S.K. Kim

New Limits on Interactions between Weakly Interacting Massive Particles and Nucleons Obtained with CsI(Tl) Crystal Detectors

The Korea Invisible Mass Search (KIMS) experiment presents new limits on the weakly interacting massive particle (WIMP)-nucleon cross section using data from an exposure of 3409 kg.d taken with low-background CsI(Tl) crystals at the Yangyang Underground Laboratory. The most stringent limit on the spin-dependent interaction for a pure proton case is obtained. The DAMA signal region for both spin-independent and spin-dependent interactions for the WIMP masses greater than 20 GeV/c2 is excluded by the single experiment with crystal scintillators.

Particle‐ and photoinduced conductivity in type‐IIa diamonds

Electrical characteristics associated with radiation detection were measured on single‐crystal natural type‐IIa diamond using two techniques: charged particle‐induced conductivity and time‐resolved transient photoinduced conductivity. The two techniques complement each other: The charged particle‐induced conductivity technique measures the product of the carrier mobility μ and lifetime τ throughout the bulk of the material while the transient photoconductivity technique measures the carrier mobility and lifetime independently at the first few micrometers of the material surface. For each technique, the μτ product was determined by integration of the respective signals. The collection distance that a free carrier drifts in an electric field was extracted by each technique. As a result, a direct comparison of bulk and surface electrical properties was performed. The data from these two techniques are in agreement, indicating no difference in the electrical properties between the bulk and the surface of the ...

Neutrino-Less Double Beta Decay Experiment Using Ca

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.

^{100}

Abstract We have studied the response of Tl- and Na-doped CsI crystals to nuclear recoils and γs below 10 keV . The response of CsI crystals to nuclear recoil was studied with mono-energetic neutrons produced by the 3 H ( p , n ) 3 He reaction. This was compared to the response to Compton electrons scattered by 662 keV γ-ray. Pulse shape discrimination between the response to these γs and nuclear recoils was studied, and quality factors were estimated. The quenching factors for nuclear recoils were derived for both CsI(Na) and CsI(Tl) crystals.Abstract We have studied the response of Tl- and Na-doped CsI crystals to nuclear recoils and γs below 10 keV . The response of CsI crystals to nuclear recoil was studied with mono-energetic neutrons produced by the 3 H ( p , n ) 3 He reaction. This was compared to the response to Compton electrons scattered by 662 keV γ-ray. Pulse shape discrimination between the response to these γs and nuclear recoils was studied, and quality factors were estimated. The quenching factors for nuclear recoils were derived for both CsI(Na) and CsI(Tl) crystals.

MoO

Abstract The Korea Invisible Mass Search (KIMS) Collaboration has been carrying out WIMP search experiment with CsI ( T l ) crystal detectors at the YangYang Underground Laboratory. A successful reduction of the internal background of the crystal was done and a good pulse shape discrimination was achieved. We report the first result on WIMP search obtained with 237 kgu2009days data using one full-size CsI ( T l ) crystal of 6.6 kg mass.

_{4}

Abstract Searches for weakly interacting massive particles (WIMP) can be based on the detection of nuclear recoil energy in CsI(Tl) crystals. We demonstrate that low-energy gamma rays down to a few keV are detected with CsI(Tl) crystal detector. A clear peak at 6xa0keV is observed using an X-ray source. Good energy resolution and linearity have been achieved down to the X-ray region. In addition, we also show that alpha particles and gamma rays can be clearly separated using the different time characteristics of the crystal.

Scintillation Crystals

The response of CsI(Tl) crystals to nuclear recoils and gammas down to 3 keV has been studied, and it has been demonstrated that the pulse shape discrimination capability for CsI(Tl) is better than that for NaI(Tl). Extensive study of internal background of CsI powder was performed and a lot of efforts have been made to reduce the internal background sources such as 137Cs and 87Rb. The 137Cs concentration of 2 mBq/kg was achieved successfully in the CsI powder using ultra-pure water for whole extraction process. The Rb concentration was successfully reduced to be less than 1 ppb by the recrystallization method. The CsI(Tl) crystal used for the experiment was grown with the ultra-low background CsI powder and a dimension of the CsI(Tl) crystal is 8times8times30 cm3 which corresponds to weight of 8.7 kg. The number of photoelectrons is measured to be about 4-6/keV with a RbCs photocathode PMT. The background of CsI(Tl) crystals were measured in the shielding of 30 cm thick mineral oil, 15 cm thick lead, 5 cm thick polyethylene and 10 cm copper at the 700 m minimum depth of underground laboratory at YangYang Pumped Storage Power Plant in Korea. The WIMP search with low background CsI(Tl) crystals has been carried out by the Korean Invisible Mass Search (KIMS) Collaboration and the lower limit on WIMP-nucleon cross section are reported.

Neutron beam test of CsI crystal for dark matter search

Several scintillation CaMoO4 crystals with size up to 28times28times220 mm3 were grown by the Czochralski method. Their scintillation properties have been evaluated. Light yield of full size crystals measured with a XP2020 PMT is about 4% relative to a small reference CsI(Tl) crystal. Radio luminescence spectrum under gamma-excitation contains single emission peak with maximum at 520 nm. Optical transmission spectra contain a weak absorption band around 420 nm, which has almost no influence on scintillation light. This allows to produce even larger scintillation elements without deteriorating the light yield. Scintillation kinetics was measured under gamma- and alpha-particle excitation both in fast (2000 ns) and slow (200 mus) time scales. Fast components - 12 ns, (0.1%); 200 ns (0.5%) were detected along with slow - 3.8 mus (3.4%); 20 mus (96%) - components. Difference in fast component contribution under gamma and alpha excitation allows to implement pulse-shape discrimination of alpha-radioactive background coming from impurities in the crystals.

First limit on WIMP cross section with low background CsI(Tℓ) crystal detector

We report here the first measurements of a diamond-tungsten sampling calorimeter. The calorimeter consisted of twenty layers of diamond with one radiation length of tungsten per layer. The diamond layers were grown by chemical vapor deposition and were 3.0 × 3.0 cm2 wafers with an average thickness of 500 μm. We measured the energy response and resolution (σE/E) of this calorimeter in 0.5–5.0 GeV electron beams and compared the results with those from a silicon calorimeter of similar construction. Our energy resolution is σE/E = (4.7 ± 2.7)%/E≍(19.13±0.86)%/√E≍(2.3±1.8)% for the diamond-tungsten calorimeter, where ⊕ indicates addition in quadrature. This is in good agreement with our result for the silicon-tungsten calorimeter of σE/E = (3.89 ± 0.87)%/E ≍ (19.73±0.19)%/√E ≍(0.0 ± 1.6)%. We also compare our data with EGS simulations.

Test of CsI (Tℓ) crystals for the dark matter search

We have constructed charged particle detectors using high quality CVD diamond. We report here the measurements of a diamond-tungsten sampling calorimeter and a diamond mustrip detector. The energy response and resolution (σEE) of the calorimeter were measured using an electron beam of energy 0.5 to 5.0 GeV, and compared with those from a silicon calorimeter of similar construction. We find σEE = (4.7 ± 2.7)%/E ⊕ (19.13 ± 0.86)%/√E ⊕ (2.3 ± 1.8)% for the diamond-tungsten calorimeter, where ⊕ indicates addition in quadrature, which is in good agreement with our result of σE/E = (3.89 ± 0.87)%/E ⊕ (19.73 ± 0.19)%/√E ⊕ (0.0 ± 1.6)% for the silicon-tungsten calorimeter. The CVD diamond mustrip detector consists of 50 μm wide strips on 100 μm centers. A signal-to-noise ratio of 6: 1 and a position resolution of 25 μm was observed during recent accelerator tests.