I. S. Hahn

Limits on WIMP-nucleon cross section with CsI(Tl) crystal detectors

The Korea Invisible Mass Search(KIMS) experiment presents new limits on WIMP-nucleon cross section using the data from an exposure of 3409 kgd taken with low background CsI(Tl) crystals at Yangyang underground laboratory. The most stringent limit on the spin dependent interaction for pure proton case is obtained. The DAMA signal region for both spin independent and spin dependent interactions for the WIMP mass higher than 20 GeV/c^2are excluded by the single experiment with crystal scintillators.

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.

Neutron beam test of CsI crystal for dark matter search

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.

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

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 kg days data using one full-size CsI ( T l ) crystal of 6.6 kg mass.

Tests on NaI(Tl) crystals for WIMP search at the Yangyang Underground Laboratory

Abstract Among the direct searches for WIMP-type dark matter, the DAMA experiment is unique in that it has consistently reported a positive signal for an annual-modulation signal with a large (9.3σ) statistical significance. This result is controversial because if it is interpreted as a signature for WIMP interactions, it conflicts with other direct search experiments that report null signals in the regions of parameter space that are allowed by the DAMA observation. This necessitates an independent verification of the origin of the observed modulation signal using the same technique as that employed by the DAMA experiment, namely low-background NaI(Tl) crystal detectors. Here, we report first results of a program of NaI(Tl) crystal measurements at the Yangyang Underground Laboratory aimed at producing NaI(Tl) crystal detectors with lower background levels and higher light yields than those used for the DAMA measurements.

Study of the internal background of CsI(Tl) crystal detectors for dark matter search

A search for particle cold dark matter with CsIðTcÞ crystal is being prepared at the Cheong-Pyeong underground laboratory in Korea. The background spectra ofCsI ðTcÞ crystal detectors in a prototype shield were obtained. The lowest background count rate ofthe test sample ofcrystals is measured to be 64 :775:1 counts/keV/kg/day in the energy range of5–20 keV : Quantitative estimation ofresidual radioactive isotope in CsI ðTcÞ was made using the GEANT4 Monte Carlo simulation. Analysis results show that CsIðTcÞ crystal could be a good candidate for direct detection of WIMPs when the contamination level ofcesium radioisotopes is reduced to under a f mBq/kg. r 2003 Elsevier Science B.V. All rights reserved.

Development of Low Background CsI(Tl) Crystals and Search for WIMP

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.

Search for solar axions with CsI(Tl) crystal detectors

A bstractThe results of a search for solar axions from the Korea Invisible Mass Search (KIMS) experiment at the Yangyang Underground Laboratory are presented. Low-energy electron-recoil events would be produced by conversion of solar axions into electrons via the axio-electric effect in CsI(Tl) crystals. Using data from an exposure of 34,596 kg · days, we set a 90 % confidence level upper limit on the axion-electron coupling, gae, of 1.39 × 10−11 for an axion mass less than 1 keV/c2. This limit is lower than the indirect solar neutrino bound, and fully excludes QCD axions heavier than 0.48 eV/c2 and 140.9 eV/c2 for the DFSZ and KSVZ models respectively.

Pulse-shape discrimination between electron and nuclear recoils in a NaI(Tl) crystal

A bstractWe report on the response of a high light-output NaI(Tl) crystal to nuclear recoils induced by neutrons from an Am-Be source and compare the results with the response to electron recoils produced by Compton-scattered 662 keV γ-rays from a 137Cs source. The measured pulse-shape discrimination (PSD) power of the NaI(Tl) crystal is found to be significantly improved because of the high light output of the NaI(Tl) detector. We quantify the PSD power with a quality factor and estimate the sensitivity to the interaction rate for weakly interacting massive particles (WIMPs) with nucleons, and the result is compared with the annual modulation amplitude observed by the DAMA/LIBRA experiment. The sensitivity to spin-independent WIMP-nucleon interactions based on 100 kg·year of data from NaI detectors is estimated with simulated experiments, using the standard halo model.

Measurement of the quenching and channeling effects in a CsI crystal used for a WIMP search

We have studied channeling effects in a cesium iodide (CsI) crystal that is similar in composition to the ones being used in a search for Weakly Interacting Massive Particles (WIMPs) dark matter candidates, and measured its energy-dependent quenching factor, the relative scintillation yield for electron and nuclear recoils. The experimental results are reproduced with a GEANT4 simulation that includes a model of the scintillation efficiency as a function of electronic stopping power. We present the measured and simulated quenching factors and the estimated effects of channeling