Search

We developed a CANDLES-III system to study the neutrino-less double beta (0$\nu\beta\beta$) decay of $^{48}$Ca. The proposed system employs 96 CaF$_{2}$ scintillation crystals (305 kg) with natural Ca ($^{\rm nat.}$Ca) isotope which corresponds 350\,g of $^{48}$Ca. External backgrounds were rejected using a 4$\pi$ active shield of a liquid scintillator surrounding the CaF$_2$ crystals. The internal backgrounds caused by the radioactive impurities within the CaF$_2$ crystals can be reduced effectively through analysis of the signal pulse shape. We analyzed the data obtained in the Kamioka underground for a live-time of 130.4\,days to evaluate the feasibility of the low background measurement with the CANDLES-III detector. Using Monte Carlo simulations, we estimated the background rate from the radioactive impurities in the CaF$_{2}$ crystals and the rate of high energy $\gamma$-rays caused by the (n, $\gamma$) reactions induced by environmental neutrons. The expected background rate was in a good agreement with the measured rate, i.e., approximately 10$^{-3}$ events/keV/yr/(kg of $^{\rm nat.}$Ca), in the 0$\nu\beta\beta$ window. In conclusion, the background candidates were estimated properly by comparing the measured energy spectrum with the background simulations. With this measurement method, we performed the first search for 0$\nu\beta\beta$ decay in a low background condition using a detector with a Ca isotope, in which the Ca present was not enriched, in a scale of hundreds of kg. The $^{48}$Ca isotope has a high potential for use in 0$\nu\beta\beta$ decay search, and is expected to be useful for the development of a next-generation detector for highly sensitive measurements.

A search has been performed for neutrinos from two sources, the $hep$ reaction in the solar $pp$ fusion chain and the $��_e$ component of the diffuse supernova neutrino background (DSNB), using the full dataset of the Sudbury Neutrino Observatory with a total exposure of 2.47 kton-years after fiducialization. The $hep$ search is performed using both a single-bin counting analysis and a likelihood fit. We find a best-fit flux that is compatible with solar model predictions while remaining consistent with zero flux, and set a one-sided upper limit of $��_{hep} < 30\times10^{3}~\mathrm{cm}^{-2}~\mathrm{s}^{-1}$ [90% credible interval (CI)]. No events are observed in the DSNB search region, and we set an improved upper bound on the $��_e$ component of the DSNB flux of $��^\mathrm{DSNB}_{��_e} < 19~\textrm{cm}^{-2}~\textrm{s}^{-1}$ (90% CI) in the energy range $22.9 < E_��< 36.9$~MeV., 11 pages, 6 figures

In the CANDLES experiment aiming to search for the very rare neutrino-less double beta decays (0νββ) using 48Ca, we introduced a new shielding system for high energy γ-rays from neutron captures in massive materials near the detector, in addition to the background reduction for 232Th decays in the 0νββ target of CaF2 crystals. The method of background reduction and the performance of newly installed shielding system are described.

High energy γ rays with several MeV produced by (n,γ) reactions can be a trouble for low background measurements in the underground laboratories such as double beta decay experiments. In the CANDLES project, which aimed to observe the neutrino-less double beta decay from 48Ca, γ rays caused by (n,γ) reactions were found to be the most significant background. The profile of the background was studied by measurements with a neutron source and a simulation with a validity check of neutron processes in Geant4. The observed spectrum of γ rays from (n,γ) reactions was well reproduced by the simulated spectra, which were originated from the surrounding rock and a detector tank made of stainless steel. The environmental neutron flux was derived by the observed event rate of γ rays from (n,γ) reactions using the simulation. The thermal and non-thermal neutron flux were found to be ( 1.3 ± 0.6 ) × 10 − 6 cm − 2 s − 1 and ( 1.1 ± 0.5 ) × 10 − 5 cm − 2 s − 1 , respectively. It is necessary to install an additional shield to reduce the background from (n,γ) reaction to the required level.

We study a strategy to reduce veto-time in the search for neutrino-less double-beta decay (0υββ) with CANDLES-III system. We develop a new likelihood analysis and apply it to our new Run010 data. We show that we can increase the un-vetoed live-time by 11.8%. Thanks to this improvements, We expect to increase a limit on the life-time of 0υββ by a factor of three by analyzing both Run009 and Run010 data.

CAlcium fluoride for the study of Neutrinos and Dark matters by Low-energy Spectrometer (CANDLES) searches for neutrino-less double beta decay of $^{48}$Ca using a CaF$_2$ scintillator array. A high Q-value of $^{48}$Ca at 4,272 keV enabled us to achieve very low background condition, however, at the same it causes difficulties in calibrating the detector's Q-value region because of the absence of a standard high-energy $\gamma$-ray source. Therefore, we have developed a novel calibration system based on $\gamma$-ray emission by neutron capture on $^{28}$Si, $^{56}$Fe and $^{58}$Ni nuclei. In the paper, we report the development of the new calibration system as well as the results of energy calibration in CANDLES up to 9 MeV., Comment: 6 pages, 7 figures

In a neutrinoless double-beta decay ($0\nu\beta\beta$) experiment, energy resolution is important to distinguish between $0\nu\beta\beta$ and background events. CAlcium fluoride for studies of Neutrino and Dark matters by Low Energy Spectrometer (CANDLES) discerns the $0\nu\beta\beta$ of $^{48}$Ca using a CaF$_2$ scintillator as the detector and source. Photomultiplier tubes (PMTs) collect scintillation photons. At the Q-value of $^{48}$Ca, the current energy resolution (2.6%) exceeds the ideal statistical fluctuation of the number of photoelectrons (1.6%). Because of CaF$_2$'s long decay constant of 1000 ns, a signal integration within 4000 ns is used to calculate the energy. The baseline fluctuation ($\sigma_{baseline}$) is accumulated in the signal integration, thus degrading the energy resolution. This paper studies $\sigma_{baseline}$ in the CANDLES detector, which severely degrades the resolution by 1% at the Q-value of $^{48}$Ca. To avoid $\sigma_{\rm baseline}$, photon counting can be used to obtain the number of photoelectrons in each PMT; however, a significant photoelectron signal overlapping probability in each PMT causes missing photoelectrons in counting and reduces the energy resolution. "Partial photon counting" reduces $\sigma_{baseline}$ and minimizes photoelectron loss. We obtain improved energy resolutions of 4.5-4.0% at 1460.8 keV ($\gamma$-ray of $^{40}$K), and 3.3-2.9% at 2614.5 keV ($\gamma$-ray of $^{208}$Tl). The energy resolution at the Q-value is estimated to be improved from 2.6% to 2.2%, and the detector sensitivity for the $0\nu\beta\beta$ half-life of $^{48}$Ca can be improved by 1.09 times., Comment: This manuscript is prepared for submission to IEEE Transaction on Nuclear Science. It contains 11 pages, 19 figures, and 1 table

The neutrino-less double beta decay (0νββ) is a powerful tool to probe the neutrino mass. The 0νββ is forbidden in Standard Model (SM) due to the violation of lepton number conservation. In experiments searching for 0νββ, the two-neutrino double beta decay (2νββ), which is an allowed process in SM, behaves as an irreducible background within the energy of interest. To discriminate the 0νββ from 2νββ, the energy resolution must be improved. The CANDLES experiment is searching for the 0νββ of 48Ca using CaF 2 (pure) scintillator as the detector and the source. Scintillation photons are collected by Photo Multiplier Tubes (PMTs) surrounding the crystals. A Flash Analog-to-Digital Converter records the waveform of each PMT. Because of the long decay time of CaF 2 (1 µsec), we make a signal integral of 4 µsec to calculate the obtained energy. Thus, the other fluctuation in the baseline is accumulated in the signal integration, and it makes the energy resolution worse. To reduce this fluctuation, we can count the number of photoelectrons in every PMT. Still, we have a significant overlapping probability of single photoelectron signals. This overlapping probability implies the photoelectrons lost, which causes terrible energy resolution. To improve the energy resolution, we introduce an alternative method named "partial photon counting" by dividing the waveform into two parts, then, perform signal integral in the prompt part and photon counting the second part. As a result, the energy resolution is reduced 4.0-3.5 % at 1460 keV (γ-ray of 40K) and 3-2.7 % at 2615 keV (γ-ray of 208Tl).

The highly radiopure NaI(Tl) was developed to search for particle candidates of dark matter. The optimized methods were combined to reduce various radioactive impurities. The $^{40}$K was effectively reduced by the re-crystallization method. The progenies of the decay chains of uranium and thorium were reduced by appropriate resins. The concentration of natural potassium in NaI(Tl) crystal was reduced down to 20 ppb. Concentrations of alpha-ray emitters were successfully reduced by appropriate selection of resin. The present concentration of thorium series and 226Ra were $1.2 \pm1.4$ $\mu$Bq/kg and $13\pm4$ $\mu$Bq/kg, respectively. No significant excess in the concentration of $^{210}$Pb was obtained, and the upper limit was 5.7 $\mu$Bq/kg at 90% C. L. The achieved level of radiopurity of NaI(Tl) crystals makes construction of a dark matter detector possible., Comment: 11 pages, 6 figures, accepted to Progress of Theoretical and Experimental Physics

The current status of laboratory for Dark Matter search at the Kamioka mine and specialized facility for production of NaI(Tl) detectors is described. Techniques developed for purification and growth of ultra-low background NaI(Tl) crystals are reported. Achieved activity levels of the 210Pb, 40K, 238U, 232 Th isotopes in the latest NaI(Tl) Dark Matter detector prototypes and the DAMA/LIBRA detectors are compared.

A new data acquisition (DAQ) system for the CANDLES experiment was developed using the SpaceWire protocol and the DAQ-Middleware framework. The CANDLES experiment uses a trigger board and Flash Analog-to-Digital Converters (FADCs). The SpaceWire helps us construct the DAQ system with a flexible and multi-path access to FADCs and the trigger board. FADCs have a ring buffer with three buffers to detect sequential decays from backgrounds. We developed the DAQ system with parallel read-out to reduce the dead time at high trigger rates using three read-out personal computers (PCs) that are connected to the three buffers in FADCs one by one. Each PC connects all FADCs and the trigger board and gathers a complete data set of one event without any event builder. The maximum DAQ speed of parallel read-out by three PCs was 2.4 times higher than that of single read-out. In order to collect event data sets from PCs, we built the network distributed DAQ system through Ethernet, which is naturally introduced with the DAQ-Middleware. To realize remote monitoring and histogram modification while the DAQ running, we also developed a dynamic online monitor system independent with the DAQ-Middleware.

CANDLES studies the double beta decay of 48Ca through CaF2 scintillation crystals. The CANDLES III detector, located in Kamioka underground laboratory, is currently running. Here we describe recent status of data analysis which includes detector performance, detector stability, and background estimation. Current sensitivity for 0 ν β β half-life is also discussed in this paper.

CANDLES aims to measure the very rare neutrino-less double beta decays using 48Ca which has the highest value among all isotope candidates. Low background techniques are the key to observe such rare phenomena. The largest background was found to be the high energy gamma rays from neutron captures on abundant material near the detector. The shielding system consisted of lead and B-contained rubber sheet was constructed to reduce the background. Performance of the constructed shielding system were described in this paper.

The Majorana Collaboration is constructing a system containing 40kg of HPGe detectors to demonstrate the feasibility and potential of a future tonne-scale experiment capable of probing the neutrino mass scale in the inverted-hierarchy region. To realize this, a major goal of the Majorana Demonstrator is to demonstrate a path forward to achieving a background rate at or below 1 cnt/(ROI-t-y) in the 4 keV region of interest around the Q-value at 2039 keV. This goal is pursued through a combination of a significant reduction of radioactive impurities in construction materials with analytical methods for background rejection, for example using powerful pulse shape analysis techniques profiting from the p-type point contact HPGe detectors technology. The effectiveness of these methods is assessed using simulations of the different background components whose purity levels are constrained from radioassay measurements.

The MAJORANA DEMONSTRATOR will search for the neutrinoless double beta decay (ββ(0ν)) of the isotope 76Ge with a mixed array of enriched and natural germanium detectors. In view of the next generation of tonne-scale germanium-based ββ(0ν)-decay searches, a major goal of the MAJORANA DEMONSTRATOR is to demonstrate a path forward to achieving a background rate at or below 1 cnt/(ROI-t-y) in the 4 keV region of interest (ROI) around the 2039-keV Q-value of the 76Ge ββ(0ν)-decay. Such a requirement on the background level significantly constrains the design of the readout electronics, which is further driven by noise and energy resolution performances. We present here the low-noise low- background front-end electronics developed for the low-capacitance p-type point contact (P-PC) germanium detectors of the MAJORANA DEMONSTRATOR. This resistive-feedback front-end, specifically designed to have low mass, is fabricated on a radioassayed fused-silica substrate where the feedback resistor consists of a sputtered thin film of high purity amorphous germanium and the feedback capacitor is based on the capacitance between gold conductive traces.

High purity germanium (HPGe) crystals will be used for the MAJORANA DEMONSTRATOR, where they serve as both the source and the detector for neutrinoless double beta decay. It is crucial for the experiment to understand the performance of the HPGe crystals. A variety of crystal properties are being investigated, including basic properties such as energy resolution, efficiency, uniformity, capacitance, leakage current and crystal axis orientation, as well as more sophisticated properties, e.g. pulse shapes and dead layer and transition layer distributions. In this paper, we will present our measurements that characterize the HPGe crystals. We will also discuss our simulation package for the detector characterization setup, and show that additional information can be extracted from data-simulation comparisons., Comment: To appear in the Proceedings of the 13th International Conference on Topics in Astroparticle and Underground Physics, TAUP 2013 (F. Avignone & W. Haxton, editors, Physics Procedia, Elsevier)

CANDLES is the project to search for neutrino-less double beta decay ( 0νββ ) of 48 Ca. The observation of 0νββ will prove existence of a massive Majorana neutrino. For the 0νββ measurement, we need a low background condition because of a low decay rate of 0νββ . Now we installed the CANDLES III system at the Kamioka underground laboratory. The CANDLES III system realizes the low background condition by a characteristic structure and data analyses for background rejection. Here we report performances of the CANDLES III system.

The neutrino-less double beta decay is acquiring great interest after the confirmation of neutrino oscillation which demonstrated nonzero neutrino mass. In order to search for neutrino-less double beta decay of $^{48}$Ca, we proposed the CANDLES project by using CaF$_{2}$(pure) scintillators. Now we installed the CANDLES III system at the Kamioka underground laboratory. The CANDLES III system realizes the low background condition by a characteristic structure and data analyses for background rejection. Furthermore we installed new shielding system in order to reduce $\gamma$-ray backgrounds from neutron capture reaction. Here we report performances of the CANDLES III system.

Direct search for dark matter is one of the most important problems in astrophysics. Significant signal for dark matter will be a hint to clarify the origin of the universe. Only DAMA/LIBRA experiment with NaI(Tl) detector has ever suggested the presence of dark matter signal. Verifying the DAMA/LIBRA result by a NaI(Tl) detector is urgent and important task. We have tried to purify NaI(Tl) crystal to search for dark matter. In this presentation, the present status of purification will be discussed. The concentration of potassium is successfully reduced to desired sensitivity. The 210Pb, which is difficult to reduce, has been reduced effectively. Present status of low background measurement in Kamioka observatory will be shown.

The PICOLON project is developing a high purity NaI(Tl) crystal by optimizing the purification methods. The reduction method for 40K was established and we got sufficiently pure NaI(Tl) crystal for 40K. On the other hand, the concentration of 210Pb is still higher than our goal. The investigation is now in progress to fix the origin of the contamination. The construction of the second phase detector system of PICOLON detector will be started in 2020 spring. The total mass of the second phase is about 54 kg. The background level by the test measurement using a single module is reported.

We have studied the neutrino-less double beta decay(0νββ) of 48Ca by using CaF2(pure) scintillators. Analysis for rejection of backgrounds(212Bi→ 212Po events and 208Tl events) was effective to reduce backgrounds in Qββ-value region. No events are observed in the Qββ-value region for the data of 131 days × 86 kg. It gives a lower limit (90% confidence level) of (math) > 6.2 × 1022 year (preliminary) for the half-life of 0νββ of 48Ca.

Recent data from the DAMA/LIBRA phase-2 confirmed detection of a signal with independent Dark Matter (DM) annual modulation signature at a 12.9 σ CL. Our attempts to verify the DAMA/LIBRA DM observation claim led to construction of underground clean rooms at the KamLAND site and specialized laboratory for production of NaI(Tl) detectors. Current status of these facilities, methods used to grow ultra-low background NaI(Tl) crystals, and radio-purity of the latest NaI(Tl) DM detector prototype are discussed.

The concentration of 7Be in surface air and rainwater at Daito, Osaka (34.71 N, 135.64 E) has been measured during the period from January 8 to October 29, 2018 and April 1 to October 31, 2018, respectively. The weekly concentration of 7Be in surface air in the range of 8.92 ± 0.08 to 1.03 ± 0.02 mBq/m3 showing seasonal trend with maxima in spring and autumn and minima in summer, which can be ascribed to the transport of air masses from low latitudes carrying low 7Be concentration into the middle latitude region. The factors influences on the concentration of 7Be were examined. It was shown that the positive and negative correlation was found in PM2.5 (cc = 0.62) and relative humidity (cc = –0.57), respectively. The weak positive correlation was found in cosmic ray intensity (cc = 0.23), the weak negative correlation was found in precipitation (cc = –0.14) and sunspot number (cc = –0.05). The deposition rate was analyzed by using the rainwater samples. It shown negative correlation between deposition rate and precipitation (cc = –0.53). High volumes of precipitation can reduce the 7Be concentration of air by wash-out.

We have developed a new background reduction method based on single photoelectron counting of scintillators for the WIMP search experiment. We introduced this method to our detector system ELEGANT VI, which has an active shield in all directions ( 4 π ) using a pulse height or, equivalently, the difference in the number of photoelectrons between two (left and right) photomultiplier tubes attached to CaF2 scintillators. The number distribution is uniquely described by a binomial function, via which we developed the unambiguous background subtraction method. This method reduces the backgrounds further compared to charge-sensitive ADC and offers improved sensitivity for the search for WIMPs.

A study of signals originating near the lithium-diffused n+ contact of p-type point contact (PPC) high purity germanium detectors (HPGe) is presented. The transition region between the active germanium and the fully dead layer of the n+ contact is examined. Energy depositions in this transition region are shown to result in partial charge collection. This provides a mechanism for events with a well defined energy to contribute to the continuum of the energy spectrum at lower energies. A novel technique to quantify the contribution from this source of background is introduced. Experiments that operate germanium detectors with a very low energy threshold may benefit from the methods presented herein.

Fallout with rain from the Fukushima reactor accident was monitored for about two months in Higashi-Hiroshima City, Hiroshima, Japan, from March 20 to May 23, 2011. Gamma-ray (γ-ray) spectra measured using a low background HPGe spectrometer showed clear evidence of fission products—131I, 137Cs, and 134Cs. The 131I was observed on March 27 and April 8, while 137Cs and 134Cs were observed on March 27, April 18, and April 22. The 131I, 137Cs, and 134Cs activity concentrations in rainwater collected in Hiroshima reached 0.44 ± 0.09 on April 8, 0.17 ± 0.01 on April 18, and 0.15 ± 0.01 Bq/L on April 18, 2011, respectively. These activity levels were compared with global results collected from the Northern Hemisphere. Several samples of rainwater that were collected in Chiba (Kashiwa) on March 21, April 11, and May 12; Tokyo (Nerima) on March 21 and April 11; Osaka (Hirano) on April 8; Nara (Kitakatsuragi) on April 9; and Fukushima (Fukushima) on April 19, were also measured by our spectrometer and compared. Among these samples of rainwater collected at different locations in Japan, the one collected at Kashiwa City, Chiba Prefecture, on March 21 showed the highest activity concentrations of 6072 ± 1, 632 ± 4, 766 ± 3, 637 ± 1, 97.6 ± 0.8, and 752 ± 2 Bq/L for 131I, 132I, 132Te, 134Cs, 136Cs, and 137Cs, respectively. The health risks due to these high activity levels were considered in terms of several regulations. The ratio of the activites for the isotopes 131I to 137Cs and 134Cs to 137Cs were compared with the other measurements and discussed.

A high purity and large volume NaI(Tl) scintillator was developed to search for cosmic dark matter. The required densities of radioactive impurities (RIs) such as U-chain, Th-chain are less than a few ppt to establish high sensitivity to dark matter. The impurity of RIs were effectively reduced by selecting raw materials of crucible and by performing chemical reduction of lead ion in NaI raw powder. The impurity of $^{226}$Ra was reduced less than 100 $\mu$Bq/kg in NaI(Tl) crystal. It should be remarked that the impurity of $^{210}$Pb, which is difficult to reduce, is effectively reduced by chemical processing of NaI raw powder down to less than 30 $\mu$Bq/kg. The expected sensitivity to cosmic dark matter by using 250 kg of the high purity and large volume NaI(Tl) scintillator (PICO-LON; Pure Inorganic Crystal Observatory for LOw-background Neutr(al)ino) is 7$\times$10$^{-45}$ cm$^{2}$ for 50 GeV$/c^{2}$ WIMPs., Comment: 6 pages, 2 Figures, Proceedings of International Symposium on Radiation Detectors and Their Uses (ISRD2016). Talk given on 19th Jan. 2016 by K.Fushimi. To be published in Proceedings will be published as JPS conference proceedings (2016)

The MAJORANA Collaboration is building the MAJORANA DEMONSTRATOR, a 60 kg array of high purity germanium detectors housed in an ultra-low background shield at the Sanford Underground Laboratory in Lead, SD. The MAJORANA DEMONSTRATOR will search for neutrinoless double-beta decay of 76Ge while demonstrating the feasibility of a tonne-scale experiment. It may also carry out a dark matter search in the 1-10 GeV/c^2 mass range. We have found that customized Broad Energy Germanium (BEGe) detectors produced by Canberra have several desirable features for a neutrinoless double-beta decay experiment, including low electronic noise, excellent pulse shape analysis capabilities, and simple fabrication. We have deployed a customized BEGe, the MAJORANA Low-Background BEGe at Kimballton (MALBEK), in a low-background cryostat and shield at the Kimballton Underground Research Facility in Virginia. This paper will focus on the detector characteristics and measurements that can be performed with such a radiation detector in a low-background environment., Comment: Submitted to NIMA Proceedings, SORMA XII. 9 pages, 4 figures

The light output responses for protons and carbon nuclei in the KamLAND liquid scintillator were precisely measured using a monochromatic neutron beam. The observed response for proton recoils is well described by Birks formula with the higher-order correction, for the recoil energy from 424 keV to 10.5 MeV. The response for carbon recoils is also well described by modifying the Birks formula with the nuclear energy loss terms, for the measured recoil energy from 171 keV to 2.2 MeV. The obtained light output responses were used to estimate the background energy spectrum of the C 13 ( α , n ) O 16 reaction in the KamLAND. The systematic uncertainty of the energy scale in the expected background spectrum was further improved with the measured light output responses.

This study demonstrates that the energy resolution of a newly developed 100 per cent pure polyvinyltoluene (PVT) plate allows its use as a base material for a plastic scintillator. The energy resolution, which is a key element for high-performance radiation detectors, was Δ E / E =8.41±0.07% (full width at half maximum (FWHM)) for 976 keV K-line conversion electrons from a 207 Bi source. On the basis of results from 207 Bi and 137 Cs sources, the observed energy resolution of the PVT plate, Δ E / E =8.2/ E 1/2 % (FWHM), was slightly better than that of a typical plastic scintillator (BC-408), Δ E / E =8.7/ E 1/2 % (FWHM), with E in units of MeV. These results prompted us to search for other new base materials for plastic scintillators. In this study, we examined polyethylene terephthalate (PET) bottles, a common source of domestic plastic waste. We demonstrated that a lump of heat-proof PET bottles is fluorescent; moreover, there is excellent compatibility of the fluorescence with the quantum efficiency of typical photomultiplier tubes. This inexpensive source of plastic appears suitable for radiation measurements and as a base material for plastic scintillators. Future studies on the radiation response of plastics should lead to the development of higher performance and more eco-friendly radiation detectors.

The environmental impacts and hazards due to the unstoppable hot mud flow by the East Java ‘LUSI’ Mud Volcano are increasing since its unexpected eruption on May 29, 2006. Analysis should be undertaken, not only to examine its impact on human health and the environment, but also to explore the potential benefits of the mud flow. One may be able to tap the mud flow as a material source for brick and cement. Recently there has been great concern about the health risks associated with exposure to natural radioactivity present in soil and building materials all over the world. In this context, measurements for natural radioactive isotopes such as 238U and 232Th series, and 40K in mud samples were carried out using the HPGe (High-Purity Germanium) detector to determine the re-usability of the mud. 226Ra, 232Th and 40K activity concentrations were found to be 13±1, 15±1 and 111±3 Bq/kg (1 Bq = 1 sec-1), respectively, and the corresponding activity index was found to be 0.16±0.02. These values were compared with previous data and our measured accuracy was improved by a factor of nine at the maximum. Radium equivalent activity, external and internal hazard indices, and annual effective dose equivalent were also evaluated and all were found to be within acceptable limits.

CANDLES is a project to search for neutrinoless double beta decay of 48Ca with CaF 2 scintillators. An effective 4 π active shield is achieved by placing the CaF 2 crystals in a liquid scintillator. Energy resolution is improved by converting the ultra-violet light of the CaF 2 scintillator into the visible region with a wavelength shifter dissolved in the liquid scintillator. The newly developed technique, “two phase system of liquid scintillator”, is presented. It will be used to exploit the unique features of the CANDLES system. Experiments to determine liquid scintillator composition will be described. Performance of CANDLES will be demonstrated with prototype detectors. Obtained results indicate that the proposed two phase technique is effective for CANDLES system which will utilize several tons of CaF 2 scintillators.

The MOON (Molybdenum Observatory Of Neutrinos) project aims at high sensitive studies of the double beta ( ββ ) decays with sensitivity to Majorana ν mass of the order of m ν > ∼ 0.03 eV and the charged-current (CC) neutrino spectroscopy of the major components of the pp and 7 Be solar ν 's. The present status of MOON for the low energy solar ν experiment is briefly discussed. The inverse β rays from solar- ν captures of 100 Mo are measured in delayed coincidence with the subsequent β decay of 100 Tc. MOON's exclusive CC value by 7 Be solar ν , together with the GNO CC data, will provide the pp solar ν flux with good accuracy.

A CaF 2 scintillation detector system (ELEGANT VI) is developed to search for neutrinoless double beta decay (0νDBD) of 48 Ca. No events were observed around the Q -value energy region after the analysis of 4.23 kg yr data. To derive the lower limit for the half-life, the expected number of background events was estimated by a Monte Carlo simulation using the measured activities of 214 Bi and 220 Rn inside CaF 2 crystals. A new lower limit is obtained to be 1.8 × 10 22 yr at the 90% C.L.

The MOON (Molybdenum Observatory Of Neutrinos) project is a hybrid ββ and solar ν experiment with 100 Mo. It aims at high sensitive studies of ββ decays with a sensitivity of m ν > ∼ 0.03 eV and real-time studies of pp and 7 Be solar ν's. The double β rays from 100 Mo are measured in prompt coincidence for the 0νββ studies, and the inverse β rays from solar-ν captures of 100 Mo are measured in delayed coincidence with the subsequent β decay of 100 Tc. Measurements with good position resolution enable one to select true signals by spatial and time correlations.

A low background HPGe detector system was developed at CANDLES Experimental Hall for multipurpose use. Various low background techniques were employed, including hermatic shield design, radon gas suppression, and background reduction analysis. A new pulse shape discrimination (PSD) method was specially created for coaxial Ge detector. Using this PSD method, microphonics noise and background event at low energy region less than 200 keV can be rejected effectively. Monte Carlo simulation by GEANT4 was performed to acquire the detection efficiency and study the interaction of gamma-rays with detector system. For rare decay measurement, the detector was utilized to detect the nature’s most stable isomer tantalum-180m (180Tam) decay. Two phases of tantalum physics run were completed with total livetime of 358.2 days, which Phase II has upgraded shield configuration. The world most stringent half-life limit of 180Tam has been successfully achieved.

CANDLES is a double beta decay experiment using 48Ca in CaF2 crystals. The measurement is being performed with prototype detector (CANDLES III) for high sensitive measurement in the future. Recent status of detector improvements and background reduction techniques are described in this paper.

The CANDLES experiment is a study of the double beta decay of $^{48}{\rm Ca}$ nuclei. The detector consists of two scintillators. One is the ${\rm CaF}_{2}$ crystal that acts as a source of double beta decay and a scintillator. The other is a liquid scintillator, which is used for gamma-veto. The total light collected from the liquid scintillator is on the same order of magnitude as that of the ${\rm CaF}_{2}$ scintillator, but the decay constant of the liquid scintillator is one or more orders of magnitude shorter than that of the ${\rm CaF}_{2}$ scintillator. Therefore, if we trigger the signal by pulse height, the energy threshold of the liquid scintillator light will be one or more orders of magnitude lower than that of the ${\rm CaF}_{2}$ scintillator. A high counting rate in the liquid scintillator causes deadtime; in order to avoid this problem, we have developed a new trigger system. In this paper, we describe the use of this trigger system in the CANDLES experiment and its performance after installation in 2013.

A search for cold dark matters has been carried out with the 662 kg NaI scintillator array of ELEGANT V at the Kamioka underground laboratory. The annual modulation of weakly interacting massive particles (WIMPs) energy spectra has been investigated. No significant modulations beyond statistical fluctuations were found. Stringent limits were obtained on masses and cross sections for both axial-vector coupled and vector coupled WIMPs.

Exclusive measurements of neutrino-less double beta decays(0νββ) of 100 Mo were made by means of ELEGANT V at Kamioka Underground Laboratory. Oto Cosmo Observatory has been constructed for underground laboratory for ELEGANT V and VI for the study of 0νββ of 100 Mo and 48 Ca, search for dark matters and so on.

Zero-degree cross sections have been measured for (t, 3 He) reactions on the light target nuclei 9 Be, 10 B, 11 B, 12 C, and 13 C at a bombarding energy of 127 MeV/A. The triton beam was produced by ( 4 He,t+p) breakup on a Be production target using a 4 He beam of 155 MeV/A. Dispersion matching was applied to obtain good energy resolution ( Δ E/E=0.002, Δ E=780 keV). The zero-degree (t, 3 He) cross sections are used to discuss Gamow-Teller strengths in 10 Be and 11 Be.

A thin plate of NaI(Tl) with the dimension of 5cm×5cm×0.05cm has been developed for WIMPs search. The thin NaI(Tl) showed the good performance for energy resolution and low energy threshold. The advantages for WIMPs search, especially, inelastic excitation of nuclei by spin-dependent interaction between WIMPs and 127I is discussed.

The MAJORANA DEMONSTRATOR is an array of natural and enriched high purity germanium detectors that will search for the neutrinoless double-beta decay of 76-Ge and perform a search for weakly interacting massive particles (WIMPs) with masses below 10 GeV. As part of the MAJORANA research and development efforts, we have deployed a modified, low-background broad energy germanium detector at the Kimballton Underground Research Facility. With its sub-keV energy threshold, this detector is sensitive to potential non-Standard Model physics, including interactions with WIMPs. We discuss the backgrounds present in the WIMP region of interest and explore the impact of slow surface event contamination when searching for a WIMP signal., Comment: To appear in the Proceedings of the 13th International Conference on Topics in Astroparticle and Underground Physics, TAUP 2013 (F. Avignone & W. Haxton, editors, Physics Procedia, Elsevier)