Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 FROB06

DEVELOPMENT OF THE SIC MUON BEAM MONITOR FOR THE
COMET EXPERIMENT
Yoshinori Fukao∗ , Yowichi Fujita, Tetsuichi Kishishita, Hajime Nishiguchi, Masayoshi Shoji,High Energy Accelerator Research Organization (KEK)
Kazutoshi Kojima, Ryouji Kosugi, Keiko Masumoto, Yasunori Tanaka,
National Institute of Advanced Industrial Science and Technology (AIST)
Abstract�������� �������A new detector to monitor a profile of a secondary muon
beam for the COMET experiment is being developed. TheCOMET experiment searches for the muon-to-electron (𝜇𝑒) conversion without emitting neutrinos. The discoveryof the 𝜇-𝑒 conversion is an evidence of the physics beyond
the standard model of the particle physics. To achieve thegoal of the experiment, it is important to suppress any fakesignals. The new beam monitor is useful to assure a stabilityof the beam operation. The detector utilizes silicon carbide
semiconductor expecting its high radiation tolerance because
the high-intensity secondary beam of COMET can cause asignificant radiation damage to the detector. We introduce
the current status of the development of the muon beam
monitor.

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�INTRODUCTION
A construction of the COMET experiment is underway
at J-PARC. The experiment aims to search for a process ofthe 𝜇-𝑒 conversion. If it is discovered, it is a solid evidenceof an existence of a new physics because the 𝜇-𝑒 conversion
is strongly prohibited in the standard model of the particle
physics.
We are newly developing a detector to monitor a secondary
muon beam. Due to a high radiation level at the detector
location, we utilize a silicon carbide (SiC) semiconductor
for a detection of the beam particles because the SiC semiconductor has higher radiation tolerance than a often-usedsilicon semiconductor. The development of the detector is
ongoing in a cooperative framework of KEK Institute ofParticle and Nuclear Studies and AIST Advanced PowerElectronics Research Center.

COMET EXPERIMENT
Figure 1 displays the COMET Phase-1 experimental setup[1]. The J-PARC proton beam is injected to a graphite Pion
Production Target. The beam energy and intensity are 8 GeV
and 2.5×1012 sec−1 , respectively. The target is positioned
in a 5-Tesla superconducting solenoidal magnet (CaptureSolenoid) to capture as many secondary pions as possible.
The pions are transported by a 3-Tesla superconducting
curved solenoidal-magnet (Transport Solenoid). During
the transportation, the pions are decayed to the muons. The
muons are injected to an aluminum Muon Stopping Target
and they form muonic atoms with the target material. A part
∗ fukao@post.kek.jp�������� ����������

Figure 1: COMET Detector.

of the muons are captured by the nucleus and the remaining
muons decay to electrons. In the standard model the decay
electrons accompanied by two neutrinos have maximum energy of about 50 MeV, while in the 𝜇-𝑒 conversion they holdabout 105 MeV which is equivalent to the muon mass. To
catch the signal of the 𝜇-𝑒 conversion, the momentum of
the electrons is measured by the COMET Cylindrical Detector (CyDet), whose main detector is a large cylindricaldrift chamber surrounding the Muon Stopping Target. The
proton beam has a bunch structure with interval of ∼1.2 𝜇sec
or 1.7𝜇sec. This feature is important because we have toavoid a beam-related prompt background in the decay electron measurements by selecting a time window between theproton beam bunches.
An intensity of the muon beam is estimated to be about
1010 sec−1 . The momentum of the muon beam is required to
be about 45 MeV/𝑐 (9.2 MeV in kinetic energy) to stop themat the Muon Stopping Target. Any detectors to measure thesecondary beam were not included at the original experimental setup because they can cause a loss of the useful muonsand a production of background particles. However a control and an optimization of the muon beam is quite difficult
without a beam monitor. We proposed the SiC muon beam
monitor with thin material to minimize above demerits.

- 224 -Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 FROB06

MUON BEAM MONITOR WITH SIC
SEMICONDUCTOR

To overcome the high radiation level, we utilize SiC semiconductor as a sensor of the detector. SiC has higher radiation tolerance than Si because it has a wide band gap of3.26 eV and a displacement threshold energy of 22–35 eV.
Our SiC semiconductor processed at AIST has a total thickness of ∼350 𝜇m and ∼50-𝜇m epitaxial layer [3]. Some
single-channel prototype were manufactured and irradiatedusing J-PARC 1 GeV/𝑐 pion beam to evaluate the performance of the SiC semiconductor. Figure 3 displays ADC
distribution and one can see clear MIP peak.1

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Figure 2: Simulated neutron fluence. The arrow indicatesthe position of the muon beam monitor.
One of the difficulties of the detector design comes froma middle-range intensity of the muon beam. Compared to anormal secondary beam, the COMET muon beam has higher
intensity and a high radiation torelance is required for thebeam monitor. On the other hand, compared to the primarybeam, its lower intensity requires higher signal gain for thedetector. The radiation level was evaluated by the MARS
simulation [2]. Figure 2 displays a neutron fluence aroundthe position of the muon beam monitor. The 1-MeV equivalent neutron fluence at the monitor was 1.6×1013 cm−2 at a
proton-on-target of 4.3×1019 , which is an expected statistics
during the COMET Phase-1 period. An ionizing damage
was mainly caused by the beam particles to be 1.2 MGy,
where about half of them is from the muons and the rest isfrom other charged particles such as electrons and positrons.

Figure 4: Schematic diagram of the muon beam monitor. Its
width is about 220 mm.
Figure 4 is a schematic diagram of the muon beam monitor. Small squares indicate SiC sensor and have a size of
5×5 mm2 . They will be mounted on the polyimide circuitboard with a thickness of about 200 𝜇m. The circuit board
will have two layers; one is for signal lines and the otheris for the high-voltage. The signal wires will run between
SiC sensors and will be connected to downstream front-end
readout electronics via 8 connectors located at the edge ofthe detector. Because the circuit board is so thin that it needs
mechanical support to self-stand, the edge region of the detector, which is outside of a red lines in Fig. 4, is thick to be
1.6 mm.

SIGNAL ESTIMATION AND SIGNALREADOUT SCHEME
Time and position distribution of the secondary beam at
the position of the muon beam monitor were evaluated using
GEANT4 simulation. Figure 5 displays time distribution
for each particle species. Most of the positively charged
particles are discarded because they are not transported by
the curved Transport Solenoid. Because many electrons andpositrons hit the beam monitor at the prompt timing, the��������� �����

1 The measurements were performed using a commercial signal amplifier.

Figure 3: ADC distribution measured with J-PARC 1-GeV/𝑐pion beam. A photo of the right hand side shows three
prototypes of single channel.

Though the signal-to-noise (S/N) was good because the amplifier has
long time constant of the order of micro-second, S/N will become worse
at the actual use because a fast amplifier of less than 100 nsec will berequired. However, due to the bunched beam, sufficient signal amplitude
will be expected at the real measurements.

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Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)����� ��� ���� ����� � ����PASJ2022 FROB06������������������������������������ � ���� � ��������� � �������� � ������� � ��������� �����������
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Figure 5: Hit time distribution of secondary particles. Awide blue line is a eye-guide of the sum of the charged
particles with 100-nsec bin.

Figure 7: The evaluation board for the ASIC.

SIGNAL LOSS AND BACKGROUND
INCREASE

monitor needs to limit a detection time window to extract
muons. Figure 6 displays horizontal position distribution
with and without timing window of 100 to 300 nsec selected.
The muon distribution can be extracted by selecting the timewindow.
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Figure 6: Horizontal hit position distribution of the secondary beam particles. The left and the right figures are
the distribution without and with 200-nsec time window
selected, respectively.

To realize applying the time window for the measurements,
we plan to record the signal as a waveform with rough time
resolution such as 100 nsec. The blue line in Fig. 5 shows
the idea. Recorded data will have a shape like the blue line
and the time window selection will be realized by selecting
the second and the third bin from the peak bin. As well as themuon sample, we will be able to extract other backgroundparticles when we choose other bins. The ADC function
described here will be implemented in ASIC (Application
Specific Integrated Circuit) with high radiation tolerance.
Because the front-end electronics must be installed near thedetector where the radiation level will be high, any other ICs
than ASIC can not be available there. The signal from theASIC will be transported to another electronics for the signal
driver that will be located outside of the Detector Solenoidin Fig. 1. The first prototype of the ASIC was produced andits evaluation is ongoing. Figure 7 is a photo of an evaluation
board for the ASIC.

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Figure 8: The setup of the GEANT4 simulation to evaluate
the harmful effects by installing the beam monitor in thephysics measurements.
The muons which is useful for the physics measurements
will be reduced by installing the muon beam monitor because
the low-momentum muons can be stopped at the detector. At
the same time, an interaction of the beam particles and themuon beam monitor can produce background particles. We
evaluated such harmful effects using GEANT4 simulation.
Figure 8 displays a setup of the GEANT4 simulation. We
assumed the beam monitor thickness as sum of 350 𝜇m-SiC,
175-𝜇m polyimide and 36-𝜇m copper. Two virtual detectors
were located in the simulation and particles which penetrate- 226 -

Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 FROB06

the virtual detectors were counted. In addition, the numberof beam particles which stopped in the Muon Stopping Target was counted. Above counts with the SiC muon beammonitor installed (Nw/ monitor ) and those without the monitor
installed (Nw/o monitor ) were recorded in the simulation. By
comparing these, the signal loss and the background increase
can be estimated.
Table 1: Simulated particle loss and background increase.
Numbers in the table is a ratio of the particle count with thebeam monitor to that without the beam monitor. Regarding
the numbers for the Virtual Detector2, particles which hit thedetector 500 nsec after the proton injection were required.
This timeing cut will be applied in the real measurements
to avoid the prompt background related to the proton beam
bunch. “-” means that no events remained with statistics ofthe simulation.
Particle

Virtual
Detector1

TargetStop𝑒−𝑒+𝜇−𝜇+𝜋−𝜋+
𝑝𝑛
𝛾

0.968
0.841
0.944
0.633
0.978
0.813
1.971
1.060
1.236

0.927
0.982
0.697
1.174
0.569
-

Virtual
Detector2(𝑡 > 500 nsec)
0.983
0.696
0.915
0.976
0.981

The ratio of Nw/ monitor / Nw/o monitor are summarized inTable 1. At the downstream of the muon beam monitor (Virtual Detector1), protons, neutrons and gammas are increased
because they are generated by muons captured by nuclei. On
the other hand, other particles are reduced. The number ofmuons which are stopped at the Muon Stopping Target isreduced to be 98.2 %, which is a little larger than 94.4 %; theratio at the Virtual Detector1, because higher-energy muons,
which were originally not stopped at the target, stopped at

the target due to a loss of the energy at the beam monitor.
The number of stopped negative pions is increased and they
can produce background particles by pion-nuclei capture
process. However, absolute yield of the negative pions isless than 1 % of the muons and the effect is not significant.
In this simulation, any particles which hit CyDet (VirtualDetector2) is the background source because 𝜇-𝑒 conversion
process is not included. It turned out that the backgroundwould not increase by installing the muon beam monitor.
The maximum ratio at the Virtual Detector2 was 98.3 %,which is very close to the loss of the stopped muons at the target. This means that there will be no significant degradationin terms of the signal-to-noise ratio.

SUMMARY AND FUTURE PROSPECTS
We are developing the new muon beam monitor with SiC
semiconductor sensors for the COMET experiment. The
monitor can be realized owing to the high radiation toleranceof SiC. It is rather new to use SiC as a particle detector. There
are several difficulties in the development other than theradiation tolerance. One of them is the mechanical feasibility
because the detector with thin film-like circuit board must
self-stand and the SiC sensors must be mounted on the board.
We are now preparing a small prototype of the detector toestablish a method of the detector fabrication.
We expect the first beam for the COMET experimentin JFY2022 where a commissioning of the proton beam
acceleration and the muon transportation by the Transport
Solenoid will be carried out. The physics measurement isexpected to be performed in JFY2024.

REFERENCES[1] The COMET collaboration, “COMET Phase-I technical design
report”, Prog. Theor. Exp. Phys. 2020, 033C01.
[2] N. V. Mokhov et al., “The MARS Code System User’s Guide
Version 15(2016)”, FERMILAB-FN-1058-APC.
[3] T. Kishishita et al., “SiC p+n Junction Diodes Toward BeamMonitor Applications”, IEEE Trans.Nucl.Sci. 68 (2021) 12,2787-2793.

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