Kensuke Yasuda

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.

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.

MOON for a next-generation neutrino-less double-beta decay experiment; present status and perspective

The performance of the MOON detector for a next-generation neutrino-less double-beta decay experiment was evaluated by means of the Monte Carlo method. The MOON detector was found to be a feasible solution for the future experiment to search for the Majorana neutrino mass in the range of 100-30 meV.

Study of Ca-48 double beta decay with CANDLES

CANDLES is the project to search for double beta decay of 48Ca by using CaF2 scintillators. If neutrinos have Majorana mass they violate lepton number conservation and neutrino-less double beta decay (OvDBD) can then take place. Therefore the study of the 0 vDBD is one of the most fundamental researches to be carried out in a coming decade. We have been studying the DBD of 48Ca using CaF2 scintillators. The Q value of 48Ca is the highest (4.27 MeV) among potential DBD nuclei. It is far above energies of γ-rays from natural radio-activities (maximum 2.615 MeV from 208TI decay), therefore we can naturally expect small backgrounds in the energy region we are interested in. Required performances for the detector are radio-purity, good background rejection efficiency and good energy resolution. We have constructed CANDLES III detector in our laboratory at sea level, which consists of 60 CaF2 crystals with the total mass of 191 kg. We are studying the basic performances of the system, including the light collection, position reconstruction and background rejection. On the bases of experiences in CANDLES III, the CANDLES project will be scaled up to several tons of calcium to have the sensitivity to the mass region of interest.

The Performance of Thin NaI(Tl) Scintillator Plate for Dark Matter Search

A thin (0.05 cm) and wide area (5×5 cm 2 ) NaI(Tl) scintillator was developed. The performance of the thin NaI(Tl) plate, energy resolution, single photoelectron energy and position sensitivity wer...A thin (0.05cm) and wide area (5cmX5cm) NaI(Tl) scintillator was developed. The performance of the thin NaI(Tl) plate, energy resolution, single photoelectron energy and position sensitivity were tested. An excellent energy resolution of 20% (FWHM) at 60keV was obtained. The single photoelectron energy was calculated to be approximately 0.42 0.02keV. Position information in the 5cmx5cm area of the detector was also obtained by analyzing the ratio of the number of photons collected at opposite ends of the detector. The position resolution was obtained to be 1cm (FWHM) in the 5cmx5cm area.

Dipole Resonances in 4He

We investigated the analogs of the giant dipole resonance (GDR) and spin‐dipole resonance (SDR) of 4He by using the 4He(7Li,7Be) reaction at an incident energy of 455 MeV and at forward scattering angles. The ΔS=0 and ΔS=1 spectra for 4He were obtained by measuring the 0.43‐MeV 7Be γ‐ray in coincidence with the scattered 7Be. From the ΔS=0 and ΔS=1 spectra thus obtained, the strength distributions of the GDR and SDR in 4He can be derived and the results are compared with the previous data.

WIMPs search by means of thin NaI(Tl) array

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.