K. Kume

Double Beta Decays of 116Cd

Double beta decays (ββ) from the 0 + ground state in 116 Cd to the 0 + ground state in 116 Sn were studied by means of ELEGANT V. A finite half-life for the two-neutrino ββ(2νββ) and an upper limit on the neutrinoless one (0νββ) were obtained as T 2ν 1/2 =2.6 +0.9 -0.5 ×10 19 y and T 0ν 1/2 >6.3×10 21 y (68% C.L.), respectively. The nuclear matrix element M 2ν ββ was deduced as 0.069±0.009 in units of ( m e c 2 ) -1 . This supports the universal quenching of the 2νββ rates in this mass region, which is consistent with that previously measured in 100 Mo.

Limits on neutrino-less double beta decay of 100Mo

Abstract Exclusive measurements of neutrino-less double beta decays (0νββ) of 100Mo were made by means of ELEGANT V. Most stringent lower limits on the half-lives for the ground-state transition were obtained for the 0νββ processes as T 1 2 0ν (m ν ) > 5.2 × 10 22 y , T 1 2 0ν (λ) > 3.9 × 10 22 y and T 1 2 0ν (η) > 5.1 × 10 22 y , for the mass term 〈mν〉, for the right-handed current terms of 〈λ〉 and 〈η〉, respectively, and as T 1 2 0νB > 5.4 × 10 21 y for the process (0νββB) followed by a Majoron (B). These limits lead to the upper limits of 〈mν〉

Limits on the annual modulation of WIMPs-nucleus scattering with large-volume NaI(Tl) scintillators

Abstract 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.

Precise measurement of the cross section ofHe3(He3,2p)He4by usingHe3doubly charged beam

The fusion cross section of 3He(3He,2p)4He at a center of mass energy of 30 to 50 keV has been measured by using helium-3 doubly ionized beam at a low-energy high current accelerator facility, OCEAN. Free from molecular interference in the beam, the measurement determines the astrophysical S-factor with better statistical and systematical errors than previous data. By using singly and doubly charged helium-3 ions the facility envisages to provide the data from high energy to Gamow energy regions.

Highly sensitive radon monitor and radon emanation rates for detector components

Abstract The radon emanation rates for materials were measured by using the electrostatic precipitation method as a radon monitor. It was found that a low level of radon was emanated from several material components in ELEGANT V. The radon monitor has been developed for the highly sensitive measurements of low-level radon concentration. The system was shown to have a sensitivity to radon concentrations as low as 1.6 mBq / m 3 for one day measurement. The system was also used as the radon concentration monitor for the gas inside the airtight box of the ELEGANT V at Oto Cosmo Observatory.

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.

Spin-isospin excitations in nuclei via (3He,t) reactions at 450 MeV

Abstract Spin-isospin excitations in nuclei have been studied by means of the ( 3 He,t) reactions at 450 MeV. The Gamow-Teller (GT) resonances are found to be strongly excited. A fine structure of the giant Gamow Teller resonances in medium-heavy nuclei is observed with an energy resolution of 130 keV. The volume integrals of the effective interactions J στ and J τ have been determined experimentally. The importance of high resolution observation of the ( 3 He,t) reactions at intermediate energies, both, to study the double β decay processes and to measure the detection sensitivities for solar neutrino experiments is discussed.

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.

Double beta decays and ELEGANT V and VI at Oto Cosmo Observatory

Abstract 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.

Determination of U-chain concentration in NaI(Tl) scintillator by delayed coincidence method

Abstract We have accurately determined U-chain concentration in a detector by means of the β–α delayed coincidence method. This method can measure the radioactivity of the order of 10 −7 Bq / kg . It corresponds to the concentration of the order of 0.01 ppt under the assumption that the U-chain is in the secular radioactive equilibrium, by a signal-to-noise ratio of one to one.