S. Umehara

Multilayer Scintillator Responses for Mo Observatory of Neutrino Experiment Studied Using a Prototype Detector MOON-1

An ensemble of multilayer scintillators is discussed as an option of the high-sensitivity detector MOON (Mo Observatory of Neutrinos) for spectroscopic measurements of neutrinoless double beta decays. A prototype detector MOON-1, which consists of 6-layer plastic scintillator plates, was built to study the photon responses of the MOON-type detector. The photon responses, i.e., the number of scintillation photons collected and the energy resolution, which are key elements for high-sensitivity experiments, are found to be 1835 � 30 photoelectrons for 976 keV electrons and � ¼ 2:9 � 0:1% (� E=E ¼ 6:8 � 0:3% in FWHM) at the Q�� � 3 MeV region, respectively. The multilayer plastic scintillator structure with high energy resolution as well as a good signal for the background suppression of � –� rays is crucial for the MOON-type detector to achieve inverted-hierarchy neutrino-mass sensitivity. It will also be useful for medical and other rare-decay experiments as well.

Mass dependence of calcium isotope fractionations in crown-ether resin chromatography

Benzo 18-crown-6-ether resin was synthesised by the phenol condensation polymerisation process in porous silica beads, of which particle diameter was ca 60μ Calcium adsorption chromatography was performed with the synthesised resin packed in a glass column. The effluent was sampled in fractions, and the isotopic abundance ratios of 42Ca, 43Ca, 44Ca, and 48Ca against 40Ca were measured by a thermo-ionisation mass spectrometer. The enrichment of heavier calcium isotopes was observed at the front boundary of calcium adsorption chromatogram. The mass dependence of mutual separation of calcium isotopes was analysed by using the three-isotope-plots method. The slopes of three-isotope-plots indicate the relative values of mutual separation coefficients for concerned isotopic pairs. The results have shown the normal mass dependence; isotope fractionation is proportional to the reduced mass difference, (M – M′)/MM′, where M and M′ are masses of heavy and light isotope, respectively. The mass dependence clarifies that the isotope fractionations are originated from molecular vibration. The observed separation coefficient ϵ is 3.1×10−3 for the pair of 40Ca and 48Ca. Productivity of enriched 48Ca by crown-ether-resin was discussed as the function of the separation coefficient and the height equivalent to the theoretical plate.

Search for WIMPs with the large NaI(Tl) scintillator of ELEGANT V

Abstract The cold dark matter search has been carried out at Oto Cosmo Observatory with the large volume NaI(Tl) scintillators of ELEGANT V. The new limits on WIMPs could be obtained by the analysis of the annual modulation.

Neutrino-less double-β decay of 48Ca studied by CaF2(Eu) scintillators

We searched for the neutrino-less double-β decay(0νββ) of 48 Ca by using CaF 2 (Eu) scintillators. Analysis of their pulse shapes was effective to reduce backgrounds. No events are observed in the Q ββ value region for the data of 3394 kg day. It gives a lower limit (90% confidence level) of T 0νββ ½ > 2.7 x 10 22 year for the half-life of 0νββ of 48 Ca. Combined with our previous data for 1553 kg day [I. Ogawa et al., Nucl. Phys. A730, 215 (2004)], we obtained a more stringent limit of T 0νββ ½ > 5.8 × 10 22 year.

Dark matter search project PICO-LON

The PICO-LON project aims at search for cold dark matter by means of highly radio-pure and large volume NaI(Tl) scintillator. The NaI powder was purified by chemical processing to remove lead isotopes and selecting a high purity graphite crucible. The concentrations of radioactive impurities of 226Ra and 228Th were effectively reduced to 58 ± 4 µBq/kg and 1.5 ± 1.9 µBq/kg, respectively. It should be remarked that the concentration of 210Pb, which is crucial for the sensitivity to dark matter, was reduced to 24 ± 2 µBq/kg. The total background rate at 10 keVee was as low as 8 keV−1kg−1day−1, which was sufficiently low to search for dark matter. Further purification of NaI(Tl) ingot and future prospect of PICO-LON project is discussed.The PICO-LON project aims at search for cold dark matter by means of highly radio-pure and large volume NaI(Tl) scintillator. The NaI powder was purified by chemical processing to remove lead isotopes and selecting a high purity graphite crucible. The concentrations of radioactive impurities of Ra and Th were effectively reduced to 58 ± 4 μBq/kg and 1.5 ± 1.9 μBq/kg, respectively. It should be remarked that the concentration of Pb, which is crucial for the sensitivity to dark matter, was reduced to 24 ± 2 μBq/kg. The total background rate at 10 keVee was as low as 8 keV −1kg−1day−1, which was sufficiently low to search for dark matter. Further purification of NaI(Tl) ingot and future prospect of PICO-LON project is discussed. 1. Outline of PICO-LON project PICO-LON (Pure Inorganic Crystal Observatory for LOw-background Neutr(al)ino) aims at search for WIMPs by means of highly radio-pure NaI(Tl) scintillator. NaI(Tl) scintillator has great advantage to searching for WIMPs because all the nuclei are sensitive to both spindependent and spin-independent interactions. The NaI(Tl) scintillator has another advantages to WIMPs search because of its low background and easy to operate under room temperature. The DAMA/LIBRA group is continuously searching for the signal of WIMPs by highly radiopure and large volume NaI(Tl) crystals [1]. They developed highly radio-pure NaI(Tl) crystal which contains only a few ppt of U and Th chain isotope impurities and less than 20 ppb ar X iv :1 51 2. 04 64 5v 1 [ as tr oph .I M ] 1 5 D ec 2 01 5 of natural potassium [2]. Many other groups are trying to develop highly radio-pure NaI(Tl) crystals to search for WIMPs, however, the sensitivity to WIMPs are suffered from a large amount of 210Pb contamination [3, 4, 5, 6]. Recently, the PICO-LON group established the method to reduce 210Pb in NaI(Tl) crystal. One of the most serious origin of background was successfully removed and further purification and low background test was done. The final set-up of the PICO-LON detector is planned to consist of 42 modules of large volume NaI(Tl) detectors, each with 12.70 cmφ×12.70 cm. The total mass of the detector system is enough to test the annual modulation signal which is reported by DAMA/LIBRA [7]. The NaI(Tl) crystal is viewed by one photomultiplier tube (PMT) in order to lower the background events from PMTs. In the following sections, we will present the recent progresses on the crystal purification and the result of test measurement of low background measurement. 2. Development of low background NaI(Tl) scintillator The purification of NaI(Tl) ingot is the most important task to develop the high sensitivity detector to search for WIMPs because radioactive impurities (RI) in the NaI(Tl) crystal reduces the sensitivity to the WIMPs seriously. The impurities of RIs in a crystal scintillator should be less than a few tens of μBq/kg in order to use the crystal for dark matter search. The contamination of 210Pb is the serious backgrounds because it emits low energy beta rays (Emax = 17 keV and 63.5 keV), the low energy gamma ray and the conversion electron (Eγ = 46.5 keV) and L-X rays below 16 keV. The 210Bi, the progeny of 210Pb, emits high energy beta ray (Emax = 1162 keV) which produces bremsstrahlung photons. All the radiations associated with 210Pb severely reduce the sensitivity to WIMPs signal. Although it is quite difficult to reduce the concentration of 210Pb, we have successfully reduced its concentration by chemical process of raw NaI powder. We tried to remove the Pb ion in the raw powder of NaI by cation exchange resin which was optimized to remove the Pb ion. The raw NaI powder was dissolved in ultra pure water with the concentration of 300 g/Liter. The NaI solution was poured into a column in which the cation exchange resin was filled. The best parameter was searched for and determined to optimize the reduction of lead ion by several trials. The processed solution was dried by rotary vacuum evaporator. The vacuum of the evaporator was broken by high purity nitrogen gas to avoid the contamination by 222Rn in the air. As a result, the concentration of 210Pb became as small as 24 ± 2 μBq/kg. The U-chain (238U and 226Ra) and Th-chain (228Th) were effectively reduced by purifying the raw material of a graphite crucible. The graphite was selected based on results of U, Th and K measurements, however, we found the purity of the graphite was not sufficiently good because a significant contamination of U-chain and Th-chain were observed. Further purification of graphite was done by baking the graphite under 3000 K. The concentration of 226Ra and 228Th were successfully reduced to 58 ± 4 μBq/kg and 1.5 ± 1.9 μBq/kg, respectively. 3. Low background measurement in Kamioka underground observatory The NaI(Tl) ingot was shaved and polished to make 7.62 cmφ×7.62 cm cylindrical shape. A quartz light guide with 4 mm in thickness was glued on the top of the cylindrical NaI(Tl) ingot. All other surfaces of the ingot was covered with 4 mm thick PTFE reflector to guide the scintillation photons to the light guide. The ingot and the light guide were covered with 0.08 cm thick oxygen free high conductive copper (OFHC). The NaI(Tl) detector was covered with 5 cm thick OFHC copper and 20 cm thick old lead passive shield. No active shield was installed in the present measurement. The minimum thickness of the lead shield was 18 cm. Fast neutrons were thermalized and absorbed by 5 cm thick borated polyethylene. Pure nitrogen gas evaporated from liquid nitrogen was flushed into the inner area of the shield to purge radon. The schematic drawing of the detector system is shown in Figure 1. Figure 1. Geometry of the present measurement in Kamioka underground observatory. The low background measurement was started in the summer of 2015 in Kamioka underground laboratory (36◦25’N, 137◦18’E) located at 2700 m water equivalent. The experiment area was placed in the area of KamLAND experiment. The air of the experimental room was controlled to keep clean as class 10 by using a HEPA filter. The flux of the cosmic ray is reduced by a factor of 10−5 relative to the flux in the surface laboratory. A low background photomultiplier tube (PMT) R11065-20 provided by Hamamatsu Photonics was attached on the light guide by optical grease. The concentrations of U and Th chain in the PMT were less than 10 mBq/module. The quantum efficiency was as large as 30 % at the wavelength of 420 nm. The PMT output pulse was introduced into the fast data acquisition system MoGURA (Module for General Use Rapid Application)[8] to digitize the pulse shape. The trigger for the data acquisition system was produced by timing filter amplifier (TFA) which integrates 200 nsec. The fast noise pulses below single photoelectron signals are effectively removed by introducing TFA and the trigger rate was reduced by about two order of magnitude. Energy calibration for higher energy range was performed by using 133Ba and 40K (KCl) sources. The energy resolution at 1.46 MeV was 6.9 % in full-width-half-maximum (FWHM). Figure 2. The energy spectra obtained by irradiating 133Ba (upper orange) and background (lower green). Low background measurement was continued for the live time of 7 days× 1.2 kg. The energy spectra of energy calibration and low background measurements are shown in Figure 2. The background energy spectrum was well reproduced by Monte Carlo simulation with the concentration of the RIs in the surrounding materials. The present energy threshold was 10 keVee and the event rate was 8 keV−1kg−1day−1at the energy threshold. 4. Future prospects We developed highly radio-pure NaI(Tl) crystal to search for cosmic dark matter. The RIs of U-chain and Th-chain were sufficiently reduced by purification of the raw NaI powder and the graphite crucible. The significant potassium impurity was observed in the low background measurement. The Monte Carlo simulation agreed with the assumption that the 2.6 ppm of potassium was contained in NaI(Tl) crystal. The concentration of potassium was too large to use the crystal to the dark matter search. The chemical process to remove the potassium in NaI raw powder is now in progress. The background from the surrounding materials is the next important issue. All the materials which will be used for the detector are selected by measuring the gamma rays from the samples. We started the collaboration with the XMASS group to lower the background from PMTs. Extensive search for the low background materials will be finished in the beginning of 2016 and low background PMT will be developed for PICO-LON in 2016. Full background simulation of 250 kg PICO-LON setup is now ongoing. The detail of the detector design is fixing by discussing with Horiba and Hamamatsu Photonics. The detector design will be optimized to ensure the background rejection by making unti-coincidence measurements of background events such as potassium, 1461 keV gamma ray and 3 keV X ray. 5. Acknowledgment The authors thank Professor S.Nakayama for fruitful discussion and encouragement. The authors also thank Kamioka Mining and Smelting Company for supporting activities in the Kamioka mine and Horiba Ltd. for making the NaI(Tl) detectors. This work was supported by Grantin-Aid for Scientific Research (B) number 24340055, Grant-in-Aid for Scientific Research on Innovative Areas number 26104008. The work was also supported by Creative Research Project in Institute of Socio, Arts and Sciences, Tokushima University. The corresponding author thanks Nogami Fund at RCNP Osaka University for the travel support to attend TAUP 2015.

Double beta decay study of 48Ca by CaF2 scintillator

Abstract 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.

Search for neutrino-less double beta decay with CANDLES

CANDLES is the project to search for neutrino-less double beta decay (0νββ) of βCa. The observation of 0νββ will prove existence of a massive Majorana neutrino. Now we installed new detector system CANDLES III. Here expected performances of the system for background rejection are presented. It is also described current status of development for the detector system.

MOON for spectroscopic studies of double beta decays and the present status of the MOON-1 prototype detector

The MOON (Molybdenum Observatory Of Neutrinos) project, as an extension of ELEGANT V, aims at spectroscopic studies of double beta decays from 100Mo with a sensitivity of the Majorana neutrino mass around 30 meV. Measurements with good energy and position resolutions enable one to select true signals and to reject background ones. A prototype MOON detector (MOON Phase-1A) with 142 g 100Mo was built and is running at the Oto underground laboratory. The present report describes briefly the outline of the MOON project and the present status of MOON-1.

High purity NaI(Tl) scintillator to search for dark matter

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

Status and future prospect of 48Ca double beta decay search in CANDLES

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