# Development of ultra-low mass and high-rate capable RPC based on Diamond-
Like Carbon electrodes for MEG II experiment

Kensuke Yamamoto<EMAIL_ADDRESS>Sei Ban  Kei Ieki111Current
address: ICRR, The University of Tokyo, 5-1-5 Kashiwa-no-Ha, Kashiwa,
277-8582, Japan Atsuhiko Ochi  Rina Onda  Wataru Ootani  Atsushi Oya  Masato
Takahashi

###### Abstract

A new type of resistive plate chamber with thin-film electrodes based on
diamond-like carbon is under development for background identification in the
MEG II experiment. Installed in a low-momentum and high-intensity muon beam,
the detector is required to have extremely low mass and a high rate
capability. A single-layer prototype detector with 2 cm $\times$ 2 cm size was
constructed and evaluated to have a high rate capability of 1
MHz/$\mathrm{cm^{2}}$ low-momentum muons. For a higher rate capability and
scalability of the detector size, the electrodes to supply high voltage were
segmented at a 1 cm pitch by implementing a conductive pattern on diamond-like
carbon. Using the new electrodes, a four-layer prototype detector was
constructed and evaluated to have a 46% detection efficiency with only a
single layer active at a rate of $\cal O$(10 kHz). The result with the new
electrodes is promising to achieve the required detection efficiency of 90% at
a rate of 4 MHz/$\mathrm{cm^{2}}$ with all the layers active.

###### keywords:

Resistive plate chamber (RPC) , Diamond-like carbon (DLC) , MEG II

††journal: Nuclear Instruments and Methods in Physics Research Section A

[u-tokyo]organization=Department of Physics, The University of
Tokyo,addressline=7-3-1 Hongo, Bunkyo-ku, state=Tokyo, postcode=113-0033,
country=Japan [icepp]organization=ICEPP, The University of
Tokyo,addressline=7-3-1 Hongo, Bunkyo-ku, state=Tokyo, postcode=113-0033,
country=Japan [kobe]organization=Department of Physics, Kobe
University,addressline=1-1 Rokkodai-cho, Nada-ku, city=Kobe, state=Hyogo,
postcode=657-8501, country=Japan

##  1 Introduction

A novel background identification detector is under development for the MEG II
experiment, aiming at a further sensitivity improvement in the $\mu\to
e\gamma$ search [1]. It detects a low-energy positron of 1–5 MeV from the
radiative muon decay (RMD) with a high-energy $\gamma$-ray of $>$48 MeV
detected by a $\gamma$ detector. An extremely low-mass design of radiation
length $X_{0}$ of 0.1% and a high rate capability of up to 4
MHz/$\mathrm{cm^{2}}$ are required since the detector is planned to be
installed in a low-momentum (28 MeV/$c$) and high-intensity ($10^{8}$/s) muon
beam. The detection efficiency of 90% for minimum ionising particle (MIP), the
timing resolution of 1 ns, and the 20 cm diameter detector size are also
required to efficiently identify the RMD.

A new type of resistive plate chamber (RPC) based on diamond-like carbon (DLC)
electrodes (DLC-RPC) shown in Fig. 1 is a promising candidate for the
detector. It has thin-film resistive electrodes based on DLC coating for the
low-mass design. The overall material budget can be suppressed to 0.095%
$X_{0}$ with a four-layer configuration. A single-layer detection efficiency
is required to be above 40% to achieve the 90% detection efficiency with four
layers.

##  2 Rate capability of DLC-RPC

Figure 1: Concept of DLC-RPC

The performance of RPCs generally degrades at a high rate due to voltage drop
in the resistive electrode [2]. In contrast to conventional glass-based RPCs,
in which an avalanche current flows through the resistive plates
perpendicularly, the current flows through the surface of the DLC layer
transversely in the DLC-RPC. Therefore, the further the current flow distance
is, the larger the voltage drop is. The voltage drop $\delta V$ for the DLC-
RPC is given by

$\nabla^{2}\delta V(x,y)=Q_{\mathrm{mean}}(V_{\mathrm{eff}})\cdot
f(x,y)\cdot\rho_{S},$ (1)

where $Q_{\mathrm{mean}}(V_{\mathrm{eff}})$ is the mean of avalanche charge at
an effective high voltage, $f(x,y)$ is the hit rate, and $\rho_{S}$ is the
sheet resistivity of DLC. The voltage drop is proportional to the square of
the current flow distance approximately and to the sheet resistivity of DLC.
These parameters are important to design the resistive electrodes.

##  3 Single-layer prototype

A single-layer prototype detector with 2 cm $\times$ 2 cm size was constructed
and its performance was evaluated [3].

Figure 2: Electrode production processes and photo of an electrode for a
single-layer prototype

###  3.1 Electrode production and construction

The resistive electrode production uses sputtering and photolithographic
technologies which are commonly used for micropattern gaseous detectors. Fig.
2 shows the electrode production processes. Firstly, DLC was sputtered on 50
µm-thick Kapton foil masked by a seal with a 3.5 cm square hole. The sheet
resistivity was 60 $\mathrm{M\Omega/sq.}$ and 7 $\mathrm{M\Omega/sq.}$ for the
anode and cathode, respectively. Secondly, 25 µm-thick insulating photoresist
film was deposited on the boundary of DLC to prevent discharge here. Finally,
photolithographic spacers with 400 µm diameter and 384 µm thickness were
formed on DLC with 2.5 mm pitch.

Figure 3: Constructed single-layer prototype [3]

The construction scheme and setups are described in [3]. Fig. 3 shows the
constructed single-layer prototype. The current flow distance is 2 cm. The
induced signal in the readout strip was amplified by 38 dB amplifiers, and fed
into DRS4 waveform digitiser [4].

###  3.2 Performance measurement at high rate

Figure 4: Setup of performance test at high rate

The performance for the MIP positron detection in a low-momentum high-rate
muon beam was measured at the $\pi$E5 beam line at Paul Scherrer Institute. As
shown in Fig. 4, the detector was exposed to a muon beam with a spread of
($\sigma_{x}$, $\sigma_{y}$) = (13 mm, 23 mm) at rates of 1
MHz/$\mathrm{cm^{2}}$ and 3.5 MHz/$\mathrm{cm^{2}}$. The readout region of the
detector was aligned to the beam centre. A part of the muons stopped in the
upstream scintillation counter and decayed to positrons. The decay positrons
that passed through the detector were selected by the coincidence of upstream
and downstream scintillation counters. The angle between the muon beam axis
and the triggered positron path was approximately 30∘, which is the typical
incident angle of the RMD positrons in the MEG II experiment.

Figure 5: Pulse height spectra for MIP positron measured at different beam
rates. The red spectrum is acquired in low-momentum muons at a rate of 1
MHz/$\mathrm{cm^{2}}$. The blue one is that at a rate of 3.5
MHz/$\mathrm{cm^{2}}$. The black one is that at a low rate of 1–10
kHz/$\mathrm{cm^{2}}$ with another setup [3]. The green one is a noise
spectrum in off-timing at a low rate.

Fig. 5 shows the pulse height spectra for MIP positrons at applied high
voltage of 2.75 kV after the selection from the hit time difference between
scintillation counters to avoid accidentally triggered events. The red and
blue histograms are the spectra in low-momentum muons at 1
MHz/$\mathrm{cm^{2}}$ and 3.5 MHz/$\mathrm{cm^{2}}$, respectively. The black
histogram is obtained at a low rate of 1–10 kHz/$\mathrm{cm^{2}}$ with another
setup [3]. The performance degradation was clearly observed with a higher beam
rate. A detection efficiency is defined as the fraction of the number of
events with pulse height over 20 mV to the number of triggered events. The
threshold of 20 mV was set according to the noise level shown in a green
histogram in Fig. 5. The fraction of noise contamination in counting signal
events at the 20 mV threshold was less than 2%. A 42% (17%) detection
efficiency was achieved at 1 MHz/$\mathrm{cm^{2}}$ (3.5
MHz/$\mathrm{cm^{2}}$).

The voltage drop was calculated to 110–170 V and 210–310 V at 1
MHz/$\mathrm{cm^{2}}$ and 3.5 MHz/$\mathrm{cm^{2}}$, respectively, using Eq.
(1), where $Q_{\mathrm{mean}}=2.3~{}\mathrm{pC}$ for 1 MHz/$\mathrm{cm^{2}}$
and $Q_{\mathrm{mean}}=1.2~{}\mathrm{pC}$ for 3.5 MHz/$\mathrm{cm^{2}}$. These
voltage drops are consistent at 10% with the observed detection efficiencies.
It is concluded that the single-layer prototype has a rate capability of up to
1 MHz/$\mathrm{cm^{2}}$ low-momentum muons.

##  4 Four-layer prototype

The single-layer prototype did not still satisfy the requirement of the rate
capability. The voltage drop should be suppressed for a higher rate
capability. As described in Sec. 2, it is necessary to shorten the current
flow distance and to make the sheet resistivity of DLC lower.

Segmented high voltage supply with 1 cm pitch and a low DLC resistivity of 10
$\mathrm{M\Omega/sq.}$ were designed to fulfill the rate capability
requirement of 4 MHz/$\mathrm{cm^{2}}$. The segmentation is also necessary for
the scalability of the detector size. The voltage drop can be suppressed to
60–80 V at 4 MHz/$\mathrm{cm^{2}}$ with these configurations.

###  4.1 Electrode production

Figure 6: Conductive and insulation pattern on DLC for high voltage supply
segmentation

The 3 cm $\times$ 3 cm electrodes with the segmented high voltage supply were
produced. Firstly, DLC was sputtered as in the electrodes for the single-layer
prototype. The sheet resistivity was 20 $\mathrm{M\Omega/sq.}$ In order to
segment the high voltage supply, a conductive pattern which consists of 50
Å-thick chromium layer and 2500 Å-thick copper layer was formed on the DLC by
the liftoff method as shown in Fig. 6. The conductive pattern was covered with
25 µm-thick insulating photoresist to avoid discharge on it. The widths of the
conductive strip and of the insulation strip were 50 µm and 200 µm,
respectively. The region on the insulation strip will be inactive due to the
charge-up effect. Implementation of conductive and insulation strips will
increase the inactive area by 2%. The quality of electric connection between
the conductive pattern and the DLC was confirmed to be connected well by
repeating measurements of the resistance between the conductive pattern and a
point in DLC several times.

Figure 7: Picture of produced electrode

Because the production of photoresist used for the spacers of the single-layer
prototype was cancelled, solder resist had to be used instead. The problem was
that 300 µm-thick spacers cannot be formed using the solder resist. As an
alternative solution, 160 µm-thick spacers were attached on both sides of the
electrode and accumulated with precise alignment to form enough thick gaps.
Fig. 7 shows a picture of the produced electrode.

The outermost resistive electrodes were made of films which were aluminised on
the outer face and DLC-coated on the inner face. Aluminium readout strips were
formed by etching.

###  4.2 Construction

Figure 8: Constructed four-layer prototype

Fig. 8 shows the assembled four-layer prototype. The outermost electrodes were
glued onto printed boards. The intermediate electrodes with resistive layers
on both sides were stacked along alignment pins. It was filled with a gas
mixture of R134a(94%) + SF6(1%) + isobutane(5%). The high voltage was supplied
by conductive glue on the conductive pattern. The signal was read out from
both ends of the aluminium strips, amplified by 38 dB amplifiers, and fed into
the DRS4 waveform digitiser [4].

###  4.3 Performance test

High voltage can be applied to each layer independently since electrodes are
decoupled by the polyimide film. Thus, the detection efficiency to $\beta$-ray
at ${\cal O}$(10 kHz) was measured for each layer independently.

An issue of quality control of gas gaps was found due to a positive pressure
inside the chamber. This caused a smaller gas gain at the centre than that at
the edge since the electrode films expanded outwards. In addition, it caused
unstable operation in three layers of four. A 46% detection efficiency was
measured using the edge of the active region in the second layer from the top
in Fig. 8 (a), which was the only layer successfully operated. It means that
there are no problems with electrode design and detector system in principle.
The issue can be solved by negative pressure in the gas gap. Further studies
are planned to evaluate the performance of the four-layer prototype.

##  5 Conclusion

A new type of RPC with DLC electrodes is under development for RMD
$\gamma$-ray identification in the MEG II experiment. Using the single-layer
prototype with $2~{}\mathrm{cm}\times 2~{}\mathrm{cm}$ size, the high rate
capability of 1 MHz/$\mathrm{cm^{2}}$ was achieved. The resistive electrode
was improved by the conductive pattern on DLC for a higher rate capability of
up to 4 MHz/$\mathrm{cm^{2}}$ and for its scalability. The four-layer
prototype with improved electrodes was tested, and promising results were
obtained although there was an issue of quality control of the gas gaps.
Further performance measurements with $\beta$-ray at ${\cal O}$(100 kHz) and
with high-intensity muon beam at up to 8 MHz/$\mathrm{cm^{2}}$ are planned,
which will be presented in the near future.

## Acknowledgement

I would like to thank H. Uehara of TRENG-F Products, Inc., whose great efforts
contributed to the production of the improved electrodes. This work is
supported by MEXT/JSPS KAKENHI Grant Number 21H04991, JSPS Core-to-Core
Program, A. Advanced Research Networks JPJSCCA20180004, and JSPS Overseas
Challenge Program for Young Researchers 202280015.

## References

  * [1] A. M. Baldini, et al., The design of the meg ii experiment, European Physical Journal C 78 (2018).  doi:10.1140/epjc/s10052-018-5845-6.
  * [2] G. Aielli, et al., Improving the rpc rate capability, Journal of Instrumentation 11 (2016) P07014–P07014.  doi:10.1088/1748-0221/11/07/P07014.
  * [3] A. Oya, et al., Development of high-rate capable and ultra-low mass resistive plate chamber with diamond-like carbon, Journal of Physics: Conference Series 2374 (2022).  doi:10.1088/1742-6596/2374/1/012143.
  * [4] S. Ritt, The drs chip: Cheap waveform digitizing in the ghz range, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 518 (2004).  doi:10.1016/j.nima.2003.11.059.