Compton camera (Figure 7.26). In this section,
the performance of the ﬂight model detector is investigated.
Figure 7.27 shows a 3D image of a typical Compton scattering event obtained by the ﬂight
model detector. In order to check the performance of the ﬂight model Compton camera, wemeasure gamma-rays using the radioactive isotopes, and also measure the background gammarays using no source with Compton camera mode. The spectra of 137 Cs and the backgroundgamma-rays are shown in Figures 7.28 and 7.29, respectively. In these spectra, the peak of
662 keV from 137 Cs is clearly seen, and the spectrum of the background gamma-rays has no
characteristic peak. On the other hand, When the position of the gamma-ray point source(137 Cs) changed, we obtained the images as shown in Figure 7.30. The source position arereconstructed correctly in these images.
The energy dependence of the energy resolution is shown in Figure 7.31, where the solidline, the dotted-line and the dashed-line are the energy resolution of the ﬂight model camera,
the GSO units and the µ-TPC, respectively. In the comparison with the prototype, the energy
resolution of the ﬂight model Compton camera is worse than that of the prototype camera, as
mentioned previously. However, it seems that the ﬂight model has an enough energy resolution,
since diﬀuse cosmic and atmospheric gamma-rays have continuum spectrum.
Figure 7.32 shows the dependence of the angular resolution of ARM and SPD on the energyof the incident gamma-rays. Those looks similar to the energy dependence of the prototype’s
ARM and SPD. However, in comparison with the prototype, the ﬂight model has the worse
90Performance of Flight Model Detector

Figure 7.24: The ﬂight model of bottom ab- Figure 7.25: The ﬂight model of side absorbersorberFigure 7.26: The photograph of the ﬂight model detector and the Al vessel

91

Chapter 7. Flight Model Detector

XY

20050818/per7 track_04-4908: source position: reconst. point: absorbed point: Compton point

E0: 639.7 keV
Eγ : 514.7 keV
Ke: 125.0 keV°

αkin: 89.9°
αgeo: 84.3°
φ: 36.3°
ψ : 53.6

15

Y [cm]

source: 137Cs 662 keV

10

5

0

-5

20

-15
-15

-10

-5

0

5

10

-10

-5

0

5

10
15
Y [cm]

15

15
X [cm]XZ

10-Z [cm]Y [cm]

-10

5
0

20

15

-5
10

-10
-15

5

-20
30
-X
[cm20
10
]

0

0
-10
-20
-30 -70

-60

-50

-40

-30

-20

0

-10

Z

10
[cm]

-5

-10
-15

×10

×10

-3

count/keV/sec

count/keV/sec

Figure 7.27: The typical gamma-ray event obtained by the ﬂight model camera

0.7
0.6
0.5

-6

90
80
70
60

0.4

50

0.3

40

without Al vessel

30
0.2
20
0.1
00

with Al vessel

10
200

400

600

800

00

1000
E0 [keV]Figure 7.28: The obtained spectrum of 137 Cs

200

400

600

800

1000
E0 [keV]Figure 7.29: The obtained background spectrum92

Y [cm]Y [cm]

Performance of Flight Model Detector

40

40

20

20

0

0

-20

-20

-40

-40

-40

-20

0

20

40
X [cm]

-40

-20

0

20

40
X [cm]

Figure 7.30: The obtained images by irradiation of gamma-rays from 137 Cs, which was put atthe distance of ∼ 50 cm from the camera window. The source position are (0cm, 0cm) and(10cm, 0cm), respectively. The energy range is 600 - 724 keV.

µ-TPC

FWHM [degree]

FWHM [%]102

SPD (measured)102

MeV camera

SPD (GEANT4)GSO

10Doppler limit+ Energy res.

ARM (measured)

10
ARM (GEANT4)
Doppler limit

1
10

102

1

3

10
E0 [keV]Figure 7.31: The energy resolution of theﬂight model camera. The dotted-line and thedashed-line are the energy resolution of the
GSO units and the ﬂight model µ-TPC, respectively. Also the solid line is that of the ﬂight
model camera expected by a calculation.

102

3

10
E0 [keV]Figure 7.32: The dependence of the ARM andthe SPD on the energy of the incident gammarays. The circles and the boxes are the FWHM
of the ARM and the SPD, respectively. The
data of a GEANT4 simulation does not include
the electron diﬀusion in the TPC and the response of the preampliﬁer.

93

Chapter 7. Flight Model Detector

effective area [cm2]efficiency

1

10-1
Compton scattering probability-2

10

for 300 keV (simulation)

10-2
for 662 keV (simulation)-3

10

obtained value
simulation (parallel)

for 662 keV

µ
¡

-4

10

simulation (point src)
prototype

-5

10

-3

10
-6

10

102

0

3

10
E0 [keV]Figure 7.33: The dependence of the detection
eﬃciency on the energy of the incident gammarays. The solid line and dashed line are the obtained eﬃciency and the simulated eﬃciency of
the ﬂight model, respectively. Also the box isthe obtained eﬃciency of the prototype.

10

20

30

40

50

60

70 80 90
Angle [degree]

Figure 7.34: The dependence of the eﬀective
area on the zenith angle. The energy range is
600 - 724 keV. The dashed line is the simulated eﬀective area for the gamma-rays of 300
keV and 662 keV.

angular resolution of both the ARM and the SPD. Since the purpose of this SMILE is not themeasurement of the celestial point sources, we consider that the ﬂight model has an enoughangular resolution.
The detection eﬃciency of the on-axis source is shown in Figure 7.33, where the circles andthe triangles are the experimental value and the simulation one, respectively. The detection
eﬃciency is ∼ 10−4 , which is roughly consistent with the simulated one. Also it was increasedby 10 times than that of the prototype due to the improvements of µ-TPC. On the other hand,
Figure 7.34 shows the variation of the eﬀective area depending on the zenith angle. This ﬁguresays the eﬀective area is decreased to a half at the zenith angle of ∼ 60◦ , thus the FOV of theﬂight model is about 3 str. This value is roughly consistent with that of the detector design.
Summarizing the performance of the ﬂight model, it has a larger eﬀective area and a larger
FOV, but the angular resolution and the energy resolution are worse than those of the prototype.
Because the increase of both the sensitivity and the FOV are most signiﬁcant improvement inthis SMILE, we consider this ﬂight model has an enough performance.

94Chapter 8

Balloon Flight
8.1

First Flight of SMILE

8.1.1

Speciﬁcation of Physical Instrument

We put our detector in a 1 mϕ × 1.4 m Al vessel, and the vessel was ﬁxed at the Al gondolawith the size of 1.20 × 1.45 × 1.55 m3 , as shown in Figure 8.1. At both sides of the gondola, thebatteries and the ballast boxes were attached, and the gondola was packed with the expanded
polystyrene. The speciﬁcations of the ﬁrst SMILE balloon are listed in Table 8.1. The total
weight is 816 kg: the balloon 263 kg, the gondola 397 kg, the ballast 130 kg, and the otherapplications (parachute, cutter and radio-buoy) 26 kg. The total power consumption is about
350 W, where the power in the vessel is ∼ 250 W and the conversion loss is ∼ 100 W. The SMILE
gondola has two clinometers and two geomagnetic aspectmeters for the posture measurement of
the gondola. Also there is a ﬁne pressure gauge outside the gondola, a general pressure gauge
inseide the vessel and a diﬀerential pressure gauge inside the TPC vessel. Also 23 temperature
sensors are set inside and outside the vessel, where 12 sensors inside of the Al vessel and 11
sensors outside. To reach a ceiling altitude of 35 km, we used a balloon with the volume of 105
m3 (B100, Fujikura Kousou). By a B100, the pressure at the expected altitude is
850 ×

819[kg]= 7.0[hPa].
105 [m3 ](8.1)

Therefore, the expected altitude is about 34.1 km.

Table 8.1: The speciﬁcations of the 1st SMILE
balloonBalloonFujikura Kousou
B100 (105 m3 )Gondola Size 120 × 145 × 155 cm3Weight816 kgBalloon
263 kgP. I.
397 kgBallast130 kgPower∼ 350 W
Commanddouble tone 15 ch
Figure 8.1: The photograph of gondola before
TelemetryBi-phase 64 kbps
the packing

95

#MENU
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14

Telemetry Test
SettingModeReset
GSO discri.
Anti. discri.
Upper discri.
Anode th.
Cathode th.
Anode V
Drift V
Bottom HVSide 1 HVSide 2 HV
Anti. HV0
Monitor data
Clear MENU
Clear MENU
Clear MENU
Clear MENU
Clear MENU
Clear MENU
Clear MENU
Clear MENU
Clear MENU
Clear MENU
Clear MENU
Clear MENU
Clear MENU
Clear MENU

1
Event data
HV default
Compton
Encoder
50 mV
50 mV
1V
−35.0 mV
35.0 mV
550 V−6.2 kV
Default
Default
Default−1.05 kV

2
Scaler data
HV oﬀ
tracking
GPS
0 mV
0 mV
0 mV
0 mV
0 mV
0V
0V
0V
0V
0V
0V

#SELECT
3
4
GPS data
Discri. default Discri. max
Shutdown
fan onfan oﬀ
15 mV
30 mV
25 mV
100 mV
200 mV
400 mV−32.5 mV
−37.5 mV
32.5 mV
37.5 mV
540 V
545 V−5.6 kV−5.8 kVdef. −10 V
def. −30 V
def. −10 V
def. −30 V
def. −10 V
def. −30 V−0.90 kV
−0.95 kV
70 mV
400 mV
600 mV−40.0 mV
40.0 mV
555 V−6.0 kVdef. −50 V
def. −50 V
def. −50 V−1.00 kV

Discri. min

5

100 mV1V
2.5 V−50.0 mV
50.0 mV
560 V−6.4 kVdef. −100 V
def. −100 V
def. −100 V−1.10 kV

6

Table 8.2: The table of the commands for the SMILE data acquisition system. The blank column means ‘Clear SELECT’.

1V

7Chapter 8. Balloon Flight96

First Flight of SMILE

Figure 8.2: The left is photograph of ﬁnal style of gondola on a launcher. The SMILE balloon
was launched from Sanriku Balloon Center at 06:11 on 1 September, 2006 (center). And werecovered the detector at 14:32 (right).

For the remote control, we use 15 telecommands using a relay switch. Six commands arefor the balloon control: 2 for the cutter, 1 for the ballast control, 1 for the exhaust control
and 2 for the balloon control system. The other 9 commands are for the SMILE system: 2 forthe standby-on/oﬀ, 4 for the power-on/oﬀ and 3 for the control of the SMILE data acquisition
system. The 3 commands for the control of the SMILE data acquisition system were named
MENU, SELECT and SET. The CPU counts the received MENU/SELECT signal. When the
CPU receives SET, the SMILE system carries out the command of the MENU/SELECT number
in Table 8.2.
All of the acquired data are saved into two local hard disk drives. However, since the local
disks may not be recovered after the experiment, we must transfer a part of the data to theground station. For the data transfer, we used the pulse-code modulation telemetry in 64 kbps.
The SMILE system sends the 384 byte HK/GPS data at every 1 sec, and sends the 320 byte
scaler data at every 10 sec. Also the system sends the 384 byte event data during the remainder.
Therefore, the maximum rate of the event data transfer is ∼ 20 Hz.

8.1.2

Flight Path and Operation Mode

The ﬁrst SMILE balloon was launched from Sanriku Balloon Center (39.16N, 141.82E) on 1st
September, 2006 (JST). The event sequence in the ﬂight is listed up in Table 8.3. We turnedon the power of the system at 05:26, and began to operate the SMILE system in the Compton
camera mode (the left of Figure 8.2). The balloon was launched at 06:11 (the center of Figure8.2). At 08:56, the balloon reached to the altitude of 35 km, and started the level ﬂight. Wechanged the operation mode to the charged particle tracking mode at 12:06, and the mode
returned to the Compton camera mode at 12:34. Then we turned oﬀ the power of system at
12:59. Finally we cut the gondola from the balloon at 13:20. The gondola landed on the sea at
13:45, and we recovered the gondola at 14:32 on the sea (the right of Figure 8.2).
Figure 8.3 shows the path of the ﬁrst SMILE ﬂight. Also, the time variation of the ﬂight
altitude of the balloon is shown in Figure 8.4, and that of the pressure of the atmosphere in
the level ﬂight is shown in Figure 8.5. In the level ﬂight from 08:56 to 10:15, the altitude wasconstant 35.0 km and the atmosphere pressure is 5.4 hPa (5.5 g/cm2 ). From 11:20 to 13:00, thealtitude is 32 km and the pressure is 8.5 hPa (8.7 g/cm2 ).
97

Chapter 8. Balloon Flight

Table 8.3: The sequence of events in the ﬂight
Eventturn on (Compton camera)launchreach to the level ﬂight
tracking mode
Compton camera modeturn oﬀ
cut from the balloon
landing
recovering

Figure 8.3: The path of the SMILE ﬂight

40000

Pressure [hPa]height [m]Time in JST
05:26
06:11
08:56
12:06
12:34
12:59
13:20
13:45
14:32

35000
30000

102

25000
20000
10
15000
10000
5000
0

6

7

8

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

6

Figure 8.4: The altitude of the gondola

7

8

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

Figure 8.5: The atmosphere pressure

8.2

Status of Detector in Balloon Flight

8.2.1

House Keeping Data

Figure 8.6 shows the time variations of the pressure of the TPC gas and the pressure inside theAl vessel. This ﬁgure says that the air tightness of the Al vessel was kept during the ﬂight.
Also the TPC gas pressure was increased gradually, and the pressure increased at the end of
the experiment was 4.5 %. The increase of the TPC gas pressure was too slight to make littlechange of the gas gain of the µ-TPC.
On the other hand, Figure 8.7 shows the temperatures of the various points. From this ﬁgure,
we inferred that the increase of the TPC gas pressure might be caused by the gas temperature
increase. The hottest point in the Al vessel is the CPU, but the temperature of the CPU was
kept under 60 ◦ C. At the outside of the Al vessel, the hottest point was the battery supplying
the most intense current, which is Battery 1 in Figure 8.7. However, because the battery works
at the temperature of −20 - 90 ◦ C, its temperature was no problem in the ﬂight.

8.2.2

Count Rate of Scintillators

The count rates of all scintillators without the veto are shown in Figure 8.8. A, B and C of
GSO units are the bottom camera. C, D, E and F are the lower line of the side camera, and
H, J, K and L are the upper line of the side camera. These count rates decreased just after thelaunch of the balloon, because the radiation from the earth decreases as the altitude increases.
The count rate has a maximum rate (the Pfotzer Maximum) at about 08:00, where the altitude
was the point of the air shower maximum, about 18 km. In the level ﬂight, the count rate wasabout the half of the maximum rate, and looked constant.
98

Pressure [hPa]

Status of Detector in Balloon Flight

1200

1000

800

600

400

Pressure [hPa]200

TPC gas

1200

1000

800

600

400

200

0

Inside of Vessel
6

7

8

9

10

11

12
13
Time on 1 September 2006 (JST) [hour]

Figure 8.6: The pressure of TPC gas (upper) and the pressure of the inside of Al vessel (lower)
in the ﬂight°

temp. [ C]100
80
Battery 1

60

CPU

40
TPC vessel

20
Battery 2

0-20

Gondola Frame

-40
-60

6

7

8

9
10
11
12
13
Time on 1 Septmber 2006 (JST) [hour]

Figure 8.7: The temperatures at the various points

99

0

100

200

300

400

500

600

700

800

900

100
200
100
0

200

100

0
6

6

6

7

7

7

8

8

8

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

GSO J

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

GSO E

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

GSO B

0

100

200

300

400

500

600

700

800

900

1000

0

100

200

300

400

500

600

700

800

900

1000

0

100

200

300

400

500

600

700

800

900

1000

6

6

6

7

7

7

8

8

8

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

GSO K

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

GSO F

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

GSO C

0

100

200

300

400

500

600

700

800

900

1000

0

100

200

300

400

500

600

700

800

900

1000

0

100

200

300

400

500

600

700

800

900

1000

6

6

6

7

7

7

8

8

8

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

GSO L

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

GSO G

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

Anti. (plastic scinti.)

Figure 8.8: The count rate of all GSO units and the plastic scintillator. A, B and C of GSO unit are the bottom camera. D, E, F and G are thelower line of the side camera. H, J, K and L are the upper line.

300

300

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

400

400

8

500

500

7

600

600

6

700

700

900

1000

800

GSO H

0

100

200

800

900

1000

0

100

200

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

400

400
300

500

500

300

600

900

1000

0

700

GSO D

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

600

8

8

700

7

7

100

200

300

400

500

600

700

800

900

1000

800

6

6

GSO A

800

900

1000

[Hz]

[Hz]

[Hz]

[Hz]
[Hz]
[Hz]

[Hz]
[Hz]
[Hz]

[Hz]
[Hz]
[Hz]

1000

Chapter 8. Balloon FlightStatus of Detector in Balloon Flight

8.2.3

Current of µ-TPC

countsCurrent [µ A]

The time variation of the current of the µ-PIC anode is shown in Figure 8.9. From this ﬁgure,it is noted that there occurred several small discharges in the ﬂight. It means that the pixels of
the µ-PIC were not broken for those several discharges. Figure 8.10 shows the discharge rate per
30 minutes during the ﬂight. The discharge rate increased after the launch of the balloon. Afterthe Pfotzer Maximum, the discharge rate looked constant, although the count rate of the GSO
units at the level ﬂight decreased in a half of that at the Pfotzer Maximum. Thus we consider
these discharges were caused by the primary heavy cosmic-rays, such as a He or C ions.
1

10

0.8
8
0.6
6
0.4

4

0.2

0

2

6

7

8

0

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

6

7

8

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

Figure 8.9: The current of µ-PIC anode in the Figure 8.10: The discharge number of µ-PIC
ﬂight
per 30 minutes

8.2.4

Dead Time & Event Rate

Figure 8.11 shows the ratio of the dead time to the real time. The ratio was 5 % on the ground.
The dead time was increased to 30 % at the Pfotzer Maximum, and 20 % in the level ﬂight
with the Compton camera mode. During the charged particle tracking mode (12:06 - 12:34),
the dead time was about 30 %. Sometimes, the ratio was increased to ∼ 100 %. That wasdue to a system bug, which the position encoder loses the acquisition end signal, and then theautomatically fail safe reseted the acquisition cycle. In the calibration run on the ground, suchan error did not happen so often. We must ﬁx the bug for the future balloon experiments. The[Hz]

Dead Time [%]

100

104

GSO Trg.

90
103

80

Anti. Trg.

70

102

Interrupt

60
10

50

Nhit > 340
130

GPS Trg.

20

10-1

10
0

6

7

8

10-2

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

6

7

8

9
10
11
12
13
Time on 1 September 2006 (JST) [hour]

Figure 8.11: The dead time ratio to the real Figure 8.12: The time variation of the various
time
ﬂags

101

Chapter 8. Balloon Flight

XY

track_00-1523

15

Y [cm]

per1

Fri Sep 1 12:08:40 2006

10

5

0

-5

20

-15
-15

15

-10

-5

0

5

10

15
X [cm]

-10

-5

0

5

10
15
Y [cm]XZ-Z [cm]-Z [cm]

-10

10

20

15

5
10

0
5

-5
15
Y [ 10
cm
]

0

5
0
-5
-10
-15 -15

-10

-5

0

5

10
X

15
[cm]

-5

-10
-15

Figure 8.13: The single track obtained in the Charged particle tracking mode

XY

track_00-1203

15

Y [cm]

per1

Fri Sep 1 12:06:29 2006

10

5

0

-5

20

-15
-15

15

-10

-5

0

5

10

15
X [cm]

-10

-5

0

5

10
15
Y [cm]XZ-Z [cm]-Z [cm]

-10

10

20

15

5
10

0
5

-5
15
Y [ 10
cm
]

0

5
0
-5
-10
-15 -15

-10

-5

0

5

10

15
]
X [cm

-5

-10
-15

Figure 8.14: The multi track obtained in the Charged particle tracking mode

102

Results of Experiment & Discussion
rates of the several DAQ ﬂag are shown in Figure 8.12, where ‘GSO Trg.’ the rate of all GSO
trigger received by the position encoder, ‘Anti. Trg.’ the plastic scintillator trigger rate received
by the encoder, ‘Interrupt’ is the rate of the coincidence of the µ-TPC and GSO, ‘Nhit > 3’ is thedata acquisition rate by GSO trigger, and ‘GPS Trg.’ is the data acquisition rate by the GPS
trigger. At the Pfotzer Maximum, the coincidence rate was ∼ 60 Hz, and the data acquisition
rate (Nhit > 3) was ∼ 10 Hz. During the level ﬂight, the coincidence rate was ∼ 25 Hz, andthe data acquisition rate was ∼ 6 Hz. The GPS trigger rate had a negative correlation with thedead time, and it was ∼ 0.7 Hz on the ground, ∼ 0.3 Hz at the Pfotzer Maximum and ∼ 0.5 Hz
during the level ﬂight.

8.3

Results of Experiment & Discussion

We analyzed the data obtained in the ﬂight, and summarize the results in this section.

8.3.1

Charged Particle Tracking Mode

The examples of the obtained tracks in the charged particle tracking mode are shown in Figures
8.13 and 8.14. Especially, Figure 8.14 shows the multi-tracks of the ‘shower event’ from GSO.
Figure 8.15 shows the relation of the energy and the track length of all events obtained in the
charged particle tracking mode. The solid line and the dashed line in this ﬁgure are calculated
dE/dX of MIPs and the stopping electrons, respectively. For the gamma-ray reconstruction, weselest the fully contained electron events under the criteria such as¯()1.8 ¯¯
¯L
K
¯
¯ e
e
− 1.8 × 10−3
Ke > 15[keV].
(8.2)¯ ≤ 2,
¯
¯
¯ cm
keV

Track Length [cm]Figure 8.15 says that fully contained electron cut rejected 95 % of the events acquired in the
charged particle tracing mode. Therefore, only using this selection, ∼ 5 % of charged particle
events might include to the gamma-ray events.
In order to obtain the dE/dX spectrum of the single track in the ﬂight, we ﬁtted the tracks
with a straight line under the position resolution (7.10), and then the resultant χ2 histogram is
Figure 8.16. We selected the events of χ2 < 1 as a single track event for which we obtained the
dE/dX spectrum as shown in Figure 8.17. In this ﬁgure, the solid line is obtained by ﬁtting witha Lorentzian having a peak at 5.4 keV/cm. Since the calculated dE/dX is about 5.3 keV/cm,
the obtained value is consistent to the calculated one.
20
16

90

14

80

14

12

70

12

10

18
16

10

60
50

8

8

40

6
6

30

4

4

20

2

2

10

0
0

20

40

60

80 100 120 140 160 180 200
Energy [keV]

0
0
0

1

2

3

4

5

6

7

8

9
χ2

10

Anode-Drift

Figure 8.15: The relation of the track enFigure 8.16: The χ2 histogram of ﬁtting the tracks
ergy and the track length in the charged
with a straight line.
particle tracking mode.

103

Chapter 8. Balloon Flight

25

20

15

10

5

0
0

2

4

6

8

10

12

14

16
18
20
dE/dX [keV/cm]

Figure 8.17: The dE/dX spectrum in the level ﬂight

8.3.2

Compton Camera Mode

Figure 8.18 shows the typical Compton scattering event obtained during the balloon ﬂight. In
order to collect such Compton events, we analyzed this ﬂight data using the same criteria usedin the ground calibration, as described in section 7.4. The sum spectra of µ-TPC and GSO
at each event selection are shown in Figure 8.19. In the Compton camera mode, 2.23 × 105
events were acquired during 7.0 hours, in which the single GSO trigger events were 1.09 × 105
events. Figure 8.20 shows the relation of the energy and the length of the obtained tracks in
the Compton camera mode. The solid line and the dashed line are the dE/dX of the MIPs andthe stopping electron in this TPC, respectivrly. The diﬀerence from Figure 8.15 is the existenceof many events along the dashed line. The fully contained electron events (8.2) were 6.46 × 103
events. After the Compton scattering kinematic ﬁt (6.3), 1.02 × 103 events remained. Then
427 events were inside the FOV. Finally, 199 events were inside the FOV during the level ﬂight
of 3.0 hours live time. On the other hand, the ratio of the single GSO trigger events to the
all acquired events is about 50 % in this experiment, and the reduction eﬃciency of charged
particle by the selection of the fully contained electron is 95 % as seen in section 8.3.1. Also the
reduction eﬃciency by the selection using the Compton scattering kinematics is > 99 %, which
was obtained by simulation. Thus, the wrong-rejection probability of charged particle is about
3 × 10−4 for the gamma-ray reconstruction. Even if all of the acquired data in the level ﬂight
were the charged particle events, the remained number of the particle events was < 20 after the
kinematical cut. Thus, the systematic error by charged particle was < 4 %. Also there are the‘not-Compton’ events, which include the events of scattering in the GSO at ﬁrst or the events ofabsorption in the GSO and absorption of irradiation GdKα in µ-TPC. A GEANT4 simulation
says that the systematic error due to such pseudo events is ∼ 20 % for 150 keV and ∼ 40 % for
600 keV.
In the level ﬂight, the energy spectrum of 199 downward gamma-rays, of which the direction
104

Results of Experiment & Discussion
XY

track_17-7926: source position: reconst. point: absorbed point: Compton point

20060901 11:02:44°

αkin: 90.7°
αgeo: 106.6°
φ: 26.3°
ψ : 64.4

E0: 536.4 keV
Eγ : 483.8 keV
Ke: 52.6 keV

15

Y [cm]

per1

10

5

0

-5

20

-15
-15

15

-10

-5

0

5

10

15
X [cm]

-10

-5

0

5

10
15
Y [cm]XZ-Z [cm]-Z [cm]

-10

10

20

15

5
10

0
5

-5
15
Y [ 10
cm
]

0

5
0
-5
-10
-15 -15

-10

-5

0

10

5

X

15
[cm]

-5

-10
-15

Figure 8.18: The event view of the Compton event in the ﬂight

104

count

count

are inside the FOV (zenith angle < 60◦ ), is shown in Figure 8.21. Since we expected to detect∼ 200 photons inside the FOV during 3 hours at the altitude of 35 km by GEANT4 simulation(Table 7.2), the experimental result, of which 199 photons were obtained during the live time of
3.0 hours at the altitude of 32 - 35 km, was well consistent. Also this spectrum has a slightly
excess at near 511 keV, which is seen in the simulation as shown in Figure 7.5. The photon
number of this excess is ∼ 10. Therefore, it is roughly explained with 511 keV gamma-rays
produced by the charged particles, because our simulation says about 10 photons are producedby such cosmic-rays at the altitude of 35 km (Table 7.2).
The time variation of the gamma-ray events is shown in Figure 8.22, which has a maximum
countrate near the Pfotzer Maximum, and thereafter it is roughly constant during the level
ﬂight, which is similar to the time variation of the hit of the GSO units.

120

single GSO trigger

Compton scattering kinematics

100
3

10

fully contained electron
80
2

10

60

in FOV

Compton scattering kinematics
40

10

20

in FOV, level ﬂight

1
0

200

400

600

800

00

1000
E0 [keV]

200

400

600

800

1000
E0 [keV]Figure 8.19: The spectra summed of µ-TPC and GSO at each event selection
105

20
18

300

16
250

14
12

Track Length [cm]Track Length [cm]Chapter 8. Balloon Flight

200

10

20

70

18
60

16
14

50

12
40
10

150

8
6

30

8
6

100

4

20

4
50

2
0
0

10

2
20

40

60

80 100 120 140 160 180 200
Energy [keV]

0

0
0

20

40

60

80 100 120 140 160 180 200
Energy [keV]

0rate [count/hour]count

Figure 8.20: The relation of the energy and the length of the obtained tracks in Compton camera
mode. The left ﬁgure is the relation of all events and the right one is that of the fully contained
events in 7 × 7 × 7 cm3 . The solid line and the dashed line are the dE/dX of the MIPs and thestopping electrons, respectively.

30
25
20

450
400
350

¾

300

Level Fight

-

250

15
200

10

150
100

5
50

00

100

200

300

400

500

600

700

800

0

900 1000E0 [keV]

6

7

8

9
10
11
12
13
Time on 1st September 2006 (JST)Figure 8.21: The spectrum of the downward Figure 8.22: The time variation of the detectedgamma-rays during the level ﬂight (after 09 : gamma-rays. The circles and the boxes are all00). The error bar is the statistical sigma.
direction and the downward gamma-rays countrate, respectively. The error bar is the statistical sigma.

Table 8.4: The number of the detected downward gamma-rays from 100 keV to 900 keV
Time
Pressure 120 - 300 keV 300 - 900 keV Real Time
(JST)
[hPa]
[hour]
8 : 00 - 8 : 12
54.6
15
16
0.144
8 : 12 - 8 : 24
31.8
7
9
0.152
8 : 24 - 8 : 48
13.8
10
20
0.324
8 : 54 - 10 : 06
5.42
17
41
1.11
10 : 06 - 11 : 00
6.69
16
32
0.752
11 : 00 - 11 : 30
8.13
9
17
0.411
11 : 30 - 12 : 55
8.46
17
27
0.767

106

Results of Experiment & Discussion
Using the obtained eﬀective area by a ground calibration (Figure 7.34), we calculated the ﬂux
of the incident gamma-rays. Figure 8.23 shows the dependence of the gamma-rays ﬂux on thezenith angle at the level ﬂight. With the events in all azimuth angle, we obtained the gamma-ray
ﬂux printed by circles in Figure 8.23. In this ﬁgure, the solid line is the calculated gamma-ray
ﬂux at 300 keV in the atmospheric depth of 7.0 g/cm2 [50]. The obtained ﬂux inside the FOV
looks consistent with the calculation. However, the obtained transverse ﬂux (60◦ < zenith angle≤ 90◦ ) is roughly two times larger. On the other hand, the dependence of the photon number
on the azimuth angle is shown in Figure 8.24. This ﬁgure says that the gamma-ray ﬂux in FOV
was statistically consistent to be uniform distribution (the resultant χ2 ﬁtting with a constant is0.68), but the transverse gamma-ray ﬂux had the azimuth angle variation. Comparing with thealignment of the accessories (the left of Figure 8.24), the directions of the high ﬂux were quiteconsistent with the position of batteries. This fact means that the batteries might become the
gamma-ray sources by the interaction with cosmic-rays. Then, we calculated the gamma-ray
ﬂux with the events of the azimuth angle between −45◦ and 75◦ , and we obtained the zenith
angle dependence of the ﬂux printed by boxes in Figure 8.23. The zenith angle dependence ofthe gamma-ray ﬂux with the azimuth angle between −45◦ and 75◦ is roughly consistent withthe calculation by Ling [50]. Therefore, we consider that the excess of the transverse ﬂux was
secondary gamma-rays by the interaction of batteries and cosmic-rays.
The detected photon number is listed in Table 8.4, and the relation of the atmospheric depth
and the photon number, which is called a growth curve, is shown by the circles in Figure 8.25.
The error bar in this ﬁgure is only statistical. In order to obtain both the ﬂuxes of diﬀuse cosmic
gamma-ray and atmospheric gamma-ray, we assumed two functions of the atmospheric depth.
Because the ﬂux of atmospheric gamma-ray is nearly proportional to the atmospheric depth[47], the ﬂux isfatmos (z) = a × (1 + k) × z,

(8.3)

where z is the atmospheric depth, k is the ratio of the gamma-rays produced by the interactionof charged particles and the vessel to atmospheric gamma-rays, and a is a free parameter. From
Figure 7.5, k is estimated to be 0.08 for 120 - 300 keV and 0.19 for 300 - 600 keV. On the other
hand, diﬀuse cosmic gamma-rays are scattered or attenuated in atmosphere. Thus, the ﬂux of
diﬀuse cosmic gamma-ray is described as
fcosmic (z) = b × (1 + p) × exp(−z/τ ),(8.4)

where τ is the mean free path of the total attenuation in the air, p is the scattered component
term (see section 7.1.1), and b is a free parameter. Then we ﬁtted the growth curve with the
combination of two function
R(z) = Aeﬀ × (1 − νvessel ) × (fcosmic + fatmos ) ,(8.5)

where Aeﬀ and νvessel are the eﬀective area of the ﬂight model detector and the attenuation
factor of the aluminium vessel, respectively. In Figure 8.25, the solid line, the dashed line
and the dotted line are obtained R(z), the cosmic component and the atmospheric component,
respectively. Obtained a is 1.27 × 10−1 ± 3.3 × 10−2 photons/(cm2 sec str MeV (g/cm2 )) for 120- 300 keV and 1.43 × 10−2 ± 3.4 × 10−3 photons/(cm2 sec str MeV (g/cm2 )) for 300 - 900 keV.
Also obtained b is 7.49 × 10−4 ± 4.59 × 10−4 photons/(cm2 sec str keV) for 120 - 300 keV and2.58 × 10−4 ± 5.6 × 10−5 photons/(cm2 sec str keV) for 300 - 900 keV. Moreover we assumed
that both diﬀuse cosmic gamma-rays and atmospheric gamma-rays have a power law spectrumof E −2.0 and E −1.7 [46, 49], respectively, and then each gamma-ray ﬂux obtained by ﬁtting is
107

Photons / (cm2 sec str MeV)

Chapter 8. Balloon Flight

10

1

10-10

10

20

30

40

50

60
70
80
90
zenith angle [degree]

count

Figure 8.23: The dependence of the gamma-ray ﬂux on the zenith angle during the level ﬂight.
The circles and the boxes were calculated with the events in all azimuth direction and in thelimited azimuth direction, respectively. The error bar is the statistical sigma. The solid line isthe calculated ﬂux for 300 keV gamma-rays at the atmospheric depth of 7.0 g/cm2 [50].

250◦ ≤ zenith ≤ 60◦

20

15

10

5

-100

-50

0

50

100

150
azimuth [degree]

20

ASD

ASD

Battery60◦ < zenith ≤ 90◦
Battery

25

-150

ASD

count

0

15

10

5

0

-150

-100

-50

0

50

100

150
azimuth [degree]

Figure 8.24: The deﬁnition of the azimuth angle (left) and the azimuth angle depending of thephoton number (right).

108[count/hour]

Results of Experiment & Discussion

104

120 - 300 keV

103

102

10

[count/hour]

1
1

102
Atmospheric depth [g/cm2]

10

104

300 - 900 keV

103

102

10

1
1

102
Atmospheric depth [g/cm2]

10

Figure 8.25: The relation of the atmospheric depth and the count rate ( the growth curve). The
circles are obtained data during this ﬂight, and the error bar is the statistical sigma. The solidline is obtained by ﬁtting of the data. The dotted-line and the dashed-line are diﬀuse cosmic
gamma-rays and atmospheric gamma-rays, respectively.

109

Flux [photons / (cm2 sec str keV)]

Chapter 8. Balloon Flight

10-1
Scho nfelder et al. (1977)
Scho nfelder et al. (1980)-2

10

Bleeker et al. (1970)
Makino et al. (1975)
Vette et al. (1970)
Tromba et al. (1973)
Kasturirangan et al. (1972)

-3

10

10-4

-5

10

-6

10

10-7

-8

10

10

102

103

104
105
Energy [keV]

Figure 8.26: The obtained ﬂux of diﬀuse cosmic gamma-rays. The red solid line is our result.
The error bar includes only the stastic error, and the systematic error is a factor of about 2. For
the comparison, we also plotted the results of past other groups [47, 51, 52, 53, 54, 55].

110

Flux [phtons / (cm2 sec str MeV (g/cm2))]

Results of Experiment & Discussion

10
Scho nfelder et al. (1980)
Ryan et al. (1977)
Kinzer et al. (1974)

1

Daniel et al. (1972)
Jacobson et al. (1968)
Peterson et al. (1972)
Mizuno et al. (2004)

-1

10

10-2

-3

10

10-4

-5

10

-6

10

10-7
10-2

10-1

1

102
10
Energy [MeV]

Figure 8.27: The obtained ﬂux of atmospheric gamma-rays. The red solid line is our result. The
error bar includes only the stastic error, and the systematic error is a factor of about 2. For thecomparison, we also plotted the results of past other groups [51, 56, 57, 58, 59].

111

Chapter 8. Balloon Flight

Effective Area [cm2]

105
104
103

OSSE

2

10

EGRET
COMPTEL

SPI
10
1

10-1
10-2

this experiment

10-3
10-4
10

-5

10-2

10-1

1

10

102

3

10
104
Energy [MeV]

Figure 8.28: The comparison of the sensitivity Figure 8.29: The comparison of the eﬀective
area [30, 60, 61]
for continuum component [38]described as follows,)
E −2.0[photons/cm2 sec str keV],
keV()
E −1.7
Fatmos (E) = (0.65 ± 0.16) ×[photons/cm2 sec str keV (g/cm2 )].
keV(

Fcosmic (E) = (41 ± 2) ×

(8.6)
(8.7)

The obtained ﬂuxes of diﬀuse cosmic gamma-rays and atmospheric gamma-rays are comparedwith the results of previous balloon/rocket experiments, as shown in Figure 8.26 and 8.27,
respectively. The systematic error is a factor of about 2 due to the uncertainty of the detection
eﬃciency as described in section 7.4. These ﬁgure says that the obtained ﬂuxes from SMILE
are nearly consistent with those of the past experiments.
Finally, we calculated the detection sensitivity of this camera. The gamma-ray, which wereobserved by this experiment, are the background, when we would observe a point gamma-ray
source. Thus, with the angular resolution of 20◦ and the observation time of 106 sec, the numberof background photons NB is expected as follows.
) (
)
( 6
0.1 str
10 secN B = Nd ×
×
,
(8.8)
3 hour3 str
where Nd is the detected photon number of this experiment. Then, the number of the photon
from the target celestial object for the signiﬁcance of 3σ is√
9 + 3 9 + 8NB
NS =
,
(8.9)
2
Therefore, the detectable minimum ﬂux isFS =NS
Aef f × 106 sec[photons / (cm2 sec)],(8.10)

where Aef f is the eﬀective area (see Figures 7.33 and 7.34). The detected photon number in theenergy range of 200 - 400 keV during the level ﬂight is 151 events, and Aef f is 2.13 × 10−2 cm2 .
Therefore, the detection sensitivity Fs is 2.2 × 10−4 MeV / cm2 sec. The detectable minimum
ﬂux and the eﬀective area of this experiment are shown in Figures 8.28 and 8.29, respectively.
From these ﬁgures, although the eﬀective area of this experiment is smaller by about three112

Results of Experiment & Discussion
order than that of COMPTEL, the detection sensitivity is smaller by only one order. This factmeans that our camera has actually a high signal to noise ratio due to the kinematic ﬁt basedon Compton scattering. On the other hand, from these ﬁgures, if a 50 × 50 × 50 cm3 detector,
which has the eﬀective area as same as COMPTEL, were realized in the future, the sensitivity
would reach ten times of that of COMPTEL.

113

Chapter 9

Summary & Future WorkFor the next generation MeV gamma-ray Compton camera, we developed an Electron Tracking
Compton Camera, which consisted of a µ-TPC as a tracker and the GSO-PSAs as absobers.
First, we constructed a prototype camera using an Ar gas ﬁlled TPC, and we revealed theability of an electron tracking Compton imaging, of which there are fully reconstruction of
the direction of the incident gamma-rays, and in particular the background rejection due tothe Compton scattering kinematics. For the 662 keV gamma-rays, the energy resolution andthe angular resolutions ARM/SPD were 14 %, ∼ 9◦ and ∼ 90◦ at FWHM, respectively. The
detection eﬃciency was ∼ 5 × 10−6 . Also the FOV of the prototype was obtained ∼ 1 str. Next,
in order to measure diﬀuse cosmic gamma-rays and atmospheric gamma-rays, we designed andconstructed a ﬂight model detector with an improvement of the detection eﬃciency for a balloon
borne experiment. In the balloon borne experiment, the detection eﬃciency is most signiﬁcant.
Therefore, we decided to use a Xe gas, although the energy resolution and the angular resolution
were little worse than those of the prototype, such as 19 %, ∼ 12◦ and ∼ 200◦ at FWHM
for 662 keV, respectively. However, by a Xe gas TPC and a larger scintillator coverage, thedetection eﬃciency was dramatically increased 10 times of that of the prototype, and the wide
FOV spreaded to 3 str. Finally, we loaded the ﬂight model on a balloon, and launched at
06:11 on September 1 2006 as the 1st ﬂight of Sub-MeV gamma-ray Imaging Loaded-on-balloon
Experiment (SMILE) from Sanriku Balloon Center (39.16N, 141.82E). The balloon reached to
35 km at 08:56, and the level ﬂight continued during 4 hours. There was no serious problem on
the balloon system and the ﬂight model detector. Also we succeeded in the detection of ∼ 200
downward gamma-rays during the 3.5 hours level ﬂight (the live time of 3.0 hours). The detected
photon number of ∼ 200 photons in 3 hours was consistent to the simulated value, Then, we
obtained the following ﬂuxes of diﬀuse cosmic gamma-rays and atmospheric gamma-rays withthe growth curve of this experiment,)
E −2.0[photons / cm2 sec str keV],
Fcosmic (E) = (41 ± 2) ×keV()
E −1.7
Fatmos (E) = (0.65 ± 0.16) ×[photons / cm2 sec str keV (g/cm2 )].
keV
(

(9.1)
(9.2)

where we assumed that the power law index of Fcosmic and Fatmos are −2.0 and −1.7, respectively.
These spectra are consistent with the previous data. Moreover, although the eﬀective area of
1st SMILE was 3 order smaller than that of COMPTEL, the sensitivity of the MeV camera ofthis experiment was about 1 order worse than the sensitivity of COMPTEL, which means thatour detector improve the signal to noise ratio by 1.5 order better than that of COMPTEL. This
experiment is the ﬁrst observation result in sub-MeV region by a Compton camera. Because
there are many backgrounds due to charged particles, the observation in this energy band isvery diﬃcult. Actually, there has existed few observations of atmospheric gamma-ray ﬂux in
115

Chapter 9. Summary & Future Work
sub-MeV region so far. Thus, it is very important that our detector obtained both the ﬂuxes
and the zenith angle dependence of diﬀuse cosmic gamma-rays and the atmospheric gamma-raysin the sub-MeV region.
The purpose of the next balloon experiment is the test of the imaging power, by the observation of a bright source like Crab nebula or Cyg X-1. Because an imaging needs a lot of photons,
we must develop a µ-TPC having a larger volume of 30 × 30 × 30 cm3 . Figure 7.6 says thatthe eﬃciency of the current analysis is 10 - 30 %, and also Figure 5.18 shows that the obtained
direction of the recoil electron is not so accurate. Therefore, the improvement of the analysis forthe reconstruction of the recoil electron would improve both the eﬀective area and the angularresolution. On the other hand, the increase of the µ-TPC volume gives an increase of the areaof the absorber. When we construct the 2nd ﬂight model similar to the 1st ﬂight detector, thereadout circuit of µ-TPC and GSO are 3 times and 10 times of those of the 1st ﬂight model,
respectively. Thus, we must also develop the low power readout circuit.
We have already started to develop a 30 × 30 × 30 cm3 TPC using a 30 × 30 cm2 µ-PIC.
However, for the tracker of the Compton camera, we will need study a gas as a Compton
scattering target. Although we used Xe gas for the 1st ﬂight model µ-TPC, the drift velocity ofelectron in Xe gas is too slow and the cross section of absorption of Xe atom is too large. Thus,
by using Xe gas, the random coincidence might be increased, and also the background event,for example the events of scattering from the GSO, might be increased. As the candidate of
the Compton target gas, we consider a CF4 gas, which has 42 electrons per one molecule andconsists of the low Z atoms. Also the drift velocity of electron in a CF4 gas is rapider than thatin an Ar gas. On the other hand, since the gas gain of a CF4 gas is not high, the problem of
the increase of the operation high voltage remains.
After these improvements, our detector would be able to obtain a few hundred photons from
Crab nebula by a 3 hours level ﬂight, because the eﬀective area will be ∼ 30 times larger thanthat of the 1st ﬂight model. Also, if a ∼ 10 days level ﬁght is realized, our camera will be able
to detect the other celestial objects, like Cen A which is a radio galaxy and its ﬂux is about 0.1
Crab [62].

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119

Acknowledgments
I am deeply grateful to Professor Toru Tanimori for his guidance and encouragements in thiswork. This thesis would not be completed without his support. I specially thanks the SMILE
team: Dr. Hidetoshi Kubo, Hironobu Nishimura, Kazuki Ueno. Also I would like to thankµ-PIC collaborators: Dr. Miuchi, Dr. Tsuchiya, Dr. Kabuki, Okada, Hattori, Kurosawa (Kyoto
Univ.), Dr. Takeda, Dr. Sekiya (ICRR), Dr. Nagayosi (Waseda Univ.), Dr. Ueno (Tokyo
Institute of Technology), Dr. Orito (Kobe Univ.), Dr. O. Bouianov (Espoo-Vantaa Institute oftechnology) and Dr. M. Bouianov (CSC-Scientiﬁc Computing Ltd.). I am grateful to all themember of the cosmic ray laboratory in Kyoto University for their continuous discussions andencouragements.
I sincerely thank Professor Yamagami, Professor Yoshida, Professor Saito, Nonaka, Mizuta,
Professor Takahashi, Dr. Nakazawa (ISAS/JAXA), Professor Gunji (Yamagata Univ.), andthe member of Sanriku Balloon Center (ISAS/JAXA) for a lot of advices and assistances of theballoon experiment. I thank Professor Nagae (KEK) and members of his laboratory, who helped
the experiment at T1 beam line in the Proton Synchrotron at KEK.
I appreciate Tomohisa Motomura (D.T. Circuit Technology Co., Ltd.) and the staﬀs of D.T.
Circuit Technology and TOSHIBA who manufactured the µ-PIC and gave us several technicalinformations about the manufacturing process. For the production of the SMILE system, I am
grateful to Ogawa, Tanaka (Arkus), Uchiyama, Shimizu (Clear Pulse), Ozawa, Okada (Hitachi
KMS), Yoribayashi(Houshin), Kochou (Max-Electronics), Matsumaru (REPIC), Oyobe (Sansei
System), Hiruta (Shinwa Denzai). I thank Mori (Furamingo) for the production of the gondola,
and I also thank Shimizu (Rikensha) for the creating the TPC vessel.
This work was supported by a Grant-in-Aid in Scientiﬁc Research of the Japan Ministryof Education, Culture, Science, Sports and Technology. Also, this work was supported by aGrand-in-Aid for the 21st Century COE “Center for Diversity and Universality in Physics”, andthe Japan Society for Promotion of Science for Young Scientists.

120