arXiv subjects

Keishi Hosokawa

Publications and source records attributed to Keishi Hosokawa.

7 recordsLinked to original sources

arXiv2026

Nonproportional Response of a GAGG Scintillator to Beta and Gamma Radiation over a Wide Energy Range in PIKACHU

Nonproportional response of scintillation light yield for sub-MeV radiations is a well-known characteristic of inorganic scintillators. In the PIKACHU experiment, a precise understanding of the nonproportional response of the GAGG ($\mathrm{Ce:Gd_3Al_2Ga_3O_{12}}$) scintillator is essential for background modeling. In this study, the nonproportional response of the GAGG scintillator was evaluated for both $β$ and $γ$ rays. The response for $β$ rays was measured using the Compton coincidence technique with a germanium detector, while that for $γ$ rays was evaluated using several monoenergetic $γ$-ray sources. The measurement results showed different responses for $β$ and $γ$ rays, with a stronger nonproportional response for $γ$ rays than for $β$ rays over the energy range of 50--2614~keV. The background model incorporating the measured nonproportional responses reproduced the measured background spectra more accurately compared with the model without the nonproportional response correction. These results demonstrate the importance of precise evaluation of the nonproportional response of the GAGG scintillator which is essential not only for background modeling in the PIKACHU experiment but also for accurate energy calibration in scintillator-based experiments.

physics.ins-det↗

arXiv2026

Performance evaluation of GYGAG crystals for neutrinoless double beta decay search

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.

physics.ins-det↗

arXiv2025

Simulation Tool Development and Sensitivity Analysis of 160Gd Double Beta Decay Search by the PIKACHU Project

Neutrinoless double beta decay (0v2b) has been investigated as a physical process that can provide evidence for the Majorana nature of neutrinos. The theoretical predictions of the 0v2b rate are subject to significant uncertainty, primarily due to nuclear matrix elements (NME). To reduce this uncertainty, experimental measurements of the half-lives of two-neutrino double beta decay (2v2b) in various nuclei are essential as a benchmark for NME calculations. The PIKACHU (Pure Inorganic scintillator experiment in KAmioka for CHallenging Underground sciences) project searches for the previously unobserved 2v2b decay of 160Gd, employing Ce-doped Gd3Ga3Al2O12 (GAGG) single crystals. In the Phase 1 experiment, we aim to improve the current lower limit on the 2v2b half-life of 160Gd by a prior study using a Ce-doped Gd2SiO5 (GSO) crystal. Ultimately, in Phase 2, the project seeks to achieve a sensitivity surpassing the theoretical prediction of 7.4 x 10^20 years, enabling the potential discovery of the 160Gd 2v2b decay. In this paper, we describe the development of background models based on GEANT4 simulations. The modeled backgrounds are contributions from uranium and thorium decay chains, 40K present in GAGG, and 40K gamma-rays from outside of GAGG. Additionally, we developed models for both 2v2b and 0v2b decay by implementing the theoretical kinematics of two-electron emission in double beta decay in the GEANT4 simulation. As a result, our background models successfully reproduced the measured background spectrum through fitting. By generating pseudo background spectra expected in Phase 1 and analyzing them with the combined background and 2v2b models, we evaluated the 2v2b sensitivity of Phase 1 to be 2.78 x 10^19 years (90% C.L.). This paper presents the development of these simulation models and the expected sensitivities for both Phase 1 and Phase 2 based on the pseudo data analyses.

hep-ex↗

arXiv2024

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

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.

hep-ex↗

arXiv2024

First Study of the PIKACHU Project: Development and Evaluation of High-Purity Gd$_3$Ga$_3$Al$_2$O$_{12}$:Ce Crystals for $^{160}$Gd Double Beta Decay Search

Uncovering neutrinoless double beta decay (0$ν$2$β$) is crucial for confirming neutrinos' Majorana characteristics. The decay rate of 0$νββ$ is theoretically uncertain, influenced by nuclear matrix elements that vary across nuclides. To reduce this uncertainty, precise measurement of the half-life of neutrino-emitting double beta decay (2$ν$2$β$) in different nuclides is essential. We have launched the PIKACHU (Pure Inorganic scintillator experiment in KAmioka for CHallenging Underground sciences) project to fabricate high-purity Ce-doped Gd$_{3}$Ga$_{3}$Al$_{2}$O$_{12}$ (GAGG) single crystals and use them to study the double beta decay of $^{160}$Gd. Predictions from two theoretical models on nuclear matrix element calculations for 2$ν$2$β$ in $^{160}$Gd show a significant discrepancy in estimated half-lives, differing by approximately an order of magnitude. If the lower half-life estimation holds true, detecting 2$ν$2$β$ in $^{160}$Gd could be achievable with a sensitivity enhancement slightly more than an order of magnitude compared to prior investigations using Ce-doped Gd$_2$SiO$_5$ (GSO) crystal. We have successfully developed GAGG crystals with purity levels surpassing previous standards through refined purification and selection of raw materials. Our experiments with these crystals indicate the feasibility of reaching sensitivities exceeding those of earlier studies. This paper discusses the ongoing development and scintillator performance evaluation of High-purity GAGG crystals, along with the anticipated future prospects of the PIKACHU experiment.

physics.ins-det↗

arXiv2019

Measurement of ambient neutrons in an underground laboratory at Kamioka Observatory and future plan

Ambient neutrons are one of the most serious backgrounds for underground experiments in search of rare events. The ambient neutron flux in an underground laboratory of Kamioka Observatory was measured using a $\mathrm{^3He}$ proportional counter with various moderator setups. Since the detector response largely depends on the spectral shape, the energy spectra of the neutrons transported from the rock to the laboratory were estimated by Monte-Carlo simulations. The ratio of the thermal neutron flux to the total neutron flux was found to depend on the thermalizing efficiency of the rock. Thus, the ratio of the count rate without a moderator to that with a moderator was used to determine this parameter. Consequently, the most-likely neutron spectrum predicted by the simulations for the parameters determined by the experimental results was obtained. The result suggests an interesting spectral shape, which has not been indicated in previous studies. The total ambient neutron flux is $(23.5 \pm 0.7 \ \mathrm{_{stat.}} ^{+1.9}_{-2.1} \ \mathrm{_{sys.}}) \times 10^{-6}$ cm$^{-2}$ s$^{-1}$. In this paper, we explain our method of the result and discuss our future plan.

physics.ins-det↗

arXiv2018

Measurement of ambient neutrons in an underground laboratory at Kamioka Observatory

Ambient neutrons are one of the most serious backgrounds for underground experiments searching for rare events. The ambient neutron flux in an underground laboratory of Kamioka Observatory was measured using a $\mathrm{^3He}$ proportional counter with various moderator setups. Since the detector response largely depends on the spectral shape, the energy spectra of the neutrons transported from the rock to the laboratory are estimated by Monte-Carlo simulations. The ratio of the thermal neutron flux to the total neutron flux was found to depend on the thermalizing efficiency of the rock. Therefore, the ratio of the count rate without a moderator to that with a moderator was used to determine this parameter. Consequently, the most-likely neutron spectrum predicted by the simulations for the parameters determined by the experimental results was obtained. The result suggests an interesting spectral shape, which has not been indicated in previous studies. The total ambient neutron flux is $(23.52 \pm 0.68 \ \mathrm{_{stat.}} ^{+1.87}_{-2.13} \ \mathrm{_{sys.}}) \times 10^{-6}$ cm$^{-2}$ s$^{-1}$. This result, especially the energy spectrum information, could be a new and important input for estimating the background in current and future experiments in the underground laboratory at Kamioka Observatory.

physics.ins-det↗