Showing 1–50 of 81 results for author: Sekiya, H

Performance evaluation of GYGAG crystals for neutrinoless double beta decay search

Authors: Hibiki Hayasaki, Keishi Hosokawa, Koichi Ichimura, Shunsuke Kurosawa, Yuta Masaki, Yusuke Mizuno, Yasuhiro Nakajima, Hiroyuki Sekiya, Akihiro Yamaji

Abstract: Neutrinoless double beta decay (0$νββ$) is a crucial for confirming neutrinos' Majorana characteristics. Previous searches for 0$νββ$ decay of $^{160}$Gd using GSO crystals was conducted, but the sensitivity was limited by background $α$-rays from radioactive impurities. To distinguish the backgrounds, PSD (Pulse Shape Discrimination) and energy resolution are essential. GAGG (Gd$_{3}$(Al, Ga)… ▽ More Neutrinoless double beta decay (0$νββ$) is a crucial for confirming neutrinos' Majorana characteristics. Previous searches for 0$νββ$ decay of $^{160}$Gd using GSO crystals was conducted, but the sensitivity was limited by background $α$-rays from radioactive impurities. To distinguish the backgrounds, PSD (Pulse Shape Discrimination) and energy resolution are essential. GAGG (Gd$_{3}$(Al, Ga)$_5$O$_{12}$) is a $^{160}$Gd-containing crystal that exhibits a higher light yield than GSO and excellent PSD capabilities. GYGAG (Gd$_{3-x}$Y$_x$(Al, Ga)$_5$O$_{12}$) is a variant crystal where a portion of Gd is substituted with Yttrium and can have enhanced light yield, motivating its investigation for 0$νββ$ decay searches. In this study, GYGAG crystals with different Yttrium concentrations were fabricated and their energy resolution, pulse shape discrimination capability between $α$ and $γ$ rays, and scintillation timing characteristics were evaluated for applications in 0$νββ$ decay searches. Y substitution enhanced the light yield, and the best energy resolution was achieved for Gd$_{2.5}$Y$_{0.5}$Al$_2$Ga$_3$O$_{12}$. GYGAG exhibited high pulse shape discrimination performance across various yttrium concentrations. △ Less

Submitted 27 May, 2026; originally announced May 2026.

The COHERENT Experiment: 2026 Update

Authors: M. Adhikari, M. Ahn, D. Amaya Matamoros, P. S. Barbeau, V. Belov, I. Bernardi, C. Bock, A. Bolozdynya, R. Bouabid, J. Browning, B. Cabrera-Palmer, N. Cedarblade-Jones, S. Chen, A. I. Colón Rivera, V. da Silva, J. Daughhetee, Y. Efremenko, S. R. Elliott, A. Erlandson, L. Fabris, M. L. Fischer, S. Foster, A. Galindo-Uribarri, E. Granados Vazquez, M. P. Green , et al. (85 additional authors not shown)

Abstract: The COHERENT experiment measures neutrino-induced recoils from coherent elastic neutrino-nucleus scattering (CEvNS) with multiple nuclear targets at the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL), USA. Several successful CEvNS measurements have been achieved in recent years with tens-of-kg detector masses, with a CsI scintillating crystal, a liquid argon single-pha… ▽ More The COHERENT experiment measures neutrino-induced recoils from coherent elastic neutrino-nucleus scattering (CEvNS) with multiple nuclear targets at the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL), USA. Several successful CEvNS measurements have been achieved in recent years with tens-of-kg detector masses, with a CsI scintillating crystal, a liquid argon single-phase detector, and high-purity germanium spectrometers. For the next phase, COHERENT aims at high-statistics detection of CEvNS events for precision tests of the standard model of particle physics, and to probe new physics beyond-the-standard model. Percent-level precision can be achieved by lowering thresholds, reducing backgrounds, and by scaling up the detector masses. It goes hand in hand with benchmarking the neutrino flux from the SNS. Further detectors will measure CEvNS in additional nuclei, including lighter target nuclei such as sodium and neon, to continue to test the expected neutron-number-squared dependence of the cross section. COHERENT can furthermore study charged-current and neutral-current inelastic neutrino-nucleus cross sections on various nuclei at neutrino energies below $\sim$50 MeV. Many of these cross sections have never been measured before, but are critical input for the interpretation of core-collapse supernova detection in large-scale neutrino experiments such as DUNE, Super-K, Hyper-K, and HALO. △ Less

Submitted 17 February, 2026; originally announced February 2026.

Comments: originally prepared in response to a request from the Neutrinos & Cosmic Messengers section of the Update of the European Strategy for Particle Physics

Upgrade of Super-Kamiokande with Gadolinium

Authors: Yusuke Koshio, Masayuki Nakahata, Hiroyuki Sekiya, Mark R. Vagins

Abstract: Super-Kamiokande [SK] was upgraded through the addition of gadolinium sulfate to its ultrapure water, initiating the SK-Gd program. This development enables efficient neutron tagging via the large capture cross section of gadolinium, greatly improving the identification of inverse beta decay events, the primary channel for detecting the diffuse supernova neutrino background [DSNB]. The upgrade als… ▽ More Super-Kamiokande [SK] was upgraded through the addition of gadolinium sulfate to its ultrapure water, initiating the SK-Gd program. This development enables efficient neutron tagging via the large capture cross section of gadolinium, greatly improving the identification of inverse beta decay events, the primary channel for detecting the diffuse supernova neutrino background [DSNB]. The upgrade also enhances sensitivity to galactic and pre-supernova neutrinos, as well as atmospheric neutrino interactions. To realize this capability, extensive work was performed, including the construction and operation of the EGADS demonstrator, the refurbishment of the SK tank, the development of radiopure gadolinium production methods, and the validation of the loading and uniformity of gadolinium in solution. Early SK-Gd operation has demonstrated high neutron-tagging efficiency, reduced backgrounds, and world-leading limits on the DSNB flux. With these advances, SK-Gd now stands at the threshold of discovering the DSNB and opens a wide range of new opportunities in astrophysics and neutrino physics. △ Less

Submitted 28 March, 2026; v1 submitted 5 November, 2025; originally announced November 2025.

Comments: 25 pages, 11 figures

First measurement of neutron capture multiplicity in neutrino-oxygen neutral-current quasi-elastic-like interactions using an accelerator neutrino beam

Authors: T2K Collaboration, K. Abe, S. Abe, R. Akutsu, H. Alarakia-Charles, Y. I. Alj Hakim, S. Alonso Monsalve, L. Anthony, M. Antonova, S. Aoki, K. A. Apte, T. Arai, T. Arihara, S. Arimoto, Y. Asada, Y. Ashida, N. Babu, G. Barr, D. Barrow, P. Bates, M. Batkiewicz-Kwasniak, V. Berardi, L. Berns, S. Bordoni, S. B. Boyd , et al. (314 additional authors not shown)

Abstract: We report the first measurement of neutron capture multiplicity in neutrino-oxygen neutral-current quasi-elastic-like interactions at the gadolinium-loaded Super-Kamiokande detector using the T2K neutrino beam, which has a peak energy of about 0.6 GeV. A total of 30 neutral-current quasi-elastic-like event candidates were selected from T2K data corresponding to an exposure of $1.76\times10^{20}$ p… ▽ More We report the first measurement of neutron capture multiplicity in neutrino-oxygen neutral-current quasi-elastic-like interactions at the gadolinium-loaded Super-Kamiokande detector using the T2K neutrino beam, which has a peak energy of about 0.6 GeV. A total of 30 neutral-current quasi-elastic-like event candidates were selected from T2K data corresponding to an exposure of $1.76\times10^{20}$ protons on target. The $γ$ ray signals resulting from neutron captures were identified using a neural network. The flux-averaged mean neutron capture multiplicity was measured to be $1.37\pm0.33\text{ (stat.)}$$^{+0.17}_{-0.27}\text{ (syst.)}$, which is compatible within $2.3\,σ$ than predictions obtained using our nominal simulation. We discuss potential sources of systematic uncertainty in the prediction and demonstrate that a significant portion of this discrepancy arises from the modeling of hadron-nucleus interactions in the detector medium. △ Less

Submitted 30 May, 2025; v1 submitted 28 May, 2025; originally announced May 2025.

Comments: 27 pages, 20 figures

Neutron multiplicity measurement in muon capture on oxygen nuclei in the Gd-loaded Super-Kamiokande detector

Authors: The Super-Kamiokande Collaboration, :, S. Miki, K. Abe, S. Abe, Y. Asaoka, C. Bronner, M. Harada, Y. Hayato, K. Hiraide, K. Hosokawa, K. Ieki, M. Ikeda, J. Kameda, Y. Kanemura, R. Kaneshima, Y. Kashiwagi, Y. Kataoka, S. Mine, M. Miura, S. Moriyama, M. Nakahata, S. Nakayama, Y. Noguchi, K. Okamoto , et al. (265 additional authors not shown)

Abstract: In recent neutrino detectors, neutrons produced in neutrino reactions play an important role. Muon capture on oxygen nuclei is one of the processes that produce neutrons in water Cherenkov detectors. We measured neutron multiplicity in the process using cosmic ray muons that stop in the gadolinium-loaded Super-Kamiokande detector. For this measurement, neutron detection efficiency is obtained with… ▽ More In recent neutrino detectors, neutrons produced in neutrino reactions play an important role. Muon capture on oxygen nuclei is one of the processes that produce neutrons in water Cherenkov detectors. We measured neutron multiplicity in the process using cosmic ray muons that stop in the gadolinium-loaded Super-Kamiokande detector. For this measurement, neutron detection efficiency is obtained with the muon capture events followed by gamma rays to be $50.2^{+2.0}_{-2.1}\%$. By fitting the observed multiplicity considering the detection efficiency, we measure neutron multiplicity in muon capture as $P(0)=24\pm3\%$, $P(1)=70^{+3}_{-2}\%$, $P(2)=6.1\pm0.5\%$, $P(3)=0.38\pm0.09\%$. This is the first measurement of the multiplicity of neutrons associated with muon capture on oxygen without neutron energy threshold. △ Less

Submitted 31 January, 2026; v1 submitted 24 February, 2025; originally announced February 2025.

In-situ high voltage generation with Cockcroft-Walton multiplier for xenon gas time projection chamber

Authors: Shinichi Akiyama, Junya Hikida, Masashi Yoshida, Kazuhiro Nakamura, Sei Ban, Masanori Hirose, Atsuko K. Ichikawa, Yoshihisa Iwashita, Tatsuya Kikawa, Yasuhiro Nakajima, Kiseki D. Nakamura, Tsuyoshi Nakaya, Shuhei Obara, Ken Sakashita, Hiroyuki Sekiya, Bungo Sugashima, Soki Urano, Sota Hatsumi, Sota Kobayashi, Hayato Sasaki

Abstract: We have newly developed a Cockcroft-Walton (CW) multiplier that can be used in a gas time projection chamber (TPC). A TPC requires a high voltage to form an electric field that drifts ionization electrons. Supplying the high voltage from outside the pressure vessel requires a dedicated high-voltage feedthrough. An alternative approach is to generate the high voltage inside the pressure vessel with… ▽ More We have newly developed a Cockcroft-Walton (CW) multiplier that can be used in a gas time projection chamber (TPC). A TPC requires a high voltage to form an electric field that drifts ionization electrons. Supplying the high voltage from outside the pressure vessel requires a dedicated high-voltage feedthrough. An alternative approach is to generate the high voltage inside the pressure vessel with a relatively low voltage introduced from outside. A CW multiplier can convert a low AC voltage input to a high DC voltage output, making it suitable for this purpose. We have integrated a CW multiplier into the AXEL (A Xenon ElectroLuminescence detector), a high pressure xenon gas TPC to search for neutrinoless double beta decay of $^{136}$Xe. It uses silicon photomultipliers to detect the ionization electrons through elecrtoluminescence, making it strong against electronic noise. Operation of the CW multiplier was successfully demonstrated; the TPC was operated for 40 days at 6.8 bar, and an energy resolution as high as (0.67 $\pm$ 0.08) % (FWHM) at 2615 keV was obtained. △ Less

Submitted 7 May, 2025; v1 submitted 14 January, 2025; originally announced January 2025.

Comments: 27 pages, 24 figures, preprint for PTEP submission, resubmitted

Journal ref: Progress of Theoretical and Experimental Physics, Volume 2025 Issue 5, 053H03

Modification on thermal motion in Geant4 for neutron capture simulation in Gadolinium loaded water

Authors: Y. Hino, K. Abe, R. Asaka, S. Han, M. Harada, M. Ishitsuka, H. Ito, S. Izumiyama, Y. Kanemura, Y. Koshio, F. Nakanishi, H. Sekiya, T. Yano

Abstract: Neutron tagging is a fundamental technique for electron anti-neutrino detection via the inverse beta decay channel. A reported discrepancy in neutron detection efficiency between observational data and simulation predictions prompted an investigation into neutron capture modeling in Geant4. The study revealed that an overestimation of the thermal motion of hydrogen atoms in Geant4 impacts the frac… ▽ More Neutron tagging is a fundamental technique for electron anti-neutrino detection via the inverse beta decay channel. A reported discrepancy in neutron detection efficiency between observational data and simulation predictions prompted an investigation into neutron capture modeling in Geant4. The study revealed that an overestimation of the thermal motion of hydrogen atoms in Geant4 impacts the fraction of captured nuclei. By manually modifying the Geant4 implementation, the simulation results align with calculations based on evaluated nuclear data and show good agreement with observables derived from the SK-Gd data. △ Less

Submitted 13 December, 2024; v1 submitted 5 December, 2024; originally announced December 2024.

Comments: 9 pages, 5 figures

Removal of Radioactive Noble Gas Radon from Air by Ag-Zeolite

Authors: H. Ogawa, Y. Takeuchi, H. Sekiya, K. Iyoki, M. Matsukura, T. Wakihara, Y. Nakano, S. Hirano, A. Taniguchi

Abstract: This paper investigates the removal of radon from purified and ambient airs by Ag-zeolite. Ag-zeolite is known to have very high performance for airborne radon removal. The dependence of zeolite type and silver content on the performance of radon removal was evaluated. The performance of radon removal by single pass and radon emanation were also evaluated. In addition, the adsorption performance d… ▽ More This paper investigates the removal of radon from purified and ambient airs by Ag-zeolite. Ag-zeolite is known to have very high performance for airborne radon removal. The dependence of zeolite type and silver content on the performance of radon removal was evaluated. The performance of radon removal by single pass and radon emanation were also evaluated. In addition, the adsorption performance due to zeolite solidification and the change in adsorption performance due to moisture in the gas were evaluated in order to investigate properties that should be considered for future practical use. These properties will be adaptable to the development of air purification systems for the large volume ultra-low radioactivity experiments. △ Less

Submitted 12 January, 2025; v1 submitted 3 November, 2024; originally announced November 2024.

Comments: 12 pages, 11 figures

Study of radon removal performance of silver-ion exchanged zeolite from air for underground experiments

Authors: T. Sone, Y. Takeuchi, M. Matsukura, Y. Nakano, H. Ogawa, H. Sekiya, T. Wakihara, S. Hirano, A. Taniguchi

Abstract: Radon is a common background source for underground astroparticle physics experiments and its removal is essential. In this study, we fabricated several prototype silver-ion exchanged zeolites, measured their radon adsorption properties, and evaluated their applicability for radon removal in underground experiments. As a result, we succeeded in producing a prototype silver-ion exchanged zeolite th… ▽ More Radon is a common background source for underground astroparticle physics experiments and its removal is essential. In this study, we fabricated several prototype silver-ion exchanged zeolites, measured their radon adsorption properties, and evaluated their applicability for radon removal in underground experiments. As a result, we succeeded in producing a prototype silver-ion exchanged zeolite that showed better radon adsorption properties than preceding studies. △ Less

Submitted 25 November, 2024; v1 submitted 7 October, 2024; originally announced October 2024.

Comments: 14 pages, 13 figures

Combined Pre-Supernova Alert System with Kamland and Super-Kamiokande

Authors: KamLAND, Super-Kamiokande Collaborations, :, Seisho Abe, Minori Eizuka, Sawako Futagi, Azusa Gando, Yoshihito Gando, Shun Goto, Takahiko Hachiya, Kazumi Hata, Koichi Ichimura, Sei Ieki, Haruo Ikeda, Kunio Inoue, Koji Ishidoshiro, Yuto Kamei, Nanami Kawada, Yasuhiro Kishimoto, Masayuki Koga, Maho Kurasawa, Tadao Mitsui, Haruhiko Miyake, Daisuke Morita, Takeshi Nakahata , et al. (290 additional authors not shown)

Abstract: Preceding a core-collapse supernova, various processes produce an increasing amount of neutrinos of all flavors characterized by mounting energies from the interior of massive stars. Among them, the electron antineutrinos are potentially detectable by terrestrial neutrino experiments such as KamLAND and Super-Kamiokande via inverse beta decay interactions. Once these pre-supernova neutrinos are ob… ▽ More Preceding a core-collapse supernova, various processes produce an increasing amount of neutrinos of all flavors characterized by mounting energies from the interior of massive stars. Among them, the electron antineutrinos are potentially detectable by terrestrial neutrino experiments such as KamLAND and Super-Kamiokande via inverse beta decay interactions. Once these pre-supernova neutrinos are observed, an early warning of the upcoming core-collapse supernova can be provided. In light of this, KamLAND and Super-Kamiokande, both located in the Kamioka mine in Japan, have been monitoring pre-supernova neutrinos since 2015 and 2021, respectively. Recently, we performed a joint study between KamLAND and Super-Kamiokande on pre-supernova neutrino detection. A pre-supernova alert system combining the KamLAND detector and the Super-Kamiokande detector was developed and put into operation, which can provide a supernova alert to the astrophysics community. Fully leveraging the complementary properties of these two detectors, the combined alert is expected to resolve a pre-supernova neutrino signal from a 15 M$_{\odot}$ star within 510 pc of the Earth, at a significance level corresponding to a false alarm rate of no more than 1 per century. For a Betelgeuse-like model with optimistic parameters, it can provide early warnings up to 12 hours in advance. △ Less

Submitted 1 July, 2024; v1 submitted 15 April, 2024; originally announced April 2024.

Comments: Resubmitted to ApJ. 22 pages, 16 figures, for more information about the combined pre-supernova alert system, see https://www.lowbg.org/presnalarm/

Second gadolinium loading to Super-Kamiokande

Authors: K. Abe, C. Bronner, Y. Hayato, K. Hiraide, K. Hosokawa, K. Ieki, M. Ikeda, J. Kameda, Y. Kanemura, R. Kaneshima, Y. Kashiwagi, Y. Kataoka, S. Miki, S. Mine, M. Miura, S. Moriyama, Y. Nakano, M. Nakahata, S. Nakayama, Y. Noguchi, K. Sato, H. Sekiya, H. Shiba, K. Shimizu, M. Shiozawa , et al. (225 additional authors not shown)

Abstract: The first loading of gadolinium (Gd) into Super-Kamiokande in 2020 was successful, and the neutron capture efficiency on Gd reached 50\%. To further increase the Gd neutron capture efficiency to 75\%, 26.1 tons of $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ was additionally loaded into Super-Kamiokande (SK) from May 31 to July 4, 2022. As the amount of loaded $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ was do… ▽ More The first loading of gadolinium (Gd) into Super-Kamiokande in 2020 was successful, and the neutron capture efficiency on Gd reached 50\%. To further increase the Gd neutron capture efficiency to 75\%, 26.1 tons of $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ was additionally loaded into Super-Kamiokande (SK) from May 31 to July 4, 2022. As the amount of loaded $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ was doubled compared to the first loading, the capacity of the powder dissolving system was doubled. We also developed new batches of gadolinium sulfate with even further reduced radioactive impurities. In addition, a more efficient screening method was devised and implemented to evaluate these new batches of $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$. Following the second loading, the Gd concentration in SK was measured to be $333.5\pm2.5$ ppm via an Atomic Absorption Spectrometer (AAS). From the mean neutron capture time constant of neutrons from an Am/Be calibration source, the Gd concentration was independently measured to be 332.7 $\pm$ 6.8(sys.) $\pm$ 1.1(stat.) ppm, consistent with the AAS result. Furthermore, during the loading the Gd concentration was monitored continually using the capture time constant of each spallation neutron produced by cosmic-ray muons,and the final neutron capture efficiency was shown to become 1.5 times higher than that of the first loaded phase, as expected. △ Less

Submitted 18 June, 2024; v1 submitted 12 March, 2024; originally announced March 2024.

Comments: 34 pages, 13 figures, submitted to Nuclear Inst. and Methods in Physics Research, A

Journal ref: Nuclear Inst. and Methods in Physics Research, A 1065 (2024) 169480

Measurement of scintillation from proportional electron multiplication in liquid xenon using a needle

Authors: P. Knights, H. Sekiya, I. Katsioulas, K. Nikolopoulos, K. Kanzawa, I. Giomataris

Abstract: Charge amplification in liquids could provide single-phase xenon time projection chambers with background discrimination and fiducialisation capabilities similar to those found in dual-phase detectors. Although efforts to achieve the high electric field required for charge amplification and proportional scintillation in liquid xenon have been previously reported, their application to large-scale d… ▽ More Charge amplification in liquids could provide single-phase xenon time projection chambers with background discrimination and fiducialisation capabilities similar to those found in dual-phase detectors. Although efforts to achieve the high electric field required for charge amplification and proportional scintillation in liquid xenon have been previously reported, their application to large-scale detectors remains elusive. This work presents a new approach to this challenge, where - instead of the thin wire approach of previous studies - a needle-like high-voltage electrode is employed to demonstrate proportional charge amplification and secondary scintillation production in liquid xenon. This is an important milestone towards the development of an electrode structure that could be utilised in a large-scale, single-phase time projection chamber with dual read-out. △ Less

Submitted 4 January, 2024; originally announced January 2024.

Comments: 10 pages, 11 figures

High-pressure xenon gas time projection chamber with scalable design and its performance at around the Q value of $^{136}$Xe double-beta decay

Authors: Masashi Yoshida, Kazuhiro Nakamura, Shinichi Akiyama, Sei Ban, Junya Hikida, Masanori Hirose, Atsuko K. Ichikawa, Yoshihisa Iwashita, Yukimasa Kashino, Tatsuya Kikawa, Akihiro Minamino, Kentaro Miuchi, Yasuhiro Nakajima, Kiseki D. Nakamura, Tsuyoshi Nakaya, Shuhei Obara, Ken Sakashita, Hiroyuki Sekiya, Hibiki Shinagawa, Bungo Sugashima, Soki Urano

Abstract: We have been developing a high-pressure xenon gas time projection chamber (TPC) to search for neutrinoless double beta ($0νββ$) decay of $^{136}$Xe. The unique feature of this TPC is in the detection part of ionization electrons, called ELCC. ELCC is composed of multiple units, and one unit covers 48.5 $\mathrm{cm}^2$. A 180 L size prototype detector with 12 units, 672 channels, of ELCC was constr… ▽ More We have been developing a high-pressure xenon gas time projection chamber (TPC) to search for neutrinoless double beta ($0νββ$) decay of $^{136}$Xe. The unique feature of this TPC is in the detection part of ionization electrons, called ELCC. ELCC is composed of multiple units, and one unit covers 48.5 $\mathrm{cm}^2$. A 180 L size prototype detector with 12 units, 672 channels, of ELCC was constructed and operated with 7.6 bar natural xenon gas to evaluate the performance of the detector at around the Q value of $^{136}$Xe $0νββ$. The obtained FWHM energy resolution is (0.73 $\pm$ 0.11) % at 1836 keV. This corresponds to (0.60 $\pm$ 0.03) % to (0.70 $\pm$ 0.21) % of energy resolution at the Q value of $^{136}Xe$ $0νββ$. This result shows the scalability of the AXEL detector with ELCC while maintaining high energy resolution. Factors determining the energy resolution were quantitatively evaluated and the result indicates further improvement is feasible. Reconstructed track images show distinctive structures at the endpoint of electron tracks, which will be an important feature to distinguish $0νββ$ signals from gamma-ray backgrounds. △ Less

Submitted 11 December, 2023; v1 submitted 30 October, 2023; originally announced October 2023.

Comments: 33 pages, 24 figures, preprint accepted by PTEP

Journal ref: Progress of Theoretical and Experimental Physics, Volume 2024 Issue 1, 013H01

Development of a low-background HPGe detector at Kamioka Observatory

Authors: K. Ichimura, H. Ikeda, Y. Kishimoto, M. Kurasawa, A. A. Suzuki, Y. Gando, M. Ikeda, K. Hosokawa, H. Sekiya, H. Ito, A. Minamino, S. Suzuki

Abstract: A new ultra-low background high-purity germanium (HPGe) detector has been installed at the Kamioka underground experimental site. The background count rate in the energy range from 40 keV to 2700 keV is about 25% lower than that of the first HPGe detector installed in 2016, which has the same detector specification and similar shielding geometry. This paper describes the shielding configuration, i… ▽ More A new ultra-low background high-purity germanium (HPGe) detector has been installed at the Kamioka underground experimental site. The background count rate in the energy range from 40 keV to 2700 keV is about 25% lower than that of the first HPGe detector installed in 2016, which has the same detector specification and similar shielding geometry. This paper describes the shielding configuration, including the cleaning of the material surface, the comparison of calibration data and simulation, the time variation of the background spectra, the sample measurement procedure, and some results of the radioactivity in the selected samples. △ Less

Submitted 4 December, 2023; v1 submitted 9 August, 2023; originally announced August 2023.

Journal ref: Progress of Theoretical and Experimental Physics, Volume 2023, Issue 12, December 2023, 123H01

Rapid Analysis of $\mathrm{^{226}Ra}$ in Ultrapure Gadolinium Sulfate Octahydrate

Authors: Y. Sakakieda, K. Hosokawa, F. Nakanishi, Y. Hino, Y. Inome, A. Sakaguchi, Y. Takaku, M. Ikeda, H. Sekiya

Abstract: Numerous particle physics experiments utilize gadolinium (Gd), a rare earth element with the most significant neutron capture cross-section among all elements, to detect anti-neutrinos via inverse beta-decays or to remove neutron-induced background events. For example, to load Gd into water Cherenkov detectors, $\mathrm{{Gd_{2}(SO_{4})_{3}\cdot 8H_{2}O}}$ is dissolved and rare event search experim… ▽ More Numerous particle physics experiments utilize gadolinium (Gd), a rare earth element with the most significant neutron capture cross-section among all elements, to detect anti-neutrinos via inverse beta-decays or to remove neutron-induced background events. For example, to load Gd into water Cherenkov detectors, $\mathrm{{Gd_{2}(SO_{4})_{3}\cdot 8H_{2}O}}$ is dissolved and rare event search experiments are required to screen for radioactive impurities in $\mathrm{{Gd_{2}(SO_{4})_{3}\cdot 8H_{2}O}}$ before dissolution. This study developed a new method to rapidly measure the radium-226($\mathrm{^{226}Ra}$) concentration in $\mathrm{{Gd_{2}(SO_{4})_{3}\cdot 8H_{2}O}}$. This method requires only three days to measure a batch of samples, as opposed to the usual method using high-purity germanium detectors, which takes approximately 20 days after arrival. The detection limit for the measurement of $\mathrm{^{226}Ra}$ is 0.32 mBq/kg. This method has been already used for $\mathrm{{Gd_{2}(SO_{4})_{3}\cdot 8H_{2}O}}$ screening at the Super-Kamiokande Gd(SK-Gd) project, and it can be applied to future experiments. △ Less

Submitted 12 June, 2023; originally announced June 2023.

Comments: 10 pages, 3 figures

Search for astrophysical electron antineutrinos in Super-Kamiokande with 0.01wt% gadolinium-loaded water

Authors: M. Harada, K. Abe, C. Bronner, Y. Hayato, K. Hiraide, K. Hosokawa, K. Ieki, M. Ikeda, J. Kameda, Y. Kanemura, R. Kaneshima, Y. Kashiwagi, Y. Kataoka, S. Miki, S. Mine, M. Miura, S. Moriyama, Y. Nakano, M. Nakahata, S. Nakayama, Y. Noguchi, K. Okamoto, K. Sato, H. Sekiya, H. Shiba , et al. (216 additional authors not shown)

Abstract: We report the first search result for the flux of astrophysical electron antineutrinos for energies O(10) MeV in the gadolinium-loaded Super-Kamiokande (SK) detector. In June 2020, gadolinium was introduced to the ultra-pure water of the SK detector in order to detect neutrons more efficiently. In this new experimental phase, SK-Gd, we can search for electron antineutrinos via inverse beta decay w… ▽ More We report the first search result for the flux of astrophysical electron antineutrinos for energies O(10) MeV in the gadolinium-loaded Super-Kamiokande (SK) detector. In June 2020, gadolinium was introduced to the ultra-pure water of the SK detector in order to detect neutrons more efficiently. In this new experimental phase, SK-Gd, we can search for electron antineutrinos via inverse beta decay with efficient background rejection and higher signal efficiency thanks to the high efficiency of the neutron tagging technique. In this paper, we report the result for the initial stage of SK-Gd with a $22.5\times552$ $\rm kton\cdot day$ exposure at 0.01% Gd mass concentration. No significant excess over the expected background in the observed events is found for the neutrino energies below 31.3 MeV. Thus, the flux upper limits are placed at the 90% confidence level. The limits and sensitivities are already comparable with the previous SK result with pure-water ($22.5 \times 2970 \rm kton\cdot day$) owing to the enhanced neutron tagging. △ Less

Submitted 30 May, 2023; v1 submitted 8 May, 2023; originally announced May 2023.

Direct dark matter searches with the full data set of XMASS-I

Authors: XMASS Collaboration, K. Abe, K. Hiraide, N. Kato, S. Moriyama, M. Nakahata, K. Sato, H. Sekiya, T. Suzuki, Y. Suzuki, A. Takeda, B. S. Yang, N. Y. Kim, Y. D. Kim, Y. H. Kim, Y. Itow, K. Martens, A. Mason, M. Yamashita, K. Miuchi, Y. Takeuchi, K. B. Lee, M. K. Lee, Y. Fukuda, H. Ogawa , et al. (7 additional authors not shown)

Abstract: Various WIMP dark matter searches using the full data set of XMASS-I, a single-phase liquid xenon detector, are reported in this paper. Stable XMASS-I data taking accumulated a total live time of 1590.9 days between November 20, 2013 and February 1, 2019 with an analysis threshold of ${\rm 1.0\,keV_{ee}}$. In the latter half of data taking a lower analysis threshold of ${\rm 0.5\,keV_{ee}}$ was al… ▽ More Various WIMP dark matter searches using the full data set of XMASS-I, a single-phase liquid xenon detector, are reported in this paper. Stable XMASS-I data taking accumulated a total live time of 1590.9 days between November 20, 2013 and February 1, 2019 with an analysis threshold of ${\rm 1.0\,keV_{ee}}$. In the latter half of data taking a lower analysis threshold of ${\rm 0.5\,keV_{ee}}$ was also available through a new low threshold trigger. Searching for a WIMP signal in the detector's 97~kg fiducial volume yielded a limit on the WIMP-nucleon scattering cross section of ${\rm 1.4\times 10^{-44}\, cm^{2}}$ for a ${\rm 60\,GeV/c^{2}}$ WIMP at the 90$\%$ confidence level. We also searched for WIMP induced annual modulation signatures in the detector's whole target volume, containing 832~kg of liquid xenon. For nuclear recoils of a ${\rm 8\,GeV/c^{2}}$ WIMP this analysis yielded a 90\% CL cross section limit of ${\rm 2.3\times 10^{-42}\, cm^{2}}$. At a WIMP mass of ${\rm 0.5\, GeV/c^{2}}$ the Migdal effect and Bremsstrahlung signatures were evaluated and lead to 90\% CL cross section limits of ${\rm 1.4\times 10^{-35}\, cm^{2}}$ and ${\rm 1.1\times 10^{-33}\, cm^{2}}$ respectively. △ Less

Submitted 1 September, 2023; v1 submitted 11 November, 2022; originally announced November 2022.

Journal ref: Phys. Rev. D 108, 083022 (2023)

Searching for neutrinos from solar flares across solar cycles 23 and 24 with the Super-Kamiokande detector

Authors: K. Okamoto, K. Abe, Y. Hayato, K. Hiraide, K. Hosokawa, K. Ieki, M. Ikeda, J. Kameda, Y. Kanemura, Y. Kaneshima, Y. Kataoka, Y. Kashiwagi, S. Miki, S. Mine, M. Miura, S. Moriyama, Y. Nagao, M. Nakahata, Y. Nakano, S. Nakayama, Y. Noguchi, K. Sato, H. Sekiya, K. Shimizu, M. Shiozawa , et al. (220 additional authors not shown)

Abstract: Neutrinos associated with solar flares (solar-flare neutrinos) provide information on particle acceleration mechanisms during the impulsive phase of solar flares. We searched using the Super-Kamiokande detector for neutrinos from solar flares that occurred during solar cycles $23$ and $24$, including the largest solar flare (X28.0) on November 4th, 2003. In order to minimize the background rate we… ▽ More Neutrinos associated with solar flares (solar-flare neutrinos) provide information on particle acceleration mechanisms during the impulsive phase of solar flares. We searched using the Super-Kamiokande detector for neutrinos from solar flares that occurred during solar cycles $23$ and $24$, including the largest solar flare (X28.0) on November 4th, 2003. In order to minimize the background rate we searched for neutrino interactions within narrow time windows coincident with $γ$-rays and soft X-rays recorded by satellites. In addition, we performed the first attempt to search for solar-flare neutrinos from solar flares on the invisible side of the Sun by using the emission time of coronal mass ejections (CMEs). By selecting twenty powerful solar flares above X5.0 on the visible side and eight CMEs whose emission speed exceeds $2000$ $\mathrm{km \, s^{-1}}$ on the invisible side from 1996 to 2018, we found two (six) neutrino events coincident with solar flares occurring on the visible (invisible) side of the Sun, with a typical background rate of $0.10$ ($0.62$) events per flare in the MeV-GeV energy range. No significant solar-flare neutrino signal above the estimated background rate was observed. As a result we set the following upper limit on neutrino fluence at the Earth $\mathitΦ<1.1\times10^{6}$ $\mathrm{cm^{-2}}$ at the $90\%$ confidence level for the largest solar flare. The resulting fluence limits allow us to constrain some of the theoretical models for solar-flare neutrino emission. △ Less

Submitted 26 October, 2022; v1 submitted 24 October, 2022; originally announced October 2022.

Comments: 36 pages, 18 figures, 9 tables (Figure 12 was replaced because it was incorrect in version 1.)

Neutron Tagging following Atmospheric Neutrino Events in a Water Cherenkov Detector

Authors: K. Abe, Y. Haga, Y. Hayato, K. Hiraide, K. Ieki, M. Ikeda, S. Imaizumi, K. Iyogi, J. Kameda, Y. Kanemura, Y. Kataoka, Y. Kato, Y. Kishimoto, S. Miki, S. Mine, M. Miura, T. Mochizuki, S. Moriyama, Y. Nagao, M. Nakahata, T. Nakajima, Y. Nakano, S. Nakayama, T. Okada, K. Okamoto , et al. (281 additional authors not shown)

Abstract: We present the development of neutron-tagging techniques in Super-Kamiokande IV using a neural network analysis. The detection efficiency of neutron capture on hydrogen is estimated to be 26%, with a mis-tag rate of 0.016 per neutrino event. The uncertainty of the tagging efficiency is estimated to be 9.0%. Measurement of the tagging efficiency with data from an Americium-Beryllium calibration agr… ▽ More We present the development of neutron-tagging techniques in Super-Kamiokande IV using a neural network analysis. The detection efficiency of neutron capture on hydrogen is estimated to be 26%, with a mis-tag rate of 0.016 per neutrino event. The uncertainty of the tagging efficiency is estimated to be 9.0%. Measurement of the tagging efficiency with data from an Americium-Beryllium calibration agrees with this value within 10%. The tagging procedure was performed on 3,244.4 days of SK-IV atmospheric neutrino data, identifying 18,091 neutrons in 26,473 neutrino events. The fitted neutron capture lifetime was measured as 218 \pm 9 μs. △ Less

Submitted 20 September, 2022; v1 submitted 18 September, 2022; originally announced September 2022.

Journal ref: JINST 17 P10029 (2022)

Development of Ultra-pure Gadolinium Sulfate for the Super-Kamiokande Gadolinium Project

Authors: K. Hosokawa, M. Ikeda, T. Okada, H. Sekiya, P. Fernandez, L. Labarga, I. Bandac, J. Perez, S. Ito, M. Harada, Y. Koshio, M. D. Thiesse, L. F. Thompson, P. R. Scovell, E. Meehan, K. Ichimura, Y. Kishimoto, Y. Nakajima, M. R. Vagins, H. Ito, Y. Takaku, Y. Tanaka, Y. Yamaguchi

Abstract: This paper reports the development and detailed properties of about 13 tons of gadolinium sulfate octahydrate, $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$, which has been dissolved into Super-Kamiokande (SK) in the summer of 2020. We evaluate the impact of radioactive impurities in $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ on DSNB searches and solar neutrino observation and confirm the need to reduce radioa… ▽ More This paper reports the development and detailed properties of about 13 tons of gadolinium sulfate octahydrate, $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$, which has been dissolved into Super-Kamiokande (SK) in the summer of 2020. We evaluate the impact of radioactive impurities in $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ on DSNB searches and solar neutrino observation and confirm the need to reduce radioactive and fluorescent impurities by about three orders of magnitude from commercially available high-purity $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$. In order to produce ultra-high-purity $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$, we have developed a method to remove impurities from gadolinium oxide, Gd$_2$O$_3$, consisting of acid dissolution, solvent extraction, and pH control processes, followed by a high-purity sulfation process. All of the produced ultra-high-purity $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ is assayed by ICP-MS and HPGe detectors to evaluate its quality. Because of the long measurement time of HPGe detectors, we have employed several underground laboratories for making parallel measurements including LSC in Spain, Boulby in the UK, and Kamioka in Japan. In the first half of production, the measured batch purities were found to be consistent with the specifications. However,in the latter half, the $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ contained one order of magnitude more $^{228}$Ra than the budgeted mean contamination. This was correlated with the corresponding characteristics of the raw material Gd$_2$O$_3$, in which an intrinsically large contamination was present. Based on their modest impact on SK physics, they were nevertheless introduced into the detector. To reduce $^{228}$Ra for the next stage of Gd loading to SK, a new process has been successfully establised. △ Less

Submitted 13 September, 2022; originally announced September 2022.

Comments: 32 pages, 9 figures

Scintillator ageing of the T2K near detectors from 2010 to 2021

Authors: The T2K Collaboration, K. Abe, N. Akhlaq, R. Akutsu, A. Ali, C. Alt, C. Andreopoulos, M. Antonova, S. Aoki, T. Arihara, Y. Asada, Y. Ashida, E. T. Atkin, S. Ban, M. Barbi, G. J. Barker, G. Barr, D. Barrow, M. Batkiewicz-Kwasniak, F. Bench, V. Berardi, L. Berns, S. Bhadra, A. Blanchet, A. Blondel , et al. (333 additional authors not shown)

Abstract: The T2K experiment widely uses plastic scintillator as a target for neutrino interactions and an active medium for the measurement of charged particles produced in neutrino interactions at its near detector complex. Over 10 years of operation the measured light yield recorded by the scintillator based subsystems has been observed to degrade by 0.9--2.2\% per year. Extrapolation of the degradation… ▽ More The T2K experiment widely uses plastic scintillator as a target for neutrino interactions and an active medium for the measurement of charged particles produced in neutrino interactions at its near detector complex. Over 10 years of operation the measured light yield recorded by the scintillator based subsystems has been observed to degrade by 0.9--2.2\% per year. Extrapolation of the degradation rate through to 2040 indicates the recorded light yield should remain above the lower threshold used by the current reconstruction algorithms for all subsystems. This will allow the near detectors to continue contributing to important physics measurements during the T2K-II and Hyper-Kamiokande eras. Additionally, work to disentangle the degradation of the plastic scintillator and wavelength shifting fibres shows that the reduction in light yield can be attributed to the ageing of the plastic scintillator. △ Less

Submitted 26 July, 2022; originally announced July 2022.

Comments: 29 pages, 18 figures. Prepared for submission to JINST

Pre-Supernova Alert System for Super-Kamiokande

Authors: Super-Kamiokande Collaboration, :, L. N. Machado, K. Abe, Y. Hayato, K. Hiraide, K. Ieki, M. Ikeda, J. Kameda, Y. Kanemura, R. Kaneshima, Y. Kashiwagi, Y. Kataoka, S. Miki, S. Mine, M. Miura, S. Moriyama, Y. Nakano, M. Nakahata, S. Nakayama, Y. Noguchi, K. Okamoto, K. Sato, H. Sekiya, H. Shiba , et al. (202 additional authors not shown)

Abstract: In 2020, the Super-Kamiokande (SK) experiment moved to a new stage (SK-Gd) in which gadolinium (Gd) sulfate octahydrate was added to the water in the detector, enhancing the efficiency to detect thermal neutrons and consequently improving the sensitivity to low energy electron anti-neutrinos from inverse beta decay (IBD) interactions. SK-Gd has the potential to provide early alerts of incipient co… ▽ More In 2020, the Super-Kamiokande (SK) experiment moved to a new stage (SK-Gd) in which gadolinium (Gd) sulfate octahydrate was added to the water in the detector, enhancing the efficiency to detect thermal neutrons and consequently improving the sensitivity to low energy electron anti-neutrinos from inverse beta decay (IBD) interactions. SK-Gd has the potential to provide early alerts of incipient core-collapse supernovae through detection of electron anti-neutrinos from thermal and nuclear processes responsible for the cooling of massive stars before the gravitational collapse of their cores. These pre-supernova neutrinos emitted during the silicon burning phase can exceed the energy threshold for IBD reactions. We present the sensitivity of SK-Gd to pre-supernova stars and the techniques used for the development of a pre-supernova alarm based on the detection of these neutrinos in SK, as well as prospects for future SK-Gd phases with higher concentrations of Gd. For the current SK-Gd phase, high-confidence alerts for Betelgeuse could be issued up to nine hours in advance of the core-collapse itself. △ Less

Submitted 17 August, 2022; v1 submitted 19 May, 2022; originally announced May 2022.

Comments: 20 pages

Journal ref: The Astrophysical Journal, Volume 935, Number 1 (2022)

Search for neutrinoless quadruple beta decay of $^{136}$Xe in XMASS-I

Authors: XMASS Collaboration, K. Abe, K. Hiraide, K. Ichimura, N. Kato, Y. Kishimoto, K. Kobayashi, M. Kobayashi, S. Moriyama, M. Nakahata, K. Sato, H. Sekiya, T. Suzuki, A. Takeda, S. Tasaka, M. Yamashita, B. S. Yang, N. Y. Kim, Y. D. Kim, Y. H. Kim, R. Ishii, Y. Itow, K. Kanzawa, K. Masuda, K. Martens , et al. (12 additional authors not shown)

Abstract: A search for the neutrinoless quadruple beta decay of $^{136}$Xe was conducted with the liquid-xenon detector XMASS-I using $\rm 327\; kg \times 800.0 \; days$ of the exposure. The pulse shape discrimination based on the scintillation decay time constant which distinguishes $γ$-rays including the signal and $β$-rays was used to enhance the search sensitivity. No significant signal excess was obser… ▽ More A search for the neutrinoless quadruple beta decay of $^{136}$Xe was conducted with the liquid-xenon detector XMASS-I using $\rm 327\; kg \times 800.0 \; days$ of the exposure. The pulse shape discrimination based on the scintillation decay time constant which distinguishes $γ$-rays including the signal and $β$-rays was used to enhance the search sensitivity. No significant signal excess was observed from the energy spectrum fitting with precise background evaluation, and we set a lower limit of the half life of 3.7 $\times$ 10$^{24}$ years at 90$\%$ confidence level. This is the first experimental constraint of the neutrinoless quadruple beta decay of $^{136}$Xe. △ Less

Submitted 5 August, 2022; v1 submitted 10 May, 2022; originally announced May 2022.

Journal ref: Physics Letters B 833 (2022) 137355

Recoil imaging for directional detection of dark matter, neutrinos, and physics beyond the Standard Model

Authors: C. A. J. O'Hare, D. Loomba, K. Altenmüller, H. Álvarez-Pol, F. D. Amaro, H. M. Araújo, D. Aristizabal Sierra, J. Asaadi, D. Attié, S. Aune, C. Awe, Y. Ayyad, E. Baracchini, P. Barbeau, J. B. R. Battat, N. F. Bell, B. Biasuzzi, L. J. Bignell, C. Boehm, I. Bolognino, F. M. Brunbauer, M. Caamaño, C. Cabo, D. Caratelli, J. M. Carmona , et al. (142 additional authors not shown)

Abstract: Recoil imaging entails the detection of spatially resolved ionization tracks generated by particle interactions. This is a highly sought-after capability in many classes of detector, with broad applications across particle and astroparticle physics. However, at low energies, where ionization signatures are small in size, recoil imaging only seems to be a practical goal for micro-pattern gas detect… ▽ More Recoil imaging entails the detection of spatially resolved ionization tracks generated by particle interactions. This is a highly sought-after capability in many classes of detector, with broad applications across particle and astroparticle physics. However, at low energies, where ionization signatures are small in size, recoil imaging only seems to be a practical goal for micro-pattern gas detectors. This white paper outlines the physics case for recoil imaging, and puts forward a decadal plan to advance towards the directional detection of low-energy recoils with sensitivity and resolution close to fundamental performance limits. The science case covered includes: the discovery of dark matter into the neutrino fog, directional detection of sub-MeV solar neutrinos, the precision study of coherent-elastic neutrino-nucleus scattering, the detection of solar axions, the measurement of the Migdal effect, X-ray polarimetry, and several other applied physics goals. We also outline the R&D programs necessary to test concepts that are crucial to advance detector performance towards their fundamental limit: single primary electron sensitivity with full 3D spatial resolution at the $\sim$100 micron-scale. These advancements include: the use of negative ion drift, electron counting with high-definition electronic readout, time projection chambers with optical readout, and the possibility for nuclear recoil tracking in high-density gases such as argon. We also discuss the readout and electronics systems needed to scale-up such detectors to the ton-scale and beyond. △ Less

Submitted 17 July, 2022; v1 submitted 11 March, 2022; originally announced March 2022.

Comments: 77 pages, 20 figures. Submitted to the Proceedings of the US Community Study on the Future of Particle Physics (Snowmass 2021)

A Next-Generation Liquid Xenon Observatory for Dark Matter and Neutrino Physics

Authors: J. Aalbers, K. Abe, V. Aerne, F. Agostini, S. Ahmed Maouloud, D. S. Akerib, D. Yu. Akimov, J. Akshat, A. K. Al Musalhi, F. Alder, S. K. Alsum, L. Althueser, C. S. Amarasinghe, F. D. Amaro, A. Ames, T. J. Anderson, B. Andrieu, N. Angelides, E. Angelino, J. Angevaare, V. C. Antochi, D. Antón Martin, B. Antunovic, E. Aprile, H. M. Araújo , et al. (572 additional authors not shown)

Abstract: The nature of dark matter and properties of neutrinos are among the most pressing issues in contemporary particle physics. The dual-phase xenon time-projection chamber is the leading technology to cover the available parameter space for Weakly Interacting Massive Particles (WIMPs), while featuring extensive sensitivity to many alternative dark matter candidates. These detectors can also study neut… ▽ More The nature of dark matter and properties of neutrinos are among the most pressing issues in contemporary particle physics. The dual-phase xenon time-projection chamber is the leading technology to cover the available parameter space for Weakly Interacting Massive Particles (WIMPs), while featuring extensive sensitivity to many alternative dark matter candidates. These detectors can also study neutrinos through neutrinoless double-beta decay and through a variety of astrophysical sources. A next-generation xenon-based detector will therefore be a true multi-purpose observatory to significantly advance particle physics, nuclear physics, astrophysics, solar physics, and cosmology. This review article presents the science cases for such a detector. △ Less

Submitted 4 March, 2022; originally announced March 2022.

Comments: 77 pages, 40 figures, 1262 references

Report number: INT-PUB-22-003

Journal ref: J. Phys. G: Nucl. Part. Phys. 50 (2023) 013001

Improvement of radon detector performance by using a large-sized PIN-photodiode

Authors: K. Okamoto, Y. Nakano, G. Pronost, H. Sekiya, S. Tasaka, Y. Takeuchi, M. Nakahata

Abstract: Radioactive noble gas radon ($\mathrm{^{222}Rn}$) is one of the major background sources below the MeV region in rare event search experiments. To precisely measure radon concentration in purified gases, a radon detector with an electrostatic collection method is widely used. In this paper, we discussed the improvements of a radon detector by installing a new PIN-photodiode ($28\times28$… ▽ More Radioactive noble gas radon ($\mathrm{^{222}Rn}$) is one of the major background sources below the MeV region in rare event search experiments. To precisely measure radon concentration in purified gases, a radon detector with an electrostatic collection method is widely used. In this paper, we discussed the improvements of a radon detector by installing a new PIN-photodiode ($28\times28$ $\mathrm{mm}$) whose surface area is $2.5$ times larger than that used previously ($18\times18$ $\mathrm{mm}$). We evaluated the detector's performance by serially connecting two radon detectors equipped with two types of PIN-photodiodes. As a result of the calibrations, we found an improvement of $(3.8\pm2.4)\%$ in the detection efficiencies below $1.0$ $\mathrm{g/m^{3}}$, while a $10$-$20\%$ improvement occurred above this level. The intrinsic background of the detector equipped with the large PIN-photodiode was measured as $0.24^{+0.09}_{-0.05}$ $\mathrm{mBq/m^{3}}$. This background level is consistent with the radon detector with the small PIN-photodiode, although we installed the large one. This improvement is useful for applications in radon emanation measurements from a material, which also emits water from its surface. △ Less

Submitted 13 December, 2021; originally announced December 2021.

Comments: 30 pages, 12 figures, 4 tables

Precise measurement of the scintillation decay constant of ZnWO$_4$ crystal

Authors: M. Shibata, H. Sekiya, K. Ichimura

Abstract: The scintillation decay time constant of a ZnWO$_4$ crystal irradiated with $α$-particles from $^{241}$Am was precisely investigated, and was found to depend on the incident direction of the $α$-particles on the crystal. The longest decay time constant ($24.3\pm0.6~μ$s) was obtained on the surface perpendicular to the b-axis of the crystal (Surface B). On Surfaces A and C, the decay constants were… ▽ More The scintillation decay time constant of a ZnWO$_4$ crystal irradiated with $α$-particles from $^{241}$Am was precisely investigated, and was found to depend on the incident direction of the $α$-particles on the crystal. The longest decay time constant ($24.3\pm0.6~μ$s) was obtained on the surface perpendicular to the b-axis of the crystal (Surface B). On Surfaces A and C, the decay constants were $20.0$ and $21.3\pm0.2~μ$s, respectively. The scintillation yield of the ZnWO$_4$ was also anisotropic and depended on the incident direction of the heavy particles. The maximum yield was achieved on surface B, suggesting a correlation between the light yield and scintillation decay time constant of ZnWO$_4$ crystals. △ Less

Submitted 4 October, 2021; originally announced October 2021.

Comments: 9 pages, 8 figures, Accepted for PTEP

Journal ref: Prog. Theor. Exp. Phys. 2022 013C01

First Gadolinium Loading to Super-Kamiokande

Authors: K. Abe, C. Bronner, Y. Hayato, K. Hiraide, M. Ikeda, S. Imaizumi, J. Kameda, Y. Kanemura, Y. Kataoka, S. Miki, M. Miura, S. Moriyama, Y. Nagao, M. Nakahata, S. Nakayama, T. Okada, K. Okamoto, A. Orii, G. Pronost, H. Sekiya, M. Shiozawa, Y. Sonoda, Y. Suzuki, A. Takeda, Y. Takemoto , et al. (192 additional authors not shown)

Abstract: In order to improve Super-Kamiokande's neutron detection efficiency and to thereby increase its sensitivity to the diffuse supernova neutrino background flux, 13 tons of $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ (gadolinium sulfate octahydrate) was dissolved into the detector's otherwise ultrapure water from July 14 to August 17, 2020, marking the start of the SK-Gd phase of operations. During the loa… ▽ More In order to improve Super-Kamiokande's neutron detection efficiency and to thereby increase its sensitivity to the diffuse supernova neutrino background flux, 13 tons of $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ (gadolinium sulfate octahydrate) was dissolved into the detector's otherwise ultrapure water from July 14 to August 17, 2020, marking the start of the SK-Gd phase of operations. During the loading, water was continuously recirculated at a rate of 60 m$^3$/h, extracting water from the top of the detector and mixing it with concentrated $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ solution to create a 0.02% solution of the Gd compound before injecting it into the bottom of the detector. A clear boundary between the Gd-loaded and pure water was maintained through the loading, enabling monitoring of the loading itself and the spatial uniformity of the Gd concentration over the 35 days it took to reach the top of the detector. During the subsequent commissioning the recirculation rate was increased to 120 m$^3$/h, resulting in a constant and uniform distribution of Gd throughout the detector and water transparency equivalent to that of previous pure-water operation periods. Using an Am-Be neutron calibration source the mean neutron capture time was measured to be $115\pm1$ $μ$s, which corresponds to a Gd concentration of $111\pm2$ ppm, as expected for this level of Gd loading. This paper describes changes made to the water circulation system for this detector upgrade, the Gd loading procedure, detector commissioning, and the first neutron calibration measurements in SK-Gd. △ Less

Submitted 15 December, 2021; v1 submitted 1 September, 2021; originally announced September 2021.

Comments: 37 pages, 19 Figures, Accepted for publication in Nucl. Instrum. Meth. A

Journal ref: Nuclear Inst. and Methods in Physics Research, A 1027 (2022) 166248

Direction-sensitive dark matter search with a low-background gaseous detector NEWAGE-0.3b''

Authors: Tomonori Ikeda, Kiseki Nakamura, Takuya Shimada, Ryota Yakabe, Takashi Hashimoto, Hirohisa Ishiura, Takuma Nakamura, Hiroshi Ito, Koichi Ichimura, Ko Abe, Kazuyoshi Kobayashi, Toru Tanimori, Hidetoshi Kubo, Atsushi Takada, Hiroyuki Sekiya, Atsushi Takeda, Kentaro Miuchi

Abstract: NEWAGE is a direction-sensitive dark matter search using a low-pressure gaseous time projection chamber. A low alpha-ray emission rate micro pixel chamber had been developed in order to reduce background for dark matter search. We conducted the dark matter search at the Kamioka Observatory in 2018. The total live time was 107.6 days corresponding to an exposure of 1.1 kg${\cdot}$days. Two events r… ▽ More NEWAGE is a direction-sensitive dark matter search using a low-pressure gaseous time projection chamber. A low alpha-ray emission rate micro pixel chamber had been developed in order to reduce background for dark matter search. We conducted the dark matter search at the Kamioka Observatory in 2018. The total live time was 107.6 days corresponding to an exposure of 1.1 kg${\cdot}$days. Two events remained in the energy region of 50-60 keV which was consistent with 2.5 events of the expected background. A directional analysis was carried out and no significant forward-backward asymmetry derived from the WIMP-nucleus elastic scatterings was found. Thus a 90% confidence level upper limit on Spin-Dependent WIMP-proton cross section of 50 pb for a WIMP mass of 100 GeV/c2 was derived. This limit is the most stringent yet obtained from direction-sensitive dark matter search experiments. △ Less

Submitted 7 April, 2021; v1 submitted 25 January, 2021; originally announced January 2021.

Journal ref: Prog. Theor. Exp. Phys. 2021, 063F01

Supernova Model Discrimination with Hyper-Kamiokande

Authors: Hyper-Kamiokande Collaboration, :, K. Abe, P. Adrich, H. Aihara, R. Akutsu, I. Alekseev, A. Ali, F. Ameli, I. Anghel, L. H. V. Anthony, M. Antonova, A. Araya, Y. Asaoka, Y. Ashida, V. Aushev, F. Ballester, I. Bandac, M. Barbi, G. J. Barker, G. Barr, M. Batkiewicz-Kwasniak, M. Bellato, V. Berardi, M. Bergevin , et al. (478 additional authors not shown)

Abstract: Core-collapse supernovae are among the most magnificent events in the observable universe. They produce many of the chemical elements necessary for life to exist and their remnants -- neutron stars and black holes -- are interesting astrophysical objects in their own right. However, despite millennia of observations and almost a century of astrophysical study, the explosion mechanism of core-colla… ▽ More Core-collapse supernovae are among the most magnificent events in the observable universe. They produce many of the chemical elements necessary for life to exist and their remnants -- neutron stars and black holes -- are interesting astrophysical objects in their own right. However, despite millennia of observations and almost a century of astrophysical study, the explosion mechanism of core-collapse supernovae is not yet well understood. Hyper-Kamiokande is a next-generation neutrino detector that will be able to observe the neutrino flux from the next galactic core-collapse supernova in unprecedented detail. We focus on the first 500 ms of the neutrino burst, corresponding to the accretion phase, and use a newly-developed, high-precision supernova event generator to simulate Hyper-Kamiokande's response to five different supernova models. We show that Hyper-Kamiokande will be able to distinguish between these models with high accuracy for a supernova at a distance of up to 100 kpc. Once the next galactic supernova happens, this ability will be a powerful tool for guiding simulations towards a precise reproduction of the explosion mechanism observed in nature. △ Less

Submitted 20 July, 2021; v1 submitted 13 January, 2021; originally announced January 2021.

Comments: 21 pages, 7 figures. Article based on thesis published as arXiv:2002.01649. v2: added references and some explanations in response to reviewer comments

Journal ref: Astrophys.J. 916 (2021) 15

Search for solar electron anti-neutrinos due to spin-flavor precession in the Sun with Super-Kamiokande-IV

Authors: Super-Kamiokande Collaboration, :, K. Abe, C. Bronner, Y. Hayato, M. Ikeda, S. Imaizumi, H. Ito, J. Kameda, Y. Kataoka, M. Miura, S. Moriyama, Y. Nagao, M. Nakahata, Y. Nakajima, S. Nakayama, T. Okada, K. Okamoto, A. Orii, G. Pronost, H. Sekiya, M. Shiozawa, Y. Sonoda, Y. Suzuki, A. Takeda , et al. (177 additional authors not shown)

Abstract: Due to a very low production rate of electron anti-neutrinos ($\barν_e$) via nuclear fusion in the Sun, a flux of solar $\barν_e$ is unexpected. An appearance of $\barν_e$ in solar neutrino flux opens a new window for the new physics beyond the standard model. In particular, a spin-flavor precession process is expected to convert an electron neutrino into an electron anti-neutrino (… ▽ More Due to a very low production rate of electron anti-neutrinos ($\barν_e$) via nuclear fusion in the Sun, a flux of solar $\barν_e$ is unexpected. An appearance of $\barν_e$ in solar neutrino flux opens a new window for the new physics beyond the standard model. In particular, a spin-flavor precession process is expected to convert an electron neutrino into an electron anti-neutrino (${ν_e\to\barν_e}$) when neutrino has a finite magnetic moment. In this work, we have searched for solar $\barν_e$ in the Super-Kamiokande experiment, using neutron tagging to identify their inverse beta decay signature. We identified 78 $\barν_e$ candidates for neutrino energies of 9.3 to 17.3 MeV in 2970.1 live days with a fiducial volume of 22.5 kiloton water (183.0 kton$\cdot$year exposure). The energy spectrum has been consistent with background predictions and we thus derived a 90% confidence level upper limit of ${4.7\times10^{-4}}$ on the $ν_e\to\barν_e$ conversion probability in the Sun. We used this result to evaluate the sensitivity of future experiments, notably the Super-Kamiokande Gadolinium (SK-Gd) upgrade. △ Less

Submitted 17 March, 2022; v1 submitted 7 December, 2020; originally announced December 2020.

Comments: 16 pages, 10 figures, 7 tables

Journal ref: Astroparticle Physics 139 (2022) 102702

The Hyper-Kamiokande Experiment -- Snowmass LOI

Authors: Hyper-Kamiokande Collaboration, :, K. Abe, P. Adrich, H. Aihara, R. Akutsu, I. Alekseev, A. Ali, F. Ameli, L. H. V. Anthony, A. Araya, Y. Asaoka, V. Aushev, I. Bandac, M. Barbi, G. Barr, M. Batkiewicz-Kwasniak, M. Bellato, V. Berardi, L. Bernard, E. Bernardini, L. Berns, S. Bhadra, J. Bian, A. Blanchet , et al. (366 additional authors not shown)

Abstract: Hyper-Kamiokande is the next generation underground water Cherenkov detector that builds on the highly successful Super-Kamiokande experiment. The detector which has an 8.4~times larger effective volume than its predecessor will be located along the T2K neutrino beamline and utilize an upgraded J-PARC beam with 2.6~times beam power. Hyper-K's low energy threshold combined with the very large fiduc… ▽ More Hyper-Kamiokande is the next generation underground water Cherenkov detector that builds on the highly successful Super-Kamiokande experiment. The detector which has an 8.4~times larger effective volume than its predecessor will be located along the T2K neutrino beamline and utilize an upgraded J-PARC beam with 2.6~times beam power. Hyper-K's low energy threshold combined with the very large fiducial volume make the detector unique, that is expected to acquire an unprecedented exposure of 3.8~Mton$\cdot$year over a period of 20~years of operation. Hyper-Kamiokande combines an extremely diverse science program including nucleon decays, long-baseline neutrino oscillations, atmospheric neutrinos, and neutrinos from astrophysical origins. The scientific scope of this program is highly complementary to liquid-argon detectors for example in sensitivity to nucleon decay channels or supernova detection modes. Hyper-Kamiokande construction has started in early 2020 and the experiment is expected to start operations in 2027. The Hyper-Kamiokande collaboration is presently being formed amongst groups from 19 countries including the United States, whose community has a long history of making significant contributions to the neutrino physics program in Japan. US physicists have played leading roles in the Kamiokande, Super-Kamiokande, EGADS, K2K, and T2K programs. △ Less

Submitted 1 September, 2020; originally announced September 2020.

Comments: 6 pages, prepared as Snowmass2021 LOI

Search for event bursts in XMASS-I associated with gravitational-wave events

Authors: XMASS Collaboration, K. Abe, K. Hiraide, K. Ichimura, Y. Kishimoto, K. Kobayashi, M. Kobayashi, S. Moriyama, M. Nakahata, H. Ogawa, K. Sato, H. Sekiya, T. Suzuki, A. Takeda, S. Tasaka, M. Yamashita, B. S. Yang, N. Y. Kim, Y. D. Kim, Y. Itow, K. Kanzawa, K. Masuda, K. Martens, Y. Suzuki, B. D. Xu , et al. (12 additional authors not shown)

Abstract: We performed a search for event bursts in the XMASS-I detector associated with 11 gravitational-wave events detected during LIGO/Virgo's O1 and O2 periods. Simple and loose cuts were applied to the data collected in the full 832 kg xenon volume around the detection time of each gravitational-wave event. The data were divided into four energy regions ranging from keV to MeV. Without assuming any pa… ▽ More We performed a search for event bursts in the XMASS-I detector associated with 11 gravitational-wave events detected during LIGO/Virgo's O1 and O2 periods. Simple and loose cuts were applied to the data collected in the full 832 kg xenon volume around the detection time of each gravitational-wave event. The data were divided into four energy regions ranging from keV to MeV. Without assuming any particular burst models, we looked for event bursts in sliding windows with various time width from 0.02 to 10 s. The search was conducted in a time window between $-$400 and $+$10,000 s from each gravitational-wave event. For the binary neutron star merger GW170817, no significant event burst was observed in the XMASS-I detector and we set 90% confidence level upper limits on neutrino fluence for the sum of all the neutrino flavors via coherent elastic neutrino-nucleus scattering. The obtained upper limit was (1.3-2.1)$\times 10^{11}$ cm$^{-2}$ under the assumption of a Fermi-Dirac spectrum with average neutrino energy of 20 MeV. The neutrino fluence limits for mono-energetic neutrinos in the energy range between 14 and 100 MeV were also calculated. Among the other 10 gravitational wave events detected as the binary black hole mergers, a burst candidate with a 3.0$σ$ significance was found at 1801.95-1803.95 s in the analysis for GW151012. However, no significant deviation from the background in the reconstructed energy and position distributions was found. Considering the additional look-elsewhere effect of analyzing the 11 GW events, the significance of finding such a burst candidate associated with any of them is 2.1$σ$. △ Less

Submitted 30 December, 2020; v1 submitted 29 July, 2020; originally announced July 2020.

Comments: 8 pages, 12 figures

Improved method for measuring low concentration radium and its application to the Super-Kamiokande Gadolinium project

Authors: S. Ito, K. Ichimura, Y. Takaku, K. Abe, M. Harada, M. Ikeda, H. Ito, Y. Kishimoto, Y. Nakajima, T. Okada, H. Sekiya

Abstract: Chemical extraction using a molecular recognition resin named "Empore Radium Rad Disk" was developed to improve sensitivity for the low concentration of radium (Ra). Compared with the previous method, the extraction process speed was improved by a factor of three and the recovery rate for $^{226}$Ra was also improved from 81$\pm$4% to $>$99.9%. The sensitivity on the 10$^{-1}$ mBq level was achiev… ▽ More Chemical extraction using a molecular recognition resin named "Empore Radium Rad Disk" was developed to improve sensitivity for the low concentration of radium (Ra). Compared with the previous method, the extraction process speed was improved by a factor of three and the recovery rate for $^{226}$Ra was also improved from 81$\pm$4% to $>$99.9%. The sensitivity on the 10$^{-1}$ mBq level was achieved using a high purity germanium detector. This improved method was applied to determine $^{226}$Ra in Gd$_2$(SO$_4$)$_3{\cdot}$8H$_2$O which will be used in the Super-Kamiokande Gadolinium project. The improvement and measurement results are reported in this paper. △ Less

Submitted 17 June, 2020; originally announced June 2020.

Comments: 8 pages, 3 figures

Journal ref: Progress of Theoretical and Experimental Physics, Volume 2020, Issue 9, September 2020, 093H02

Development of low-background photomultiplier tubes for liquid xenon detectors

Authors: XMASS Collaboration, K. Abe, Y. Chen, K. Hiraide, K. Ichimura, S. Imaizumi, N. Kato, Y. Kishimoto, K. Kobayashi, M. Kobayashi, S. Moriyama, M. Nakahata, K. Sato, H. Sekiya, T. Suzuki, A. Takeda, S. Tasaka, M. Yamashita, B. S. Yang, N. Y. Kim, Y. D. Kim, Y. H. Kim, R. Ishii, Y. Itow, K. Kanzawa , et al. (14 additional authors not shown)

Abstract: We successfully developed a new photomultiplier tube (PMT) with a three-inch diameter, convex-shaped photocathode, R13111. Its prominent features include good performance and ultra-low radioactivity. The convex-shaped photocathode realized a large photon acceptance and good timing resolution. Low radioactivity was achieved by three factors: (1) the glass material was synthesized using low-radioact… ▽ More We successfully developed a new photomultiplier tube (PMT) with a three-inch diameter, convex-shaped photocathode, R13111. Its prominent features include good performance and ultra-low radioactivity. The convex-shaped photocathode realized a large photon acceptance and good timing resolution. Low radioactivity was achieved by three factors: (1) the glass material was synthesized using low-radioactive-contamination material; (2) the photocathode was produced with $^{39}$K-enriched potassium; and (3) the purest grade of aluminum material was used for the vacuum seal. As a result each R13111 PMT contains only about 0.4 mBq of $^{226}$Ra, less than 2 mBq of $^{238}$U, 0.3 mBq of $^{228}$Ra, 2 mBq of $^{40}$K and 0.2 mBq of $^{60}$Co. We also examined and resolved the intrinsic leakage of Xe gas into PMTs that was observed in several older models. We thus succeeded in developing a PMT that has low background, large angular acceptance with high collection efficiency, good timing resolution, and long-term stable operation. These features are highly desirable for experiments searching for rare events beyond the standard model, such as dark matter particle interactions and neutrinoless double beta decay events. △ Less

Submitted 18 August, 2020; v1 submitted 1 June, 2020; originally announced June 2020.

First limits from a 3d-vector directional dark matter search with the NEWAGE-0.3b' detector

Authors: Ryota Yakabe, Kiseki Nakamura, Tomonori Ikeda, Hiroshi Ito, Yushiro Yamaguchi, Ryosuke Taishaku, Miki Nakazawa, Hirohisa Ishiura, Takuma Nakamura, Takuya Shimada, Toru Tanimori, Hidetoshi Kubo, Atsushi Takada, Hiroyuki Sekiya, Atsushi Takeda, Kentaro Miuchi

Abstract: The first directional dark matter search with three-dimensional tracking with head-tail sensitivity (3d-vector tracking analysis) was performed with a gaseous three-dimensional tarcking detector, or the NEWAGE-0.3b' detector. The search was carried out from July 2013 to August 2017 (Run14 to Run18) at the Kamioka underground laboratory. The total livetime is 434.85 days corresponding to an exposur… ▽ More The first directional dark matter search with three-dimensional tracking with head-tail sensitivity (3d-vector tracking analysis) was performed with a gaseous three-dimensional tarcking detector, or the NEWAGE-0.3b' detector. The search was carried out from July 2013 to August 2017 (Run14 to Run18) at the Kamioka underground laboratory. The total livetime is 434.85 days corresponding to an exposure of 4.51 kg$\cdot$days. A 90 % confidence level upper limit on spin-dependent WIMP-proton cross section of $4.3 \times10^{2}$ pb for WIMPs with the mass of 150 GeV/$c^2$ is obtained. △ Less

Submitted 18 September, 2020; v1 submitted 11 May, 2020; originally announced May 2020.

Evaluation of radon adsorption efficiency values in xenon with activated carbon fibers

Authors: Y. Nakano, K. Ichimura, H. Ito, T. Okada, H. Sekiya, Y. Takeuchi, S. Tasaka, M. Yamashita

Abstract: The radioactive noble gas radon-222 ($\mathrm{^{222}Rn}$, or Rn) produced in the uranium series is a crucial background source in many underground experiments. We have estimated the adsorption property of Rn with Activated Carbon Fibers (ACFs) in air (Air), argon (Ar), and xenon (Xe) gas. In this study, we evaluated six ACFs, named A-7, A-10, A-15, A-20, A-25, and S-25, provided from UNITIKA Ltd.… ▽ More The radioactive noble gas radon-222 ($\mathrm{^{222}Rn}$, or Rn) produced in the uranium series is a crucial background source in many underground experiments. We have estimated the adsorption property of Rn with Activated Carbon Fibers (ACFs) in air (Air), argon (Ar), and xenon (Xe) gas. In this study, we evaluated six ACFs, named A-7, A-10, A-15, A-20, A-25, and S-25, provided from UNITIKA Ltd. We measured intrinsic radioactivity of these ACF samples, and found A-20's radioactivity of the uranium series is $<5.5$~$\mathrm{mBq/kg}$ with $90\%$ confidence level. In Air and Ar gas, we found ACF A-15 has the adsorption efficiency of $1/10000$ reduction at maximum before saturation of Rn adsorption, and more than $97\%$ adsorption efficiency after the saturation. In Xe gas, we found ACF A-20 has the best Rn adsorption ability among tested ACFs. We also found S-25, A-25, and A-15 have similar Rn adsorption performance. △ Less

Submitted 6 August, 2020; v1 submitted 25 March, 2020; originally announced March 2020.

Comments: 17 pages, 9 figures

Journal ref: PTEP 2020, 113H01 (2020)

Design and performance of a high-pressure xenon gas TPC as a prototype for a large-scale neutrinoless double-beta decay search

Authors: S. Ban, M. Hirose, A. K. Ichikawa, Y. Iwashita, T. Kikawa, A. Minamino, K. Miuchi, T. Nakadaira, Y. Nakajima, K. D. Nakamura, K. Z. Nakamura, T. Nakaya, S. Obara, K. Sakashita, H. Sekiya, B. Sugashima, S. Tanaka, K. Ueshima, M. Yoshida

Abstract: A high-pressure xenon gas time projection chamber, with a unique cellular readout structure based on electroluminescence, has been developed for a large-scale neutrinoless double-beta decay search. In order to evaluate the detector performance and validate its design, a 180~L size prototype is being constructed and its commissioning with partial detector has been performed. The obtained energy res… ▽ More A high-pressure xenon gas time projection chamber, with a unique cellular readout structure based on electroluminescence, has been developed for a large-scale neutrinoless double-beta decay search. In order to evaluate the detector performance and validate its design, a 180~L size prototype is being constructed and its commissioning with partial detector has been performed. The obtained energy resolution at 4.0~bar is 1.73 $\pm$ 0.07% (FWHM) at 511 keV. The energy resolution at the $^{136}$Xe neutrinoless double-beta decay Q-value is estimated to be between 0.79 and 1.52% (FWHM) by extrapolation. Reconstructed event topologies show patterns peculiar to track end-point which can be used to distinguish $0νββ$ signals from gamma-ray backgrounds. △ Less

Submitted 19 February, 2020; v1 submitted 9 January, 2020; originally announced January 2020.

Comments: 24 pages, 25 figures, 1 table. Preprint paper for PTEP

Journal ref: Progress of Theoretical and Experimental Physics, Volume 2020 Issue 3, 033H01

Anisotropic Response Measurements of ZnWO$_4$ Scintillators to Neutrons for Developing the Direction-Sensitive Dark Matter Detector

Authors: Juan W. Pedersen, Hiroyuki Sekiya, Koichi Ichimura

Abstract: The scintillation yields of ZnWO$_4$ crystals change depending on the incident direction of particles. This property can be used as a direction-sensitive dark matter detector. So, we investigated the ZnWO$_4$ light yields ratio of neutron-induced nuclear recoils to gamma-ray induced electron recoils(quenching factor). Two surfaces almost perpendicular to the crystal axis of ZnWO$_4$ were irradiate… ▽ More The scintillation yields of ZnWO$_4$ crystals change depending on the incident direction of particles. This property can be used as a direction-sensitive dark matter detector. So, we investigated the ZnWO$_4$ light yields ratio of neutron-induced nuclear recoils to gamma-ray induced electron recoils(quenching factor). Two surfaces almost perpendicular to the crystal axis of ZnWO$_4$ were irradiated with a quasi-monochromatic neutron beam of 0.885MeV, and the quenching factors of both surfaces for the oxygen-nucleus recoil in the ZnWO$_4$ crystal were measured. The obtained quenching factors of the two surfaces were 0.235 $\pm$ 0.026 and 0.199 $\pm$ 0.020, respectively confirming 15.3$\%$ of anisotropy. △ Less

Submitted 7 January, 2020; v1 submitted 20 December, 2019; originally announced December 2019.

Measurement of the radon concentration in purified water in the Super-Kamiokande IV detector

Authors: Y. Nakano, T. Hokama, M. Matsubara, M. Miwa, M. Nakahata, T. Nakamura, H. Sekiya, Y. Takeuchi, S. Tasaka, R. A. Wendell

Abstract: The radioactive noble gas radon can be a serious background source in the underground particle physics experiments studying processes that deposit energy comparable to its decay products. Low energy solar neutrino measurements at Super-Kamiokande suffer from these backgrounds and therefore require precise characterization of the radon concentration in the detector's ultra-pure water. For this purp… ▽ More The radioactive noble gas radon can be a serious background source in the underground particle physics experiments studying processes that deposit energy comparable to its decay products. Low energy solar neutrino measurements at Super-Kamiokande suffer from these backgrounds and therefore require precise characterization of the radon concentration in the detector's ultra-pure water. For this purpose, we have developed a measurement system consisting of a radon extraction column, a charcoal trap, and a radon detector. In this article we discuss the design, calibration, and performance of the radon extraction column. We also describe the design of the measurement system and evaluate its performance, including its background. Using this system we measured the radon concentration in Super-Kamiokande's water between May 2014 and October 2015. The measured radon concentration in the supply lines of the water circulation system was $1.74\pm0.14~\mathrm{mBq/m^{3}}$ and in the return line was $9.06\pm0.58~\mathrm{mBq/m^{3}}$. Water sampled from the center region of the detector itself had a concentration of $<0.23~\mathrm{mBq/m^{3}}$ ($95\%$ C.L.) and water sampled from the bottom region of the detector had a concentration of $2.63\pm0.22~\mathrm{mBq/m^{3}}$. △ Less

Submitted 26 June, 2020; v1 submitted 9 October, 2019; originally announced October 2019.

Comments: 20 pages, 10 figures

Journal ref: Nucl. Instrum. Meth. A 977, 164297 (2020)

AXEL: High-pressure Xe gas TPC for BG-free $0\nu2β$ decay search

Authors: S. Obara, S. Ban, M. Hirose, A. K. Ichikawa, T. Kikawa, K. Z. Nakamura, T. Nakaya, S. Tanaka, M. Yoshida, Y. Iwashita, H. Sekiya, Y. Nakajima, K. Ueshima, K. Miuchi, K. D. Nakamura, A. Minamino, T. Nakadaira, K. Sakashita

Abstract: AXEL is a high-pressure xenon gas time projection chamber for neutrinoless double-beta decay (0n2b) search. The AXEL has a unique readout system called ELCC which has a cellular structure and photosensors to detect electroluminescence light produced by ionization electrons. We demonstrated the performance of the ELCC with a small prototype detector (AXEL-HP10L). The obtained energy resolution corr… ▽ More AXEL is a high-pressure xenon gas time projection chamber for neutrinoless double-beta decay (0n2b) search. The AXEL has a unique readout system called ELCC which has a cellular structure and photosensors to detect electroluminescence light produced by ionization electrons. We demonstrated the performance of the ELCC with a small prototype detector (AXEL-HP10L). The obtained energy resolution corresponds to 0.82 -- 1.74% (FWHM) at the 0n2b Q-value. We are constructing a new prototype (AXEL-HP180L) in order to study the energy resolution at the Q-value of 0n2b with a new design of ELCC with unit structure, newly developed electronics board, field-shaping electrodes, and Cockcroft-Walton-type high voltage power supply. For a future 1-ton scale large AXEL detector, we are developing new background-reduction techniques; topology identification with machine learning, positive-ion detection, and active-shield options. △ Less

Submitted 30 September, 2019; v1 submitted 20 September, 2019; originally announced September 2019.

Comments: 10 pages, 6 figures, Proceedings of the 15th Vienna Conference on Instrumentation (VCI2019)

Evaluation of Gadolinium's Action on Water Cherenkov Detector Systems with EGADS

Authors: Ll. Marti, M. Ikeda, Y. Kato, Y. Kishimoto, M. Nakahata, Y. Nakajima, Y. Nakano, S. Nakayama, Y. Okajima, A. Orii, G. Pronost, H. Sekiya, M. Shiozawa, H. Tanaka, K. Ueno, S. Yamada, T. Yano, T. Yokozawa, M. Murdoch, J. Schuemann, M. R. Vagins, K. Bays, G. Carminati, N. J. Griskevich, W. R. Kropp , et al. (43 additional authors not shown)

Abstract: Used for both proton decay searches and neutrino physics, large water Cherenkov (WC) detectors have been very successful tools in particle physics. They are notable for their large masses and charged particle detection capabilities. While current WC detectors reconstruct charged particle tracks over a wide energy range, they cannot efficiently detect neutrons. Gadolinium (Gd) has the largest therm… ▽ More Used for both proton decay searches and neutrino physics, large water Cherenkov (WC) detectors have been very successful tools in particle physics. They are notable for their large masses and charged particle detection capabilities. While current WC detectors reconstruct charged particle tracks over a wide energy range, they cannot efficiently detect neutrons. Gadolinium (Gd) has the largest thermal neutron capture cross section of all stable nuclei and produces an 8 MeV gamma cascade that can be detected with high efficiency. Because of the many new physics opportunities that neutron tagging with a Gd salt dissolved in water would open up, a large-scale R&D program called EGADS was established to demonstrate this technique's feasibility. EGADS features all the components of a WC detector, chiefly a 200-ton stainless steel water tank furnished with 240 photo-detectors, DAQ, and a water system that removes all impurities in water while keeping Gd in solution. In this paper we discuss the milestones towards demonstrating the feasibility of this novel technique, and the features of EGADS in detail. △ Less

Submitted 26 February, 2020; v1 submitted 30 August, 2019; originally announced August 2019.

Comments: 25 pages, 11 figures, 1 table

J-PARC Neutrino Beamline Upgrade Technical Design Report

Authors: K. Abe, H. Aihara, A. Ajmi, C. Alt, C. Andreopoulos, M. Antonova, S. Aoki, Y. Asada, Y. Ashida, A. Atherton, E. Atkin, S. Ban, F. C. T. Barbato, M. Barbi, G. J. Barker, G. Barr, M. Batkiewicz, A. Beloshapkin, V. Berardi, L. Berns, S. Bhadra, J. Bian, S. Bienstock, A. Blondel, S. Bolognesi , et al. (360 additional authors not shown)

Abstract: In this document, technical details of the upgrade plan of the J-PARC neutrino beamline for the extension of the T2K experiment are described. T2K has proposed to accumulate data corresponding to $2\times{}10^{22}$ protons-on-target in the next decade, aiming at an initial observation of CP violation with $3σ$ or higher significance in the case of maximal CP violation. Methods to increase the neut… ▽ More In this document, technical details of the upgrade plan of the J-PARC neutrino beamline for the extension of the T2K experiment are described. T2K has proposed to accumulate data corresponding to $2\times{}10^{22}$ protons-on-target in the next decade, aiming at an initial observation of CP violation with $3σ$ or higher significance in the case of maximal CP violation. Methods to increase the neutrino beam intensity, which are necessary to achieve the proposed data increase, are described. △ Less

Submitted 14 August, 2019; originally announced August 2019.

Development of a method for measuring rare earth elements in the environment for future experiments with gadolinium loaded detectors

Authors: S. Ito, T. Okada, Y. Takaku, M. Harada, M. Ikdeda, Y. Kishimoto, Y. Koshio, M. Nakahata, Y. Nakajima, H. Sekiya

Abstract: Demand to use gadolinium (Gd) in detectors is increasing in the field of elementary particle physics, especially neutrino measurements and dark matter searches. Large amounts of Gd are used in these experiments. Therefore, to access the impacts of Gd onto the environments, it is becoming important to measure the baseline concentrations of Gd in the environments. The measurement of the baseline con… ▽ More Demand to use gadolinium (Gd) in detectors is increasing in the field of elementary particle physics, especially neutrino measurements and dark matter searches. Large amounts of Gd are used in these experiments. Therefore, to access the impacts of Gd onto the environments, it is becoming important to measure the baseline concentrations of Gd in the environments. The measurement of the baseline concentrations, however, is not easy due to interferences by other elements. In this paper, a method for measuring the concentrations of rare earth elements including Gd is proposed. In the method, an inductively coupled plasma-mass spectrometry is utilized after collecting the dissolved elements in chelating resin. Results of the ability to detect anomalous concentrations of rare earth elements in river water samples in the Kamioka and Toyama areas are also reported. △ Less

Submitted 26 May, 2019; originally announced May 2019.

Comments: 10 pages, 5 figures

Journal ref: Progress of Theoretical and Experimental Physics, Volume 2019, Issue 6, June 2019, 063H03

Measurement of the $ν_μ$ charged-current cross sections on water, hydrocarbon, iron, and their ratios with the T2K on-axis detectors

Authors: K. Abe, R. Akutsu, A. Ali, C. Andreopoulos, L. Anthony, M. Antonova, S. Aoki, A. Ariga, Y. Ashida, Y. Awataguchi, Y. Azuma, S. Ban, M. Barbi, G. J. Barker, G. Barr, C. Barry, M. Batkiewicz-Kwasniak, F. Bench, V. Berardi, S. Berkman, R. M. Berner, L. Berns, S. Bhadra, S. Bienstock, A. Blondely , et al. (292 additional authors not shown)

Abstract: We report a measurement of the flux-integrated $ν_μ$ charged-current cross sections on water, hydrocarbon, and iron in the T2K on-axis neutrino beam with a mean neutrino energy of 1.5 GeV. The measured cross sections on water, hydrocarbon, and iron are $σ^{\rm{H_{2}O}}_{\rm{CC}}$ = (0.840$\pm 0.010$(stat.)$^{+0.10}_{-0.08}$(syst.))$\times$10$^{-38}$cm$^2$/nucleon, $σ^{\rm{CH}}_{\rm{CC}}$ = (0.817… ▽ More We report a measurement of the flux-integrated $ν_μ$ charged-current cross sections on water, hydrocarbon, and iron in the T2K on-axis neutrino beam with a mean neutrino energy of 1.5 GeV. The measured cross sections on water, hydrocarbon, and iron are $σ^{\rm{H_{2}O}}_{\rm{CC}}$ = (0.840$\pm 0.010$(stat.)$^{+0.10}_{-0.08}$(syst.))$\times$10$^{-38}$cm$^2$/nucleon, $σ^{\rm{CH}}_{\rm{CC}}$ = (0.817$\pm 0.007$(stat.)$^{+0.11}_{-0.08}$(syst.))$\times$10$^{-38}$cm$^2$/nucleon, and $σ^{\rm{Fe}}_{\rm{CC}}$ = (0.859$\pm 0.003$(stat.) $^{+0.12}_{-0.10}$(syst.))$\times$10$^{-38}$cm$^2$/nucleon respectively, for a restricted phase space of induced muons: $θ_μ<45^{\circ}$ and $p_μ>$0.4 GeV/$c$ in the laboratory frame. The measured cross section ratios are ${σ^{\rm{H_{2}O}}_{\rm{CC}}}/{σ^{\rm{CH}}_{\rm{CC}}}$ = 1.028$\pm 0.016$(stat.)$\pm 0.053$(syst.), ${σ^{\rm{Fe}}_{\rm{CC}}}/{σ^{\rm{H_{2}O}}_{\rm{CC}}}$ = 1.023$\pm 0.012$(stat.)$\pm 0.058$(syst.), and ${σ^{\rm{Fe}}_{\rm{CC}}}/{σ^{\rm{CH}}_{\rm{CC}}}$ = 1.049$\pm 0.010$(stat.)$\pm 0.043$(syst.). These results, with an unprecedented precision for the measurements of neutrino cross sections on water in the studied energy region, show good agreement with the current neutrino interaction models used in the T2K oscillation analyses. △ Less

Submitted 21 April, 2019; originally announced April 2019.

Journal ref: Prog Theor Exp Phys (2019)

T2K ND280 Upgrade -- Technical Design Report

Authors: K. Abe, H. Aihara, A. Ajmi, C. Andreopoulos, M. Antonova, S. Aoki, Y. Asada, Y. Ashida, A. Atherton, E. Atkin, D. Attié, S. Ban, M. Barbi, G. J. Barker, G. Barr, M. Batkiewicz, A. Beloshapkin, V. Berardi, L. Berns, S. Bhadra, J. Bian, S. Bienstock, A. Blondel, J. Boix, S. Bolognesi , et al. (359 additional authors not shown)

Abstract: In this document, we present the Technical Design Report of the Upgrade of the T2K Near Detector ND280. The goal of this upgrade is to improve the Near Detector performance to measure the neutrino interaction rate and to constrain the neutrino interaction cross-sections so that the uncertainty in the number of predicted events at Super-Kamiokande is reduced to about 4%. This will allow to improve… ▽ More In this document, we present the Technical Design Report of the Upgrade of the T2K Near Detector ND280. The goal of this upgrade is to improve the Near Detector performance to measure the neutrino interaction rate and to constrain the neutrino interaction cross-sections so that the uncertainty in the number of predicted events at Super-Kamiokande is reduced to about 4%. This will allow to improve the physics reach of the T2K-II project. This goal is achieved by modifying the upstream part of the detector, adding a new highly granular scintillator detector (Super-FGD), two new TPCs (High-Angle TPC) and six TOF planes. Details about the detector concepts, design and construction methods are presented, as well as a first look at the test-beam data taken in Summer 2018. An update of the physics studies is also presented. △ Less

Submitted 14 October, 2020; v1 submitted 11 January, 2019; originally announced January 2019.

Comments: 196 pages

Report number: CERN-SPSC-2019-001 (SPSC-TDR-006)

A measurement of the scintillation decay time constant of nuclear recoils in liquid xenon with the XMASS-I detector

Authors: XMASS Collaboration, K. Abe, K. Hiraide, K. Ichimura, Y. Kishimoto, K. Kobayashi, M. Kobayashi, S. Moriyama, M. Nakahata, H. Ogawa, K. Sato, H. Sekiya, T. Suzuki, O. Takachio, A. Takeda, S. Tasaka, M. Yamashita, B. S. Yang, N. Y. Kim, Y. D. Kim, Y. Itow, K. Kanzawa, K. Masuda, K. Martens, Y. Suzuki , et al. (13 additional authors not shown)

Abstract: We report an in-situ measurement of the nuclear recoil (NR) scintillation decay time constant in liquid xenon (LXe) using the XMASS-I detector at the Kamioka underground laboratory in Japan. XMASS-I is a large single-phase LXe scintillation detector whose purpose is the direct detection of dark matter via NR which can be induced by collisions between Weakly Interacting Massive Particles (WIMPs) an… ▽ More We report an in-situ measurement of the nuclear recoil (NR) scintillation decay time constant in liquid xenon (LXe) using the XMASS-I detector at the Kamioka underground laboratory in Japan. XMASS-I is a large single-phase LXe scintillation detector whose purpose is the direct detection of dark matter via NR which can be induced by collisions between Weakly Interacting Massive Particles (WIMPs) and a xenon nucleus. The inner detector volume contains 832 kg of LXe. $^{252}$Cf was used as an external neutron source for irradiating the detector. The scintillation decay time constant of the resulting neutron induced NR was evaluated by comparing the observed photon detection times with Monte Carlo simulations. Fits to the decay time prefer two decay time components, one for each of the Xe$_{2}^{*}$ singlet and triplet states, with $τ_{S}$ = 4.3$\pm$0.6 ns taken from prior research, $τ_{T}$ was measured to be 26.9$^{+0.7}_{-1.1}$ ns with a singlet state fraction F$_{S}$ of 0.252$^{+0.027}_{-0.019}$.We also evaluated the performance of pulse shape discrimination between NR and electron recoil (ER) with the aim of reducing the electromagnetic background in WIMP searches. For a 50\% NR acceptance, the ER acceptance was 13.7${\pm}$1.0\% and 4.1${\pm}$0.7\% in the energy ranges of 5--10 keV$_{\rm ee}$ and 10--15 keV$_{\rm ee}$, respectively. △ Less

Submitted 24 December, 2018; v1 submitted 16 September, 2018; originally announced September 2018.

Comments: 18 pages, 10 figures

Journal ref: JINST,13, P12032 (2018)

Search for sub-GeV dark matter by annual modulation using XMASS-I detector

Authors: M. Kobayashi, K. Abe, K. Hiraide, K. Ichimura, Y. Kishimoto, K. Kobayashi, S. Moriyama, M. Nakahata, H. Ogawa, K. Sato, H. Sekiya, T. Suzuki, A. Takeda, S. Tasaka, M. Yamashita, B. S. Yang, N. Y. Kim, Y. D. Kim, Y. Itow, K. Kanzawa, K. Masuda, K. Martens, Y. Suzuki, B. D. Xu, K. Miuchi , et al. (11 additional authors not shown)

Abstract: A search for dark matter (DM) with mass in the sub-GeV region (0.32-1 GeV) was conducted by looking for an annual modulation signal in XMASS, a single-phase liquid xenon detector. Inelastic nuclear scattering accompanied by bremsstrahlung emission was used to search down to an electron equivalent energy of 1 keV. The data used had a live time of 2.8 years (3.5 years in calendar time), resulting in… ▽ More A search for dark matter (DM) with mass in the sub-GeV region (0.32-1 GeV) was conducted by looking for an annual modulation signal in XMASS, a single-phase liquid xenon detector. Inelastic nuclear scattering accompanied by bremsstrahlung emission was used to search down to an electron equivalent energy of 1 keV. The data used had a live time of 2.8 years (3.5 years in calendar time), resulting in a total exposure of 2.38 ton-years. No significant modulation signal was observed and 90% confidence level upper limits of $1.6 \times 10^{-33}$ cm$^2$ at 0.5 GeV was set for the DM-nucleon cross section. This is the first experimental result of a search for DM mediated by the bremsstrahlung effect. In addition, a search for DM with mass in the multi-GeV region (4-20 GeV) was conducted with a lower energy threshold than previous analysis of XMASS. Elastic nuclear scattering was used to search down to a nuclear recoil equivalent energy of 2.3 keV, and upper limits of 2.9 $\times$10$^{-42}$ cm$^2$ at 8 GeV was obtained. △ Less

Submitted 22 December, 2018; v1 submitted 19 August, 2018; originally announced August 2018.

Comments: 7 pages, 5 figures

Development of low radioactivity photomultiplier tubes for the XMASS-I detector

Authors: XMASS Collaboration, K. Abe, K. Hiraide, K. Ichimura, Y. Kishimoto, K. Kobayashi, M. Kobayashi, S. Moriyama, M. Nakahata, T. Norita, H. Ogawa, K. Sato, H. Sekiya, O. Takachio, A. Takeda, S. Tasaka, M. Yamashita, B. S. Yang, N. Y. Kim, Y. D. Kim, Y. Itow, K. Kanzawa, R. Kegasa, K. Masuda, H. Takiya , et al. (17 additional authors not shown)

Abstract: XMASS-I is a single-phase liquid xenon detector whose purpose is direct detection of dark matter. To achieve the low background requirements necessary in the detector, a new model of photomultiplier tubes (PMTs), R10789, with a hexagonal window was developed based on the R8778 PMT used in the XMASS prototype detector. We screened the numerous component materials for their radioactivity. During dev… ▽ More XMASS-I is a single-phase liquid xenon detector whose purpose is direct detection of dark matter. To achieve the low background requirements necessary in the detector, a new model of photomultiplier tubes (PMTs), R10789, with a hexagonal window was developed based on the R8778 PMT used in the XMASS prototype detector. We screened the numerous component materials for their radioactivity. During development, the largest contributions to the reduction of radioactivity came from the stem and the dynode support. The glass stem was exchanged to the Kovar alloy one and the ceramic support were changed to the quartz one. R10789 is the first model of Hamamatsu Photonics K. K. that adopted these materials for low background purposes and provided a groundbreaking step for further reductions of radioactivity in PMTs. Measurements with germanium detectors showed 1.2$\pm$0.3 mBq/PMT of $^{226}$Ra, less than 0.78 mBq/PMT of $^{228}$Ra, 9.1$\pm$2.2 mBq/PMT of $^{40}$K, and 2.8$\pm$0.2 mBq/PMT of $^{60}$Co. In this paper, the radioactive details of the developed R10789 are described together with our screening methods and the components of the PMT. △ Less

Submitted 29 January, 2019; v1 submitted 10 August, 2018; originally announced August 2018.

Comments: 7 pages, 3 figures

Journal ref: Nuclear Inst. and Methods in Physics Research Section A, 922 (2019) 171-176

Development of new radon monitoring systems in the Kamioka mine

Authors: Guillaume Pronost, Motoyasu Ikeda, Taku Nakamura, Hiroyuki Sekiya, Shigeki Tasaka

Abstract: Radioactivity from radon is a major threat for high-precision low energy physics experiments like the ones in the Kamioka Mine. We developed a new high sensitive radon monitoring system and conducted systematic radon concentration measurements for the first time in Kamioka. The system consists of portable radon detectors with a capacity of 1 L and new electronics based on Raspberry Pi. These radon… ▽ More Radioactivity from radon is a major threat for high-precision low energy physics experiments like the ones in the Kamioka Mine. We developed a new high sensitive radon monitoring system and conducted systematic radon concentration measurements for the first time in Kamioka. The system consists of portable radon detectors with a capacity of 1 L and new electronics based on Raspberry Pi. These radon detectors measure the radon in air with electro-static collection and a PIN photodiode. We measured the absolute humidity dependence of the 1-L radon detector for air to be $C_{F} (A_{H}) = (12.86 \pm 0.40) - (1.66 \pm 0.19) \sqrt{A_{H}}$ $\mathrm{(counts/day)/(Bq/m^3)}$. The background level of the 1-L radon detector is $0.65\pm0.15$ (stat.) count/day. This corresponds to a detection limit of $\sim0.4$ Bq/m$^3$ in a one-day measurement. Data was collected for a period of more than one year with twenty one 1-L radon detectors in the Kamioka mine. They indicate seasonal and day-night variations in radon concentration within the mine. These results also allow us to confirm the stability of the new Raspberry Pi electronics. △ Less

Submitted 3 June, 2019; v1 submitted 29 July, 2018; originally announced July 2018.

Comments: 21 pages, 22 figures, to be published in PTEP Erratum on 2019/03/06: Figures 15 and 16 were corrected, this doesn't change our conclusions

Journal ref: Progress of Theoretical and Experimental Physics, Volume 2018, Issue 9, 1 September 2018, 093H01