# The JSNS2 Detector

S. Ajimura  M. Botran  J. H. Choi  J. W. Choi  M. K. Cheoun  T. Dodo  H.
Furuta  J. Goh  K. Haga  M. Harada  S. Hasegawa  Y. Hino  T. Hiraiwa  H. I.
Jang  J. S. Jang  M. C. Jang  H. Jeon  S. Jeon  K. K. Joo  J. R. Jordan  D. E.
Jung  S. K. Kang  Y. Kasugai  T. Kawasaki  E. J. Kim  J. Y. Kim  S. B. Kim
<EMAIL_ADDRESS>W. Kim  H. Kinoshita  T. Konno  D. H. Lee  S. Lee  I. T. Lim  E.
Marzec  T. Maruyama  S. Masuda  S. Meigo  S. Monjushiro  D. H. Moon  T. Nakano
M. Niiyama  K. Nishikawa111Deceased M. Nomachi  M. Y. Pac  J. S. Park  S. J.
M. Peeters  H. Ray  G. Roellinghoff  C. Rott  K. Sakai  S. Sakamoto  T. Shima
C. D. Shin  J. Spitz  I. Stancu  Y. Sugaya  F. Suekane  K. Suzuya  M. Taira
R. Ujiie  Y. Yamaguchi  M. Yeh  I. S. Yeo  C. Yoo  I. Yu  A. Zohaib
Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL,
35487, USA  Brookhaven National Laboratory, Upton, NY, 11973–5000, USA
Department of Physics, Chonnam National University, Gwangju, 61186, KOREA
Laboratory for High Energy Physics, Dongshin University, Chonnam 58245, KOREA
Department of Physics, University of Florida, Gainesville, FL, 32611, USA
Department of Physics and Optical Science, Gwangju Institute of Science and
Technology, Gwangju, 61005, KOREA  J-PARC Center, JAEA, Tokai, Naka Ibaraki
319–1195, JAPAN  Advanced Science Research Center, JAEA, Tokai, Naka Ibaraki
319–1195, JAPAN  Division of Science Education, Jeonbuk National University,
Jeonju, 54896, KOREA  High Energy Accelerator Research Organization (KEK),
Tsukuba, Ibaraki 305–0801, JAPAN  Department of Physics, Kitasato University,
Sagamihara, Kanagawa 252–0373, JAPAN  Department of Physics, Kyoto Sangyo
University, Kyoto 603–8555, JAPAN  Department of Physics, Kyung Hee
University, Seoul 02447, Korea  Department of Physics, Kyungpook National
University, Daegu 41566, KOREA  Department of Physics, University of Michigan,
Ann Arbor, MI, 48109, USA  Research Center for Nuclear Physics, Osaka
University, Osaka 565–0871, JAPAN  Department of Fire Safety, Seoyeong
University, Gwangju 61268, KOREA  School of Liberal Arts, Seoul National
University of Science and Technology, Seoul, 139–743, KOREA  Department of
Physics, Soongsil University, Seoul 06978, KOREA  Department of Physics,
Sungkyunkwan University, Suwon 16419, KOREA  Department of Physics and
Astronomy, University of Sussex, BN1 9QH, Brighton, UK  Research Center for
Neutrino Science, Tohoku University, Sendai, Miyagi 980–8577, JAPAN
Department of Physics and Astronomy, University of Utah, Salt Lake City, UT,
84112, USA

###### Abstract

The JSNS2 (J-PARC Sterile Neutrino Search at J-PARC Spallation Neutron Source)
experiment aims to search for oscillations involving a sterile neutrino in the
eV2 mass-splitting range. The experiment will search for the appearance of
electron antineutrinos oscillated from muon antineutrinos. The electron
antineutrinos are detected via the inverse beta decay process using a liquid
scintillator detector. A 1 MW beam of 3 GeV protons incident on a spallation
neutron target produces an intense and pulsed neutrino source from pion, muon,
and kaon decay at rest. The JSNS2 detector is located 24 m away from the
neutrino source and began operation from June 2020. The detector contains 17
tonnes of gadolinium (Gd) loaded liquid scintillator (LS) in an acrylic
vessel, as a neutrino target. It is surrounded by 31 tonnes of unloaded LS in
a stainless steel tank. Optical photons produced in LS are viewed by 120 R7081
Hamamatsu 10-inch Photomultiplier Tubes (PMTs). In this paper, we describe the
JSNS2 detector design, construction, and operation.

###### keywords:

Sterile neutrino , Neutrino source from decay at rest , Neutrino detector ,
Liquid scintillator

††journal: Journal of Nuclear Instruments and Methods in Physics Research A

##  1 Introduction and Overview

The existence of sterile neutrinos remains an open question. Some experimental
results may not be explained by the three active flavor neutrino framework and
suggest the existence of sterile a neutrino with a mass-squared splitting
around 1 eV2 [1, 2, 3, 4, 5]. The JSNS2 experiment will search for sterile
neutrino oscillations at the J-PARC Materials and Life science experimental
Facility (MLF). With a 3 GeV proton beam from the Rapid Cycling Synchrotron
(RCS) and a spallation neutron target, an intense and 25 Hz pulsed neutrino
source is available from pion, muon, and kaon decays at rest [6]. The
experiment will look for appearance of $\overline{\nu}_{e}$ oscillated from
$\overline{\nu}_{\mu}$ that has a well-known spectrum and a 52.8 MeV end point
energy. The $\overline{\nu}_{\mu}$ to $\overline{\nu}_{e}$ oscillation results
in $\overline{\nu}_{e}$ spectral modulation. The produced $\overline{\nu}_{e}$
is detected by the inverse beta decay (IBD) interaction
$\overline{\nu}_{e}+p\rightarrow e^{+}+n$, followed by $\sim$8 MeV gammas from
neutron capture on Gd. The $\overline{\nu}_{e}$ energy is reconstructed by the
positron visible energy plus 0.8 MeV. In particular, the delayed signal
identification by neutron capture on Gd allows for a significant reduction of
ambient $\gamma$-ray backgrounds below 3 MeV.

A coincidence requirement of 100 $\mu$s between prompt and delayed events
further reduces the background event rate substantially. The requirements of
the time coincidence and their spatial correlation reduces the accidental
background by a factor of approximately $10^{4}$. By taking advantage of the
short pulse width and low frequency of the RCS proton beam, a timing gate
requirement of a prompt candidate event between 1 and 10 $\mu$s from the beam
starting time eliminates most of the neutrinos from pion and kaon decays and
beam-induced fast neutrons, and also reduces the cosmic-ray induced background
by $\sim$$10^{5}$.

The detector is located on the third floor of the MLF at a baseline distance
of 24 m from the neutrino source. Figure 1 shows the overall layout of the
JSNS2 experiment. Data collected with the JSNS2 experiment will be sensitive
to eV-scale sterile neutrinos and can be used to perform an ultimate and
direct test of the LSND anomaly.

Figure 1: Layout of JSNS2 experiment. The detector is located on the third
floor of the MLF at a baseline distance of 24 m from the neutrino source.

The JSNS2 detector consists of three cylindrical layers including an innermost
neutrino target, an intermediate gamma-catcher, and an outermost veto. The
neutrino target is made of 17 tonnes of Gd loaded LS (Gd-LS) stored in an
acrylic vessel, 3.2 m ($D$) $\times$ 2.5 m ($H$). The gamma-catcher and veto
are filled with 31 tonnes of Gd unloaded LS in a stainless steel vessel, 4.6 m
($D$) $\times$ 4.0 m ($H$), and optically separated. The gamma-catcher is 25
cm wide while the veto has a thickness of 45 cm and 25 cm for the barrel side
and for the top and bottom sides, respectively. Optical photons produced via
scintillation in the target and gamma-catcher are viewed by an array of 96
10-inch PMTs. Scintillation light in the veto can be detected by an array of
24 10-inch PMTs. A schematic view of the detector is shown in Fig 2.

Figure 2:  Schematic view of the JSNS2 detector.

The third floor of the MLF is normally used as a maintenance area for the
spallation neutron target and its ancillary systems. During the maintenance
period, typically from July to September, the JSNS2 detector must be removed
from this area in order to avoid possible interference with necessary
maintenance activity. Therefore, the detector has been designed for a
convenient relocation, including liquid filling and extraction. The detector
is assembled and stored in a detector storage building at J-PARC. The detector
is transported to the first floor of the MLF using a low-bed trailer. There it
is filled with LS and Gd-LS. The 70-tonne detector is lifted to the third
floor by a crane and placed on top of a shielding layer. Lead blocks and iron
plates are installed underneath the detector in order to shield against gamma
rays produced in the concrete floor by neutron captures coming from the beam
target. During the maintenance period, the detector and shielding materials
are removed from the third floor. The detector is lifted back to the first
floor where the liquid is extracted into storage tanks. Next, it is
transported back to the detector storage building.

The detector construction began in 2017 and was completed in 2020. The first
data-taking period in a commissioning mode lasted for ten days in June 2020.
Data taking was resumed with a 0.6 MW proton beam in January 2021. The beam
power was increased to 0.7 MW in April 2021 and is expected to reach 1 MW in
the near future. In this paper, we describe the design, assembly, and
operation of the JSNS2 detector.

##  2 Detector Design

As mentioned in the previous section, the detector consists of three
cylindrical layers of neutrino target, gamma-catcher and veto. This section
presents a detailed description of the detector design.

###  2.1 Overall Design

A schematic view of the JSNS2 detector is shown in Fig 2. From inner to outer
detector, the experiment is structured as follows: an acrylic vessel holding
the target liquid, an array of 10-inch PMTs, an optical separation wall of
black boards, a veto layer, a stainless steel tank holding the gamma-catcher
and veto LS, and a spill tank. Each of them is described in more detail in the
following sections.

Figure 3:  Gamma-ray shielding layers consisting of iron plates and lead
blocks. The 70 tonne detector with Gd-LS and LS was transported by a crane and
installed on the shielding layers.

Figure 3 shows a picture of a gamma-ray shielding layer underneath the
detector. The gamma-rays are produced in beam neutron capture reactions inside
the concrete floor. Iron plates are assembled into three layers of
approximately 6000 $\times$ 9000 $\times$ 66 mm3. Lead blocks are put together
to form two disks each with thickness of 50 mm: a disk of 5 m in diameter in
the lower layer and a disk of 3.7 m in diameter in the upper layer. This
shielding structure reduces the gamma-ray background by a factor of more than
100 in the 7 to 12 MeV region.

###  2.2 Stainless Steel Tank

The 5 mm thick stainless steel tank was constructed by the Morimatsu company
in the period 2017 to 2018. Water leak tests were done to confirm that the
tank had no leaks. Figure 4 shows a schematic drawing of the stainless steel
tank and the spill tank. The detector size is limited by the size of the MLF
entrance, which is 7 m wide and 5 m high.

Figure 4:  A schematic drawing of the stainless tank and the spill tank. The
red circles indicate an LS buffer space of a ring shape in order to minimize
changes of the liquid level due to thermal expansion of the LS.

The detector is filled with liquid above the top-lid level in order to
minimize LS sloshing during detector relocation by a crane. The detector also
has a compact liquid level stabilization system using a reverse siphon system
with a minimal pipe length. As indicated by the red circles in Fig. 4, a ring-
shaped LS buffer space is included to provide a large liquid surface and
minimize the change of liquid level due to the thermal expansion of LS. More
details of the stainless steel tank are given in Ref. [7].

###  2.3 Spill Tank

To adhere to the local safety regulations, the stainless steel tank is
surrounded by a spill tank. The tank’s diameter is 6.3 m and the inner height
is 2.7 m. If all the LS leaked out of the stainless steel tank, the liquid
level in the spill tank would still be around 1 m below the top of the tank. A
more detailed description of the spill tank can be found in Ref. [7].

###  2.4 Moving Equipment

The 70 tonne detector with Gd-LS and LS needs to be transported using a crane
of 130 tonne capacity during the MLF maintenance period, as shown in Fig. 3. A
balance jig is used for the crane work as shown in Fig. 5. It can withstand a
load of 80 tonnes, sufficient for the filled detector.

Figure 5:  A schematic drawing of the balance jig for lifting the detector. It
shows a top view (top), a front side view (bottom left), and the other side
view (bottom right). The dimensions are given in mm.

Four slings are used to bind the jig and the detector together and connect
them to the crane, as shown in Fig. 3. The sling is tied with a shackle that
is linked to a metal ring welded to the detector, as shown in Fig. 6.

Figure 6:  A shackle tied to the detector. Four shackles are used to lift the
detector.

###  2.5 Acrylic Vessel

A schematic drawing, as well as a picture, of the acrylic vessel are shown in
Fig. 7. A tapered design for its top and bottom plates is adopted to avoid air
bubbles in the LS and provide mechanical strength.

Figure 7:  Schematic drawing (top) and picture (bottom) of the acrylic vessel.
The dimensions are given in mm.

The thicknesses of the acrylic plates are 20, 25, and 30 mm for the barrel,
top/bottom, and chimney parts, respectively. Figure 8 shows measured
transmittances of the 25 mm thick acrylic plates as a function of wavelength.
The UV transparent acrylic vessel was manufactured by the Nakano company in
Taiwan in 2018 and delivered to J-PARC in 2019. A leak test using nitrogen gas
was successfully completed before delivery. A more detailed description of the
acrylic vessel can be found in Ref. [8].

Figure 8:  Measured transmittance of acrylic plates with 25 mm in thickness.
The transmittance is better than 90% at wavelengths longer than 400 nm.

###  2.6 Veto

The veto layer, filled with LS, surrounds the central target and gamma-catcher
regions. The thicknesses of the veto layers are 25 cm at top and bottom, and
45 cm at barrel. The veto region is optically separated from the inner gamma-
catcher by black acrylic boards.

The veto layer is useful for identifying charged particles coming from outside
the detector and providing additional shielding from external backgrounds. The
surfaces of the veto layer are covered with reflective sheets of REIKO
LUIREMIRROR [9] to maximize the number of photons received by PMTs. The
measured reflectance of the sheet is greater than 94$\%$ for wavelengths
longer than 440 nm. Figure 9 shows the veto region, with the reflective sheets
clearly visible.

Figure 9:  Pictures of the veto region, barrel (left) and top-lid (right).

###  2.7 Liquid Level Stabilization System

The thermal expansion coefficient of Linear Alkyl Benzene (LAB) is
$\Delta\,V/V=9\times 10^{-4}$ / ∘C [10]. Since the maximum temperature
variation of the experimental area is expected to be $\pm$5 degrees, the
liquid volume changes as much as $\pm$0.45$\%$. The volume change leads to a
change in the Gd-LS level in the acrylic chimney by approximately 14 cm/∘C. A
liquid level stabilization system is therefore necessary. Note that a change
of the Gd unloaded LS level is minimized by the buffer space along the top
edge of the stainless steel tank as shown in Fig. 4.

A total of eight acrylic containers, four identical pairs, are mounted inside
the detector lid as shown in Fig. 10 to ensure a minimum change of the Gd-LS
level and keep its level equal to the LS. Each pair of two containers has a
volume of 480 $\times$ 660 $\times$ 240 mm3 and 1100 $\times$ 330 $\times$ 240
mm3.

Figure 10:  Liquid level stabilization system consisting of eight acrylic
containers and an inverse siphon. A pair of containers are deployed on each
section of the detector lid (top left). The red dashed lines indicate pipe
connections between the acrylic containers and to the siphon system.

Two sets of four containers are connected together by pipes and also to the
acrylic vessel inside the detector via an inverse siphon system, as shown in
Fig. 11.

Figure 11:  Conceptual design of the liquid level stabilization system. The
nominal liquid level is above the top lid indicated by the dashed line. The
vacant spaces in the upper corner of the stainless steel tank, stabilization
acrylic containers, and chimney are filled with the nitrogen gas supplied from
a generator at a flow rate of 3 L/min.

The effective surface area of the Gd-LS is expanded from 0.14 m2 to
approximately 2.5 m2 by this system. As a result, the variation of the Gd-LS
liquid level is only 0.7 cm/∘C. The acrylic stabilization containers and
chimney are partially filled with Gd-LS. The stainless steel tank is also
partially filled with LS, leaving a vacant space in the upper corner as shown
by red circles in Fig. 4. The remaining space is filled with nitrogen gas to
prevent oxygen contamination from the outside. A positive pressure of 0.24 kPa
is maintained by the supplied nitrogen gas at a flow rate of 3.4 L/min.

###  2.8 PMT Cable Flange

The PMT cables come out of the detector through four flanges. The PMT cable
flanges were especially designed to ensure air-tightness. Figure 12 shows a
schematic drawing of one of the flanges.

Figure 12:  A schematic drawing of the PMT cable flange. A flange consists of
two layers, a lower (left) layer and an upper (right) layer. There are a total
of 81 cable holes in the flange. The dimensions are given in mm.

Figure 13 shows a flange with its PMT cables fed through. The lower layer is
filled with an epoxy resin after the PMT cables go through the flange holes
and the top layer is used to close the flange.

Figure 13:  PMT cable flanges. Left: A flange consists of double layers.
Center: The lower layer is filled with the epoxy resin. Right: The top layer
is used to close the flange.

##  3 Photomultiplier Tubes

JSNS2 employs 120 R7081 [11, 12] Hamamatsu 10 inch oil-proof PMTs, of which
most were used previously by the RENO and Double Chooz experiments [13, 14].
The photocathode material is a bialkali metal with a spectral response range
between 300 nm to 650 nm and a maximum response at 420 nm. The typical quantum
efficiency at 390 nm is 25%. The characteristics of PMT R7081 are given in
Table 1.

Gain | Applied HV | Quantum eff. | Dark rate | Peak to
---|---|---|---|---
| (V) | at 390 nm (%) | (kHz) | valley ratio
1.0 $\times$ 107 | 1500 | 25 | 7 $-$ 15 | 1.5 $-$ 2.8
Table 1: Basic characteristics of the PMT R7081.

The high voltage (HV) is supplied to each PMT using CAEN A1535 modules [15]
with the photocathode potential set to ground. A single coaxial cable is used
for both high voltage supply and PMT signal readout. Double Chooz splitter
boards, for decoupling the HV and PMT signal, are reused, and are described in
more detail in Section 5.

A linear dynamic range of 1$-$500 photoelectrons (p.e.) is required to
maintain full efficiency up to $\sim$60 MeV, the maximum prompt energy of an
IBD event. The gain linearity is measured using laser and scintillation light.
Typically, a PMT saturates at roughly 600 p.e. at $5\times 10^{6}$ gain.

Figure 14: Gain curve of a typical PMT (left) and HV required to achieve a
gain of 107 (right). The solid histogram is for PMTs from RENO and Double
Chooz and the dashed histogram for purchased new PMTs.

For the JSNS2 experiment 40 PMTs were newly purchased, while the remaining
PMTs were obtained from the RENO and Double Chooz experiments. All PMTs were
carefully calibrated before being deployed in the detector. For the PMT
calibration, we used a 440 nm nanosecond-pulse laser and a 14-bit ADC board to
measure the gain, peak to valley ratio of the single photoelectron
distribution, and the dark hit rate. A total of eight PMTs were simultaneously
calibrated by using a light splitter and a dark box with 8 PMT mounting ports.
The calibration apparatus was developed by Double Chooz [14]. Figure 14 shows
the gain curve of a typical PMT, used to find the required HV for a gain of
107. The peak-to-valley ratios of the evaluated PMTs were found to be larger
than 2 at that gain. The dark rate at a threshold of 0.3 p.e. was less than 3
kHz. The results obtained from this calibration are consistent with those
provided by Hamamatsu Photonics [16]. Following our testing, 123 PMTs were
identified to fulfill all quality criteria required for integration into the
JSNS2 experiment. The detailed description of the PMT calibration is available
in Ref. [16]. Once installed in the detector, the timing performance of the
PMTs is further calibrated using in-situ measurements provided by the LED
system described in Section 9.

A total of 120 PMTs are installed in the detector. Each PMT is assembled with
an optical separation cover of black PET film, a magnetic shield of FINEMET
[17] and a cone-shaped reflection cover. The black PET film [18] is used to
reduce the background light coming from light flashes originating from inside
of the PMT base circuit. The FINEMET film improves the collection efficiency
of photoelectrons by shielding the PMT from the geo-magnetic field [19]. The
veto wall and the backside of inner PMTs are covered by reflective sheets of
REIKO LUIREMIRROR with a reflectance of 94% at 440 nm wavelength [9]. Figure
15 shows the PMT assembly process before mounting it on the detector.

Figure 15: PMT assembly process consisting of (a) black PET, (b) FINEMET, (c)
reflector, (d) black PET for PMT base and cable, (e) reflector, (f) fixing the
reflector, and (g) L-type angle support frame.

An assembled PMT is mounted to a SUS L-type angle support frame that is welded
to the inner detector wall, as shown in Fig. 16. A black acrylic board is
attached to the PMT support structure for optical separation between the
gamma-catcher and veto. At the detector barrel side, five PMTs are mounted to
a barrel layer of support frame unit. In total, 60 PMTs are mounted to twelve
barrel support frame units. At the detector top and bottom sides, 18 PMTs are
mounted to two circular layers of the support frame unit. For light detection
in the veto region, 24 PMTs are installed in the upper and lower corners of
the cylindrical detector. Twelve PMTs are mounted on the detector top and
directed downward, and the other twelve are mounted on the detector bottom
circumference with six of them directed radially inward and the remaining six
directed upward.

Figure 16: Front (left) and backside (right) views of a barrel PMT support
structure module.

##  4 Liquid Scintillator

The JSNS2 LS needs to differentiate the electron antineutrino signal from
accidental and neutron backgrounds. It must produce sufficient light to allow
for precise energy measurement, such that the spectral modulations expected
from sterile neutrino oscillations can be observed. It must also have
appropriate chemical properties to satisfy the Japanese fire law and maintain
stable detection efficiency for several years.

To satisfy these requirements, the JSNS2 experiment uses 31 tonnes of unloaded
LS, produced by the RENO experiment’s refurbished facility [20]. ISO
(International Organization for Standardization) tanks, made of stainless
steel (SUS316), are used for LS storage and transportation from the RENO site
in Korea to the J-PARC MLF in Japan. A detailed description of the LS
production and its optical properties is given in Ref. [20].

The JSNS2 LS is composed of LAB (${\rm C_{n}H_{2n+1}}$-${\rm C_{6}H_{5}}$,
$\mathrm{n}=10\sim 13$) as a base solvent, 3 g/L of PPO (2,5-diphenyloxazole)
as a primary fluor, and 30 mg/L of bis-MSB (1,4-bis(2-methylstyryl) benzene)
as a secondary wavelength shifter. LAB is chosen as an organic solvent because
it is environmentally friendly, nontoxic, and has a high flash point (152 °C).
It also has a long attenuation length, $>$10 m at 430 nm, and has a light
yield of $\sim$8,000 photons per MeV [21]. The Isu Chemical Company in South
Korea produces high quality LAB and can deliver it in a clean storage
container. LAB suffers from a non-linear energy response due to quenching
effects at low energies. The RENO experiment measured the Birk’s constant for
LS as 0.0117$\pm$0.0003 cm/MeV using a Ge detector [21].

The JSNS2 experiment uses 17 tonnes of Gd-LS, as a neutrino target, which was
donated by the Daya Bay experiment [22]. The IBD reaction is utilized to
detect antineutrino appearance from sterile neutrino oscillations. A neutron
coming from the IBD reaction captures on either Gd or hydrogen (H). While the
neutron capture on H produces a 2.2 MeV gamma-ray, which cannot be
distinguished from natural radioactive backgrounds, the $\sim$8 MeV gamma-rays
from the neutron capture on Gd can be easily identified above the background.
The mean neutron-capture time on 0.1% Gd is $\sim$30 $\mu$s, which is more
than 6 times shorter than that on H, and can result in significant reduction
of accidental backgrounds.

In accordance with Japanese fire law, the MLF has stringent safety
requirements for how the flammable LS and Gd-LS must be treated. To comply
with these safety requirements, JSNS2 has taken precautions to ensure the
scintillator cannot spill or leak from the detector. The temperature of the
Gd-LS is monitored by eight sensors installed inside the detector. The liquid
level in the detector is monitored by ultrasonic sensors. Several acrylic
containers are mounted on top of the detector and provide a buffer volume for
the Gd-LS. The buffer volume helps maintain a constant liquid level in the
detector, and prevents overflow as the liquid volume changes due to thermal
expansion. The liquid levels in the containers are also monitored by
ultrasonic sensors. The detector is surrounded by a spill tank; any liquid
spill can be observed by ultrasonic sensors and web cameras. Section 7
contains a more detailed description of the LS and Gd-LS monitoring.

A liquid handling system has been developed to complete liquid filling or
extraction within a few days. It consists of two pumps, several flexible
pipes, two frequency inverters, and two flow meters. An FM3104-PD-XP/K flow
meter [23] is used to measure the liquid flow rate into the detector. The
frequency inverter is used to adjust the pump speed to modulate the flow rate.
The liquid filling and extraction takes place at the first floor of the MLF
and are both carefully performed to maintain similar Gd-LS and LS levels by
controlling liquid flow rates with the inverter. Three ISO tanks with heat
insulation are used for the liquid storage and transportation.

One of the most serious backgrounds for the sterile neutrino search comes from
fast neutrons produced by cosmic muons. An energetic neutron loses its energy
through interactions with protons in the scintillator, and the recoil protons
can produce sufficient scintillation light to fake an IBD prompt signal event.
The neutron eventually thermalizes and captures on Gd or H. Thus, a fast
neutron can mimic an IBD prompt and delayed signal. The JSNS2 detector, not
being underground, suffers significantly from the cosmic muon induced fast
neutrons. The charge waveform of a scintillation signal generated by a recoil
proton is wider than that of a positron due to difference in the dE/dx between
the two particles. By exploiting this difference in waveforms, the fast
neutron background can be distinguished from an IBD prompt event using a pulse
shape discrimination (PSD) algorithm. In order to improve the PSD performance,
an additional solvent, DIN (di-isopropylnaphthalene), EJ-309 by Eljen [24],
was dissolved into the Gd-LS before the second run, in December 2020. The DIN
concentration is 8% of the Gd-LS, and an additional 2% will be dissolved in
July 2021. The different charge waveforms for neutron and electron
interactions are shown in Fig. 17. It is expected that 90% of the fast neutron
background can be removed using PSD.

Figure 17: Comparison of the average waveforms for fast neutron and electron
events. The fast neutron’s average waveform is wider than that of an electron.

Nitrogen purging is used to remove oxygen and moisture from the Gd-LS. Oxygen
in the Gd-LS causes degradation of its fluorescence and transparency. The
nitrogen purging system consists of spherical Teflon bubblers (35 mm diameter)
with many 10 $\mu$m size holes and a Cosmo CS MicroMini 1 nitrogen generator
[25]. The bubblers are installed at the bottom corner of the inner target
vessel, as shown in Fig. 18. The generator outside the detector provides
nitrogen gas at a rate of 0.8 L/min, through a 3/8” Teflon tube attached to
the vessel wall. During nitrogen purging, a NIKUNI 32NPD07Z nitrogen-
regenerative turbopump [26] is used for circulating the Gd-LS at a rate of 28
L/min. The liquid is extracted at the chimney and supplied back to the target
volume, next to the bubbler, through a 1/2” Teflon tube. When the nitrogen
purging system runs, the oxygen concentration in the chimney air increases
because the dissolved oxygen is expelled from the Gd-LS. The oxygen in the
chimney is removed by the nitrogen gas flow. Roughly five days of nitrogen
purging and Gd-LS circulation removes most of the oxygen in the Gd-LS and
increases the observed light yield.

Figure 18: Nitrogen purging system. Four Teflon bubblers are installed at the
bottom corner of the inner acrylic vessel, indicated by the yellow circles
(right). The nitrogen gas is provided by a generator (top left) through a
Teflon tube and injected into Gd-LS via the bubbler holes (bottom left). The
target liquid is circulated by a turbopump. The yellow solid arrows indicate
the nitrogen gas flow and the Gd-LS circulation.

##  5 Electronics

A schematic overview of the DAQ system is shown in Fig. 19. The HV supply
modules, HV splitter components, front-end analog processing board and
digitization modules were all reused from the Double Chooz experiment.

Figure 19: Schematic overview of the JSNS2 front end and DAQ electronics. The
“Analog Sum” signal for the trigger system is described in Section 6.

HV is supplied to each PMT individually from six CAEN A1535 modules, all
housed within a purchased CAEN SY4527LC power supply crate [27]. Each A1535
module supplies HV up to 3.5 kV to 24 PMTs, each with a maximum current of 3
mA. The read-back voltage, supply current, and temperature of each of the
modules are monitored as part of the JSNS2 slow control and HV monitoring
system.

The next component of the DAQ system is a HV splitter, responsible for
separating the high speed PMT signal from the DC HV. The splitter was
developed by CIEMAT for the Double Chooz experiment. The signal is separated
from the HV using a single 6.8 nF capacitor, rated up to 2500 V. An additional
noise-filter circuit is included as well. The circuit diagram is available in
Ref. [16]. The splitter circuit board is housed within a grounded metal box.
Each splitter handles a single PMT.

The analog PMT signal from the splitter is further processed by Front-End
Electronics (FEE). The FEE modules were originally developed by Drexel
University and Lawrence Livermore National Lab for the Double Chooz DAQ
system. Each FEE board processes eight PMT signals. It produces as output a
high-gain and low-gain copy of each input signal, and also provides an analog
sum of high gain outputs.

We introduced one significant modification to the FEE compared to the Double
Chooz experiment. The gain of the low-gain signal path is modified to reduce
the output signal amplitude by a factor of $3.9$ compared to the Double Chooz
configuration. The modification is made to increase the dynamic range of the
DAQ system to achieve an increased sensitivity to high energy interactions.
All FEE measurements and values discussed here include that modification.

Twenty-seven CAEN V1721 FADC modules [28] provide digitization and readout for
PMT signals after the FEE’s processing. These modules digitize signals at 500
M samples per second across a 1 V dynamic range with 8-bit precision. Each
FADC module receives a trigger signal and a global synchronization clock from
a special trigger FADC. Each FADC module has an internal 125 MHz counter that
is used to reconstruct the time of an event relative to the other events and
other FADCs. The FADCs are all readout by a single DAQ PC over an optical
fiber connection.

The effective gain and dynamic range of the FEE’s high and low gain signal
path is of primary importance for JSNS2. The dynamic range limits our
sensitivity to high energy interactions and precise knowledge of the gain is
necessary for reliable energy reconstruction. Measurements have shown that the
high gain channels saturate at an output voltage of approximately $1700$ mV
with a corresponding input voltage of approximately $110$ mV. Similar
measurements for the low gain channels showed saturation at an output voltage
of $1500$ mV and a corresponding input voltage of approximately $3000$ mV. For
the typical PMT gain in the JSNS2, the electronics saturate at approximately
20 and 550 p.e. for the high gain and low gain, respectively.

Figure 20: Distribution of gain values across all FEE channels, both high
(left) and low (right) gain.

The gains of the FEE’s high and low gain signal paths are measured to have
average values of 16.0 and 0.59, respectively. The gain of each FEE channel
used in the detector is measured individually, and determined using both ex-
situ experimental setups and in-situ measurements. Figure 20 shows the
distributions of measured gain values for all channels in the detector.

##  6 Trigger and Data Acquisition

The JSNS2 experiment aims to collect physics signals coming from the collision
of the 3 GeV proton beam accelerated by the J-PARC RCS with a mercury target.
The Radio-Frequency (RF) module of the accelerator provides a timing signal
synchronized with extraction of the proton beam from the RCS. The timing
signal is useful to detect the arrival of the beam since it arrives at JSNS2
100 $\mu$s before the real beam.

###  6.1 Trigger Scheme

Three different types of trigger are utilized to collect physics signals from
the beam collision with the mercury target and for detector calibration. The
first type is a 25 Hz periodical trigger produced from the RF module and
synchronized with the beam collision, named the “kicker trigger”. A “beam-off
trigger” is introduced by copying the kicker trigger with a timing delay. The
delay is occasionally changed from 11 $\mu$s to 30 ms, to monitor events in
the beam-off period. The second type is a detector “self-trigger” formed from
the analog sum of the inner PMT signals. The third type is a “LED trigger”
implemented for the LED calibration described in Section 9.

One of the FADC boards is used for making a trigger decision, as the CAEN
V1721 is able to issue a NIM signal to the other FADC boards using its own
internal discriminator. We call this board the “trigger FADC” board. The
hardware trigger logic is summarized in Fig. 21. The waveforms of the trigger
signals are recorded by the trigger FADC board. The waveforms of the analog
sum and NIM signals are also recorded for offline analysis. The kicker signal
is always produced by the RCS, regardless of whether the proton beam really
arrives at the mercury target or not. In order to determine if the proton beam
is injected from the RCS accelerator to MLF, we use the reference signals from
the current transformers (CTs) located at the beam line. The CT signal is
discriminated and transported to the experimental groups in MLF. The CT signal
is approximately 2.7 $\mu$s delayed with respect to the real beam [29].

Figure 21: A block diagram of the trigger scheme. All boxes except for the
PMTs indicate the hardware electronics. The arrows show the paths of analog or
NIM signals. The inner and veto PMTs are summed by the FEE and the FAN-IN/the
FAN-OUT modules to issue NIM signals by the discriminator (Discri. in the
diagram). The NIM signals of the veto PMTs are connected to the discriminator,
for the inner PMTs to veto the self trigger. The kicker and CT signals are 25
Hz periodical NIM signals connected to the trigger FADC board. The beam off
trigger is created as a copy of the kicker signal with flexible delay time.
The CT and analog signals of the inner and veto PMTs are not used for trigger,
but these waveforms are recorded for reference.

The analog sum of the veto PMTs is used to form a 100 ns veto-gate signal,
called “online veto”, to suppress the trigger rate due to cosmic rays during
the self-trigger data taking. The online veto is also applied during dedicated
calibration data taking runs. The veto analog sums are also recorded and read
out with each event.

A special trigger scheme, consisting of two successive trigger signals, is
implemented for the sterile neutrino search, in order to suppress the
background rate and maximize the IBD candidate rate. A prompt trigger is
formed if the target PMT analog sum is above 250 mV, which is 100% efficient
at 15 MeV, in a roughly 10 $\mu$s timing window issued at 1.1 $\mu$s after the
first beam bunch. A delayed trigger is formed after a prompt trigger if an
analog sum is found above 100 mV, which is 100% efficient at 5 MeV, in a
subsequent $\sim$12 ms timing window. The LED and beam-off trigger channels
shown in Fig. 21 are used for the prompt and delayed triggers during the
normal data-taking with the special trigger. The special trigger is optimized
to take a maximum number of IBD candidate events by adjusting the energy
threshold of the prompt trigger and the timing width of the delayed trigger.

###  6.2 Data Acquisition

All waveforms are collected by four daisy chains of the optical links among
the FADC boards. Each of the FADC boards provides meta information about each
trigger, such as a 32-bit event counter, and a trigger timestamp. We use the
event counter and timestamp to validate the trigger synchronization across the
FADC boards. Figure 22 shows an overall diagram of the data acquisition (DAQ)
system.

Figure 22: A diagram of the DAQ system. The boxes indicate hardware
electronics and PC, while the solid and the dashed arrows show analog or
digitized waveforms from the PMTs and the trigger NIM signals, respectively.
Each of the FEE connected to an inner PMT has two outputs with the higher and
lower gain amplitudes, while one FEE of a veto PMT has only a single output.
The waveforms from the PMTs and the trigger sources are finally digitized and
transferred to the DAQ PC. The NIM out of the trigger FADC is distributed to
the FADC boards, including the trigger FADC itself.

The clocking system for the FADC boards also uses a daisy chain, each FADC’s
clock output is connected to the clock input of the neighboring FADC board.
Therefore, there is a fixed time difference due to the clock delay between
neighboring boards. The relative delay for each PMT, including the board-to-
board delay, is measured using LED calibration data and corrected for in
offline analysis. The fixed board-to-board timing offsets are corrected for
using the FADC’s internal delay functionality.

The FADC board has the capability to change its waveform readout time window
size. For the kicker and beam-off trigger data taking, a 10 $\mu$s time
window, equivalent to 5000 FADC samples per channel, is used, followed by
another trigger with a 10 $\mu$s window. A 0.5 $\mu$s, 250 sample
configuration is used for self-trigger data taking. The maximum sustainable
self-trigger rate for our readout system is 550 Hz. A 2 $\mu$s, 1000 sample
window is used for the special trigger.

The threshold settings used for typical self-trigger data taking runs can
produce trigger rates beyond the limitation of the data bandwidth from the
FADCs to the readout PC. When this occurs, and the FADC memory buffers are
full, we pause the trigger and read out the data until the FADC’s memory
buffers are empty, then resume normal data taking.

###  6.3 Data Acquisition Control and Monitoring

The JSNS2 run control software is based on the EPICS (Experimental Physics and
Industrial Control System) framework. Both the readout and DQM (Data Quality
Monitoring) PCs have EPICS client programs to launch their DAQ process. A GUI,
built using Control System Studio, is used to start, stop, and reconfigure
each DAQ processes. Run information, such as run start and stop time, and run
type, are recorded into a MySQL database running on the DQM PC. File
information, such as timestamp and data size, are also recorded into the
database. The trigger rate and event size are also recorded into the database.
These slow monitoring data are visualized on a custom webpage, built using the
Grafana framework [30].

##  7 Slow Control and Monitoring System

A slow control and monitoring system (SCMS) is implemented to provide reliable
control of detector operation and quick monitoring of both operational status
and environmental conditions. The SCMS issues an alarm if any of its monitored
the values exceed a preset acceptable range. The system includes monitoring of
the liquid level in the detector and rapid alert of any possible liquid
overflow and leakage. It also monitors the temperature and air pressure inside
the detector, the humidity in the experimental area, and the liquid flow rate
during filling and extraction. The SCMS is also responsible for monitoring and
controlling the HV supplied to each PMT.

The data acquired by sensors are collected by readout modules and forwarded to
a LabVIEW-based client program [31] for monitoring and storing. The monitored
data are visualized for displaying detector operational status using a Grafana
based graphical user-interface tool. The data are also stored in a MySQL
database [32] every 30 seconds. Figure 23 shows a block diagram of the JSNS2
SCMS to show its overall configuration.

Figure 23: Conceptual diagram of the JSNS2 slow control and monitoring system.

The LabVIEW program queries sensors every 5 seconds and displays them over an
8-hour timespan. The LabVIEW display includes liquid levels, liquid leak in
the spill tank, detector temperature, detector pressure, and humidity in the
detector area.

A LabVIEW-based HV control and monitoring program integrated into the SCMS is
responsible for controlling HV to each PMT, storing preset HV values, and
reading applied HV values and the temperatures of the HV modules. The CAEN
operational process control (OPC) server is used to control and communicate
with the HV modules [33]. The LabVIEW program obtains data from the OPC server
via Ethernet and displays the status of individual HV supplied to each PMT in
an exploded view of the detector. Figure 24 shows a screenshot of the PMT HV
display where the color represents the difference between the preset and
currently supplied HV values.

Figure 24: A screenshot of HV control and monitor display. The green circle
indicates that its PMT HV is within 10 V range of its preset value. The
temperature of a HV module is also displayed.

The Gd-LS and LS levels are measured with SICK ultrasonic sensors [34]. A
long-range UM30-215113 sensor is used for liquid filling and extraction and a
short-range UM30-213113 sensor is used during normal data-taking. The measured
level generates an analog voltage value, 0 to 10 V, which is read out by a
National Instruments (NI) 9201 module [35]. Four US-015 [36] ultrasonic
sensors are used to monitor for a possible liquid leak inside the spill tank,
and two additional US-015 sensors are used to monitor the liquid level inside
the stabilization container; these sensors are read out by an Arduino Uno Rev3
[37].

The detector is sealed to avoid oxygen contamination into the LS and Gd-LS.
This allows a pressure difference between the inside and outside of the
detector to develop. For monitoring this pressure difference, two types of
relative pressure meters are installed. The GC-31 [38] and GC-62 [39] both
provide voltage outputs proportional to the pressure difference. These
voltages are read out by an NI 9201 module.

A total of eight PT100 RTD sensors measure temperatures in the detector veto
region. The temperature outputs are read out by an NI 9216 module [40].

The liquid flow rate during filling into or extraction from the detector is
measured by a flow meter. The flow meter provides an analog current
proportional to the flow rate. An NI 9203 module [41] reads and digitizes the
current output.

An NI cDAQ-9178 crate [42] houses our three NI readout modules, and is
connected to the SCMS PC via a USB cable. The SCMS PC runs a LabVIEW readout
program to receive and record sensor values. LabVIEW was selected because it
interfaces well with the NI modules.

A TR-73U ambient sensor [43] monitors environmental conditions around the
detector such as temperature, humidity, and atmospheric pressure. Its recorded
data are read out and delivered to the LabVIEW program running on the SCM PC
via USB.

The SCMS, as described here, has been successfully tested, commissioned, and
deployed. Using this system liquid filling and extraction for the JSNS2
detector has been completed without significant trouble. In addition to its
continued use to support reliable data-taking, this demonstrates that the SCMS
provides sufficient monitoring and safety assurance for the experiment. A more
detailed description of the SCMS can be found in Ref. [44].

##  8 Data Processing

The JSNS2 data processing system is responsible for transporting the waveform
data from the DAQ system to the KEK Computing Center (KEKCC), and for
performing a quick data analysis routine as part of the DQM system. The data
processing system consists of two parts: (1) processing at the on-site PCs as
preparation for data transfer and (2) processing at the KEKCC by performing
file type conversion and a quick analysis to save into the disk. Figure 25
shows the data processing scheme.

For the initial part of data processing the DQM PC receives data from the FADC
boards via 1 Gbps Ethernet and dumps them into binary files in 8 TB RAID-5
disk as temporary storage, sufficient for a week of data taking. A parallel
job system is used on the DQM PC to compress each binary files. The
compression factor is about 3 to 6. The files are finally transferred to KEKCC
via rsync with the SSH protocol; the DQM PC is connected to KEKCC by a 1 Gbps
Ethernet connection using J-PARC’s internal LAN.

As the second part of the data processing, the compressed files are posted at
KEKCC’s GPFS HDD based file system. The DQM PC submits jobs to KEKCC to
convert the data files into ROOT. The produced ROOT files contain ROOT TTree
objects and are used for offline analysis. The conversion is done in a few
minutes on average. Finally, the compressed files are moved to the tape drive
system in KEKCC for long term storage, while the ROOT files are on the GPFS
for user analysis.

Figure 25: A schematic of the data processing scheme. The boxes show PCs and
files. The solid lines show the data transfer between processes over the
network, while the dashed lines show the data paths to the storage systems.

###  8.1 Data Quality Monitor

In parallel to the file transfer, the online data and the recorded files are
analyzed at the on-site DQM PC and at KEKCC, respectively, to extract several
detector status values. Since the CPU power in the DQM PC is limited, a brief
analysis routine is integrated with the online data stream on the DQM PC to
produce information, such as raw waveforms of PMTs. A further analysis is done
in KEKCC using the converted root files, which extracts more information.
Basic information such as trigger bit rate is obtained in an analysis to
monitor the beam injection consistency. The results from both the on-site PC
and KEKCC are formatted as histograms in ROOT files and displayed on a
webpage. The webpage uses JSROOT to read the histogram files.

###  8.2 Data Production

The waveforms in the data files are quickly analyzed to extract signal pulses
as charge and timing information for each PMT. We developed a common framework
for data analysis named JADE (JSNS2 Analysis Development Environment), which
reconstructs PMT pulses and identifies events in the long waveforms. For
higher level analysis, JADE is also used to do vertex position and energy
reconstruction. Data are processed in parallel using JADE and KEKCC’s batch
job system.

##  9 Calibration System

Three sources are used to calibrate the response of the detector. An LED
system is used to calibrate the relative timing of the individual PMTs as well
as the gain. A neutron source is used to calibrate the energy scale at several
positions in the detector, with the 8 MeV energy peak coming from neutron
capture on Gd. Finally, the Michel electron spectrum from stopping cosmic
muons is used to calibrate the energy response and the energy resolution up to
the 52.8 MeV end-point.

In this section, the three kinds of calibration are described in more detail.

###  9.1 LED system

The JSNS2 LED system is based on the LED systems that have been used in Double
Chooz and SNO+ [45]. For those systems, where ensuring low-radioactivity
backgrounds is a key requirement, the electronics were installed outside the
detector, and light pulses enter the detector via optical fibers. As accurate
timing is a key requirement for JSNS2 and the energy depositions from the
physics signals are above the natural radioactivity backgrounds, the LEDs in
this system are embedded in a sealed acrylic housing and mounted on the PMT
array. This enables an LED optical pulse to enter the detector without any
degradation to the pulse width. Lab tests have demonstrated that the fast
signals in the electronics inside the detector do not produce any interference
with the PMT signals. In addition, the Kapustinsky-based LED drivers [46] used
by SNO+ have been much improved for this system [47]: The optical pulses have
a rise time of 0.55 ns, as shown in Fig. 26. These measurements have been
directly obtained using the method described in [48].

Figure 26: Pulse profiles of the 355 (left) and 420 nm (right) LEDs.

The JSNS2 LED system consists of twelve 420 nm LEDs and two 355 nm LEDs. An
overview of the system, as well as a photo of the driver circuit of the LEDs
inside the detector, is shown in Fig. 27.

Figure 27: A schematic overview of the JSNS2 LED system (left). The (two)
purple points indicate the positions of the 355 nm LEDs, the (twelve) blue
points the positions of the 420 nm LEDs. All LEDs point inwards. The numbers
B1–B6 indicated the different branches, which connect two or three LEDs. IV
represents the Inner Veto volume and ID represents the Inner Detector volume.
The LED driver circuit (without the acrylic housing) is shown on the right,
with the flattened LED at the top.

Standard 5 mm LEDs (T-1 3/4 package) [49] are used. To ensure the maximum
opening angle, the curved front is removed and the remaining flat end is
polished. This enables full coverage of the detector for the 420 nm LEDs, for
which the scintillator is effectively transparent. The light of two 355 nm
LEDs gets absorbed and re-emitted uniformly inside the target. LED drivers can
be individually addressed. A selected driver returns a trigger pulse in-sync
with the LED pulse to the controller box. After calibration, the controller
box provides central data-acquisition a trigger signal in-sync with the LED
pulse. The timing error between the LEDs and jitter is significantly less than
the rise time of the optical pulse. The intensity can be controlled and ranges
from less than 50 to over $10^{6}$ photons per pulse.

###  9.2 Neutron source

Figure 28: The JSNS2 252Cf calibration source inside an acrylic container

The neutron calibration source is used for energy calibration, in particular
to convert the observed number of photoelectrons to energy, at several
positions along the central axis of the detector. Neutrons are produced by a
252Cf radioactive source with outer dimensions of 20 mm in length and 5 mm in
diameter, and with a calibrated activity of $3.589\times 10^{6}$ Bq on the
10th of August 1983. The dominant capture is expected to be on 157Gd having
15.65% natural abundance and a capture cross-section of 254,000 b, resulting
in 7.937 MeV total energy in gammas. A weaker second peak is expected to be
observed at 8.536 MeV from the capture on 155Gd having 14.80% natural
abundance and a capture cross-section of 60,900 b [50]. A short run of several
minutes provides sufficient events for a precise calibration.

The neutron source is contained in an acrylic and cylindrical holder, as shown
in Fig. 28, with dimensions of 60 mm in diameter and 16 mm in thickness. The
neutron source, in the acrylic container, can be deployed at any detector
position along the central axis. A source deploying device is made of a disc-
shaped reel with a diameter of 100 mm, a stainless steel wire, and a stepping
motor as shown in Fig. 29. The source can be deployed at a desired location by
rotating the reel using the stepping motor and adjusting the wire length
accordingly. The motor is controlled by a micro-controller using a PC. In
order to avoid oxygen and moisture contamination, a glove box filled with
nitrogen gas is installed as a calibration port above the acrylic chimney.
Figure 29 shows the glove box where the neutron source is deployed into the
detector.

Figure 29: Glove box for deploying a neutron source. It is filled with
nitrogen gas and deploys a 252Cf source inside an acrylic container using a
stepping motor controlled by a PC.

###  9.3 Michel electrons

Michel electrons coming from cosmic muons stopping in the JSNS2 detector
provide a copious event sample for the detector calibration. They are useful
for understanding the detector response in the entire energy range of the IBD
prompt signal. Because of their uniform event vertices, they can be used for
surveying a position dependence of the detector response. During a recent
commissioning run, $\sim 100$ Hz of stopping muons were observed inside the
central volume with a rate of $\sim 2400$ Hz cosmic muons. The high statistics
of Michel electrons allow monitoring the stability of the detector response
over time as well as a possible in-situ measurement of the optical attenuation
length of Gd-LS.

##  10 Summary

The JSNS2 detector is designed and built to be sensitive to
$\overline{\nu}_{e}$ of energies less than 52.8 MeV, motivated by the sterile
neutrino search. The construction of the JSNS2 detector was completed in the
spring of 2020. Following the completion of the detector the experiment was
ready for a search for sterile neutrinos using the neutrino beam from muon
decay at rest. During June 2020 the first 10 days of physics data with the
neutrino beam was acquired. After data taking, the Gd-LS and LS liquids were
extracted back to the ISO tanks, and the detector was relocated and stored at
the detector storage building for a maintenance period, until December 2020.
For the improved PSD performance, an additional solvent DIN was dissolved into
Gd-LS before the second run began in January 2021. The PMT gain and electronic
dynamic range are adjusted for efficient IBD event detection. For reliable and
accurate event reconstruction, the detector is calibrated using an LED system,
a Cf neutron source, and Michel electrons produced by cosmic muons. A slow
control and monitoring system was implemented to provide reliable control of
detector operation and quick monitoring of operational status and
environmental conditions. Based on the first and second runs, we have
evaluated the detector operation including triggering, data acquisition, and
data processing. Necessary modifications and improvements have been performed
to carry out a successful search for sterile neutrino oscillations.

##  11 Acknowledgement

We thank the J-PARC staff for their support. We acknowledge the support of the
Ministry of Education, Culture, Sports, Science, and Technology (MEXT) and the
JSPS grants-in-aid (Grant Numbers 16H06344, 16H03967, and 20H05624), Japan.
This work is also supported by the National Research Foundation of Korea (NRF)
Grant No. 2016R1A5A1004684, 2017K1A3A7A09015973, 2017K1A3A7A09016426,
2019R1A2C3004955, 2016R1D1A3B02010606, 2017R1
A2B4011200, 2018R1D1A1B07050425, 2020K1A3A7A09080133 and 2020K1A3
A7A09080114. Our work has also been supported by a fund from the BK21 of the
NRF. The University of Michigan gratefully acknowledges the support of the
Heising-Simons Foundation. This work conducted at Brookhaven National
Laboratory was supported by the U.S. Department of Energy under Contract DE-
AC02- 98CH10886. The work of the University of Sussex is supported by the
Royal Society grant no.IESnR3n170385. We also thank the Daya Bay Collaboration
for providing the Gd-LS liquid, the RENO collaboration for providing the LS
liquid and PMTs, CIEMAT for providing the splitters, Drexel University for
providing the FEE circuits, and Tokyo Inst. Tech for providing FADC boards.

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