Master Thesis

Development and commissioning of MEG II
Radiative Decay Counterby
Ryoto Iwai

March, 2017
The University of Tokyo
Department of Physics, School of ScienceAbstract
The MEG experiment has been searching for the lepton flavor violating process, µ+ → e+ γ,
which can be a clear evidence of new physics beyond the Standard Model if it is discovered.
The upgrade experiment (MEG II) is currently being prepared to achieve one order higher
branching ratio sensitivity ∼ 4 × 10−14 by using the world’s most intense muon beam up to∼ 108 µ+ /s and upgraded detectors with considerably improved performance. One of the key
for the upgrade is to suppress the background significantly increased with the higher muon
decay rate. Radiative Decay Counter (RDC) will be newly installed in MEG II to identify
the background photon from the radiative muon decay (RMD). The concept of the RDC is todetect the low momentum positron associated with RMD with two detectors installed on the
beam axis at both upstream and downstream of the muon stopping target.
The downstream detector was successfully developed and the final detector was constructed
and tested in the muon beam. We demonstrated the capability of the identification of the
background photon from RMD. The upstream detector, which is more difficult, is still underdevelopment. The main issues such as the influence on the muon beam and the detection
efficiency for the RMD positrons were studied. We concluded that the influence on the muon
beam transportation and the muon stopping rate is expected to be small. We evaluated thedetection efficiencies expected in various configurations and the final sensitivities for the possible
scenarios.

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Contents
Abstracti

1 Introduction
1.1 Standard Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.1 Flavor conservation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.2 Muon decay modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.3 Neutrino oscillation and µ → eγ decay . . . . . . . . . . . . . . . . . . .
1.2 Beyond the Standard Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 History of µ → eγ search . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2 MEG II experiment
2.1 Signal and background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1 Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Requirements for the µ+ → e+ γ search . . . . . . . . . . . . . . . . . . . . . . .
2.3 Experimental apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1 Proton accelerator complex and πE5 beam line . . . . . . . . . . . . . .
2.3.2 Beam transport system . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.3 Target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.4 MEG II detector overview . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.5 Positron Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.6 Photon detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.7 Radiative Decay Counter . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.8 DAQ boards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Expected performance and sensitivity . . . . . . . . . . . . . . . . . . . . . . . .

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3 Radiative Decay Counter3.1 Principle of background identification . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Concepts of the RDC detector . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Downstream detector design . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1 Timing Counter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.2 Calorimeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4 Upstream detector design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4 Development of LYSO calorimeter
4.1 Mass test of the LYSO crystals . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Afterglow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.1 Study with room light . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2.2 Study with β-ray source . . . . . . . . . . . . . . . . . . . . . . . . . . .

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CONTENTS

4.3

4.2.3 Influence on the energy resolution . . . . . . . . . . . . . . . . . . . . . . 39
4.2.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Optimization of optical coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

5 Commissioning of the Downstream Detector
5.1 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.1 RDC installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.2 BGO photon detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.3 DAQ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1.4 Signal check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1 Non-linearlity of the WaveDREAM . . . . . . . . . . . . . . . . . . . . .
5.2.2 Energy scale calibration . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Data taking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.1 Event selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.2 Timing coincidence of the RDC and BGO detector . . . . . . . . . . . .
5.4.3 Energy cut with the calorimeter . . . . . . . . . . . . . . . . . . . . . . .
5.5 Issues and prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.1 Problematic channels in the timing counter . . . . . . . . . . . . . . . . .
5.5.2 Uncertainty of energy scale of the BGO detector . . . . . . . . . . . . . .

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6 Development of the Upstream Detector
6.1 Influence on the muon beam . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.1 Beam spot size on the target . . . . . . . . . . . . . . . . . . . . . . . . .
6.1.2 Effect of the larger beam spot . . . . . . . . . . . . . . . . . . . . . . . .
6.1.3 Muon stopping rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2 Effect of pileup muon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.1 Capability to distinguish two waveforms . . . . . . . . . . . . . . . . . .
6.2.2 Effect of size and position of the fiber bundle . . . . . . . . . . . . . . . .
6.2.3 Total inefficiency for RMD positrons of all bundles . . . . . . . . . . . .
6.2.4 Prospects for reducing the inefficiency . . . . . . . . . . . . . . . . . . .
6.3 Total detection efficiency for RMD positrons . . . . . . . . . . . . . . . . . . . .
6.3.1 Light yield of the scintillating fiber . . . . . . . . . . . . . . . . . . . . .
6.3.2 Detection efficiency and sensitivity in different configurations . . . . . . .

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7 Summary and Outlook
7.1 Radiative Decay Counter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2 Downstream detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3 Upstream detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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A Characterization of SiPM
A.1 Principle of photon detection . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.2 Current-Voltage curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.3 Dark noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4 Crosstalk and after-pulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.5 Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Bibliography86

Acknowledgements

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List of Tables
1.1

Major decay modes of µ+ in the Standard Model [3]. . . . . . . . . . . . . . . .

2.1

Resolutions and efficiencies of the upgraded detector. . . . . . . . . . . . . . . . 17

3.1
3.2
3.3

Properties of BC-418 [15]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Properties of S13360-3050PE [16]. . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Properties of the LYSO crystal. [17]. . . . . . . . . . . . . . . . . . . . . . . . . 25

4.1
4.2
4.3

Obtained parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Fitting parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Comparison of the parameters for different source intensities. . . . . . . . . . . . 39

5.1
5.2

Light yields of the MiP peak of the cosmic-rays (45.4 MeV). . . . . . . . . . . . 60
Peak of the charge distributions in two configurations. . . . . . . . . . . . . . . . 61

6.1
6.2
6.3
6.4
6.5
6.6

Beam spot size at the target position . . . . . . . . . . . . . . . . . . . . . . . .
Fiducial volume parameters of the target . . . . . . . . . . . . . . . . . . . . . .
Efficiency for the signal positrons (%) . . . . . . . . . . . . . . . . . . . . . . . .
Hit rate in the inner most wire . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Reconstructed momentum resolution . . . . . . . . . . . . . . . . . . . . . . . .
Optimal degrader thickness and muon stopping rate . . . . . . . . . . . . . . . .

2

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A.1 Basic properties of S13360-1350CS [16]. VOP is breakdown voltage + 3.0 V. The
measurements were performed at 25 ◦ C. . . . . . . . . . . . . . . . . . . . . . . . 82

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List of Figures
1.1
1.2
1.3
1.4
1.5

Elementary particles in the Standard Model . . . . . . . . . . . . . . . . . . . .
Relative energy to the half of the muon mass (52.8 MeV). . . . . . . . . . . . . .
Feynman diagram of the µ → eγ decay through the neutrino oscillation. . . . . .
Branching ratio of µ → eγ and τ → µγ processes [5]. mN 3 is the mass of theright-handed Majonara neutrino. . . . . . . . . . . . . . . . . . . . . . . . . . .
History of the upper limits at 90% C.L. on the muon CLFV channels [9]. The
arrows represent target branching ratio sensitivities of the future experiments.
Λ shows the energy scale of the new physics models. . . . . . . . . . . . . . . . .

2.1 Signal event. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Prompt background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Accidental background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Source of the background photons (Eγ > 48 MeV). . . . . . . . . . . . . . . . .
2.5 Aerial view of the 590 MeV ring cyclotron [10]. . . . . . . . . . . . . . . . . . . .
2.6 Schematic view of the cyclotron facility and the beam lines at PSI [11]. . . . . .
2.7 Schematic view of the beam line from the πE5 channel to the MEG II detector.
2.8 Target in the MEG experiment. . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.9 Schematic view of the MEG II detector. . . . . . . . . . . . . . . . . . . . . . .
2.10 Schematic view and photograph of the COBRA. . . . . . . . . . . . . . . . . . .
2.11 Distribution of the magnetic field. . . . . . . . . . . . . . . . . . . . . . . . . . .
2.12 Schematic view of one of the cathode plane. . . . . . . . . . . . . . . . . . . . .
2.13 Drift chamber under construction. . . . . . . . . . . . . . . . . . . . . . . . . . .
2.14 Schematic view of the pixelated timing counter. Only half part is shown. . . . .
2.15 Timing counter. Left : Single counter. Right : Assembled 128 counters before
the installation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.16 Liquid-xenon photon detector. Left : Cryostat. Right : Inner view after installing the PMTs and SiPMs. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.17 DRS chip. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.18 Schematic view of the waveform sampling. . . . . . . . . . . . . . . . . . . . . .
2.19 Pictures of the WaveDREAM. Left : Single board for 16 channels. Right : Cratefor the boards. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.20 Simplified scheme of the WaveDREAM board. . . . . . . . . . . . . . . . . . . .
2.21 90% C.L. upper limit sensitivity as a function of DAQ time. . . . . . . . . . . .
3.1
3.2

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18

Schematic view of MEG II detectors . . . . . . . . . . . . . . . . . . . . . . . . 19
Expected hit time difference of the downstream RDC and the detected signal photon assuming timing resolution of 100 ps. (red) Accidental background.
(blue) Positron from Michel decay. The last bin shows the events which have no
hit in the RDC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20ix

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LIST OF FIGURES
3.3

Expected energy deposit in the downstream detector. (red) RMD with Eγ > 48
MeV. (blue) Michel decay. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.4 Schematic view of the downstream detector. The timing counter is placed in front. 21
3.5 Schematic view of the downstream detector prototype. . . . . . . . . . . . . . . 21
3.6 Constructed downstream detector. The timing counter is visible in front. . . . . 22
3.7 Timing counter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.8 Left : PCB for a scintillator with a width of 2 cm. Right : PCB for three
scintillators with a width of 1 cm. . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.9 Left : Plastic scintillator glued with 3 SiPMs. Right : Short and long PCBs
after being wrapped with the reflector. . . . . . . . . . . . . . . . . . . . . . . . 24
3.10 Schematic view of the setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.11 Measured distribution of T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.12 LYSO crystals contained in a holder. . . . . . . . . . . . . . . . . . . . . . . . . 25
3.13 Decay scheme of 176 Lu and energy spectrum of the LYSO intrinsic radioactivity[18]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.14 PCB in the calorimeter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
3.15 Support structure in the calorimeter. Left : Between the crystals and the timing
counter. Right : Between the crystals and the PCBs. . . . . . . . . . . . . . . . 27
3.16 Chassis to fix the springs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.17 Aluminum cover for the light shielding. In order to prevent light leakage, black
tapes are attached at the corner. . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.18 CG image of the upstream detector. . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.19 Schematic view of the MEG II detector. The upstream detector is installed
between the drift chamber end cap and the COBRA end cap where a small
space is available (r ∼ 20 cm, z ∼ 2 cm). . . . . . . . . . . . . . . . . . . . . . . 29
3.20 Provisional mechanical design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.21 Prototype of the bundled fibers (64 fibers ×2). . . . . . . . . . . . . . . . . . . . 30
4.1
4.2
4.3
4.4

4.5

4.6

4.7
4.8
4.9

Schematic view of the setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Energy spectrum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Distribution of the light yield and energy resolution. . . . . . . . . . . . . . . . .
Setup of the afterglow measurement with the room light. Left : 76 crystals
exposed to the room light. Right : Schematic view of the setup for the current
measurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Measured sensor current of the SiPM. The 76 crystals were measured according
to the order of their serial numbers. Note that several serial numbers are skipped
because we replaced several crystals in which we found some defects. The red andblue plots are corresponding to the results of the next measurement (Figure 4.6).
Measured sensor current of the SiPM and the serial number of the crystal. The
red and blue plots show the result of the first and the second measurement,
respectively. The order of the serial numbers corresponds to the order of themeasurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Observed sensor current of the SiPM after exposing to the room light. The values
over 100 µA are not recorded due to the current limitation of the power supply.
Schematic view of the setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The observed current of the SiPM during the β-ray irradiation. Left : Region
that the current exponentially increases. Left : Whole measurement. . . . . . . .

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LIST OF FIGURES
4.10 Expected number of emitted scintillation photons with afterglow (black). The
red line shows the fitting with the below function. . . . . . . . . . . . . . . . . .
4.11 Setup for the reproducibility test and the measured sensor current of the SiPM.
Current jumps during irradiation are due to a technical problem of the power
supply. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.12 Examples of fitting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.13 Sensor current of the SiPM during the irradiation with 37 MBq 90 Sr. The orderof the figures is corresponding to the order of the measurement. . . . . . . . . .
4.14 Left : ∆Imax vs. τ . Right : ∆Imax vs. measured sensor current after exposing
to the room light. The black plot shows the additional measurement with serial
number 28. The current with the room light was measured three times in the
red plots. The error bar is the standard deviation of the three results. . . . . . .
4.15 Expected energy resolution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.16 Candidates for the optical coupling. . . . . . . . . . . . . . . . . . . . . . . . . .
4.17 Light yield of the crystal and the over voltage of the SiPM. . . . . . . . . . . . .
4.18 Comparison of the grease and the pad. Left : Comparison of the light yield. The
arrows represent the order of the measurement. Right : The energy resolution
and the light yield. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.1
5.2
5.3
5.4
5.5
5.6

Schematic view of the setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Stage of the downstream RDC. . . . . . . . . . . . . . . . . . . . . . . . . . . .
Positions of the detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Feed-through PCB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Detector after cabling and light shielding. . . . . . . . . . . . . . . . . . . . . . .
BGO detector. Left : 16 BGO crystals. A front cover of the crystals is not
attached. Right : BGO crystal attached with the PMT. . . . . . . . . . . . . . .
5.7 WavedDREAM boards after cabling. The left half part was used for the RDC
and the BGO detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.8 Boards assignment for each detector and the gain of the pre-amplifiers. . . . . .
5.9 Problematic channels in the timing counter (red parts). . . . . . . . . . . . . . .
5.10 Schematic view of the setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.11 Schematic view of the input pulse. Several pulse heights were used. . . . . . . .
5.12 Input pulse height and the the measured pulse height of the output waveform. .
5.13 Example of waveform correction (calorimeter channel). Red and black shows the
waveform before and after correction, respectively. . . . . . . . . . . . . . . . . .
5.14 Simulated energy deposit (left) and measured spectrum (right). . . . . . . . . .
5.15 Left : Expected energy spectrum (blue). The red and the green parts show thephoto-peaks of the gamma-rays and the β-decay spectrum, respectively. The Qvalue of the β-decay spectrum is 1192 keV [20]. Right : Example of the measured
spectrum in the single channel. . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.16 Measured spectrum with the 88 Y. . . . . . . . . . . . . . . . . . . . . . . . . . .
5.17 Simulated energy deposit (left) and measured spectrum (right). The energy scaleof the measured spectrum is calibrated with the gamma-rays of 88 Y. . . . . . . .
5.18 Total energy deposit in the BGO detector of all the triggered events. The energy
scale is defined by the calibration data with the 88 Y. . . . . . . . . . . . . . . .
5.19 Event display of the RDC and the BGO detector. The marker color and thediameter represent the hit timing and the energy deposit respectively. . . . . . .
5.20 Fiducial volume of the BGO detector (red shaded part). . . . . . . . . . . . . .

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xii

LIST OF FIGURES
5.21 Total energy deposit in the BGO detector. The black and red spectra shows thebeam on and off data, respectively. . . . . . . . . . . . . . . . . . . . . . . . . .
5.22 Total energy deposit in the BGO detector. The blue and red part shows before
and after the event selections, respectively. . . . . . . . . . . . . . . . . . . . . .
5.23 Timing difference of the BGO detector and the RDC. . . . . . . . . . . . . . . .
5.24 Total energy deposit in the calorimeter. . . . . . . . . . . . . . . . . . . . . . . .
5.25 Timing difference of the BGO detector and the RDC after the energy cut withthe calorimeter( > 4 MeV). The black and the red show the events before andafter the cut, respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.26 Schematic view of the connections in the timing counter. . . . . . . . . . . . . .
5.27 PCB (top) and SiPMs (bottom) after disassembling. The red arrows show theglued areas before disassembling. . . . . . . . . . . . . . . . . . . . . . . . . . .
5.28 Reflow soldering processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.29 Schematic view of the tested counter. The length of the scintillator is 11 cm. . .
5.30 Example of waveforms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.31 Comparison of the light yields. The red and black show the channel 1 and 2,
respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.32 Schematic views of the setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.33 Measured spectra (channel A). . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.34 Schematic view of the setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.35 Measured spectrum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.1
6.2
6.3

RDC mockup (230 µm thick Mylar foil). . . . . . . . . . . . . . . . . . . . . . .
Schematic view of the setup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Distribution of the muon vertex on the target. Black solid line and red dashedline represent the fiducial area of the nominal and bigger target, respectively. . .
6.4 Number of hit wire cells before crossing the outer shell in the drift chamber. . .
6.5 Number of hit tiles in the timing counter. . . . . . . . . . . . . . . . . . . . . . .
6.6 Distribution of the hit wire ID. The last 192 wires are corresponding to the innermost wires. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.7 Difference of the reconstructed momentum and the true momentum in the simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.8 Muon stopping rate with several degrader thicknesses. . . . . . . . . . . . . . . .
6.9 Definition of ∆T . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.10 Prototype detector. Left : Fiber and PCB. Right : Detector at the measurement
position after light shielding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.11 Examples of waveform mixing. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.12 Probability to find a positron signal after mixing. . . . . . . . . . . . . . . . . .
6.13 Hit distribution in the upstream RDC. . . . . . . . . . . . . . . . . . . . . . . .
6.14 Example of the probability of the pileup in two different positions (9 fibers bundle, y = 2, 4 cm). The orange area represents the high muon hit rate region. . .
6.15 Probability of the pileup calculated with a step size of 250 µm . . . . . . . . . .
6.16 Distribution of the inefficiency for the RMD positrons. . . . . . . . . . . . . . .
6.17 Best configuration of the bundle width. Smaller widths are used in more central
regions of the detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.18 Total number of bundles and inefficiency for RMD positrons with the best
bundling configuration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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6.19 Principle of the staggered readout. When there is a pileup event in the same
bundle in (b) it is distinguishable in the right side of the SiPM. . . . . . . . . .
6.20 Distribution of the number of the photoelectrons observed at single side. . . . .
6.21 Simulated energy deposit of the positrons from RMD (Eγ > 48 MeV). . . . . . .
6.22 Schematic view of the transmission of the scintillation photons. . . . . . . . . . .
6.23 Calculated number of scintillation photons at the single side. The probability todetect no photons is also included in the first bin. . . . . . . . . . . . . . . . . .
6.24 Light yields and the probabilities to detect a signal with the SiPMs. . . . . . . .
6.25 Hit patterns of the muon in the standard readout. The probabilities to detecta signal are shown in the red boxes. The fraction of each conditional branch isalso shown. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.26 Hit patterns of the muon in the staggered readout. Details of the hit position
patterns are shown in the next figure. . . . . . . . . . . . . . . . . . . . . . . . .
6.27 Four hit position patterns when there are muon hits within ∆T . Case A, C :Positron cannot be detected at both ends. Case B : Positron can be detected
only at one end. Case D : Positron can be detected at both ends. . . . . . . . .
6.28 Expected detection efficiency for the RMD positrons. . . . . . . . . . . . . . . .
6.29 Expected sensitivity of the MEG II experiment. . . . . . . . . . . . . . . . . . .
A.1 PCB with 36 SiPMs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.2 Schematic diagram of avalanche multiplication [23]. . . . . . . . . . . . . . . . .
A.3 I-V curve of the SiPM. Left : Negative bias voltage. Right : Positive bias voltage.
Due to the current limitation, we did not use the software below −1 V, but weread the value on the display of the picoammeter. . . . . . . . . . . . . . . . . .
A.4 Example of fitting I-V curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.5 Dark rate vs. threshold of the discriminator with different bias voltages (at 24◦
C). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.6 Dark rate vs. bias voltage with different temperatures. The threshold of thediscriminator was always set to the signal amplitude of the half photoelectrons. .
A.7 Dark rate vs. temperature with different bias voltages. . . . . . . . . . . . . . .
A.8 Spectrum of the dark noise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.9 Measured spectrum at 24 ◦ C. The bias voltage is 53.9 V. The left peak corrrenponds to a pedestal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.10 Gain vs bias voltage with different tempertures. . . . . . . . . . . . . . . . . . .

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xiv

Chapter 1
Introduction
The MEG experiment has been searching for the charged lepton flavor violating (CLFV)
process, µ+ → e+ γ. This chapter introduces the physics motivations of the µ+ → e+ γ search.
We first review the flavor conservation in the Standard Model and decay modes of the muonin its framework. Next, the CLFV process in the new physics models are discussed. The lastsection describes a historical overview of the CLFV search in the muon channels.

1.1

Standard Model

1.1.1

Flavor conservation

In the modern particle physics, the Standard Model has been the most successful theory todescribe the elementary particles and their interactions. Figure 1.1 summarizes the elementary
particles in the Standard Model . The constituents of the matter are called fermions (particles
with half integer spin), which is divided into six quarks and six leptons. The interaction of
the particles are caused by four gauge bosons (particle with integer spin). In addition to theseparticles, the Higgs boson, which was discovered several years ago [1][2], is responsible for themasses of the particles.

Leptons

QuarksThree generation of Fermions

Ⅰ

Ⅱ

Ⅲ

Gauge Bosoon

u

c

t

γ

d

s

b

g

e

μ

τ

Z

νe νμ ντW

Higgs Bosoon±

H

Figure 1.1: Elementary particles in the Standard Model12Chapter 1. Introduction

As shown above, the quarks and leptons are classified into the three flavor generations. In
the quark sector, it is known that the flavor mixing occurred as :d0
Vud Vus Vub
d
 0 



 s  =  Vcd Vcs Vcb   s  ,b0
Vtd Vts Vtb
b










(1.1)

where (d0 , s0 , b0 ) represents the flavor eigenstates vector and (d, s, b) represents the mass
eigenstates of each quark. The mixing angle is described by the unitary matrix V , which iscalled Cabibbo-Kobayashi-Maskawa (CKM) matrix. The probability of the transition can be
written as the square of the matrix element. In the Standard Model, it is considered that theflavor violation in the lepton sector is forbidden. This is because the neutrinos are masslessand their mixing matrix is thereby diagonal. Thus, the µ+ → e+ γ is not allowed.

1.1.2

Muon decay modes

In the Standard Model, the decay of the muon is induced only by electroweak interactions.
Decay modes of the µ+ are listed in the Table 1.1. The decay modes of the µ− can be expressedby conjugating their electric charges.
Decay mode
µ + → e + ν µ νe
µ + → e + ν µ νe γ

Branching ratio∼ 100%(1.4 ± 0.4)%

Table 1.1: Major decay modes of µ+ in the Standard Model [3].
The most dominant decay mode (µ+ → e+ ν µ νe ) is called Michel decay. Another process(µ → e+ ν µ νe γ) is called Radiative Muon Decay (RMD), which can be regarded as an inner
bremsstrahlung process of the Michel decay. Their contributions to the high energy positronsand photons are important in µ+ → e+ γ search. Figure 1.2 shows the energy spectra of thepositrons from Michel decay and the photons from RMD.
+(a) Positrons from Michel decay(b) Photons from RMD

Figure 1.2: Relative energy to the half of the muon mass (52.8 MeV).

1.2. Beyond the Standard Model

1.1.3

3

Neutrino oscillation and µ → eγ decay

Although the flavor violation in the lepton sector is forbidden in the Standard Model,
several neutrino experiments observed the violation through the neutrino oscillations. Hence,
the Standard Model which assumes the massless neutrinos is regarded as incomplete. If we takethe neutrino masses into account, the neutrinos flavors are mixed through the non-diagonal
mixing matrix as :










νe
Ue1 Ue2 Ue3
ν1





 νµ  =  Uµ1 Uµ2 Uµ3   ν2  ,ντUτ 1 Uτ 2 Uτ 3
ν3

(1.2)

where the vector (νe , νµ , ντ ) and (ν1 , ν2 , ν3 ) are representing the flavor eigenstates andthe mass eigenstates, respectively. The mixing matrix U is called Pontecorvo-Maki-NakagawaSakata (PMNS) matrix. Consequently, µ → eγ can occurre through the νµ → νe transition asshown in Figure 1.3.

Figure 1.3: Feynman diagram of the µ → eγ decay through the neutrino oscillation.
The branching ratio of the µ+ → e+ γ can be written by using the corresponding elementsof the PMNS matrix as :
2

3α X
∆m2
B(µ → e γ) =Uµi ∗ Uei 2i1 ,32π i=2,3
MW
+

+

(1.3)

where MW is the mass of the W -boson and ∆m2 is the square neutrino mass difference.
Experimentally, the neutrinos are known to have very tiny masses [4]. The branching ratio of theµ+ → e+ γ will be thereby very small (B(µ+ → e+ γ) ∼ 10−54 ) and it cannot be experimentallyobserved.

1.2

Beyond the Standard Model

Although the Standard Model has been strongly supported both experimentally and theoretically, several phenomena in the particle physics are not described in this framework. For

4Chapter 1. Introduction

example, the presence of gravitational interactions, dark matter and matter-antimatter asymmetry cannot be explained with the Standard Model. It is believed that more fundamental
theory of the particle physics exists and the Standard Model is just an low-energy approximation. Various scenarios of the new physics models beyond the Standard Model are considered.
The grand unification theory (GUT) is a theory in which all the interactions are unified at
the energy scale of O(1016 ) GeV. Supersymmetric model (SUSY) assumes the supersymmetric partners of each elementary particle. In these new physics models, the CLFV process are
considered to be induced at a sizable rate, which is even reachable with the experiment. Therefore, searching for the charged lepton flavor violating process is equivalent to searching for anevidence of the new physics.
For example, the SUSY-SeeSaw is a model which predicts both supersymmetric particles
and the tiny masses of the neutrinos by the See-Saw mechanism. Since no supersymmetricparticles in the quark and lepton sector have been discovered at the same mass scale, it isconsidered that the SUSY is violated at the higher energy scale. In order to explain theviolation, the SUSY-SeeSaw model assumes the presence of a non-diagonal mass matrixes of thesupersymmetric particles, and thus, the CLFV rate is enhanced. Figure 1.4 shows the branchingratios of the CLFV processes for the several cases. Recently, relatively large neutrino mixing
angle of θ13 ∼ 8.4◦ was measured [3]. As a consequence, the branching ratio of the µ → eγ
decay is more enhanced in the model. The current best upper limit of the branching ratio ofthe τ → µγ decay is O(10−8 ). On the other hand, the branching ratio of the µ → eγ decay
already reaches O(10−13 ) as described below.

Figure 1.4: Branching ratio of µ → eγ and τ → µγ processes [5]. mN 3 is the mass of theright-handed Majonara neutrino.

1.3. History of µ → eγ search

1.3

5History of µ → eγ search

The CLFV process has been searched for in various experiments for many decades. Particularly, the muon channels have been widely used since the way to produce a lot of muons
has been established. There are also other CLFV processes induced in the new physics models
such as µ → eee or µ − e coherent conversion. Figure 1.5 shows the history of the upper
limit of the branching ratio in the muon CLFV processes. The current best upper limit on the
branching ratio of µ+ → e+ γ decay is B < 4.2 × 10−13 (90% C.L.) [6]. This was set by the
final result of the MEG experiment, which was performed in the period of 2008-2011 at thePaul Scherrer Institut (PSI) in Switzerland. PSI has the world’s most intense µ+ beam, which
can provide 108 µ+ /s. In the MEG experiment, 7.5 × 1014 muons were stopped in the target in
total. Currently, an upgrade experiment, called MEG II experiment, is being prepared at PSI
aiming at one order higher branching ratio sensitivity of O(10−14 ) [8]. The details of the MEG
II experiment are described in the next chapter.

Figure 1.5: History of the upper limits at 90% C.L. on the muon CLFV channels [9]. The
arrows represent target branching ratio sensitivities of the future experiments. Λ shows theenergy scale of the new physics models.

Chapter 2
MEG II experiment
Currently, the preparation for the upgrade experiment (MEG II experiment) is in progress.
It aims to achieve one order higher sensitivity by using the world’s most intense muon beamat the PSI and detectors with significantly improved performance. This chapter first explains
signatures of the signal and background events. Section 2.3 summarizes requirements for theµ+ → e+ γ search. Next, section 2.4 details the experimental apparatus of MEG II including
the beam line and the detectors. The features of the upgrades from the previous experimentare also described. In the last section, the expected performance of the MEG II detectors andthe sensitivity are described.

2.1

Signal and background2.1.1

SignalThe signature of µ+ → e+ γ is a simple 2-body decay at the rest frame (Figure 2.1). A
positron and a photon are emitted back-to-back (Θeγ = 180◦ ) and coincident in time (teγ = 0).
The energies of the emitted photon and positron are both equal to half of the muon mass (Eγ= Ee = mµ /2 = 52.8 MeV).

Figure 2.1: Signal event.
The expected number of signal events Nsig depends on the muon stopping rate on thetarget (Rµ ) , the total acquisition time (T ) and the efficiency to detect the signal (sig ) as
shown in the following equation.

Nsig ∝ Rµ × T × sig
6

2.1. Signal and background

2.1.2

7

Background

There are two types of background events for the µ+ → e+ γ signal, “prompt background”
and “accidental background”. The prompt background is the Radiative Muon Decay (RMD ;µ+ → e+ ν µ νe γ) where both photon and positron carry away large energy (Figure 2.2). In the
MEG II experiment, the fraction of the “The prompt background” will be smaller than that of
MEG due to the improved energy resolutions of the photon and positron detectors.

Figure 2.2: Prompt background.
Another background is the accidental coincidence of an energetic positron from Michel
decay (µ+ → e+ ν µ νe ) and an overlapping photon (Figure 2.3). The source of the photon iseither RMD or Annihilation In Flight (AIF) of a positron.

(a) Michel decay + RMD(b) Michel decay + AIF

Figure 2.3: Accidental background.
The accidental background rate is proportional to the square of the Rµ , it is thereby the mostdominant background source in the MEG II experiment. The number of this background events(Nacc ) is written as
Nacc ∝ Rµ2 × ∆Eγ2 × ∆Pe × ∆Θ2eγ × ∆teγ × T
where (∆Eγ , ∆Pe , ∆Θeγ , ∆teγ ) are the experimental resolutions for the photon energy, thepositron momentum, the opening angle and the relative timing, respectively. Therefore, inorder to distinguish the signal and the background, the good resolution of the timing, energy,
and emission angle are important for both the positron and the photon detector.
Figure 2.4 describes the fraction of the gamma-ray sources in the accidental background.
The two sources in the analysis region (Eγ > 48 MeV) were almost the same in the previous

8

Chapter 2. MEG II experimentexperiment. On the other hand, the fraction of AIF becomes much smaller in MEG II. This ismainly due to the reduced material in the tracking volume of the new positron spectrometer(Figure 2.9). In addition, further identification of the AIF is possible in the analysis. Therefore,
accidental pileup of RMD and Michel decay is the most dominant background event in the MEG
II experiment.

RMD

RMD

AIF

AIF

Figure 2.4: Source of the background photons (Eγ > 48 MeV).

2.2

Requirements for the µ+ → e+γ search

In order to reach the target branching ratio sensitivity B < 4.3 × 10−14 with three years
physics data taking, it is important to have a high muon decay rate. Meanwhile, because theaccidental background rate would be significantly increased due to the high muon decay rate, apulsed muon beam is not suitable for the µ+ → e+ γ search. Moreover, negative charged muons
are not suitable because they will be captured by the nuclei and will form muonic atoms. For
this reason, the MEG II experiment requires a high intensity continuous positive charged muon
beam. The world’s most intense muon beam, which can provide ∼ 108 µ+ /s, is available at theπE5 beam line at PSI.
In the high muon decay rate environment, the suppression of the background rate is
crucial. For this reason, the previous experiment was not operated with the full beam intensity(∼ 3.0 × 107 µ+ /s). Therefore, much better resolutions of the photon and positron detectors
are required in the MEG II experiment. As shown in Figure 1.2, the energy spectrum of the
background photon from RMD falls quickly near the signal energy region. Therefore, it isespecially important to have a good energy resolution for the photon detector.
In order to suppress the AIF background rate, the muon stopping target and the positron
spectrometer are required to have a small amount of materials. As shown in section 2.3.3, MEG
II will use a thinner target by maintaining the muon stopping efficiency. Reducing the materialsof the tracking volume of the spectrometer also helps to suppress the multiple scattering or theenergy loss of the positrons. Moreover, the positron spectrometer has to be operational in thehigh decay rate positrons.

2.3. Experimental apparatus

2.3

9

Experimental apparatus

This section details the MEG II apparatus in PSI. The high intensity muon beam at theπE5 line is produced by a proton accelerator complex. The muons are transported to the MEG
II detector system (Figure 2.9) and stopped by a thin stopping target. The photons and thepositrons from the µ+ → e+ γ decays are detected by a liquid xenon calorimeter and a positron
spectrometer, respectively. For further improvement of the sensitivity, the MEG II experimentintroduces a new detector (Radiative Decay Counter) for an active background identification

2.3.1

Proton accelerator complex and πE5 beam line

The high intensity muon beam is produced by a proton accelerator complex in following
ways.
1. A Cockcroft-Walton accelerator first injects 870 keV protons into a 72 MeV cyclotron(Injector 2).
2. The protons are then injected into the 590 MeV ring cyclotron (Figure 2.5). The 590
MeV proton beam with the current of 2.2 mA is produced.
3. The protons are delivered to the pion and muon production targets (Target M and TargetE). The produced particles are transported to each sub-beam lines.

Figure 2.5: Aerial view of the 590 MeV ring cyclotron [10].
A schematic view of the cyclotron facility and the beam lines are described in Figure 2.6.
The MEG II experiment uses the πE5 beam line which collects “surface” muons produced at
the target E (4 cm thick graphite). The surface muons are produced from the decays of thepions stopped at the target surface Such a muon has a momentum of 29.8 MeV/c and a small
momentum spread.

2.3.2

Beam transport system

The 28 MeV/c muons provided by the πE5 channel are delivered to the stopping target
through the beam transport system. As shown in Figure 2.7, it consists of several components.

10

Chapter 2. MEG II experiment

Figure 2.6: Schematic view of the cyclotron facility and the beam lines at PSI [11].

2.3. Experimental apparatus

11

The first part is composed of two sets of quadrupole triplet magnets (Triplet I,II) and a separator
sandwiched between Triplet I and II. The separator applies perpendicular electric and magnetic
fields and eliminates the positron contamination. After being refocused by the Triplet II, themuons are brought to the beam transport solenoid (BTS) through a collimator. The BTS
transports the muon beam to the stopping target in the COBRA spectrometer magnet. It also
has a degrader made of a 300 µm thick MYLAR foil in order to reduce the muon momentum.

Figure 2.7: Schematic view of the beam line from the πE5 channel to the MEG II detector.

2.3.3

Target

The muon stopping target is placed inside the positron spectrometer. In the previous
experiment, the target made of an ellipsoidal polyethylene-polystyrene foil (Figure 2.8). The
target has the major axis of 200.5 mm and the minor axis of 79.8 mm. The thickness was 205µm and the Rohacell frame was equipped for the mechanical support. The target had a slanted
angle of 20.5◦ with respect to the beam axis in order to reduce the background due to thepositron annihilation and to minimize multiple scattering. Moreover, the target had several
holes to allow a calibration of the vertex reconstruction or an alignment of the target position.
The MEG II experiment uses a thinner target with smaller slanted angle of 15◦ . Since thedeformation of the target was found in the previous experiment, several scenarios concerning
the sizes and the materials are being considered, such as 140 µm thick polyethylene or 90 µm
thick beryllium. In addition to this, a target made of a 130 µm thick scintillation film is being
developed in order to monitor the muon beam profile.

12

Chapter 2. MEG II experiment

Figure 2.8: Target in the MEG experiment.

2.3.4

MEG II detector overview

The MEG detector principally consists of a liquid-xenon photon detector and a positron
spectrometer. The positron spectrometer system, including a COBRA magnet, drift chamber
and timing counter detects positrons. Photons are detected with a liquid-xenon photon detector.
In MEG II, these detectors are upgraded. In addition, the Radiative Decay Counter is newly
introduced on the beam axis.
A schematic view of the MEG II detector is shown in Figure 2.9 with the definition of thecoordinate system. The positive z-axis is defined as the muon beam direction. x and y-axis aredefined in accordance with a right-handed coordinate system, where the positive y-axis points
upward. The distance from the z-axis is defined as r.

yz
XFigure 2.9: Schematic view of the MEG II detector.

2.3.5

Positron Spectrometer

The positrons are detected by the positron spectrometer which consists of several components. The two detectors are placed in a gradient magnetic filed produced by a superconducting
solenoid. A cylindrical drift chamber and a pixelated timing counter detects the track and thetiming of the positrons, respectively. The detail of each component is described in the following
section.

2.3. Experimental apparatus

13

A. COBRA magnet
The COBRA (COnstant Bending RAdius) magnet is a superconducting magnet with agradient magnetic field specially developed for the MEG experiment (Figure 2.10). The total
length is 2.8 m along the beam axis. In order to produce the gradient magnetic field, it consistsof five coils with the three different radii. Additionally, it has two compensation coils to reduce
the stray magnetic field which affects stable operations of the PMTs of the liquid-xenon photon
detector. The wall of the magnet made of a thin layer (0.197 X0 ) in order to transmit photons
emitted from the target in the warm bore of the COBRA magnet.
The produced gradient magnetic filed ranges from 1.27 T at the center to 0.49 T at theboth ends (Figure 2.11). Due to the gradient magnetic field, the positrons emitted from thetarget are quickly swept away regardless of their emission polar angles. Only high momentum
positrons can enter the drift chamber and the timing counter. Therefore, the stable operationof the positron detectors is possible with the high beam intensity..

Figure 2.10: Schematic view and photograph of the COBRA.

Figure 2.11: Distribution of the magnetic field.
B. Drift chamber
The tracks of the positrons are measured by the drift chamber. In the previous experiment,
sixteen drift chamber modules were used. In MEG II, they will be totally replaced with a singlevolume cylindrical drift chamber. The length of the new drift chamber is 193 cm along the beamaxis. It consist of multiple-planes of anode and cathode wires (Figure 2.12). In total, morethan 10,000 wires are used. By alternating wires with stereo angles of 6-8◦ , the detector iscomposed of ten layers of drift cells. Thanks to the small drift cells (6-9 mm), the detector canbe finely segmented and reduce occupancies in the high hit rate environment. Moreover, the14Chapter 2. MEG II experimentdetector is required to have a high transparency in order to suppress the Coulomb multiple
scattering of the positrons. For this reason, the chamber gas with a very low mass mixture(helium : isobutane = 85:15) will be used. Currently, the wiring is in progress in a clean room
by using a semi-automatic wiring machine (Figure 2.13).

Figure 2.12: Schematic view of one of the Figure 2.13: Drift chamber under construccathode plane.
tion.
C. Timing counter
Since the drift chamber is not able to measure the timing of the positrons precisely, thetiming counter is placed outside of the drift chamber. In the previous experiment, 128 plastic
scintillator bars with a double-side PMT readout
In the MEG II experiment, they will be replace with 512 scintillator tiles as shown in
Figure 2.14. The size of the plastic scintillator tile is 100 × (40 or 50) × 5 mm3 typically. The
scintillation light is collected at both ends with multiple-SiPMs, which are connected in series(Figure 2.15). In the laboratory test, a good timing resolutions of 60-70 ps was obtained in eachcounter. Moreover, the timing counter can obtain a much better timing resolution by using themultiple tile hits. In the simulation study, a single positron hits 9.2 tiles on average and an
overall timing resolution of ∼ 30 ps can be obtained.

Figure 2.14: Schematic view of the pixelated timing counter. Only half part is shown.

2.3. Experimental apparatus

15

Figure 2.15: Timing counter. Left : Single counter. Right : Assembled 128 counters before theinstallation.

2.3.6

Photon detector

In the MEG II experiment, the liquid-xenon detector precisely measures the timing, energy
and position of the 52.8 MeV signal photons Thanks to the high light yield and the fast decay
time of the liquid-xenon, a good energy and timing resolution can be obtained. Moreover, theliquid-xenon is superior in terms of the high stopping power and the uniformity. The scintillation
light of the liquid-xenon has a peak around 175 nm in the region of the vacuum ultra violet(VUV). Therefore, special 2-inches VUV-sensitive PMTs were developed in collaboration with
Hamamatsu Photonics.
In the previous experiment, about 900 ` liquid-xenon was surrounded by the 846 PMTs.
However, there was an issue that the energy resolution was deteriorated for the events where
the photon conversion happens near the PMTs on the photon entrance face due to the positiondependent light collection efficiency
In the MEG II experiment, the 216 PMTs at the entrance face will be replaced with 4092
VUV-sensitive SiPMs, each of which has an active area of 12 × 12 mm2 (Figure 2.16). The sizeof the single SiPM is 12 × 12 mm2 . Thanks to the high granularity in the entrance face, theposition and the energy resolution will be improved by factor of two. The detection efficiency
will be also improved by ∼ 10% thanks to the reduced materials of the entrance face. In
addition, the photon entrance face is extended by ∼ 20% to reduce the energy leakage for thephotons entering near the lateral faces.

Figure 2.16: Liquid-xenon photon detector. Left : Cryostat. Right : Inner view after installing
the PMTs and SiPMs.

16

Chapter 2. MEG II experiment

2.3.7

Radiative Decay Counter

In addition to the upgrades of the positron spectrometer and photon detector, the Radiative Decay Counter will be newly installed for the active background identification. On the
beam axis, two detectors will be installed at both upstream and downstream of the stopping
target. The detail of the detector is described in the following chapter.

2.3.8

DAQ boards

The waveforms of each detector are stored in order to measure the pulse timing or charge
precisely even with possible pileups at the high rate environment. For the waveform digitizer,
the MEG collaboration adopted DRS4 (Domino Ring Sampler) chip, which was developed at
PSI (Figure 2.17).
The principle of the waveform sampling is shown in Figure 2.18. The single DRS chip
contains eight sampling channels, a channel for a clock signal and a channel for a trigger signal.
In each channel, the waveform data is stored in a ring of 1024 capacitors. During the operation,
the 1024 capacitors are continuously storing the waveform data at a fixed sampling frequency.
The maximum sampling frequency is ∼5 GHz. When the trigger signal stops the sampling, thestored data is readout by a shift register and digitized by a commercial 12 -bits FADC.

Figure 2.17: DRS chip.

Figure 2.18: Schematic view of the waveform sampling.

The DAQ of the MEG II experiment will be also performed with the DRS chips. In order toconstruct a compact DAQ system for the increased number of the channels, the new DAQ board
named “WaveDREAM” is adopted (Figure 2.19). As shown in Figure 2.20, the WaveDREAM
combines both the waveform digitizing technology of the DRS chip and the trigger system [12].
It also contains a bias voltage supply for the SiPM, pre-amplifier and waveform shaper in eachchannel.

Figure 2.19: Pictures of the WaveDREAM. Left : Single board for 16 channels. Right : Cratefor the boards.

2.4. Expected performance and sensitivity

17

Figure 2.20: Simplified scheme of the WaveDREAM board.

2.4

Expected performance and sensitivity

The expected detector performance is summarized in the Table 5.8. Note that two energy
resolutions of the photon detector are shown depending on the conversion points of the photons.
The w is defined as the distance of the conversion point and the entrance face of the photon
detector.
Currently, each detector is being prepared toward the engineering run in 2017. As shownin Figure 5.23, the MEG II experiment is expected to reach the sensitivity goal of ∼ 5 × 10−14
after three years data taking.
Resolution (RMS)e+ energy (keV)e+ vertex z position (mm)γ energy (%) (w < 2 cm)
γ energy (%) (w > 2 cm)
γ z position (mm)γ-e+ timing (ps)
Efficiency (%)
γ
e+

MEG
306
2.4
2.4
1.7
5
122

MEG II
130
1.6
1.1
1.0
2.6
84

63
40

69
88

Table 2.1: Resolutions and efficiencies of the upgraded detector.

18

Chapter 2. MEG II experiment

Branching ratiowb

10− 13MEG II in 3 years

10− 14
0

20

40

60

80

100
weeks

Figure 2.21: 90% C.L. upper limit sensitivity as a function of DAQ time.

Chapter 3
Radiative Decay Counter
The Radiative Decay Counter (RDC) is the new detector in the MEG II experiment which
is able to improve the sensitivity by identifying significant part of the background photons from
RMD. In this chapter, the detail of the RDC is described.

3.1

Principle of background identification

The concept of the RDC is illustrated in Figure 3.1. As previously mentioned, a positron
emitted from the target follows a trajectory along the gradient magnetic field, which is producedby the COBRA magnet. When a high energy photon is emitted from RMD, a low momentum positron of typically 2-5 MeV is also emitted. This positron does not enter the positron
spectrometer but it is swept away along the beam axis. The bending radius of these positrons
are smaller than 6 cm when the energy of the gamma-ray is greater than 48 MeV. Therefore,
the background photons from RMD can be identified by detecting the time-coincident low momentum positrons on the beam axis. The detectors can be installed at both upstream anddownstream of the muon stopping target. Figure 3.2 shows the expected hit timing differenceof the RDC and the photon detector. The timing peak in the red line is corresponding to the
RMD events. The spread of the 6 ns (FWHM) mainly comes from the fluctuation of the timeof-flight of positrons. According to the simulation result, 41% of total background photons canbe identified by installing two RDC detectors and thus the sensitivity is improved by 22%.

Figure 3.1: Schematic view of MEG II detectors

19

20

Chapter 3. Radiative Decay Counter
fraction (%)

1

10−1

10−2

−3

10

−9

×10
0

10

20

30

40

T RDC - T γ (s)Figure 3.2: Expected hit time difference of the downstream RDC and the detected signal
photon assuming timing resolution of 100 ps. (red) Accidental background. (blue) Positronfrom Michel decay. The last bin shows the events which have no hit in the RDC.

3.2

Concepts of the RDC detector

Because a lot of positrons from Michel decays hit each detector (∼10 MHz), it has to be
operational in such a high hit rate environment. Moreover, there is a high hit rate of thebeam muons in the upstream detector (∼100 MHz). The detectors thereby consist of a fast
plastic scintillators which should be finely segmented in the high rate region. As a result, thelarge number of readout channels are required. Meanwhile, in order to be installed inside thesuperconducting solenoid, the size of the detector has to be as compact as possible (∼20 cm).
For this reason, the scintillation light is collected by using a SiPM because it is small andinsensitive to the magnetic field. Since pileup positrons from Michel decays are distinguishable
by measuring their energies (Figure 3.3), the downstream detector also has a calorimeter basedon LYSO crystals with SiPM readout. In addition to this, the upstream detector is required tohave a minimum influence on the muon beam transportation. Therefore, it consists of a thinlayer of scintillating fibers.

10−1
10−2
10−3
10−4
10−5
10

20

30

40
50
Energy (MeV)

Figure 3.3: Expected energy deposit in the downstream detector. (red) RMD with Eγ > 48
MeV. (blue) Michel decay.

3.3. Downstream detector design

3.3

21

Downstream detector design

A schematic view of the downstream detector is shown in Figure 3.4. It consists of thetiming counter and the calorimeter.

Figure 3.4: Schematic view of the downstream detector. The timing counter is placed in front.
The principle of the background identification was first demonstrated with a small prototype of the downstream detector in 2013 [13]. The prototype detector consists of two plastic
scintillators and four GSO crystals (Figure 3.5). The size of the detector was ∼1/20 of the
final design. It was operated with a high intensity muon beam expected in MEG II and a clear
timing peak of RMD was successfully observed by triggering with the liquid-xenon detector.
Because of the successful demonstration at the beam test with the prototype, the downstream
detector was approved by the MEG II collaboration.

Figure 3.5: Schematic view of the downstream detector prototype.
The early work [14] reported on the development and the construction. Figure 3.6 shows
the downstream detector after the construction. In order to be installed in a strong magnetic
field, all of the support parts are made of non-magnetic materials such as aluminum or brass.
The detail design of the timing counter and the calorimeter are described in this section. The
commissioning of the downstream detector by using the high intensity muon beam is reported
in chapter 5.

22

Chapter 3. Radiative Decay Counter

Figure 3.6: Constructed downstream detector. The timing counter is visible in front.

3.3.1

Timing Counter

The timing counter consists of 12 plastic scintillator bars with multiple-SiPM readout(Figure 3.7). Each scintillator bar has a 5 mm of thickness and several different lengths (719 cm). There are two types of widths (1, 2 cm) and the smaller width is used for central 6
scintillators where the hit rate is relatively high. The scintillation light is collected at the twoends by using a 3×3 mm2 SiPM. In order to obtain a good timing resolution, it is important
to collect as many scintillation photons as possible. For this reason, more than one SiPMs areused at each end. Two and three SiPMs are used at the end of 1 cm and 2 cm width scintillator
bars, respectively.

Figure 3.7: Timing counter.
In order to obtain a good timing resolution, a high light yield of the plastic scintillator is3.3. Downstream detector design23

required. For this reason, BC-418 from Saint-Gobain is selected for the plastic scintillator. This
scintillator also has a very short rise and decay time (Table 3.1). For collecting the scintillation
light, S13360-3050PE from Hamamatsu Photonics is selected for the SiPM because of its highphoton detection efficiency and high gain (Table 3.2). The basic properties of the SiPMs aredescribed in appendix.

Rise time
Decay time
Wavelength of Max. emission

0.5 ns
1.4 ns391 nmTable 3.1: Properties of BC-418 [15].

Effective area
Pixel pitch
Number of pixels
Peak sensitivity wavelength
Dark countGain

3×3 mm2
50 µm
3600
450 nm
500 kHz
1.7×106

Table 3.2: Properties of S13360-3050PE[16].
At each end of the plastic scintillators, the SiPMs are connected in series to reduce thenumber of readout channels. The series connection is superior to the parallel connection interms of the shape of the waveform. When they are connected in series, a rising time of the
waveform becomes shorter thanks to smaller total capacitance of the sensor. As a result, bettertiming resolution can be obtained. In order to operate the series connected SiPMs properly,
SiPMs in close electrical properties were grouped by measuring current-voltage response curvesof all the SiPMs. Figure 3.8 shows the SiPMs mounted on the Printed Circuit Board (PCB).
Six SiPMs in the central 3 scintillators are mounted on the single long PCB because of thelimitation of the space. The SiPMs were mounted by using conductive epoxy (CW2400 from
Circuit-Works) to avoid the possible damage due to heat during soldering. The SiPM and thescintillator were glued with optical cement (Figure 3.9). Each counter was wrapped with areflector and light shielding.

Figure 3.8: Left : PCB for a scintillator with a width of 2 cm. Right : PCB for three
scintillators with a width of 1 cm.
As shown in Figure 3.10, the performance of each counter was checked by using a betaray source (90 Sr). The signal of the SiPMs were saved with a waveform digitizer (DRS4) bytriggering with a small counter based on a plastic scintillator and a SiPM. The waveform data
was analyzed with a software developed for MEG. The timing of the pulses are obtained witha constant fraction method, which calculates the timing where the waveform reaches the 15%of the full pulse amplitude. The timing resolution is defined as sigma of the distribution of
T = (TL − TR )/2 − Ttr , where TL and TR are timing of the left and the right channel of thecounter. Ttr is the timing of a trigger counter which has a 30 ps timing resolution in sigma.
Figure 3.11 shows the measured distribution of the T . For each counter, the timing resolutionof ∼90 ps was obtained, which is good enough for tagging the RMD event.

24

Chapter 3. Radiative Decay Counter

Figure 3.9: Left : Plastic scintillator glued with 3 SiPMs. Right : Short and long PCBs after
being wrapped with the reflector.

Figure 3.10: Schematic view of the setup.
(mppc.cftime[2]+mppc.cftime[3])/2 - mppc.cftime[1]

450
h

400

Entries

27832

Mean

1.614e − 09

350

RMS

1.29e− 10

χ2 / ndf

167.1 / 188300

Constant

423.2 ± 3.2

Mean1.614e −09 ± 7.717e − 13

250

Sigma

1.254e −10 ± 5.352e − 13

200
150
100
50
0
0

1

2

3

4

×10
5

−9

Figure 3.11: Measured distribution of T .

3.3. Downstream detector design

3.3.2

25

Calorimeter

The calorimeter part is placed just behind the timing counter. It consists of 76 LYSO
(Lutetium Yttrium Orthosilicate) crystals (Figure 3.12). The LYSO crystal is a 20 mm cube.
They were produced by Shanghai Institute of Ceramics. Each crystal was wrapped with a 65
µm thick reflector (ESR from 3M), which was produced by laser cutting.

Figure 3.12: LYSO crystals contained in a holder.
The properties of the LYSO crystal are summarized in Table 3.3. It is suitable for high hit
rate operation because of its short decay time. The LYSO crystal has an intrinsic radioactivity
due to the contained radio isotope 176 Lu (2.59 % natural abundance, Figure 3.13). It has along life time of 3.6 × 1010 years and makes an energy peak around 600 keV. Because the rateof the intrinsic radioactivities is low (∼2 kHz), the accidental pileup with the positron signal
would be negligible. On the other hand, the intrinsic radioactivity can be used for the energy
scale calibration of each channel.
Density
Radiation length
Decay constantEmission peakLight yield

7.4 g/cm3
1.14 cm
42 ns
420 nm3 × 104 photons/MeV

Table 3.3: Properties of the LYSO crystal. [17].
Before the construction, the performances of all the crystals were measured by using a
gamma-ray source (60 Co). The detail is described in chapter 4.1. Moreover, the LYSO crystal
has a feature called afterglow, which induces an increase of the sensor current of the SiPM.
Because there are large individual differences among crystals, the effect of the afterglow wasinvestigated for all crystals. The detail is described in chapter 4.2.
Each crystal is readout with a single 3×3 mm2 SiPM (S12572-025P from Hamamatsu
Photonics). Since the saturation of the pixels would be a problem due to the high light yield of
the crystals, the small pixel size of 25 µm was selected. Figure 3.14 shows one of a PCB in thecalorimeter. The PCB has several flexible branches where the SiPMs are mounted. The SiPM ispressed on backside of the crystal with a spring and they are not glued for ease of maintenance.

Anode Signall (526Figure 2. PreLude 420 response to
511 keV with R3241 PMT

Chapter 3. Radiative Decay Counter

Counts/s/keV/cc

Counts/s/keV/cct (ns)

Figure 2. PreLudeTM 420 response to
511 keV with R3241 PMT
E (keV)E (keV)

Figure 4. Beta distributions

Figure 3.13: Decay scheme of 176 Lu and energy spectrum of the LYSO intrinsic radioactivity[18].

Figure 4. Beta distributions

Figure 3.14: PCB in the calorimeter.
½ mm

Therefore, the optical coupling of the crystals and the SiPMs need to be optimized. The detailis described in chapter 4.3.
Energy (keV)
In order to absorb the stress of the springs (∼2.5 kg in total), several support plates areinserted (Figure 3.15). The material thickness has to be as small as possible between theProperty
crystals and the timing counter. Therefore, the support plate consists ½ofa thick Rohacell platemm(3.3 mm) and two thin carbon fiber plates (CFRP : 0.2 mm × 2). In addition, a thinDensity
aluminum[g/cm3]
Table comparing principal properties
plate (0.1 mm) is inserted for the light shield of the crystals.
On the other side of the crystals,
Attenuation length fof PreLudeTM 420 versus BGO and LSO
a carbon fiber plate and two Derlin plates are inserted.
Energy (keV)
Decay
The springs are fixed on a chassis (Figure 3.16). After cabling, all the cables are
fixedtimeon [ns]the chassis not to touch the springs. For light shielding, an aluminum holder is equippedtoEnergy resolution
Property
cover the whole calorimeter (Figure 3.17).
Light output, photo
Density [g/cm3]
Table comparing principal properties
Average temperatur
Attenuationof PreLudeTM 420 versus BGO and LSO
25 to 50oClength(%/oC)forDecay time [ns]Energy resolution

Light under
output,photons
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25 to 50 C (%/otheC)right

is a registered tra®420

Saint-Gobain Crystals
www.crystals.saint-gobain.com

Manufacturer reserves the ri©2004-2016
Saint-Gobain
CProtected
under patents:
US66244

Management Systems of Saint-Gob
registered to ISO 9001:2008.
Manufacturer reserves the right tois a registered trade®420

Saint-Gobain Crystals
www.crystals.saint-gobain.com

Manufacturer reserves the righ©2004-2016 Saint-Gobain Cer

3.3. Downstream detector design

27

Figure 3.15: Support structure in the calorimeter. Left : Between the crystals and the timing
counter. Right : Between the crystals and the PCBs.

Figure 3.16: Chassis to fix the springs.

Figure 3.17: Aluminum cover for the light shielding. In order to prevent light leakage, black
tapes are attached at the corner.

28

3.4

Chapter 3. Radiative Decay Counter

Upstream detector design

Presently, the installation of the upstream RDC detector has not been approved yet because it requires additional R&D concerning the operation in the high intensity muon beam(∼ 108 µ+ /s). In the provisional design, the upstream detector consists of a thin layer of scintillation fibers (Figure 3.18). We will use a double-clad square shaped scintillating fiber with athickness of 250 µm. In order to cover the full beam pipe, about 780 fibers will be used.

Figure 3.18: CG image of the upstream detector.
When the upstream detector is not installed, a 300 µm thick Mylar foil degrades themomentum at a beam waist in order to stop the 28 MeV/c of muons at the target. In principle,
the upstream detector can be installed just by thinning the degrader. However, because it is
not installed at the waist position, the upstream detector could affect the beam property.
In addition, the detection efficiency loss due to pileup beam muons is a potential issue.
According to the simulation study, the hit rate of the muons is ∼ 500 kHz at the central fiber.
To minimize the effect of the pileup, the detector has to be finely segmented in the high hit rate
region. On the other hand, the upstream detector has to be installed inside a limited space asshown in Figure 3.19. Therefore, the fibers are grouped into a few tens of bundles and they are
bent at right angles. Each bundle end is readout with a single SiPM. A series of the studies onabove issues are reportred in chapter 6.
Figure 3.20 illustrates the provisional mechanical design of the detector with a flangeof the beam pipe. Each SiPM is contained in a house in order to fix the alignment andreduce the damage due to the irradiation. Because the detector will be installed in the helium
atmosphere, the signals are transmitted with feed-through connectors. In provisional design,
the total number of the bundles is 18, which is limited by the available space for the readout
SiPMs.
The upstream detector is desired to have a timing resolution of ∼500 ps in sigma by the
double side readout. Figure 3.21 shows the bundle of fibers which was produced for a test.
In order to make the detector as plane as possible, the fibers are glued together with opticalcement by using a plane mold.

3.4. Upstream detector design

29(a) Whole view of the detector(b) Upstream side

Figure 3.19: Schematic view of the MEG II detector. The upstream detector is installed
between the drift chamber end cap and the COBRA end cap where a small space is available(r ∼ 20 cm, z ∼ 2 cm).

30

Chapter 3. Radiative Decay Counter

Figure 3.20: Provisional mechanical design.

Figure 3.21: Prototype of the bundled fibers (64 fibers ×2).

Chapter 4
Development of LYSO calorimeter
Before constructing the downstream RDC, several studies on the calorimeter were required.
In order to evaluate the performance of the calorimeter, we first studied on the properties of all
the LYSO crystals. The light yield and energy resolution was measured one by one. During themeasurement, we observed an afterglow of the crystals which is one of the features of inorganic
scintillators. Since afterglow could affect the performance of the calorimeter, we studied it byusing a room light and β-ray source. The details of the studies are described in section 4.2.
Because the crystal and the SiPM are not glued in the calorimeter, it is necessary to optimize
the optical coupling. We investigated the best way of the coupling by comparing the light yieldof the crystal. The detail is summarized in section 4.3.

4.1

Mass test of the LYSO crystals

We measured the light yield and the energy resolution with all the LYSO crystals by using
the setup shown in Figure 4.1. The crystals were wrapped with a reflector and tested one byone. The SiPM was fixed on the crystal by using spring pins. A gamma-ray source (60 Co) was
set just beside the crystal. The signal of the SiPM was transmitted to a waveform digitizer(DRS4 evaluation board). Figure 4.2 shows the observed light yield spectrum. Two photopeaks of the gamma-ray (1.17, 1.33 MeV) are overlapping with the spectrum of the intrinsic
radioactivity.
-1*charge[0]hEntries36053
Mean 2.084
RMS

1000

800

600

400

200

0
0

1

2

3

4

5

Light yield (a.u.)

Figure 4.1: Schematic view of the setup.
Figure 4.2: Energy spectrum.
31

6

1.05

32

Chapter 4. Development of LYSO calorimeter

The light yield and energy resolution were obtained by fitting the photo-peaks with gaussians in which beta-decay spectrum was convoluted. The energy resolution R was defined as
R = σ/Eγ , where Eγ and σ are the mean and sigma of each gaussian. Figure 4.3 shows theresult for the 76 crystals. Thanks to the high light yield of the crystal, good energy resolutions
were obtained in most of the crystals (∼6% at 1 MeV).

Resolution at 1 MeV (%)

8
7.5
7
6.5
6
5.5
5
4.5
4
6

7

8

9

10

11

Light yield (a.u.)

Figure 4.3: Distribution of the light yield and energy resolution.

4.2

Afterglow

During the mass test of the crystals, a large variation of a sensor current of the SiPM wasobserved. The reason of the variation was found to be “afterglow” of the crystal which is also
called “phosphorescence” [19]. The afterglow has been observed in several types of inorganic
scintillators including the LYSO. Some excited electrons in the scintillator are trapped in latticedefects, such as oxygen vacancies. Afterwards, these trapped electrons induce late scintillation
photons which usually have a long time constant (typically few hours). When the crystal is
exposed to a room light, a lot of electrons are excited and trapped. Therefore, the sensor
current of the SiPM was significantly increased due to the large number of random scintillation
photons. However, the afterglow due to exposing to the room light would not be a problem if
we keep the calorimeter in a dark place before the operation. On the other hand, the afterglow
due to the high hit rate of positrons (∼600 kHz at the central crystal) may not be negligible.
The influence on the energy resolution needs to be properly understood.

4.2.1

Study with room light

We studied on the afterglow with the all 76 crystals by using the room light. The sensor
current of the SiPM was measured with a setup shown in Figure 4.4. Before the measurement,
all the crystals were kept in a dark place over 48 hours. Afterward, the crystals were placed inseveral rows and they were uniformly exposed to the room light over 24 hours. After exposing

4.2. Afterglow

33

to the room light, the crystals were taken out and tested one by one. We applied the bias
voltage to the SiPM by using a picoammeter (Keithley, 6485) and read the sensor current.
Each measurement was carried out immediately after taking out the crystal (∼ 30 s).

Figure 4.4: Setup of the afterglow measurement with the room light. Left : 76 crystals exposedto the room light. Right : Schematic view of the setup for the current measurement.
As a result, the large variation of the current was observed (Figure 4.5). When the crystals
are kept in a dark place for long time (12 hours∼), the sensor current of the SiPM would be
typically a few micro-amperes. On the other hand, after exposing to the room light, the current
was increased in most of the crystals. Especially several crystals induced the very large current
which is over a hundred micro-amperes. It seems that the crystals in close serial numbers have
similar tendency.
250

Current ( µA)

200

150

100

50

0

0

10

20

30
40
50
60
Serial number of the LYSO crystal

70

80

90

Figure 4.5: Measured sensor current of the SiPM. The 76 crystals were measured according
to the order of their serial numbers. Note that several serial numbers are skipped because
we replaced several crystals in which we found some defects. The red and blue plots arecorresponding to the results of the next measurement (Figure 4.6).
In order to check the reproducibility, the measurement was repeated twice for the five crystals (serial number : 1, 8, 35, 45, 62) by using the same setup. In the first measurement, the34

Chapter 4. Development of LYSO calorimeter

160

160

140

140

120

120
Current (µA)

Current (µA)

order of the measurement was 35→62→8→45→1, and it was inverted in the second measurement. The placement while exposing to the room light was also inverted in two measurements.
The results show that the variation of the current values is independent of the order of themeasurement (Figure 4.6). Hence, it can be seen that the current values are correlating to the
production serial numbers of the crystals.

100
80
60

100
80
60

40

40

20

20

0

35
1

62
45
38
4
2
Serialnumberof the
LYSO
crystalSerialnumberof thecrystal

0

51

1
1

62
45
8
4
2
3
Serial number of the crystal

35
5

Figure 4.6: Measured sensor current of the SiPM and the serial number of the crystal. The
red and blue plots show the result of the first and the second measurement, respectively. The
order of the serial numbers corresponds to the order of the measurement.

We also measured how the increased sensor current changes after putting back the crystal
to the dark place. This measurement was performed for the one crystal (serial number 54) inwhich we observed the largest current. The current was recorded every fixed period by using
a power supply slot card of the SCS-2000, which is developed in PSI. As shown in Figure 4.7,
we observed that the current was exponentially decreasing after stopping exposure to the room
light.

80
70

Current (µA)

60
50
40
30
20
10
0
0

0.5

1

1.5

2

2.5

3

Time (hours)

Figure 4.7: Observed sensor current of the SiPM after exposing to the room light. The values
over 100 µA are not recorded due to the current limitation of the power supply.

4.2. Afterglow

4.2.2

35

Study with β-ray source

As previously mentioned, it is necessary to consider the afterglow due to the high hit rateof the positrons. In order to investigate the influence on the performance of the calorimeter, aseries of measurements were performed by using a β-ray source (90 Sr). We firstly monitored thecurrent during the β-ray irradiation with the one crystal (serial number 45). After keeping the
setup in a dark place for long time, 3.7 MBq of 90 Sr was set beside the crystal and the current
was monitored with the SCS2000 (Figure 4.8). During the measurement, the temperature
was kept at 26 ◦ C by using a thermal chamber. The temperature and the bias voltage of
the SiPM were also monitored. The measurement was carried out for almost 20 days. As aresult, we observed that the current was exponentially rising in the first few hours (Figure 4.9).
Afterwards, it had kept increasing very slowly for 20 days.

Figure 4.8: Schematic view of the setup.

Figure 4.9: The observed current of the SiPM during the β-ray irradiation. Left : Region thatthe current exponentially increases. Left : Whole measurement.
On the other hand, we can expect that the number of the emitted scintillation photons
will be exponentially increased with the afterglow by assuming as follows. By assuming thata certain number of electrons are excited and some of them are trapped in a time interval∆t, the time of the de-excitation follows a geometric distribution with ∆t → 0. Figure 4.10
shows the expected number of scintillation photons with this assumption. The similar behavior

36

Chapter 4. Development of LYSO calorimeter

was reproduced. However, the reason why the current had kept increasing for 20 days is notunderstood.
χ 24.882e+04
/ ndf
/ 598

p0 1e+04 ± 0.656

0.009795
p1± 0.0003771

N photonp2

468.8 ± 17.16

10040
10020
10000
9980
9960
9940
9920
9900
0

200

400

600

800

1000

Time (sec)Figure 4.10: Expected number of emitted scintillation photons with afterglow (black). The redline shows the fitting with the below function.
Next, we checked the reproducibility of the current changes due to the afterglow by repeating the irradiation and shielding in turn (Figure 4.11). The same crystal (serial number45) was used in the measurement. During the irradiation, the distance of the β-ray source andthe crystal was always fixed. The β-rays were shielded by inserting a ∼5 mm thick lead plate
between the source and the crystal. As a result, the similar behaviors were observed in each
irradiation or shielding period.
For further quantitative evaluation of the result, we characterized the measured current I
as a function of time t as :I = C1 ± exp(−

t − C2)τ(4.1)

where τ and Ci are arbitrary constants. The constant τ characterizes the rise or ta time. By
assuming both the rise and tail are asymptotic, the maximum variation of the current can be
expressed as ∆Imax = exp(C2 /τ ). Figure 4.12 exemplifies the fitting of the rising and tailingregions in Figure 4.12 and Table 4.1 summarizes the fitting parameters. As a result, similar τand ∆Imax were obtained in either rising or tailing region.

Irradiation
Shielding∆Imax (µA)
τ (hours)
1.502 ± 0.007 1.771 ± 0.013-1.490 ± 0.001 1.733 ± 0.003

Table 4.1: Obtained parameters.
Finally, we checked if there were any individual differences among crystals as well as we
had observed in the measurement with the room light. The current measurement during theirradiation was performed with the six crystals. Five of them had been previously measured withthe room light for three times. In addition, another crystal (serial number 28) was measured.
In this measurement, more powerful β-ray source (37 MBq of 90 Sr) was used.

4.2. Afterglow

37(a) Irradiation(b) Sheilding

98Current (µA)

7
6
5
4
3
2
1
0

0

20

40

60

80

120

100

140

160

Time (hours)

8.8

2.4

8.6

2.2

8.4
8.2
8
7.8

χ2 / ndf
950.2 / 477
p08.637 ± 0.003
p1
0.5645 ± 0.0043
p2
0.7203 ± 0.0064

7.6
7.4
7.2
7
0

Current (µ A)

Current (µ A)

Figure 4.11: Setup for the reproducibility test and the measured sensor current of the SiPM.
Current jumps during irradiation are due to a technical problem of the power supply.

χ2 / ndf
5709 / 477
p00.8136 ± 0.0004
p1
0.577 ± 0.001
p2
0.6912 ± 0.0011

2
1.8
1.6
1.4
1.2
1
0.8

2

4

6

8

Time (hours)

10

12

14

0

2

(a) Irradiation

Figure 4.12: Examples of fitting.

4

6

8

Time (hours)(b) Shielding

10

12

14

38

Chapter 4. Development of LYSO calorimeter

The observed current and fitting parameters are summarized in Figure 4.13 and Table 4.2.
The result shows that each crystal has unique τ and ∆Imax . As shown in Figure 4.14, nocorrelation was found between τ and ∆Imax . On the other hand, ∆Imax showed a correlation
with the measured current values after exposing to the room light.
As shown in Figure 4.12 and Figure 4.13, the same crystal (serial number 45) was measured
with two sources with different intensities. In each measurement, the hit rate of the β-rays was
also measured by using a discriminator and scaler. The threshold of the discriminator was
adjusted as small as possible, but large enough not to trigger on the dark noise of the SiPM.
As summarized in Table 4.3, ∆Imax became larger with the higher hit rate.

33

29.8

32

29.4

Current (µ A)

Current (µ A)

29.6
29.2

31
χ2 / ndf

30

p0
p1
p2

29

29

28.8

956.2 / 477
32.71 ± 0.00
0.5434 ± 0.0025
2.661 ± 0.012

28.6

χ2 / ndf
789.7 / 477
p0
29.01 ± 0.00
p1
0.6503 ± 0.0447
p2−1.65 ± 0.14

28.4
28.2
28

28
0

2

4

6

8

Time (hours)

10

12

0

14

2

(a) Serial number 45

4

6

8

Time (hours)

10

12

14

(b) Serial number 1
26.8

43.2
26.6
43

Current (µ A)

Current (µ A)

26.4

42.8

26.2

42.6

42.4

42
0

25.8

χ2 / ndf
970.4 / 477
p042.84 ± 0.00
p1
0.7532 ± 0.0312
p2−0.6046 ± 0.0477

42.2

2

4

6

8

Time (hours)

10

12

26
χ2 / ndf
1096 / 477
p0
26.17 ± 0.00
p1
0.5875 ± 0.0332
p2−1.704 ± 0.117

25.6

14

25.4
0

2

(c) Serial number 35

4

6

8

Time (hours)

10

12

14

(d) Serial number 62
47.5
47
46.5

Current (µ A)

Current (µ A)

39.5

39

46

45.5

38.5

χ2 / ndf
p0
p1
p2

38

45

44.5

1077 / 477
39.43 ± 0.00
2.503 ± 0.053
0.2188 ± 0.0043

χ2 / ndf
p0
p1
p2

44

43.5

931 / 477
46.93 ± 0.00
0.805 ± 0.006
1.64 ± 0.01

43
37.5
0

2

4

6

8

Time (hours)

10(e) Serial number 8

12

14

0

2

4

6

8

Time (hours)

10

12

14

(f) Serial number 26

Figure 4.13: Sensor current of the SiPM during the irradiation with 37 MBq 90 Sr. The orderof the figures is corresponding to the order of the measurement.

4.2. Afterglow

39
Serial number
45
1
35
62
8
26

∆Imax (µA)
4.246 ± 0.028
0.342 ± 0.002
0.634 ± 0.004
0.367 ± 0.002
1.729 ± 0.012
3.744 ± 0.025

τ (hours)
1.840 ± 0.009
1.538 ± 0.011
1.328 ± 0.055
1.702 ± 0.096
0.400 ± 0.008
1.242 ± 0.009

4

4

3

3

∆ I max (µA)

∆ I max (µA)

Table 4.2: Fitting parameters.

2

1

0

2

1

0.5

1

1.5

τ (hours)

0
0

50

100

150

Measured current with the room light (µA)

Figure 4.14: Left : ∆Imax vs. τ . Right : ∆Imax vs. measured sensor current after exposing to
the room light. The black plot shows the additional measurement with serial number 28. The
current with the room light was measured three times in the red plots. The error bar is thestandard deviation of the three results.
Radioactivity of the 90 Sr (MBq)
3.7 (nominal)
37

Measured hit rate (kHz)161
372∆Imax (µA)
1.502 ± 0.007
4.246 ± 0.028

τ (hours)
1.771 ± 0.013
1.840 ± 0.009

Table 4.3: Comparison of the parameters for different source intensities.

4.2.3

Influence on the energy resolution

The influence on the energy resolution was estimated by calculating the number of photoelectrons contained in the waveform of the SiPM. Let us consider the case where the waveform
contains photoelectrons induced by afterglow. The total number of photoelectrons in a single
waveform can be expressed as :Nall = Nsig + NAG ,

(4.2)

40

Chapter 4. Development of LYSO calorimeter

where Nsig and NAG are the number of photoelectrons from the signal positron and the afterglow,
respectively. Due to the photoelectrons from afterglow, the mean value of the photo-peak
becomes larger. Here, the energy resolution is defined as the ratio of the variation of Nall (σall )and the absolute energy of the signal. Hence, the energy resolution R can be written as :

σall
,
Nsig

R =

(4.3)

where σall is the variation of Nall . By assuming that the number of photoelectrons follows a
Poisson distribution, the energy resolution can be written as :q

R =Nsig + NAG
Nsig

.

(4.4)

Therefore, the contribution of afterglow can be calculated with the energy Nsig and NAG .
Let us consider the 6% energy resolution at 1 MeV, where Nsig can be approximately estimatedas ∼280. On the other hand, NAG can be calculated as :NAG =∆Imax × W
,
G×e

(4.5)

where W is a width of the signal waveform and G is a gain of the SiPM. e is an elementarycharge (1.6 × 10−19 C). The typical width of the waveform is ∼300 ns. The gain of the SiPM isapproximately 6.0 × 105 according to the spec sheet [16]. Because ∆Imax is calculated as thedifference of the two current values, the effect of the dark current of the SiPM is not included.
Figure 4.15 shows the expected energy resolution as a function of ∆Imax .
Energy resolution (%)

100*(TMath::Sqrt(280+((x/1e6)*300e-9)/(6.0e5*1.6e-19)))/(280)

7.4
7.2
7
6.8
6.6
6.4
6.2
6
0

5

10

15

20

25

30

35

40

45
50
∆I maxFigure 4.15: Expected energy resolution.

4.3. Optimization of optical coupling

4.2.4

41

Discussion

As a conclusion of the study, we can conclude that the influence of afterglow is expected
to be small. The reasons are as follows :• The crystal (serial number 45) induced the largest sensor current of the SiPM in all the
crystals after exposing to the room light. (Figure 4.6)
• The measured current in two results (with the room light and β-ray source) are correlated(figure 4.14).
• ∆Imax in the serial number 45 can be considered as less than 10 µA even if it is placed at
the central area of the calorimeter where there is a high hit rate of ∼600kHz (Table 4.3).
Therefore, as shown in Figure 4.15, the influence on the energy resolution is expected to beless than 1% even if the crystal with a large afterglow is placed at the center of the calorimeter.
Further reduction of the influence is possible by placing the crystal with a small afterglow at
the center.
On the other hand, as shown in Figure 4.9, we observed that the current had kept increasingfor 20 days. However, because of its very slow change, the total variation of the current is
expected to be 50 µA at most even if we keep operating the calorimeter for 200 days. Therefore,
we concluded that the influence on the energy resolution due to the slow component is expected
to be also small (less than 1.5%).

4.3

Optimization of optical coupling

Because the crystal and the SiPM are not glued in the calorimeter, the optical couplingneeds to be optimized. We compared three possibilities as shown in Figure 4.16 for the optical
coupling. The pad made of soft elastic materials which has a thickness of 1 mm. It is commonlyused for the optical coupling of a scintillator and a PMT. One the other hand, the film is muchthinner (20 µm) since it is developed for the coupling between light fibers.

(a) Pad (Saint-Gobain, BC634A) (b) Film (Tomoegawa, FW205)(c) Grease (Eljen, EJ550)

Figure 4.16: Candidates for the optical coupling.
The best way of the optical coupling was investigated by comparing the light yields ofthe crystal. The same setup as shown in Figure 4.1 was used. In addition to those three
configurations, the measurement without any optical coupling materials was also performed.
Figure 4.17 shows the measured light yield at the 1.17 MeV peak of 60 Co with different over

42

Chapter 4. Development of LYSO calorimeter
−9

0.22

×10

Light yield (C)0.2

No material

0.18

Pad

0.16

Film

0.14

Grease

0.12
0.1
0.08
0.06
0.04
3

3.2

3.4
3.6
3.8
Over Voltage of the SiPM (V)4Figure 4.17: Light yield of the crystal and the over voltage of the SiPM.
voltages of the SiPM. The significant improvement of the light yield was observed by using thepad or grease.
As can be seen in the result, the grease is a little superior to the pad. In order to check the
reproducibility, we repeated same measurement only with the grease and the pad (Figure 4.18).
As a result, a larger light yield was always obtained by using the grease. Therefore, we concluded
that using the grease would be the best way for the optical coupling.
−9

×10
0.15

4.8

Pad

0.14

Pad

Grease

4.6
GreaseResolution (%)

Light yield (C)

0.13
0.12
0.11
0.1
0.09

4.4

4.2

4

0.08
0.07
0

2

4

6

8

10

12

Measurement number

14

16

18

3.8
0.08

0.09

0.1

0.11

0.12

0.13

×10−9
0.14

Light yield (C)Figure 4.18: Comparison of the grease and the pad. Left : Comparison of the light yield. The
arrows represent the order of the measurement. Right : The energy resolution and the lightyield.

Chapter 5
Commissioning of the Downstream
Detector
In July, 2016, the first commissioning of the downstream detector was carried out by using
a high intensity muon beam at the πE5 beam line. This chapter describes the details of thecommissioning from the setup to the data analysis. We successfully demonstrated the capabilityof the identification of the RMD events. At the same time, several issues were revealed. The
details of the issues and the prospects are discussed in the last section.

5.1

Setup

The setup of the commissioning is shown in Figure 5.1. As the same condition as the MEG
II experiment, the high intensity muon beam (∼ 108 µ+ /s) was transported to the stopping
target. The downstream RDC detector was installed at end cap of the COBRA cryostat.
The low momentum positrons were swept along the beam axis by a gradient magnetic field
produced by the COBRA magnet. As a substitution for the liquid-xenon photon detector, adetector based on BGO crystals with PMT readout was used. The positron spectrometer was
not used in this commissioning. The details of the apparatus and DAQ setup are described
below.

Figure 5.1: Schematic view of the setup.

43

44

5.1.1

Chapter 5. Commissioning of the Downstream Detector

RDC installation

During the MEG II data taking, the downstream RDC is required to be movable. This isbecause a calibration system for the photon detector is needed to be inserted on the beam axis
from the downstream side. Therefore, the downstream detector is mounted on a moving arm.
Figure 5.2 shows the stage with the moving arm, which was developed by the detector groupin PSI. In order to be operational in the strong magnetic field, most of the components are
made of aluminum. On the other hand, several parts in which there are heavy loads, such as
the shaft of the arm, are made of titanium. The detector is moved by two plastic water pistonsbehind the stage. The position of the detector can be monitored with two end switches.

(a) front(b) back

Figure 5.2: Stage of the downstream RDC.

The mechanism of the moving arm was tested in the laboratory with the RDC detector
mounted. As shown in Figure 5.3, the detector can take two positions. While the calibration
system is inserted, the detector stays in the parking position. The moving system was successfully tested in the laboratory, however, during the commissioning, the detector was always set
to the measuring position due to a geometrical conflicts between the front part of the detector
and the end cap of the COBRA.

(a) parking position(b) measuring position

Figure 5.3: Positions of the detector.

5.1. Setup

45

Because the detector is installed in the helium atmosphere, all the cables are connected
to outside electronics by using two feed-through PCBs. The feed-through PCB was originallydeveloped for the MEG II photon detector. It is designed to have a high density signal transmission from vacuum and realize a low noise environment. The single PCB can contain 60 channels
at most (Figure 5.4). The downstream detector has 100 signal channels (timing counter : 24
channels, calorimeter : 76 channels) in total. In addition, two thermometers (PT100 : 2 channels × 2) and two end switches (2 channels × 2) were connected to the same PCB. The one
thermometer was equipped inside the calorimeter, near the SiPMs. The other one was placedoutside the main frame of the LYSO crystals. The temperatures had been always monitored
during the commissioning because the gain of the SiPMs are affected by the change of thetemperature.
Figure 5.5 shows the detector after the cabling and the light shielding. All the cables were
fixed on the stage, in order to avoid the interruption with the moving arm.

Figure 5.4: Feed-through PCB.

5.1.2

Figure 5.5: Detector after cabling andlight shielding.

BGO photon detector

The photon detector consists of 16 BGO (Bismuth Germanium Oxide) crystals readout
with PMTs (Figure 5.6). The BGO detector was originally developed as a tagging detector
for the energy calibration of the liquid-xenon detector. It was designed to detect high energy
photons (54.9, 82.9 MeV) from π 0 → γγ, which follows the charge exchange reaction (π − p →π 0 n). The BGO crystal is superior in terms of a short radiation length (1.12 cm) and a small
Molière radius (2.23 cm). The crystals are readout with fine-mesh-type PMTs (H8409-70, from
Hamamatsu Photonics), which are operational in the strong magnetic field. Therefore, we
considered that the BGO detector would efficiently detect the high energy gamma-rays fromRMD (Eγ > 48 MeV). For the original purpose of the BGO detector, it was placed at the
opposite side of the liquid-xenon detector. The detector was aligned to face forward the center
point of the target.

46

Chapter 5. Commissioning of the Downstream Detector

Figure 5.6: BGO detector. Left : 16 BGO crystals. A front cover of the crystals is not
attached. Right : BGO crystal attached with the PMT.

5.1.3

DAQ

The signals from the RDC and BGO detector were transmitted to the WaveDREAMboards (Figure 5.7). The WaveDREAM board contains a power supply, waveform shaper, preamplifier and waveform digitizer. Because each board contains 16 channels, eight boards wereprepared for the RDC and the BGO detector. The boards assignment for each detector and theconfigurations of the pre-amplifiers are summarized in Table 5.8. Each waveform was digitizedwith a sampling frequency of 2 GHz. Since each channel of the BGO detector requires a highbias voltage (∼2 kV), another power supply was prepared.

Nchannel

NboardGain

Timing counter

24

2

100

Calorimeter

76

5

1

BGO detector

16

1

1

Figure 5.8: Boards assignment for each detecFigure 5.7: WavedDREAM boards after ca- tor and the gain of the pre-amplifiers.
bling. The left half part was used for the RDC
and the BGO detector.

5.1.4

Signal check

Before the data taking, we checked the signals of all the channels in each detector. For
the RDC detector, the waveforms were checked with an oscilloscope by applying the bias
voltages with a picoammeter (Keithley, 6485). The waveforms of the cosmic-rays and intrinsic
radioactivities of the LYSO crystal were checked for the timing counter and the calorimeter,
respectively.
No strange waveforms were observed in the calorimeter. On the other hand, we could notobserve any waveforms in four channels in the timing counter (Figure 5.9). In these channels,
the current of the SiPMs were nearly zero even if we applied the bias voltages. After thecommissioning, the cause of the problematic channels was investigated. The detail is discussed
in section 5.5.

5.2. Calibration

47

Figure 5.9: Problematic channels in the timing counter (red parts).
For the BGO detector, the signals of the cosmic-rays were checked. No problematic channels were found. In order to optimize the bias voltage of each PMT, the waveform data was
acquired beforehand. The details of the calibration of the BGO detector are described in thenext section.

5.2

Calibration

We firstly measured the timing offset of the RDC and the BGO detector in order toproperly set the timing window for the RDC. The offset mainly comes from the difference ofthe cable lengths. Figure 5.10 shows the schematic view of the set up for the measurement.
We prepared a trigger counter based on the plastic scintillator and SiPMs, whose cable lengthis same as the RDC detector. The trigger counter was put on the top of the BGO detector
and same cosmic-ray events were acquired. The timing offset was measured to be 30.8 ns by
calculating the constant fraction timing for each waveform.

(a) Commissioning(b) Timing offset measurement

Figure 5.10: Schematic view of the setup.

5.2.1

Non-linearlity of the WaveDREAM

During the calibration, a non-linearity of the readout amplitude of the WaveDREAM wasfound. In order to know the correct energy scales with the calibration, the response functions

48

Chapter 5. Commissioning of the Downstream Detector

were measured. By using a function generator (Agilent, 81150A), we made an input pulse
which is similar to the that of the LYSO crystals (Figure 5.11). We compared the input pulse
height and the readout amplitude which was calculated as a difference of the peak amplitude
and the baseline amplitude.

Figure 5.11: Schematic view of the input pulse. Several pulse heights were used.

[mV]

Figure 5.12 shows the response functions which were measured by changing the input pulse
height. All the channels in the calorimeter and the BGO detector were measured. Because thetiming counter does not require the precise energy calibration, the common response function
was used for all the timing counter channels. Before the waveform analysis, the amplitude dataof each bin was corrected with these response functions. The example of the corrected waveform
is shown in Figure 5.13. In the final design of the WaveDREAM board, the non-linearity will
be fixed.

0

− 200

− 400

− 600

− 800
−500

−400

−300

−200

−100

0

[nsec]

Figure 5.13: Example of waveform correction(calorimeter channel). Red and black shows the
waveform before and after correction, respecFigure 5.12: Input pulse height and the the tively.
measured pulse height of the output waveform.

5.2. Calibration

5.2.2

49

Energy scale calibration

In order to equalize the gain and to obtain the absolute energy scale of each channel, eachdetector was calibrated with the different calibration sources.

A. RDCThe timing counter was calibrated by using the positrons from Michel decay. During thecommissioning, the calibration data was acquired by triggering on any hit of the timing counter.
Figure 5.14 shows the simulated actual energy deposit and the measured spectrum fitted with a
Landau’s function. According to the simulation study with GEANT4, the mean energy depositof the Minimum Ionizing Particle (MIP) is about 0.8 MeV.

ch15
title

400

1400

350

1200

χ2 / ndf = 366.3 / 78
1000

7476 ± 114.6

Constant
MPV

800

0.7946 ± 0.0010

Sigma 0.04406 ± 0.00052

600

χ2 / ndf

300
250

50.5 / 34
2285 ± 50.9
0.127 ± 0.001
0.02057 ± 0.00047

200
150

400

100

200

50

0
0

Constant
MPV
Sigma

0
0.5

1

1.5

2

2.5

3

3.5

4
4.5
Energy (MeV)

0

0.2

0.4

0.6
0.8
charge (a.u.)

Figure 5.14: Simulated energy deposit (left) and measured spectrum (right).

As previously mentioned, the calorimeter is calibrated by using a peak of the intrinsic radio
activity of the LYSO crystals. During the commissioning, the calibration data was acquiredby triggering on any hit of the crystal with the beam turned off. As shown in figure 5.15, thespectrum has both photo-peaks of the gamma-ray and β-decay spectra. We fitted the photopeak corresponding to ∼ 600 keV. The fitting function consists of a photo-peaks and a β-decay
spectra, which are smeared with the energy resolution.

50

Chapter 5. Commissioning of the Downstream Detector

Figure 5.15: Left : Expected energy spectrum (blue). The red and the green parts show thephoto-peaks of the gamma-rays and the β-decay spectrum, respectively. The Q-value of the
β-decay spectrum is 1192 keV [20]. Right : Example of the measured spectrum in the single
channel.

B. BGO detector
The BGO detector was calibrated with two calibration sources. In order to equalize thegain of each channel, the cosmic-ray data was acquired before the physics data taking. The
muon beam was turned off during the calibration. The data was acquired by triggering on the
coincidence hit of any top and bottom crystals. The second calibration source was gamma-raysof the 88 Y source (0.9, 1.8 MeV). This calibration was performed for the one channel in orderto know the absolute energy scale (figure 5.16).
-bgocrystal.charge[12]300Mean 4.20e-01 ± 2.13e-03
Sigma 6.13e-02 ± 3.08e-03

Mean 8.11e-01 ± 2.46e-03
Sigma 8.69e-02 ± 3.79e-03

250

h
200

Entries
Mean
RMS

30183
0.64860.2756

1.6

1.8

150

100

50

0

0.2

0.4

0.6

0.8

1

1.2

1.4

2

charge (a.u.)

Figure 5.16: Measured spectrum with the 88 Y.
According to the simulation study, the energy deposit of the cosmic-ray is expected to
be around 45.4 MeV. However, if we assume that the energy scale calibration with the 88 Y iscorrect, the measured energy deposit of the cosmic-rays is about 17% lower (figure 5.17). The
detail of the discrepancy between the two calibration results is discussed in the last section.

5.3. Data taking

51
ch12
title

400

χ2 / ndf
Constant
MPV
Sigma

50

350

χ / ndf = 195.3 / 215
2

300

MPV

45.39 ± 0.05

Sigma

2.334 ± 0.027

200

40

2121 ± 32.2

Constant

250

12.87 / 12294.8 ± 18.8
38.33 ± 0.26
2.227 ± 0.243

30
20

150
100

10
50
0
0

20

40

60

80

100

0

120
140
Energy (MeV)

10

20

30

40

50

60

70

80

90

100

E

Figure 5.17: Simulated energy deposit (left) and measured spectrum (right). The energy scaleof the measured spectrum is calibrated with the gamma-rays of 88 Y.

5.3

Data taking

The physics data had been acquired by triggering on any hit of the BGO crystals. The
trigger rate was about 15 Hz. The threshold of each crystal was around 35 MeV if we assume
that the calibration with the 88 Y is correct. About 680000 events were acquired in total. The
data acquisition rate was about 5 Hz due to the limited transmission speed of the trigger board.
Most of the triggered events have the energy deposit greater than 52.8 MeV, which are triggeredby the cosmic-rays (figure 5.18).

30000

25000

Entries 679571

20000

15000

10000

5000

0

0

50

100

150

200

250
E γ (MeV)

Figure 5.18: Total energy deposit in the BGO detector of all the triggered events. The energy
scale is defined by the calibration data with the 88 Y.

5.4

Analysis

Each waveform data was analyzed by using a software developed for MEG II. The pulse
height was calculated as the difference of the peak amplitude and the baseline which was
calculated by averaging amplitudes in the region before the pulse. The timing of the pulse
was calculated with the constant fraction method. The timing where the waveform reached

52

Chapter 5. Commissioning of the Downstream Detector

15% of the full pulse height was measured. The light yield was calculated by integrating the
waveforms.

5.4.1

Event selection

Because most of the events were triggered by the cosmic-rays, we first applied the event
selections in following two steps. First, the event was selected with the hit positions in the
BGO detector. As shown in figure 5.19, the cosmic-ray track makes several hits from the top
to the bottom crystals. Moreover, if the gamma-ray from RMD hits the crystal in the edge,
the energy deposit could not be measured correctly. Therefore, we defined the fiducial volumeof the BGO detector as the central four crystals (figure 5.20) and selected the event which had
the largest energy deposit in this area.
Second, the events were selected with the total energy deposit in the BGO detector. As
previously mentioned, the event whose energy deposit is greater than 52.8 MeV are considered
to be triggered by the cosmic-rays. Figure 5.21 compares two spectra of the energy deposit of
the BGO detector, which were acquired either with or without the muon beam. We set theenergy threshold to 55 MeV to cut the cosmic-ray events.
Figure 5.22 shows the energy deposit distribution of the BGO detector after applying two
cuts. Most of the cosmic-ray events were rejected (98%).
(x+100e-9)*49/200e-9)
−9

× 10

−6

0.1

× 10

80
0.08
60

0.06

40

0.04
0.02

20

0

0

− 0.02−20− 0.04−40− 0.06−60− 0.08
−80

−6

× 10
− 0.1
−−−−
0.08
0.1
0.06
0.04
0.02
0.04
0.06
00.1
0.08

(a) Candidate of the cosmic-ray event(x+100e-9)*49/200e-9)
−9

× 10

−6

0.1

× 10

80
0.08
60

0.06

40

0.04
0.02

20

0

0

− 0.02−20− 0.04−40− 0.06−60− 0.08
−6

× 10
− 0.1
−−−−
0.08
0.1
0.06
0.04
0.02
0.04
0.06
00.1
0.08

−80

(b) Candidate of the RMD event

Figure 5.19: Event display of the RDC and the BGO detector. The marker color and thediameter represent the hit timing and the energy deposit respectively.

5.4. Analysis

53

Figure 5.21: Total energy deposit in the
Figure 5.20: Fiducial volume of the BGO deBGO detector. The black and red spectra
tector (red shaded part).
shows the beam on and off data, respectively.

6000

5000

4000

3000

2000
Entries 15447
1000

0 0

50

100

150

200

250
E γ (MeV)

Figure 5.22: Total energy deposit in the BGO detector. The blue and red part shows before
and after the event selections, respectively.

54

5.4.2

Chapter 5. Commissioning of the Downstream Detector

Timing coincidence of the RDC and BGO detector

After the event selection, the coincidence between the BGO detector and the RDC waschecked. Figure 5.23 shows the timing differences of the two detectors. As a result, we successfully observed a clear timing peak, which is corresponding to the RMD events. At the sametime, the accidental hits of the positrons from Michel decay were observed, which corresponded
to the flat region of the Figure 5.23.

350
300
250
200
150
100
50
0
−20

−9

−15

−10

−5

0

5

×10
15
20
10
T RDC - T BGO - offset (s)Figure 5.23: Timing difference of the BGO detector and the RDC.

5.4.3

Energy cut with the calorimeter

As previously mentioned, the accidental positrons from Michel decay are distinguishable
because of their larger energy deposit than that of the positrons from RMD. Figure 5.24 shows
the total energy deposit in the calorimeter after the event selection.

10h_Elyso
Entries 15447
Mean
7.664
RMS
8.182

3

102

10

0

5

10

15

20

25

30

35
40
E (MeV)

Figure 5.24: Total energy deposit in the calorimeter.
As a demonstration, we cut the events which have the energy deposits greater than 4 MeV.
Figure 5.25 compares the timing difference of the BGO detector and the RDC before and after5.4. Analysis

55

the energy cut. As a result, the fraction of the flat regions were decreased to ∼ 1/10. At thesame time, the fraction of the timing peak of the RMD events was also decreased to ∼ 1/3.
This was probably due to the low energy threshold of the BGO detector (Eγ >∼ 35 MeV). The
relatively high energy positrons from RMD were also cut.

350

300

250
200

150
100

50

0
− 20

−9

−15

− 10

−5

0

5

×10
15
20
T RDC - T BGO - offset (s)10Figure 5.25: Timing difference of the BGO detector and the RDC after the energy cut withthe calorimeter( > 4 MeV). The black and the red show the events before and after the cut,respectively.

56

Chapter 5. Commissioning of the Downstream Detector

5.5

Issues and prospects

As shown in the previous section, the first commissioning of the downstream RDC wascompleted. At the same time, it revealed following issues.
• A geometrical conflict between the front part of the RDC and the COBRA end cap was
found.
• We found four problematic timing counter channels in the signal check. Because thesechannels did not work at all, ∼ 10% of the timing counter acceptance was lost.
• We calibrated the energy scale of the BGO detector with two calibration sources (gammarays of the 88 Y and cosmic-rays), however, the MIP peak of the cosmic-rays are 17%
smaller than expectation. Due to the discrepancy of the two results, the correct energy
scales of the photons are uncertain accordingly.
The first issue can be solved by modifying some detector layouts because it requires only
few millimeters of extra z space in front of the RDC. Other two issues were investigated after
the commissioning. The details and prospects are described in the following sections.

5.5.1

Problematic channels in the timing counter

After the commissioning, we investigated the problematic channels in the timing counterby disassembling several counters. Because of the following reasons, the cause was considered
as bad connections around the PCB.
• The three of the four problematic channels were placed near the MMCX connector in thelong PCB, which had larger loads when connection or disconnection of the cables.
• When we push the PCB to the scintillator with applying the bias voltage, we again
observed the signals.
The scheme of the assembly of the scintillator, SiPMs and PCB are illustrated in Figure 5.26. The cause was found to be a mechanical detachment between the PCB and the
SiPMs where the conductive epoxy was used. The mechanical detachment was observed insome of the problematic channels after we removed the light shielding. Figure 5.27 shows thesurfaces of the PCB and the SiPMs after we disassembled one of the counter by hand. The
conductive epoxy sticks on the both PCB and SiPMs surfaces and it can be seen that theconnection is not strong enough.

Figure 5.26: Schematic view of the connections in the timing counter.

5.5. Issues and prospects

57

Figure 5.27: PCB (top) and SiPMs (bottom) after disassembling. The red arrows show theglued areas before disassembling.
As a solution to this problem, we are going to solder the SiPMs on the PCB instead ofusing the conductive epoxy. For the surface mount SiPMs, this process should be done by the
reflow soldering. This was tested with a help of the electronics group in PSI. One PCB andthree SiPMs were soldered in following steps.

Figure 5.28: Reflow soldering processes.

58

Chapter 5. Commissioning of the Downstream Detector
1. A solder paste which has a diameter of ∼1 mm was put on the PCB by using a fine needle(Figure 5.28 (a), (b)).
2. The three SiPMs were aligned on the PCB by using a microscope.
3. The PCB and the SiPMs were heated with a reflow oven, which can gradually change thetemperatures (Figure 5.28 (c)). The temperature was changed in accordance with thehandling manual of the SiPMs
peak temperature was set below the limitation of the SiPM (Peak temperature ∼ 240◦ C)[16]. It took about 10 minutes for the heating and the cooling. During this process, thetemperatures were monitored with several thermometers (the one is attached to the PCB,
Figure 5.28 (d)).

As long as we checked by eyes, neither a deformation nor a change of the color of the SiPMs
due to heating were observed (Figure 5.28 (e)). Moreover, the alignment of the SiPMs were
kept even after the reflow.
Next, we checked if there were no changes in the performance by heating the SiPMs. We
made a test counter where the soldered PCB was glued in one side (Figure 5.29). By using thesame setup as shown in Figure 3.10, we acquired the waveform data with the 90 Sr. Since weapplied the same bias voltage on each channel, the gains of two channels were not exactly equal.
However, as long as we compared the waveforms (Figure 5.30) and the light yields (Figure 5.31),
no significant difference was observed in the soldered PCB. Moreover, we obtained ∼ 85 ps ofthe timing resolution which is good enough. Therefore, the reflow soldering can be solution
for the detachment problem. We are going to reproduce all the timing counters in this way.
Moreover, we will prepare some spare counters so that we can quickly replace with them even
if problematic channels are found during the operation.

Figure 5.29: Schematic view of the tested counter. The length of the scintillator is 11 cm.

(a) ch1(b) ch2

Figure 5.30: Example of waveforms.

5.5. Issues and prospects

59

Figure 5.31: Comparison of the light yields. The red and black show the channel 1 and 2,
respectively.

5.5.2

Uncertainty of energy scale of the BGO detector

For the quantitative evaluation of the capability of the background identification, thecorrect energy scale of the BGO detector has to be known. It is necessary to investigate thediscrepancy between the two calibration results (gamma-ray of the 88 Y and cosmic-ray). For
the possible reasons of this discrepancy, we considered following four possibilities.
I. Currently, the energy deposit of the cosmic-rays is calculated only for the case that thecosmic-rays are always penetrating perpendicularly at the center. On the other hand, thecalibration data was acquired by triggering on the coincidence hit of any top and bottom
crystals. Therefore, the actual energy deposit might have a smaller peak than 45.4 MeV
by glancing the crystal.
II. We corrected the non-linearity of the readout amplitude of the WaveDREAM. However,the correction may not be perfect.
III. Because we placed the 88 Y in front of the crystal, the light yields of the gamma-rays could
become larger if the response of the detector changes with different position and incident
angle.
VI. There could be some non-linearity in the scintillation properties of the BGO crystal or
the PMT.
In order to check the first two possibilities (I, II), we took the calibration data again after
the commissioning by using a DRS evaluation board. As shown in figure 5.32, we used four
crystals placed in a row. In the commissioning, two calibrations were performed with a channel
A (Figure 5.16, 5.17).
This time, the 88 Y data was acquired in each of four channels. The source was set in frontof the crystal as we did in the commissioning. While retaking the cosmic-ray data, two small
trigger counters (1.5 cm cube) based on plastic scintillators and SiPMs were placed above thecrystals to select the cosmic-rays with vertical incident angles. Moreover, for the analysis, we
required coincidence hits in the bottom crystal (channel D).

60

Chapter 5. Commissioning of the Downstream Detector(a) Gamma-rays of the 88 Y(b) Cosmic-rays

Figure 5.32: Schematic views of the setup.
Figure 5.33 shows the example of the measured spectra. By using two photo-peaks of the
gamma-rays (0.9, 1.8 MeV) in the 88 Y data, we calculated the MIP peak of the cosmic-rays.
Table 5.1 summarizes the results of the calculations and the measured values by fitting. As aresult, all the peaks of the cosmic-rays were measured to be smaller than their expectations.
We concluded that the first two possibilities in the list were not the case.
ch12

ch12

50

1800
Mean 2.33e+00 ± 3.56e-03

1600

Sigma 2.95e-01 ± 3.93e-03

χ2 / ndf
Constant
MPV
Sigma

40

1400
1200

30

1000
Mean 4.83e+00 ± 7.39e-03

800

Sigma 3.94e-01 ± 9.00e-03

76.4 / 81
181.7 ± 12.2
105.2 ± 0.6
6.5 ± 0.3

20

600
400

10

200
0

2

4

6

8

10
12
charge (a.u.)(a) Gamma-rays of the 88 Y

0

100

200

300
charge (a.u.)(b) Cosmic-rays

Figure 5.33: Measured spectra (channel A).

ChannelA
B
C
D

Expected from 88 Y data
126.1 ± 0.4
157.4 ± 0.4
155.9 ± 0.4
155.9 ± 0.4

Measured value
105.2 ± 0.6
122.5 ± 0.7
132.9 ± 0.8
122.4 ± 0.7

Discrepancy (%)
16.6 ± 0.6
22.2 ± 0.5
14.8 ± 0.6
21.5 ± 0.5

Table 5.1: Light yields of the MiP peak of the cosmic-rays (45.4 MeV).

5.5. Issues and prospects

61

Next, we checked the position dependence of the gamma-rays by changing the positions of
the source. As shown in Figure 5.34, we acquired the data by placing the source on the top of
the channel A. The measured spectrum is shown in Figure 5.35. We compared the two photopeaks to the peaks observed in Figure 5.33 (a). Table 5.2 shows the energy scales calculatedby the photo-peaks in two configurations. Consequently, a large difference which may explain
the discrepancies were not observed.
ch12 top

1400

Mean 2.24e+00 ± 4.36e-03
Sigma 2.78e-01 ± 5.67e-03

1200
1000
800

Mean 4.71e+00 ± 7.20e-03
Sigma 3.65e-01 ± 9.50e-03

600
400
200
0

Figure 5.34: Schematic view of the setup

88

Y position
Front
Top

2

4

6

8

10
12
charge (a.u.)

Figure 5.35: Measured spectrum.

Charge at 1 MeV (a.u.)
2.78 ± 0.01
2.74 ± 0.01

Table 5.2: Peak of the charge distributions in two configurations.
So far, we investigated all the possibilities on the list except VI. As long as we refer otherexperiments which uses the BGO detector, such a non-linearity is not reported. Although thefine-mesh-type PMT is known to have a good linearity, some non-linearity of the light yieldand the readout amplitude are reported [21]. Because this could be a reason in our case, wewill check it next.

Chapter 6
Development of the Upstream Detector
As previously mentioned, the influence of the upstream detector on the muon beam transportation has to be minimized. First of all, the influence on the beam spot was investigatedby using a mockup RDC. According to the measured beam spot size with the mockup, theeffect of the different beam size was evaluated in the simulation study. Moreover, the effect on
the muon stopping rate was studied in the simulation. Section 6.2 describes the effect of thepileup beam muon. Because of the high hit rate of the beam muons (∼100 MHz), the detection
efficiency loss due to pileup is not negligible. The configuration of the bundle widths can bealso optimized to minimize the inefficiency due to pileup. Finally, according to the measured
light yield or the inefficiency due to pileup, the total detection efficiency for RMD positrons
was evaluated. Section 6.3 compares the efficiencies in several configurations.

6.1

Influence on the muon beam

6.1.1

Beam spot size on the target

The influence on the muon beam was studied with a help of the beam group of PSI. A
mockup RDC with the same material thickness as expected for the scintillating fiber layer of the
upstream RDC was used (Figure 6.1). The beam spot size at the target position was measured
with two configurations as shown in Figure 6.2. When the upstream RDC is not installed, adegrader with a 300 µm Mylar foil is inserted at a beam waist inside the BTS to reduce themuon momentum. By thinning the degrader, the RDC with the same amount of the material
thickness can be installed.
Nowmal Running

BTS degrader
300μm

vaccum window
190μm

RDC Test

Figure 5: (Left) Shows mounting flange and “Dummy” RDC foil. (Right)“Dummy”
RDC mountedvaccum window RDC mockup
BTS
degraderFigure 6.1:
RDC
mockup (230 µm thickat US End-cap position.
85μm
190μm
230μm
Mylar foil). The comparative conditions for “normal” running and those used in the RDC test are

shown in Table 2, together with the BTS optimized excitation curves in Figure 6. The higher
value in the case of the RDC setup is expected since a thinner degrader is used, leading to ahigher momentum exiting the BTS. Hence to achieve the same focus at the centre of COBRA ahigher excitation current is needed.

Figure 6.2: Schematic view of the setup.
62

Table 2: Comparison of material parameters for the different running conditions during thePre-engineering Run

Parameters
Beam Momentum
Degrader thickness(U = 1.395 g/cm3)
Beam window

Normal Running
28 MeV/c
MYLAR 300 Pm

RDC Test
28 MeV/c
MYLAR 85 Pm

MYLAR 190 Pm

MYLAR 190 Pm

6.1. Influence on the muon beam

63

The beam profile was measured in two dimensions at the target position by moving theposition of a thick depletion-layer APD with a step size of 5 mm. Table 6.1 summarizes a resultof gaussian fitting for each x and y distribution. The product of σx and σy increased by 16%
with the mockup. Concerning the muon stopping rate, the comparison in a simulation study
is described in section 6.1.3.

without mockup
with mockup

σx (mm)10.7 ± 0.2
11.5 ± 0.2

σy (mm)10.4 ± 0.2
11.2 ± 0.2

Table 6.1: Beam spot size at the target position

6.1.2

Effect of the larger beam spot

As previously mentioned, a positron emitted from the target follows a spiral trajectory in
the gradient magnetic field. The gradient magnetic field ranges from 1.27 T at the center to0.49 T at the cryostat end (|z| = 1.42 m). If the beam spot becomes larger, the muon vertex
will be spread more in z-direction due to the slanted angle of the target. As a consequence, thetrajectory of the positron could be significantly affected with the larger beam spot. To study the
influence on the positron tracking performance, a series of simulation studies were performedby using GEANT4. In order to simplify the study, the muon beam was not simulated, but thesimulation started from a muon decay on the target. Table 6.2 summarizes the geometricalparameters concerning a size of the target in two configurations. In addition to the nominal
size of the target, the bigger target was simulated. The bigger target is able to cover both (6σx ,6σy ) of the larger beam spot when it is placed with the slanted angle of 15◦ . The muon vertex
distribution was input to the simulation according to the result of the mockup test (Figure 6.3).

nominallargermajor-axis (mm)260
267minor-axis (mm)7070

thickness (µm)
120120Table 6.2: Fiducial volume parameters of the target

Chapter 6. Development of the Upstream Detectory (cm)

64

4

50

2

40

0

30

−2

20
10

−4
−10

0

10

x/sin15° (cm)0y (cm)(a) without RDC
50

4

45
40

2

35
30

0

25
20

−2

15
10

−4

5

−10

0

10

x/sin15° (cm)0(b) with RDC

Figure 6.3: Distribution of the muon vertex on the target. Black solid line and red dashed line
represent the fiducial area of the nominal and bigger target, respectively.

6.1. Influence on the muon beam

65

A. Detection efficiency for signal positrons
We firstly studied on the detection efficiencies of the drift chamber and the timing counterfor the signal positrons.. In this study, µ+ → e+ γ events were simulated on the target. The
emission angle of the photon was selected according to an acceptance of the MEG II liquidxenon detector (r = 64.97 cm, |z| < 23.9 cm, |φ| < 1.05 rad).
In the drift chamber, the signal positron track intersects multiple wire cells by making
several turns. Once it crosses the outer shell of the drift chamber, the track will be interrupted.
Figure 6.4 shows the distribution of the number of hit wire cells before crossing the outer shellin the drift chamber. The track with less than ∼50 hits are interrupted by the outer shell in
the first turn. In the track reconstruction, a long track is important in order to obtain the goodangular and momentum resolutions. Table 6.3, I shows the efficiencies of the drift chamber. For
three different configurations, we first counted the number of events generated on the fiducial
area of the target. The efficiency was then calculated as the probability not to be interrupted
in the first turn. Figure 6.5 shows the distribution of the number of the hit tiles in the timing
counter. The distribution does not seem different with the bigger beam spot. The efficiency of
the timing counter was calculated as the probability to cross at least one counter tile (Table 6.3,
II).
As summarized in Table 6.3, the product of two results represents the total efficiency. It
was changed by −0.79% (standard target) and −0.83% (bigger target) with the bigger beam
spot. As a result, it can be seen that the change of the efficiency for the signal positrons would
be small with the upstream RDC.

mcspx.npixel {((pow(mckine.xvtx[0]/(sin(3.141592/12)*13.0),2) + pow(mckine.yvtx[0]/3.5,2) <1)&&(TMath::Abs( mckine.zvtx[0] + mckine.zmom[0]*-1*(mckine.xvtx[0]*mckine.xmom[0] + mckine.yvtx[0]*mckine.ymom[0] + sqrt( pow(mckine.xvtx[0]*mckine.xmom[0] + mckine.yvtx[0]*mckine.ymom[0],2) - (mckine.xmom[0]*mckine.xmom[0] + mckine.ymom[0]*mckine.ymom[0])*(mckine.xvtx[0]*mckine.xvtx[0] + mckine.yvtx[0]*mckine.yvtx[0] - 64.97*64.97)) /(mckine.xmom[0]*mckine.xmom[0] + mckine.ymom[0]*mckine.ymom[0]) )) < 23.9))&&(TMath::Abs(TMath::ATan2(mckine.yvtx[0] + mckine.ymom[0]*-1*(mckine.xvtx[0]*mckine.xmom[0] + mckine.yvtx[0]*mckine.ymom[0] + sqrt( pow(mckine.xvtx[0]*mckine.xmom[0] + mckine.yvtx[0]*mckine.ymom[0],2) - (mckine.xmom[0]*mckine.xmom[0] + mckine.ymom[0]*mckine.ymom[0])*(mckine.xvtx[0]*mckine.xvtx[0] + mckine.yvtx[0]*mckine.yvtx[0] - 64.97*64.97)) /(mckine.xmom[0]*mckine.xmom[0] + mckine.ymom[0]*mckine.ymom[0]) ) , -1*(mckine.xvtx[0] + mckine.xmom[0]*-1*(mckine.xvtx[0]*mckine.xmom[0] + mckine.yvtx[0]*mckine.ymom[0] + sqrt( pow(mckine.xvtx[0]*mckine.xmom[0] + mckine.yvtx[0]*mckine.ymom[0],2) - (mckine.xmom[0]*mckine.xmom[0] + mckine.ymom[0]*mckine.ymom[0])*(mckine.xvtx[0]*mckine.xvtx[0] + mckine.yvtx[0]*mckine.yvtx[0] - 64.97*64.97)) /(mckine.xmom[0]*mckine.xmom[0] + mckine.ymom[0]*mckine.ym

595 pxRDCRDC,
off, nominal
targetw/onominal target

7000
20000

6000

2nd turn

5000

15000

10000

5000

RDC
on,nominal
nominaltarget
targetw/
RDC,
RDC
on,bigger
biggertarget
targetw/
RDC,

Entries

75934

Mean

73.58

Std Dev

25.29

Entries

4000
3000

75934

Mean

11

Std Dev

5.207

2000

1st turn

1000
0
0

50

100

150

200

Figure 6.4: Number of hit wire cells before
crossing the outer shell in the drift chamber.

without RDC, nominal target
with RDC, nominal target
with RDC, bigger target00

N wire1020

30

N counter

Figure 6.5: Number of hit tiles in the timing counter.

I Drift Chamber
98.25 ± 0.05
97.65 ± 0.06
97.53 ± 0.06

II Timing Counter
95.33 ± 0.08
95.11 ± 0.08
95.09 ± 0.08

Table 6.3: Efficiency for the signal positrons (%)

I×II
93.73 ± 0.09
92.99 ± 0.09
92.95 ± 0.09

66

Chapter 6. Development of the Upstream Detector

B. Hit rate in the drift chamber
Due to the high muon decay rate of ∼ 7 × 107 Hz, the positron spectrometer is operated insuch a heavy irradiation environment. Particularly, there is a high positron hit rate (∼1 MHz)in the inner most wire of the drift chamber, which is placed at r = 18 cm. The aging of thewire is thereby a potential issue. The previous study shows a yearly gain drop of ∼25% in the
inner most wires [9]. If the beam spot becomes bigger, more positrons may hit the inner mostwires. Therefore, the increase of the hit rate could be a problem.
To study on the hit rate in the inner most wire, Michel decay events were simulated on
the target. The emission angle of the positron was isotropic. Figure 6.6 shows the distributionof hit wire ID in the drift chamber. Because it is uniformly distributed in the same layer, theaverage hit rate in each wire can be calculated by using the muon decay rate (7 × 107 Hz) andthe number of wires (192 wires in a layer). The result is summarized in Table 6.4. The changewith the bigger beam spot is +0.79% with either standard or bigger target. We concluded thatthe influence on the hit rate in the drift chamber would be negligibly small.

50000

5000

40000

4000
Entries 484163

30000

3000

Entries 3698905

20000

2000

10000

1000

0
0

500

1000

1500

2000
wire ID(a) Wires in the whole drift chamber

0

1750

1800

1850

1900
wire ID(b) Inner most wires

Figure 6.6: Distribution of the hit wire ID. The last 192 wires are corresponding to the innermost wires.

without RDC, nominal target
with RDC, nominal target
with RDC, bigger target

hit rate (MHz)1.765 ± 0.003
1.779 ± 0.003
1.779 ± 0.003

Table 6.4: Hit rate in the inner most wire
C. Reconstructed momentum resolution
The reconstructed momentum resolution with the drift chamber was estimated by simulating the signal events. Track fitting was performed by using an algorithm based on Kalman-filter
developed for positron tracking in MEG. Figure 6.7 shows the difference of the reconstructed
momentum and the true momentum in the simulation. The low momentum tail is corresponding
to energy loss of positrons, which mostly comes from a bremsstrahlung of multiple scattering.
The distribution was fitted with a double-gaussian where the reconstructed momentum resolution σP is expressed as sigma of the main peak. Table 6.5 summarizes the reconstructed6.1. Influence on the muon beam

67

momentum resolution. In the upgrade proposal of the MEG II, the momentum resolution of∼ 130 keV was obtained [9]. We obtained the better resolution due to the improved track fitting
algorithm, which can associate more hits to the track. Moreover, the contribution of the electronic is not considered in our simulation. As a result, the resolution changed by +1.30% withthe bigger beam spot and the bigger target. We concluded that the reconstructed momentum
resolution would not be significantly affected with the upstream RDC.
P

P

7000

hP

6000
5000
4000
3000
2000
1000
0
−1

−0.5

0

hP

7000

Entries

60474

Mean

−0.01002

RMS

0.1117

χ2 / ndf

723.1 / 90

p0976.1 ± 9.4

p1

0.006698 ± 0.000375

p2

0.0622 ± 0.0005

p3

205.7 ± 8.6

p4−0.06511 ± 0.00332

p5

0.1681 ± 0.0036

0.5
1
P - Ptrue [MeV]6000

Entries

61457

Mean

−0.009072

RMS

5000

χ2 / ndf

4000
3000
2000
1000
0
−1

(a) without RDC, nominal target

−0.5

0

0.11
790.4 / 85

p0977.3 ± 10.3

p1

0.007154 ± 0.000380

p2

0.06135 ± 0.00051

p3

223.3 ± 9.5

p4−0.05613 ± 0.00306

p5

0.1617 ± 0.0034

0.5
1
P - Ptrue [MeV](b) with RDC, bigger target

Figure 6.7: Difference of the reconstructed momentum and the true momentum in the simulation.

without RDC, nominal target
with RDC, bigger target

σP (keV)61.4 ± 0.4
62.2 ± 0.4

Table 6.5: Reconstructed momentum resolution

6.1.3

Muon stopping rate

Because it is difficult to measure the actual muon stopping test in the beam test, it wasstudied in the simulation. The muon beam of 28 MeV/c was generated in the simulation at theentrance of the beam transport solenoid. The muons were transported to the stopping target
after passing the beam degrader and 190 µm thick vacuum window. A layer of the scintillation
fibers was placed between the vacuum window and the target in the configuration with the
upstream RDC. The stopping rate was calculated by counting the number of muon decay
vertices inside a fiducial area of the nominal target. The muon beam intensity was assumed tobe 108 µ/s.
Figure 6.8 shows the stopping rate in two configurations. The thickness of the degrader
was changed with a step size of 10 µ m. The degrader thickness was optimized by maximizing
the stopping rate in each configuration. Table 6.6 summarizes the stopping rate and the optimaldegrader thickness. The change of the stopping rate is small (−0.54%) with the upstream RDC.

68

Chapter 6. Development of the Upstream Detector
6

6

×10

80

70

70

60

60

µ stopping rate (Hz)µ stopping rate (Hz)

80

50
40
30

×10

50
40
30

20

20

10

10
50 100 150 200 250 300 350 400
degrader thickness (µm)(a) without RDC

50 100 150 200 250 300 350 400
degrader thickness (µm)(b) with RDC

Figure 6.8: Muon stopping rate with several degrader thicknesses.

without RDC
with RDC

degrader thickness (µm)
350130Stopping rate (MHz)69.20 ± 0.16
68.82 ± 0.14

Table 6.6: Optimal degrader thickness and muon stopping rate

6.2

Effect of pileup muon

Because of the high muon hit rate (∼500 kHz at the central fiber), the effect of the pileup
is not negligible. In order to estimate the detector performance, the detection efficiency loss
for the RMD positrons due to the pileup was studied. The inefficiency due to the pileup isestimated by calculating the following two factors. The first one is the capability to distinguishmuon and positron pulses in the waveform analysis. The second one is the hit rate of muon andpositron from RMD at each channel. The hit rate depends on the width of the fiber bundle andits position. In other words, the bundle widths can be optimized to minimize the inefficiency.
Finally, section 6.2.4 presents several prospects to reduce the inefficiency due to the pile up.

6.2.1

Capability to distinguish two waveforms

As mentioned above, the inefficiency depends on the capability to distinguish muon andpositron pulses in waveform analysis. Let us first define the minimum time difference to distinguish the muon and positron pulse as ∆T (Figure 6.9). To the first approximation, ∆T can beconsidered as a sum of muon and positron pulse widths. However, there are after-pulses of the
SiPM associated with the main pulse. The muon waveform usually has a several after-pulses
due to its large amplitude of the main pulse. Therefore, the pileup with these after-pulses needs
to be also considered.

6.2. Effect of pileup muon

69

Figure 6.9: Definition of ∆T .
A. Data taking with a prototype
In order to measure actual ∆T , we acquired muon and positron waveform data individuallyat the πE5 beam line in PSI. The momentum of the beam particles were tuned to be 28 MeV/c.
The prototype detector consists of the scintillation fiber and SiPM (Figure 6.10). Squareshape fiber with a size of 250 µm (Saint-Gobain, BCF-12) are placed with a pitch of 5 mm.
The length of the fiber is about 20 cm. In order to reduce the light yields loss, the fiber iscoated with a 100 nm aluminum. Because this is also a prototype of a two dimensional beam
monitoring device, it has another perpendicular fiber layer. The scintillation light is collected
at two fiber ends with 1.3×1.3 mm2 SiPMs (Hamamatsu, S13360-1350CS). The fiber and SiPM
are coupled with optical grease. The position of the fiber end is fixed on the PCB. Before datataking, the characteristics of all the SiPM were studied. The detail is described in appendix.
After cabling and light shielding, the detector was placed between the BTS and the steering
magnet. The signal was transmitted to the WaveDREAM which was placed just in front of thedetector. The waveform was acquired by triggering on a coincidence hit between the signals of
two fiber ends.

Figure 6.10: Prototype detector. Left : Fiber and PCB. Right : Detector at the measurement
position after light shielding.
B. Waveform analysis
For each waveform, timing and amplitude of the pulse were calculated. ∆T was determinedwith following steps.
1. We first selected good sample waveforms. The waveform which has only one main pulse in
the time window was selected. Examples of the selected waveforms are show in Figure 6.11(a) and (b).

70

Chapter 6. Development of the Upstream Detector
2. One positron and one muon waveform were randomly selected. The two waveforms weremixed by summing up amplitude data in a whole time window. The same positron
waveform data was reused by shifting the timing by 20 ns so that we could obtain enoughstatistics.
The example of the mixed waveform is shown in Figure 6.11 (c). By mixing waveforms,
the noise level was increased by 54%.
3. The mixed waveforms were analyzed again. Timing and amplitude were calculated foreach pulse.
4. According to the timing and amplitude information of the positron pulse before themixing, the positron signal was searched for the corresponding timing (± 10 ns) andamplitude (± 10 mV) region.
Graph

0.01
0

Graph

0.1

amplitude (V)amplitude (V)amplitude (V)

Graph
0.02

0

−0.1

0.1

0

−0.1

−0.01
−0.02
−0.03

−0.2

−0.2

−0.3

−0.3

−0.04

−0.5

−0.4

−0.4

−0.05
−0.4

−0.3

−0.2

×10−6
0

−0.1

−6

−0.5

−0.4

−0.3

−0.2

×10
0

−0.1

time (s)

−6

−0.5

−0.4

−0.3

−0.2

(a) Positron

×10
0

−0.1

time (s)time (s)(b) Muon(c) Positron + Muon

Figure 6.11: Examples of waveform mixing.
Figure 6.12 shows the probabilities to find the positron signal in the mixed waveform.
The decrease of the probability was observed for ∼120 ns around the region where the positron
waveform overlaps muon waveform. Because of the noises, the probabilities are always below
100% even if there should be no pileup. The measured value is much larger than the sum of thepositron and the muon waveform widths. This is because the after-pulses are generated only
randomly after the main pulse, the positron signal could be detected even if it comes within
120 ns after the main pulse. However, in following sections, we consider the worst case where∆T is 120 ns.
histogram title

1

0.8

0.6

0.4

0.2

0
−0.5

−0.4

−0.3

−0.2

−0.1

0

0.1

0.2

0.3

0.4

×10
0.5

−6

Figure 6.12: Probability to find a positron signal after mixing.

6.2. Effect of pileup muon

6.2.2

71

Effect of size and position of the fiber bundle

The hit rate of the beam muons and the positrons from RMD at each fiber bundle wereobtained in a simulation study. Figure 6.13 shows the hit distributions in the upstream RDC.
By using the hit rate and the value of ∆T , the probability of pileup can be calculated as :Probability =Number of pileup events.
Number of detected RMD events(6.1)

As previously mentioned, the probability depends on the width and position of the bundle.
The probability will be larger if the bundle is placed near the beam axis (Figure 6.14). We
assumed that the width of each bundle is square number of fibers (from 4 to 100) in order toefficiently fix at the end. The probabilities for all possible configurations were calculated asshown in Figure 6.15.
title

2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
−15

×10

3

Entries

59976

χ2 / ndf

406.4 / 77

Constant

500− 0.07088 ± 0.01157

Sigma

2.813 ± 0.009

46884

χ2 / ndf

1.255e+06 / 591

Constant
4.715e+05 ± 5.961e+01

2106 ± 11.0

Mean

Entries400

Mean

0.005876 ± 0.000209

Sigma

2.039 ± 0.000

300
200
100
−10

−5

0

5

10

15
y (cm)(a) Positron from RMD (Eγ > 48 MeV)

0
−15

−10

−5

0

5

10

15
y (cm)(b) Beam muon

Figure 6.13: Hit distribution in the upstream RDC.

Figure 6.14: Example of the probability of the pileup in two different positions (9 fibers bundle,
y = 2, 4 cm). The orange area represents the high muon hit rate region.

72

Chapter 6. Development of the Upstream Detector

1
0.9
0.8

bundle width ( N fibers )prob. of pileup

0.7
0.6
0.5

4

9

16

25

36

49

64

81

100

0.4
0.3
0.2
0.1
0
0

1

2

3

4

5

6

7

8

9

10

y (cm)

Figure 6.15: Probability of the pileup calculated with a step size of 250 µm

6.2.3

Total inefficiency for RMD positrons of all bundles

The inefficiency due to the pileup was calculated by integrating the probability of the pileup
over the whole detector. We estimated the inefficiency by assuming following two conditions.
• Total y length of the upstream RDC is within 19.6 ± 0.25 cm.
• The maximum number of bundles is 18 due to the limitation of the space for the SiPMs.
There are over a thousand configurations of the bundle widths, which satisfy above conditions.
We calculated the inefficiency for all these possible configurations as shown in Figure 6.16.
Table 6.17 shows the best configuration of the bundle width, which minimizes the inefficiency.
The inefficiency due to the pileup is 50.4% with the best configuration.
90
80
70
1404
Entries
0.5829Mean
RMS 0.03543

60
50
40
30
20

Bundle width

Nbundle

100

4

49

2

36

2

25

2

16

8

10
0

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

Inefficiency

Figure 6.16: Distribution of the inefficiency
for the RMD positrons.

Figure 6.17: Best configuration of the bundle width. Smaller widths are used in more
central regions of the detector.

6.2. Effect of pileup muon

6.2.4

73

Prospects for reducing the inefficiency

The inefficiency due to the pileup is large even by using the best bundling configuration.
In order to reduce the inefficiency, several possibilities are being considered. The first idea
is to increase the total number of bundles as shown in Figure 6.18. This might be possible
by modifying the detector layout. The second idea is to make a probability density function
related to the after-pulse and implement it in likelihood analysis of MEG II. In provisional
estimation, we assumed that the after-pulses are always present in the timing region of 120
1mmns. Therefore, further reduction of the inefficiency is possible by taking into account thatthe after-pulses are generated only randomly after the main pulse. In order to construct a
probability density function, the characteristic of the after-pulse needs to be fully understood.
The third idea is to use a staggered readout as shown in Figure 6.19. The inefficiency due tothe pileup can be reduced by using this method. However, a high light yield at the single side
is required to efficiently detect a positron signal.
Graph
0.6

inefficiency

0.5
0.4
0.3
0.2
0.1

20

30

40

50

60

70

80

90 100 110

N bundle

Figure 6.18: Total number of bundles and inefficiency for RMD positrons with the best bundlingconfiguration.

(a) standard readout(b) staggered readout

Figure 6.19: Principle of the staggered readout. When there is a pileup event in the same
bundle in (b) it is distinguishable in the right side of the SiPM.

74

Chapter 6. Development of the Upstream Detector

6.3

Total detection efficiency for RMD positrons

Based on the studies in the previous sections, we next evaluated the detection efficiency
for the RMD positrons. When the position enter the acceptance of the detector, the signal
could not be detected due to following reasons.
A The positron goes through a fiber cladding and does not hit a fiber core.
B There is a pileup of beam muon.
C No scintillation photons are detected due to the small light yields of the scintillating fiber.
According to the catalog [22], the cladding of the 250 µm thick square shaped fiber is 5%(first cladding 3%, second cladding 2%). Therefore, we can assume that the probability of A is
about 10% and the efficiency has to be calculated by considering mainly B and C.

6.3.1

Light yield of the scintillating fiber

The light yields of the positrons that we obtained in the prototype beam test is shownin Figure 6.20. If we assume that the number of the photoelectrons follows the Poisson distribution, the probability to detect no photoelectron can be calculated as 29%. By taking into
account this, the mean of the number of photoelectrons is estimated to be about 1.2.
-(height+1.01538999999999935e-02)/4.29626000000000036e-02
1200

1000

h
Entries
Mean
RMS

800

8121
1.755
0.9907

600

400

200

0
0

1

2

3

4

5

6

7

8

9

10

N p.e.

Figure 6.20: Distribution of the number of the photoelectrons observed at single side.
However, this value could not be realistic because we used the positron beam with thedifferent momentum. Moreover, the optical coupling of the scintillating fibers and the SiPMs
could be optimized. Therefore, we calculated the ideal number of the scintillation photons
at fiber ends when the positron from RMD penetrates the center of the square shaped fiber.
Following conditions were taken into account.
• Energy deposit of the positrons from RMD is ∼ 38.5 keV (Figure 6.21).
• Light yields of the fiber is 8000 photons at 1 MeV [22].
• The photons are lost if their incidence angles to the cladding wall are less than 78.6◦ [22](Figure 6.22).
• Attenuation length of the scintillating fiber is ∼ 20 cm.
• Photon detection efficiency of the SiPM is 40% [16].

6.3. Total detection efficiency for RMD positrons

75

title

300

Entries60553
χ2 / ndf
473.7 / 372
Constant 1517 ± 11.3
MPV
38.52 ± 0.05
Sigma
4.556 ± 0.035

250
200
150
100
50
0
0

50

100

150
200
energy deposit (keV)

Figure 6.21: Simulated energy deposit of
Figure 6.22: Schematic view of the transmisthe positrons from RMD (Eγ > 48 MeV).
sion of the scintillation photons.
We calculated the number of photons event by event with above conditions by assumingthat the emission angles of the scintillation photons were isotropic. The reflections at the
cladding wall were calculated until the photon was reflected twice. This was because even if
the photon was reflected at the cladding wall once, it could not be reflected at the vatical wall.
As a result, about 4.3 photons were obtained on average at single side (Figure 6.23). It is known
that attenuation length of the scintillating fiber has both short (15-60 cm) and long (1 m∼)components. However, the short component, which largely affect the light yields, has not been
precisely measured. Therefore, the result of the light yield calculation has a large uncertainty(∼ 40%).
Because of the uncertainty of the light yields, it was calculated in different configurations.
Based on the ideal case (4.3 photons at the single side), the other cases with smaller light yields
were calculated with additional factors. The range of the factors is from 0.2 to 1.0 and it scaled
the probabilities to detect a signal event by event. In the real case (∼ 1.2 photons at the singleside), the factor is about 0.3.
Figure 6.24 shows the probability to detect a signal at least one photon with the SiPMs.
The probability to detect at the single side is written as PSingle . POR represents the probability
to detect a signal at the either side of the fiber. PAND represents the probability to detect asignal at both ends. If we use both fiber sides, the signal can be efficiently detected even if
the mean light yields at the single side is small such as ∼1.2. However, the light yield or theattenuation could be degraded due to the radiation damage. Currently, irradiation test of thescintillating fiber is being planned at PSI in order to measure the effect.

6.3.2

Detection efficiency and sensitivity in different configurations

Next, the detection efficiency was calculated in several configurations of the light yields.
The standard readout and staggered readout were compared in terms of the efficiency. In
addition to the probabilities shown in the previous section, the effect of the pileup muons was
also taken into account. By assuming the muon hit rate of ∼ 108 Hz, the muon hit bundles andthe timing were simulated. The hit bundles were decided by the position distribution, which
was obtained in the simulation study (Figure 6.13). As described in section 6.2.3, we considered
the case that the bundle widths were optimized to minimize the inefficiency due to the pileup,
where the number of the bundles is 18. The efficiency was defined as :76

Chapter 6. Development of the Upstream Detector

1.1

1800
1600

1

1400

0.9

Entries
Mean
RMS

1200

10000
4.253
3.392

0.8
Prob.

1000
800
600

P Single

0.6

P OR

0.5

400

P AND

0.4

200
0
0

0.7

0.3
5

10

15

20

25

30

35

40

45

50

N photon

0.2
1

Figure 6.23: Calculated number of scintillation photons at the single side. The probability to detect no photons is also includedin the first bin.

Efficiency =

1.5

2

2.5

3

3.5

4

Light yields at single side (N photon)

Figure 6.24: Light yields and the probabilities to detect a signal with the SiPMs.

Number of detected events by the SiPMs (either side).
Number of entered positrons in the upstream RDC(6.2)

In principle, the efficiency can be calculated by using the probabilities which were shown in
Figure 6.24. However, the probability that we should consider (PSingle or POR ) depends on thehit position and timing patterns of the muon. Following section describes how the efficiencies
can be calculated in either standard or staggered readout.
Standard readout
In the standard readout, we can consider the muon hit patterns as shown in Figure 6.25.
In the simulated RMD positron hit event, we first checked if there was a muon hit within ∆T =120 ns in the whole detector. If there was no muon hit within ∆T , we regarded the probability
to detect a signal as POR . Actually, 99.7% of the RMD positron hit events had at least one
muons within ∆T . For these case, we next checked the hit bundle of the muons. If there wasno pileup muon in the same bundle, we regarded the probability as POR .

Figure 6.25: Hit patterns of the muon in the standard readout. The probabilities to detect asignal are shown in the red boxes. The fraction of each conditional branch is also shown.

6.3. Total detection efficiency for RMD positrons

77

Staggered readout
The muon hit patterns in the staggered readout is more complicated than that in thestandard readout (Figure 6.26). When there is a muon hit within ∆T = 120 ns in the wholedetector, we can consider four hit position patterns as illustrated in Figure 6.27.

Figure 6.26: Hit patterns of the muon in the staggered readout. Details of the hit position
patterns are shown in the next figure.

Figure 6.27: Four hit position patterns when there are muon hits within ∆T . Case A, C :Positron cannot be detected at both ends. Case B : Positron can be detected only at one end.
Case D : Positron can be detected at both ends.
Figure 6.28 compares the efficiency for the RMD positrons with the different light yields
between the standard readout and the staggered readout. Given the fact that the staggered
readout can reduce the effect of the pileup (Figure 6.19), its total efficiencies are superior tothat of the standard readout.

78

Chapter 6. Development of the Upstream Detector

10.9Standard readout

Detection eficiency

0.8
Staggered readout

0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

1

1.5

2

2.5

3

3.5

4

Light yields at single side (N photon)

Figure 6.28: Expected detection efficiency for the RMD positrons.
Based on the calculated detection efficiency, we finally estimated the MEG II sensitivity
with the upstream RDC. Previously, the sensitivity was calculated by making an ensemble ofpseudo experiments to to extract the upper limit at 90% C.L. in three years data taking. If
the upstream RDC is not installed, the sensitivity will be 4.3 × 10−14 . On the other hand,
if the upstream RDC is installed and the detection efficiency is 100%, the sensitivity willbe 3.9 × 10−14 . The sensitivity including the efficiency loss was calculated by assuming thatthe sensitivity becomes worse depending on the number of the remaining background events.
Approximately, the change of the sensitivity is in proportion to the square root of the numberof the remaining background events. Figure 6.29 shows the estimated sensitivities with above
assumption in several configurations of the light yields.

Standard readout

Sensitivity (× 10-14)

4.2

Staggered readout

4.15

4.1

4.05

1

1.5

2

2.5

3

3.5

Light yields at single side (N photon)

4

Figure 6.29: Expected sensitivity of the MEG II experiment.

Chapter 7
Summary and Outlook
7.1

Radiative Decay Counter

The Radiative Decay Counter is one of the key detectors of the MEG II experiment.
It identifies the dominant source of the background photons from RMD by detecting timecoincident low momentum positrons. Two detectors are installed on the beam axis at both
the downstream and the upstream of the stopping target. According to the simulation study,
41% of total background photons can be identified and thus the sensitivity is improved by
22% if both the downstream and the upstream detectors are installed. The installation of thedownstream RDC detector was approved by the collaboration and the construction was alreadyfinished. The first commissioning was successfully performed by using a high intensity muon
beam. Based on the issues revealed in the commissioning, the optimization of the downstream
detector is in progress toward the final installation. The installation of the upstream detector
has not been approved yet because it requires additional R&D concerning the operation in themuon beam. A series of studies has been performed in various ways as summarized in thefollowing sections.

7.2

Downstream detector

We started from optimizing the calorimeter, which consists of the LYSO crystals and
SiPMs. Performances of all the LYSO crystal was checked and we obtained the good energy
resolution of ∼ 6% for all channels. On the other hand, we observed afterglow of the LYSO
crystals, which could affect the energy resolution. By studying with a room light and β-ray
source, we concluded that the influence on the energy resolution would be small. We also
optimized the optical coupling of the crystals and the SiPMs. By comparing the light yields ofthe crystal, we concluded that the coupling with grease is the best.
The first commissioning of the constructed detector was performed by using a high intensity
muon beam and a BGO photon detector. The detector installation with the moving arm, trigger
and DAQ were tested. The calibration of each detector was performed before the data taking.
By triggering on the hit in the BGO detector, we observed a clear timing peak of the RMD
events after event selections. Although the commissioning was successfully accomplished, few
issues were found at the same time. Due to the discrepancy of the two calibration resultsof the BGO detector, we could not quantitatively evaluate the capability of the background
identification. The cause has not been fully understood yet. In addition, we found that theconnection of the PCBs and SiPMs in the timing counter were very fragile. Due to this, we79

80

Chapter 7. Summary and Outlook

could not detect the signals in four channels. However, we can improve the mechanical strengthwith the reflow soldering.
Towards start of the physics data taking of MEG II, we will fix all the issues found in thecommissioning. All the timing counters will be reassembled with the reflow soldering instead of
the conductive epoxy. We are also going to prepare some spare counters so that we can quickly
replace even if some problematic channels are found. Moreover, the detector layout will be
modified in order to have a sufficient clearance for the movement of the detector.
For further quantitative analysis of the data acquired in the first commissioning, it isnecessary to understand the correct energy scale of the BGO detector. We will check thelinearity of each component such as the BGO crystal and the PMT.

7.3

Upstream detector

First, the influence on the muon beam was studied with a mockup RDC. The increaseof the spot size was measured to be 16%. However, according to the simulation study, theincreased beam spot does not significantly affect the performance of the positron spectrometer.
The efficiency loss for the signal positron is less than 1% and the momentum resolution would
not be changed. Moreover, the loss of the muon stopping rate would be less than 1%.
Another potential issue is the detection efficiency loss for the RMD positrons due to thepileup muons. Because there are after-pulses of the SiPM associated with the main pulse of
the muon, the effect of the pileup with these after-pulses was also considered. In order tomeasure the minimum time difference to distinguish the muon and positron waveform (∆T ),we acquired each muon and positron waveform data by using a prototype detector. By mixing
two waveforms randomly and analyzing it, ∆T was estimated to be 120 ns. The inefficiency
due to the pileup was calculated with ∆T and the expected hit rate of muon and positronfrom RMD at each bundle, which was obtained in the simulation study. The configuration of
the bundle widths was also optimized to minimize the inefficiency. Due to the high muon hit
rate (∼ 500 kHz at the central fiber) and large ∆T , the inefficiency is large (∼ 50%) even byusing the best bundling configuration. However, a further reduction of the pileup is possible inseveral ways.
We finally calculated the total detection efficiency for the RMD positrons and the sensitivity. Because the light yield of the scintillating fiber has not been completely understood yet,the efficiency was estimated with several possible configurations.
In order to evaluate the light yields more accurately, the attenuation length of the fiber isbeing measured. After the measurement, we will be able to calculate the efficiency and evaluate
the sensitivity. Moreover, light yields loss due to the radiation damage has to be investigated.
For this purpose, the irradiation test of the scintillating fiber is planned at PSI.

7.4

Outlook

The engineering run of the MEG II experiment is expected to start in 2017, which will befollowed by the physics data taking. The downstream detector will be installed by fixing all theissues which were found in the commissioning. The performance evaluation of the upstream
detector will be finished in this year, including the irradiation test and the attenuation length
measurement of the scintillating fibers. After that, we will propose to install the upstream
detector, and as soon as it is approved by the MEG II collaboration, we will start the construction. The MEG II experiment is expected to reach the sensitivity goal of ∼ 4 × 10−14 after7.4. Outlook
three years data taking with both RDC detectors.

81Appendix A
Characterization of SiPM
This chapter reviews the basic properties of the SiPM and introduces several measurements
which were performed by using a S13360-1350CS from Hamamatsu Photonics. This SiPM wasused in the prototype of the upstream RDC (Figure A.1). It has a small active area of 1.3×1.3
mm2 and the crosstalk probability is suppressed compared to the previous model (S12825050C). The basic properties are summarized in Table A.1. Although a lot of the SiPMs aremounted on the PCB, one of the SiPM was used in all of the following measurements.

Figure A.1: PCB with 36 SiPMs.

Pixel pitch
Breakdown voltage
Photon detection efficiency
Gain at VOPFill factor
Dark rate at VOP
Crosstalk probability at VOP
Temperate coefficient
Terminal capacitance

50 µm
53 ± 5 V40%
1.7 × 10674%
90 ∼ 270 kcps
1%
54 V/◦ C
60 pF

Table A.1: Basic properties of S13360-1350CS [16]. VOP is breakdown voltage + 3.0 V. The
measurements were performed at 25 ◦ C.

82

A.1. Principle of photon detection

A.1

83

Principle of photon detection

The SiPM is a semiconductor photodiode in which multi-pixels are connected in parallel. Each pixel is operated in “Geiger mode” with the bias voltage which is larger than the
breakdown voltage. This causes an avalanche signal and enables to detect photons at very low
light level from a single photon. As illustrated in Figure A.2, each pixel has a structure of the
p-n junction. When a photon hits the active area of the pixel and its energy is higher thanthe band gap energy, an electron is excited into the conduction band and thus it produces anelectron-hole pair. The electron is accelerated with the bias voltage toward the avalanche region
where the higher bias voltage is applied. When the energy of the election is large enough, it
triggers an avalanche of the further electron-hole pair creations. Once the avalanche starts, thecurrent of the quenching resistance starts to increase and the voltage of the pixel starts to go
down. The avalanche stops when the bias voltage becomes below the break down voltage.

Figure A.2: Schematic diagram of avalanche multiplication [23].

A.2

Current-Voltage curve

0

×103

Current[nA]

Current[nA]

Measuring a current-voltage curve (I-V courve) is one of the important characterizationof the SiPM in order to extract parameters such as breakdown voltage or quenching resistance.
The measurement was performed by using a picoammeter (Keithley, 6485) and recording software (LabView). The temperature was kept at 24 ◦ C with a thermal chamber. Figure A.3
shows the measured I-V curves in both negative and positive bias voltage regions.

−500

180
160
140
120
100

−1000

80
χ2 / ndf
8.133e+07 / 4
p0
1.32e+06 ± 1.36e+04
p1
2.07e+06 ± 1.078e+04

−1500

60
40
20
0

−2000

−1.6

−1.4

−1.2

−1

−0.8

−0.6

−0.4

−0.2

0

Voltage[V]

0

10

20

30

40

50

60

Voltage[V]Figure A.3: I-V curve of the SiPM. Left : Negative bias voltage. Right : Positive bias voltage.
Due to the current limitation, we did not use the software below −1 V, but we read the valueon the display of the picoammeter.

84

Appendix A. Characterization of SiPM

Current[nA]

When we apply the negative bias voltage which corresponds to the forward voltage, we canmeasure the quenching resistance of each pixel from the slope. In Figure A.3, the quenching
resistance was measured to be 322.2 kΩ. Moreover, the recovery time of the pixel can becalculated as a product of the combined quenching resistances of all the pixels and the terminal
capacitance. It was measured to be 29 ns. By applying the positive bias voltage, we can extract
the breakdown voltage of the SiPM (Figure A.4). The curve was fitted with two functions
independently and we can extract the breakdown voltage at the crossing point. For example
in Figure A.3, the breakdown voltage was measured to be 51.5 V which is equivalent to that inFull I-V curve (new)the spec sheet.
102

10

1

42

44

46

48

50

52

54

56

58

Voltage[V]Figure A.4: Example of fitting I-V curve.

A.3

Dark noise

The electron-hole pair is not only created by the incident photon, but it is also thermallygenerated. It triggers the avalanche and induces an undistinguishable signal from the photon
signal. This is called a dark noise. Although the dark noise is induced randomly, the rate of thedark noise is related to the temperature and the bias voltage. Figure A.5,A.6 and A.7 show theresults of the dark rate measurements with different configuration. It was measured by using a
discriminator (PHILIPS, 708), scaler (CAEN, Mod. M145) and 30 dB pre-amplifier which wasdeveloped in PSI. The temperature was controlled with a thermal chamber.
new at 24degree
80

Dark rate (kHz)

70

54.1V

60

54.6V

50

55.1V

40
30
20
10
0

40

60

80
100
Threshold (mV)

120

Figure A.5: Dark rate vs. threshold of the discriminator with different bias voltages (at 24 ◦ C).

A.4. Crosstalk and after-pulse

85
Graph

180
160
140
Dark rate (kHz)

120

8 °C
16 °C
20 °C
24 °C
28 °C
32 °C

100
80
60
40
20
0

54 54.2 54.4 54.6 54.8 55 55.2
Bias voltage (V)Figure A.6: Dark rate vs. bias voltage with different temperatures. The threshold of thediscriminator was always set to the signal amplitude of the half photoelectrons.
darkrate with defferent bias voltage
160
140
54.0V
Dark rate (kHz)

120

54.6V

100

55.2V

80
60
40
20
0

10

15

20
Temp.(°C)

2530

Figure A.7: Dark rate vs. temperature with different bias voltages.

A.4

Crosstalk and after-pulse

A crosstalk and an after-pulse are also important characteristics of the SiPM. The crosstalkis occurred when a triggered pixel induces an avalanche in neighboring pixels. In the S133601350CS, the crosstalk probability is suppressed in the S13360-1350CS by inserting an isolation
trench between the pixels. On the other hand, the after-pulse is occurred when a carrier in the
primary avalanche is trapped and released after quenching.
The upper limit of the probability to induce the crosstalk and after-pulse can be measuredby acquiring the spectrum of the dark noise. Figure A.8 shows the charge spectrum which

86

Appendix A. Characterization of SiPM

was acquired with a DRS evaluation board in the room temperature. The upper limit can becalculated as :Probability =Fraction of > 1.5 p.e.
.
Fraction of > 0.5 p.e.

(A.1)

Note that the probability of the accidental coincidence of the dark signals is also includedin the definition. It was calculated to be about 1.3%.
htemp

area.ch1

Entries
Mean
RMS

1000002.949e+05
4.17e+04

104

103

102

10

1

3

200

400

600

800

1000

1200

×10

Figure A.8: Spectrum of the dark noise.

A.5

Gain

area.ch1

Thanks to the operation with Geiger mode, the SiPM obtains a high gain which enables
the photon counting. We measured the gain with different bias voltages and temperaturesby acquiring the charge spectrum. In principle, the gain can be calculated as the distance oftwo photo-peaks in the spectrum. However, it is difficult to use the dark spectrum because
the fraction of the second or third photo-peak is very small. Therefore, we used a LED andpulse generator (TTI, TGP100-10 MHz) to collect actual photons. The waveform data was
acquired with the DRS evaluation board by triggering on the synchronization signal of thepulse generator. An example of the spectrum is shown in Figure A.9. By fitting first two
photo-peaks, the gain was calculated. Figure A.10 shows the gain which was measured withdifferent bias voltages and temperatures. The fluctuation at the high temperature probably
comes from the fluctuation of the temperature.

A.5. Gain

87

h0
h0

4000

Entries
Mean
RMS

3500

50000
3.256
2.829

4000
3500

3000

3000

2500

2500

2000

2000

1500

1500

1000

1000

500

500

0− 5

0

5

0
−5

15

10

0

Figure A.9: Measured spectrum at 24 ◦ C. The bias voltage is 53.9 V. The left peak corrrenponds
h4
to a pedestal.
3500

Entries
Mean
RMS

3000

h4

50000
3.97
3.271

3500
3000

2500

2500

2000

2000

1500 Temperture ( °C)

1500

3.51000

8

1000

16

500

gain(a.u.)

3 0
−5

500

20
24
28

0

5

10

0
−5

15

0

32

2.5

h8

3500

3500

h8
Entries
Mean
RMS

23000

50000
4.383
3.678

3000

2500

2500

1.52000

2000

1500

1500

1000
500

54

54.5

55

55.5

bias voltage(V)

1000
500

0− 5
5
10 different15tempertures.
Figure A.10:
Gain 0vs bias voltage
with

0
−5

h12
3000
2500

Entries
Mean
RMS

h12

50000
4.93
4.01

3000
2500

0

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91

Acknowledgements
First of all, I would like to thank my supervisor, Prof. Wataru Ootani for supporting me all thetime. I am really delighted to keep studying under his supervision in the future. I would like tothank Prof. Toshinori Mori for giving me an opportunity to join the MEG II experiment. I amheartily thankful to the people in the team RDC for guiding my work with detailed advices. I
learned so many things from Dr. Ryu Sawada and Dr. Kei Ieki. I would like to thank Mr. Shota
Nakaura for inviting me to the MEG II experiment and the team RDC. I am very grateful forthe people in the scintillating fiber group, especially, Dr. Angela Papa, Dr. Emanuele Ripiccini
and Ms. Giada Rutar for giving me an opportunity to study in their group. I would like tothank all the MEG II collaborators, especially, Dr. Toshiyuki Iwamoto, Dr. Yusuke Uchiyama,
Dr. Peter-Raymond Kettle, Dr. Malte Hildebrandt, Dr. Daisuke Kaneko, Mr. Felix Berg, Ms.
Miki Nishimura, Mr. Shinji Ogawa, Mr. Kohei Yoshida, and Mr. Mitsutaka Nakao, for a great
help. I would like to thank Mr. Naoya Shibata for giving me a lot of advices. I also thank to
all friends in Switzerland for helping me a lot in a daily life. Finally, I am deeply thankful tomy family for supporting me all the time.