Search

Reactor experiment for neutrino oscillation (RENO) began data-taking from August 2011. It successfully observed reactor antineutrino disappearance in April 2012 to measure the smallest mixing angle of θ 13 . Two identical detectors, one at near location and the other at far location, are constructed at the Yonggwang nuclear power plant in South Korea, to compare the observed reactor neutrino fluxes. Each RENO detector is filled with 16 mass tons of Gadolinium loaded liquid scintillator (GdLS) in the neutrino target region, and with 28 mass tons of unloaded liquid scintillator (LS) in the γ - catcher region surrounding the target. LS was developed to satisfy chemical, physical, optical properties, and safety requirements. Linear alkyl benzene (LAB) was chosen as a solvent because of its high flash-point, sufficient light yield, and being environmentally friendly. GdLS is carefully developed to keep a long attenuation length and high light yield for a long time period. In this paper, we report the characteristics and mass production of the RENO LS and GdLS.

In order to understand the scintillation characteristics of nuclear recoil events in CsI(Tl) crystals and for application to the analysis of Dark Matter (DM) searches, we study quenching and channeling effects with simulations of recoil ion motion in CsI(Tl) and experiments using neutrons impinging on the crystal. The electronic energy loss versus penetration depth of recoil ions in a CsI(Tl) target are simulated by the SRIM and MARLOWE programs and modified by the application of a measured scintillation efficiency function. They are used to produce quenching factors as a function of recoil energy. The application of the MARLOWE code to a monocrystalline target yields a light yield distribution including tails which are correlated with ions' enhanced ranges. The 2.4 MeV pulsed neutrons impinging on a monocrystalline CsI(Tl) target and an array of neutron detectors located at different scattering angles are used to measure the light yield distribution and the status of the measurement is reported.

Scintillation properties and radioactive contamination of calcium molybdate scintillating crystals depleted in 48Ca and enriched in 100Mo (40Ca100MoO4 ) have been studied to prepare a 100Mo-based neutrinoless double beta decay experiment. The prospects of a neutrinoless double beta decay experiment using 40Ca100MoO4 crystals are discussed.

The search for neutrinoless double beta decay (0 νββ ) is a key experiment for evaluating the mass of the neutrino and for determining the Majorana/Dirac nature of neutrinos. A scintillating CaMoO 4 crystal is a good material for use in investigating 0 νββ with cryogenic detectors. The high transition energy ( Q = 3034 keV) and nearly 10% natural abundance of 100 Mo, together with the scintillating property of the crystal, provide favorable conditions to search for 0 νββ of 100 Mo. We report a prototype experiment operating at low temperatures using a CaMoO 4 crystal as a particle absorber. Prior to testing of isotopically-enriched 40 Ca 100 MoO 4 crystals, a CaMoO 4 crystal with Mo of natural isotopic composition was tested. Using a metallic magnetic calorimeter as a temperature sensor, we achieved high energy resolutions for alpha particles and low-energy gamma rays. This experiment shows the feasibility of scaling up the crystal size to perform a high sensitivity search for 0 νββ decay of 100 Mo.

Search for neutrino-less double beta decay of 100Mo is proposed using active method with Ca 100MoO4 scintillation crystals which show the brightest scintillation light among variety of inorganic scintillation materials containing Mo. Study of X-ray luminescence and scintillation properties such as energy response, number of photoelectrons/keV, absolute light yield, decay time, pulse shape discrimination and radioactive contamination of CaMoO4 crystals grown by the Czochralski method with different conditions are presented. Further R&D of resolution optimization, crystal quality improvement and background reduction are underway. Significant improvement of sensitivity to neutrino-less double beta decay can be achieved by using 10 Mo enriched Ca 100MoO4 crystals with good energy resolution and low background. Further reduction of background induced by 48 Ca two neutrino double beta decay can be achieved by using 48Ca depletion. We are planning to install several kilograms of Ca 100MoO4 crystals depleted in 48Ca at underground laboratory for the neutrino-less double beta decay experiment in the near future.

A search for the neutrinoless double-beta decay of 124 Sn was carried out using the tin-loaded liquid scintillator for an active source-detector technique. Tin (32.6%) in weight was successfully loaded into the liquid scintillator, and light output was as high as 57% of the unloaded liquid scintillator. A tin-loaded liquid scintillator with 1.1 l volume was installed at the 700 m underground laboratory in YangYang, and data were taken for 5285 h. No evidence for the 0 ν β β decay was found and a lower limit on the 124 Sn half-life was obtained to be 2.0 × 10 19 year with 90% C.L. The new limit represents a significant improvement with respect to those previously available for 124 Sn .

We have studied the energy resolution, α / β ratio, temperature dependence of the scintillation properties, and the radioactive contamination of CaMoO4 crystal scintillators. We have also examined the use of pulse-shape discrimination to distinguish γ rays and α particles. A high sensitivity experiment to search for the 0 ν 2 β decay of Mo 100 using CaMoO4 scintillators is discussed.

Results of searches for the β + / EC decays of 64 Zn and 112 Sn, and the ββ decay of 124 Sn to excited states in 124 Te are presented. This work was done at the ChyungPyung Underground Laboratory. An ultra-low background HPGe detector with/without CsI(T l ) crystal was used for gamma tagging. No evidence of any of these decay modes has been found. Lower limits on the half-lives on the 124 Sn decay to different excited channels of ββ decay are set in the range of 10 18 – 10 19 years by 90% C.L. Lower limits of the half-lives of β + / EC decay of 64 Zn, 112 Sn are obtained to be 9.1 × 10 17 years and 1.3 × 10 20 years by 90% C.L., respectively.

Tin-loaded liquid scintillator offers some advantages as a detector for double beta decay experiment. A research program was carried out with the objective of developing such scintillators. Several organometals were loaded into pseudocumene-based liquid scintillator (LS) and light output variation was studied. Up to 50% of tetramethyltin (32.6% w of tin) was successfully loaded into LS and light output was as high as 57% of unloaded LS. We have studied the characteristics and stability of the one liter tin-loaded LS contained in a Teflon vessel. Several γ -ray radioactive sources were used for energy calibration and calibration data were compared with Monte Carlo simulation. Data-taking at the 700 m underground laboratory in Yangyang is ongoing for the search of 0 ν β β decay of 124 Sn.

Current–voltage and low frequency excess electrical noise characteristics of two different—Schottky diode and n-i-n diode—GaAs structures embedded with self-assembled In(Ga)As quantum dots are reported. We find the growth of quantum dots induces defects not only near the quantum dot but also extended to quite a distance toward the growth direction. In Schottky diode structure, comparing with the reference sample without the quantum dot layer, the current dependence of the low frequency noise spectral density indicated that the noise is from the generated interface states with the density increasing towards the band tail. Also the crystal quality of the Schottky diode including the quantum dot layer, deduced from the Hooge parameter, was slightly worse than that of the reference sample. For n-i-n diode structure, the current–voltage relation was linear, and a quadratic current dependence of the noise spectral density was observed. The Hooge parameter for the n-i-n structure was determined to be on the order of unity indicating the general degradation of the structure.

We report some comparative results on the normal-incidence device characteristics accomplished with a couple of self-assembled InAs/GaAs quantum-dot infrared photodetectors (QDIPs) doped in InAs QDs and GaAs barriers. The peak values of responsivity and detectivity for the barrier-doped device are 650 mA/W and 3.2 × 108 cm Hz1/2/W (18 K) at λp ≅ 5 μm, respectively, which are approximately two and ten times higher than those for the QD-doped one. In addition, while there is no spectral response over 6 μm in the QD-doped structure, a strong photoresponse is extended up to around 10 μm in the barrier-doped one. Although the direct doping in InAs QDs is effective for blocking the dark current, the doping in GaAs barriers has better device performance of QDIP.

We have extensively studied the mechanism of the 137 Cs and 134 Cs contamination in CsI compounds. The nuclei 137 Cs and 134 Cs in various samples have been studied using a HPGe detector at deep underground. The main source of the 137 Cs contamination in CsI powder was water dissolved 137 Cs used in the chemical process of cesium extraction at company side. We also found that 134 Cs was produced mainly from 133 Cs by neutron capture. We have successfully produced CsI powder containing no more than 2 mBq/kg of 137 Cs by using only ultra-pure water for whole extraction

Scintillation properties of calcium molybdate are investigated. The emission spectrum is broad with a maximum at 520 nm. At room temperature the light yield is 4% of CsI(Tl) (400 photoelectrons/MeV) and the decay time is 17 /spl mu/s. During cooling the light yield and decay time increase. Prospects of optimization of this scintillator are also discussed. The feasibility of the material for the search of neutrinoless double beta decay of /sup 100/Mo is analyzed. A half life limit of T/sub 1/2/=1/spl middot/10/sup 25/ years (68% CL) seems feasible with CaMoO/sub 4/ scintillator, running with 10 kg of /sup 100/Mo for 5 years. The background due to two neutrino double beta decay of /sup 48/Ca is also considered. Requirements for new scintillators for double beta decay search are formulated.

A coupled-channel method was used for the estimation of nucleon optical potential parameters for the actinide region based on the axial rigid-rotor model built on a saturated coupling scheme. A new global form of optical potential was proposed to incorporate energy dependences guided by physical principles. Best-fit potential parameters were searched for to reproduce experimental nucleon scattering angular distributions, low-energy observables (strength functions and scattering radii) and neutron total cross sections of 238U and 232Th. It was found that the present form of the optical potential could reproduce both the neutron- and proton-data for a wide energy range, namely from 1 keV to 200 MeV. Available experimental data for other actinides were also described to a good accuracy with the same set of parameters only by adjusting the deformation parameters for each nucleus.

In order to explore a correlation between superconductivity and the electronic structure of Ni-based superconducting quaternary compounds, a systematic investigation of the electronic structures for ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$X (X=B, C, N, and O) is carried out, by employing the linearized muffin-tin orbital band method. It is found that the Ni 3d density of states (DOS) in ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$C is broader than in fcc Ni metal, and so N(${\mathit{E}}_{\mathit{F}}$) becomes small enough to make the system nonferromagnetic and then superconducting. A rigid-band-like shift of the Fermi level is observed as atom X varies. In the case of ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$C, the Fermi level is located right at the van Hove--like singular DOS peak, which originates from saddle-point extremal band crossing \ensuremath{\Gamma}. This singular DOS peak at ${\mathit{E}}_{\mathit{F}}$ is expected to be related to the superconductivity observed in ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$C. A crude estimate within the framework of the simple rigid-ion approximation indicates that the superconductivity in ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$C can be properly described by the conventional phonon mechanism.

New limits are presented on the cross section for Weakly Interacting Massive Particle (WIMP) nucleon scattering in the KIMS CsI(T) detector array at the Yangyang Underground Laboratory. The exposure used for these results is 24524.3 kg\cdotdays. Nuclei recoiling from WIMP interactions are identified by a pulse shape discrimination method. A low energy background due to alpha emitters on the crystal surfaces is identified and taken into account in the analysis. The detected numbers of nuclear recoils are consistent with zero and 90% confidence level upper limits on the WIMP interaction rates are set for electron equivalent energies from 3 keV to 11 keV. The 90% upper limit of NR event rate for 3.6-5.8 keV corresponding to 2-4 keV in NaI(T) is 0.0098 counts/kg/keV/day which is below the annual modulation amplitude reported by DAMA. This is incompatible with interpretations that enhance the modulation amplitude such as inelastic dark matter models. We establish the most stringent cross section limits on spin-dependent WIMP-proton elastic scattering for the WIMP masses greater than 20 GeV/c2., 5 pages, 6 figures

The GaAs quantum dots in AlGaAs barriers were grown by droplet epitaxy, emitting around 700 nm in wavelength which is compatible with low cost Si based detectors. The excitation power dependent and time resolved micro‐photoluminescence measurements identified optical characteristics of exciton and biexciton states which are attributed to good quantum confinements in GaAs QDs.

The magnetization of parabolic quantum wells and wires in strong tilted magnetic fields has been calculated analytically within the single-particle picture. The results deviate considerably from the usual dHvA oscillations and exhibit the superposition of smooth large-period oscillations and abrupt small-period oscillations as a function of the chemical potential characterized by two hybrid eigenfrequencies. The coupling Hamiltonian ∼B x B z plays a significant role in determining the magnetization despite its insignificant contribution to the energy spectrum

Liquid xenon is a suitable material for a dark matter search. For future large scale experiments, single phase detectors are attractive due to their simple configuration and scalability. However, in order to reduce backgrounds, they need to fully rely on liquid xenon's self-shielding property. A prototype detector was developed at Kamioka Observatory to establish vertex and energy reconstruction methods and to demonstrate the self-shielding power against gamma rays from outside of the detector. Sufficient self-shielding power for future experiments was obtained., 8 pages, 8 figures

We developed a SrCl 2 crystal scintillator for 0υ Electron Capture/β+ decay search, and 4π CsI(Tl) tagging detector to reduce the background. This experimental system was installed at the YangYang underground laboratory which is for the WIMP search by KIMS group. The system ran for 60 days and we have finished analyzing for 2 hits back to back coincidence signal selection.

KIMS is a dark matter search experiment using low background CsI(TB) crystals at Yangyang Underground Laboratory in Korea. With a total exposure of 3409 kg⋅d data, we set a new limit on WIMP‐nucleon cross section. We achieved the most stringent limit on the spin‐dependent interaction for a pure proton case. We were able to exclude the DAMA signal region for both spin‐dependent and spin‐independent interaction for the WIMP mass greater than 20 GeV/c2. KIMS experiment is upgraded with 12 CsI(TB) crystals corresponding to a total mass of 104 kg and accumulating data since Jan. 2008.

We measured the thermal neutron capture cross section for the $^{180}\mathrm{W}$ nucleus. There is only one previous measurement with regard to this cross section, and it yielded a value of ${30}_{\ensuremath{-}100%}^{+300%}$ b. To determine whether $^{181}\mathrm{W}$ is an appropriate low energy neutrino source, the thermal neutron capture cross section should be measured more precisely to estimate the production rate of $^{181}\mathrm{W}$ inside a nuclear reactor. We measured the cross section of $^{180}\mathrm{W}$ using a natural tungsten foil and obtained a value of $22.6\ifmmode\pm\else\textpm\fi{}1.7$ b.

We demonstrate electron transport in an enhancement mode in‐plane‐gate quantum dot transistor (IPGQDT). Deeply etched trenches allow large positive biases on the IPG with negligible leakage current Strong negative differential resistance (NDR) peaks and single electron tunneling are observed in a wide gate bias window. The position of the NDR peaks systematically moves with the change of the gate bias, suggesting that they originate from resonant tunneling through QD quantum states.

The surface electronic structures of ${\mathrm{MgCNi}}_{3}(001)$ with both the MgNi terminated (MgNi-Term) and the CNi terminated (CNi-Term) surfaces were investigated using the all-electron full-potential linearized augmented plane-wave method within the generalized gradient approximation to density-functional theory. The calculated work function of MgNi-Term $(4.17\mathrm{eV})$ is lower than that of CNi-Term $(5.16\mathrm{eV}\mathrm{}).$ The number of total electrons at the surface layer of MgNi-Term is much decreased, while that of CNi-Term is less decreased than that of MgNi-Term, with respect to their center layers. The number of $\mathrm{Ni}(S)$ d electrons of MgNi-Term is calculated to be 0.08 electrons more than that of CNi-Term. The layer projected l-decomposed local density of states (DOS) show that the difference in the number of $\mathrm{Ni}(S)\ensuremath{-}d$ electrons is due to the strong $\mathrm{C}\ensuremath{-}p$ and $\mathrm{Ni}\ensuremath{-}d$ hybridization at the surface layer of CNi-Term. The peak just below the Fermi level ${(E}_{\mathrm{F}})$ in bulk ${\mathrm{MgCNi}}_{3}$ is broadened substantially at the $\mathrm{Ni}(S)$ of CNi-Term, while that peak survives at $\mathrm{Ni}(S)$ of MgNi-Term. By analyzing the charge density belonging to a very narrow energy window just below ${E}_{\mathrm{F}},$ such considerable modifications of the DOS peak at CNi-Term is seen to be due to the broken local symmetry of the CNi layer at the surface. It is considered that the behavior of the modification of the peak near ${E}_{\mathrm{F}}$ resembles p-band hole doping through C-site substitution, supported by the stability against ferromagnetism determined from total-energy calculations. Superconductivity of the ${\mathrm{MgCNi}}_{3}(001)$ surface is discussed briefly in relation with the modifications of the DOS peak at ${E}_{\mathrm{F}}.$

AMoRE (Advanced Mo based Rare process Experiment) collaboration is going to use calcium molybdate crystals as cryogenic scintillation detector in a search for neutrinoless DBD of 100Mo isotope. Simultaneous detection of phonons and light will be used to reject internal background. A FWHM resolution of 0.2% in the phonon channel has been achieved with a 0.5 cm3 crystal. Several 40Ca100MoO4 crystals (≈ 0.5 kg) have been developed from enriched 100Mo and depleted 40Ca materials. The light yield of these crystals has been shown to be comparable with reference CaMoO4 scintillators for temperatures ranging down to 8 K. The content of dangerous radioisotopes in the crystals is under measurement. The projected sensitivity of the experiment for a 250 kg × years exposure is lim T1/2 ~ 3 × 1026 years, which corresponds to the effective Majorana neutrino mass mv ~ 0.02 – 0.06 eV.