Kamioka Observatory Research Articles

CANDLES-III detector: Low-background spectrometer for studying neutrino-less double [formula omitted] decay of 48Ca

The CANDLES-III detector was installed at the Kamioka Observatory in Japan to investigate the double β decay of 48Ca. This system combines an undoped calcium fluoride scintillator with a long attenuation length (larger than 100 cm), and a two-phase liquid scintillator, thereby serving as a larger-mass detector of 305 kg (contains 350 g of 48Ca) while maintaining a high energy resolution (σ=2.6% at Q-value of 48Ca double beta decay). Low-background conditions are achieved in the energy region of interest by improving the light yield and collection efficiency of the detector system, allowing to access a high energy resolution, as well as by installing passive and active radiation shielding, and using pulse-shape analysis. This paper describes the details of the detector including its site and data acquisition system and presents its performance characterization.

Development of a low-background HPGe detector at Kamioka Observatory

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, 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.

Direction-sensitive dark matter search with 3D-vector-type tracking in NEWAGE

Abstract NEWAGE is a direction-sensitive dark matter search experiment with a 3D tracking detector based on a gaseous micro time projection chamber. A direction-sensitive dark matter search was carried out at Kamioka Observatory with a total live time of 318.0 days resulting in an exposure of 3.18 kg·days. A new gamma-ray rejection and a head–tail determination analysis were implemented for this work. No significant non-isotropic signal from the directional analysis was found and a 90% confidence level upper limit on a spin-dependent weakly interactive massive particle (WIMP)–proton cross section of 25.7 pb for a WIMP mass of 150 GeV/c2 was derived. This analysis marks the most stringent upper limit in the direction-sensitive dark matter searches.

Search for exotic neutrino-electron interactions using solar neutrinos in XMASS-I

In recent years, the larger size and lower background of dark matter search detectors have opened up a new frontier of searching for new physics other than dark matter using these detectors. One of them is the search for new properties of neutrinos by observing the interaction of neutrinos at low energy. XMASS is a multi-purpose experiment using xenon exclusively in ites liquid xenon (LXe) and is located at the Kamioka Observatory in Japan. We searched for exotic neutrino-electron interactions that could be produced by either a neutrino millicharge, a neutrino magnetic moment, or by dark photons, which might affect the intearction cross section of solar neutrinos in XMASS. We analyzed data taken between November 2013 and March 2016 amounting to 711 live days. No significant signal has been found above the predicted background level in detector. We obtained an upper limit for neutrino millicharge of 5.4 × 10–11e for all flavors of neutrino. We also set individual flavors limits for υμ and ν τ, which are the best limits obtained by direct detection. We also obtain an upper limit for the neutrino magnetic moment of 1.8× 10–10μ B . In addition, we obtain upper limits for the coupling constant of dark photons in the U(1) B–L model. This result almost excludes the possibility to understand the muon g — 2 anomaly by dark photons.

Direction-sensitive dark matter search with the low-background gaseous detector NEWAGE-0.3b”

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 d, corresponding to an exposure of 1.1 kg$\>$d. Two events remained in the energy region of 50–60 keV, which was consistent with the expected background of 2.5 events. 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 the spin-dependent WIMP-proton cross-section of 50 pb for a WIMP mass of 100 GeV$\>c^{-2}$ was derived. This limit is the most stringent yet obtained from direction-sensitive dark matter search experiments.

Development of a dual-phase xenon TPC with a quartz chamber for direct dark matter searches

Abstract The idea of a hermetic quartz chamber in a dual-phase xenon time projection chamber (TPC) has the potential to improve the detector sensitivity for direct dark matter searches in the future. A major challenge facing TPC detectors in future dark matter experiments will be the reduction of the internal background such as $^{222}$Rn and the deterioration of the ionization signal due to electronegative impurities. The hermetic quartz chamber can isolate the TPC’s sensitive volume from external interference and is thus expected to prevent contamination caused by radioactive and electronegative impurities, which originate from the outer detector materials. At the Kamioka Observatory in Japan, we have developed a TPC with a quartz chamber that contains a ⌀$ 48 \times 58$ mm volume of liquid xenon. At this development stage, we have not aimed for perfect hermeticity of the quartz chamber. Our aim here is twofold: first, to demonstrate via the use of a calibration source that the presence of quartz materials in the TPC does not impact its operation; and second, to perform quantitative measurements of the TPC’s characteristics. We successfully measured electron drift velocities of 1.2–1.7 mm/$\mu$s in liquid xenon under electric fields ranging from 75–384 V/cm, and also observed small S2 signals produced by a single ionized electron with a light yield of 16.5 $\pm$ 0.5 PE. These results were consistent with the expected values; therefore, our demonstrations provide a proof of principle for TPCs incorporating a quartz chamber.

Radon monitoring in the Kamioka mine

Radioactivity from radon is one of the main backgrounds for low energy experiments, like Super-Kamiokande and the other experiments hosted in Kamioka Observatory. In addition, it is an issue for the safety of the researchers and technicians working in underground laboratories. In the Kamioka Observatory, a total of 29 1-L radon detectors are deployed in different areas in order to monitor the radon concentration and ensure it stays below a certain limit. Here we briefly describe the 1-L radon detector system and their Raspberry Pi-based data acquisition system. The evolution of the tunnel’s radon concentration over the last 3 years is then shown and discussed.

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 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 [1]. In this paper, we explain our method of the result and discuss our future plan.

Search for dark matter in the form of axion-like particles and hidden photons in the XMASS detector

Axion-like Particles (ALPs) and Hidden Photons (HPs) are candidates of cold dark matter. These bosons are experimentally interesting because they are absorbed by materials through an interaction analogous to a photoelectric effect, transferring the energy equivalent to their rest mass to recoil electrons. XMASS is an experiment aiming at direct detection of dark matter, using single-phase liquid-xenon scintillator at the Kamioka Observatory in Japan. With its low background environment, the XMASS detector has a good sensitivity to the electron recoil signals from ALPs and HPs. Analyzing 800 live-days of XMASS data with 327 kg liquid xenon in the fiducial volume, we set the most stringent upper limits on the coupling constant gAe of ALPs and the parameter for kinetic mixing α′/α of HPs in the mass range from 40 to 120 keV/c2.

Results of a directional dark matter search from the NEWAGE experiment

In this study we present the results of a directional dark matter search from the NEWAGE experiment. A low-background gaseous detector, namely a low alpha-ray emitting μ-PIC (LA μ -PIC), was developed. The LA μ -PIC was installed in Kamioka Observatory and the NEWAGE-0.3” detector with a detection volume of 37 L was operated from June to November 2018. A total exposure of 1.1 kg·days was obtained and a reduction of background by 30 times comparing with a previous result was confirmed. The remaining backgrounds were studied using a Monte Carlo simulation. The main background in the energy region of 50 ≤ E ≤ 100 keV was found to be alpha-rays emitted from the surface of the LA μ -PIC.

WIMP search from XMASS-I fiducial volume data with background prediction

XMASS-I is a single-phase liquid xenon detector located underground in Kamioka Observatory mainly aiming direct detection of dark matter. We performed a WIMP search in a fiducial volume containing 96.5 kg liquid xenon using 705.9 live days of data between November 2013 and March 2016. All events remaining in the fiducial volume after data reduction were consistent with only background prediction from careful evaluation of it. In conclusion, a 90% confidence level upper limit on the spin-independent WIMP-nucleon cross section of 2.2×10−44 cm2 for a WIMP mass of 60 GeV/c2 was derived.

A direct dark matter search in XMASS-I

A search for dark matter using an underground single-phase liquid xenon detector was conducted at the Kamioka Observatory in Japan, particularly for Weakly Interacting Massive Particles (WIMPs). We have used 705.9 live days of data in a fiducial volume containing 97kg of liquid xenon at the center of the detector. The event rate in the fiducial volume after the data reduction was (4.2±0.2)×10−3day−1kg−1keVee−1 at 5keVee, with a signal efficiency of 20%. All the remaining events are consistent with our background evaluation, mostly of the “mis-reconstructed events” originated from 210Pb in the copper plates lining the detector's inner surface. The obtained upper limit on a spin-independent WIMP-nucleon cross section was 2.2×10−44cm2 for a WIMP mass of 60GeV/c2 at the 90% confidence level, which was the most stringent limit among results from single-phase liquid xenon detectors.

Measurement of ambient neutrons in an underground laboratory at the Kamioka Observatory

Abstract Ambient neutrons are one of the most serious backgrounds for underground experiments searching for rare events. The ambient neutron flux in an underground laboratory at the 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. 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 $(23.5 \pm 0.7 \ \mathrm{_{stat.}} ^{+1.9}_{-2.1} \ \mathrm{_{sys.}}) \times 10^{-6}$ 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 the Kamioka Observatory.

Recent results and future prospects of Super-Kamiokande

Abstract Super-Kamiokande (SK) is a water Cherenkov detector located 1000 m underground in Kamioka Observatory , ICRR, University of Tokyo in Japan. It consists of a cylindrical stainless steel tank, 50,000 ton of purified water, and 11,000 of 20-inch PMTs. The fiducial volume of the SK detector is 22.5 kton. The experiment was started in April 1996, and currently phase IV (SK-IV) is running. SK has been the world largest water Cherenkov detector with purified water in underground since 1996. SK has a variety of measurement or physics targets, namely, solar neutrinos , atmospheric neutrinos , supernova neutrinos, neutrinos produced by the accelerator in J-PARC (as T2K experiment), and search for nucleon decay. In near future, we are going to move to the next phase, that is SK-Gd. In the SK-Gd phase, we are planning to add 0.1% of gadolinium to the current SK detector in order to enhance neutron tagging efficiency, and then try to achieve the first observation of supernova relic neutrinos (SRN) (or diffuse supernova neutrino background (DSNB)). The initial work in this upgrade project was started in June 2018. This paper reports a brief summary of the SK experiment, recent solar neutrino results, and recent atmospheric neutrino results. As a future plan, the purpose of SK-Gd and status of the upgrade work are reported.

Search for double electron capture on 124Xe with the XMASS-I detector

The XMASS project is a multi-purpose experiment using highly-purified liquid xenon scintillator located underground at the Kamioka Observatory in Japan. A search for two-neutrino double electron capture on 124Xe is performed using 165.9 days of data collected with the XMASS-I detector. No significant excess above background was observed and we set a lower limit on the half-life as 4.7 × 1021 years at 90% confidence level.

XMASS 1.5: The next step in Kamioka, Japan

The XMASS program at Kamioka Observatory uses self-shielding liquid xenon scintillation detectors to search for dark matter, double beta decay, and ultimately low energy solar neutrinos. For the next stage in this program, we envision building alt fiducial volume detector: XMASS 1.5. The upgraded detector design reflects our experience with the current XMASS detector. The new design requires PMTs that can intercept light traveling parallel to the inner detector surface. The first batch of these new PMTs has passed the performance verification test.

The data acquisition system of the XMASS experiment

The XMASS experiment aims to detect dark matter, pp and 7Be solar neutrinos, and neutrinoless double beta decay using highly purified liquid xenon located underground in the Kamioka Observatory in Japan. Construction of the detector was completed in September 2010 and data-taking started in December 2010. This paper discusses the design of the XMASS data acquisition system and its capabilities.

Design of Low-energy Calibration Sources for Liquid Xenon Dark Matter Detectors

XMASS experiment is ongoing at Kamioka observatory in Japan, and one of its purposes is mainly for the dark matter search using 835kg liquid xenon as a target. Low-energy X-ray calibration-sources using characteristic X-rays are planning to be developed for the XMASS calibration.

Search for inelastic WIMP nucleus scattering on 129Xe in data from the XMASS-I experiment

A search for inelastic scattering of Weakly Interacting Massive Particles (WIMPs) on the isotope $^{129}$Xe was done in data taken with the single phase liquid xenon detector XMASS at the Kamioka Observatory. Using a restricted volume containing 41 kg of LXe at the very center of our detector we observed no significant excess of events in 165.9 days of data. Our background reduction allowed us to derive our limits without explicitly subtracting the remaining events which are compatible with background expectations and derive for e.g. a 50 GeV WIMP an upper limit for its inelastic cross section on $^{129}$Xe nuclei of 3.2 pb at the 90% confidence level.

Low Energy Investigations at Kamioka Observatory

At Kamioka Observatory many activities for low energy rare event search are ongoing. Super-Kamiokande(SK), the largest water Cherenkov neutrino detector, currently continues data taking as the fourth phase of the experiment (SK-IV). In SK-IV, we have upgraded the water purification system and tuned water flow in the SK tank. Consequently the background level was lowered significantly. This allowed SK-IV to derive solar neutrino results down to 3.5 MeV energy region. With these data, neutrino oscillation parameters are updated from global fit; Δm212 = 7.44+0.2−0.19 × 10−5eV2, sin2 θ12 = 0.304 ± 0.013, sin2 θ13 = 0.0301+0.017−0.015. NEWAGE, the directional sensitive dark matter search experiment, is currently operated as "NEWAGE-0.3a" which is a 0.20 × 0.25 × 0.31 m3 micro-TPC filled with CF4 gas at 152 Torr. Recently we have developed "NEWAGE-0.3b". It was succeeded to lower the operation pressure down to 76 Torr and the threshold down to 50 keV (F recoils). XMASS experiment is looking for scintillation signals from dark matter interaction in 1 ton of liquid xenon. It was designed utilizing its self-shielding capability with fiducial volume confinement. However, we could lower the analysis threshold down to 0.3 keVee using whole volume of the detector. In February 2012, low threshold and very large exposure data (5591 kg·days) were collected. With these data, we have excluded some part of the parameter spaces claimed by DAMA/LIBRA and CoGeNT experiments.