Dark Matter Direct Search with Liquid-Gas Double Phase Xenon Detector

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Following modifications were done to improve the detector performance.* Protection wires were installed to keep the constant electric field of the detector without a vibration of the anode wires.* Krytox was introduced between photomultipliers and MgF2 windows to improve light collection efficiency.* Additional purification device was introduced to improve with the purity of xenon.We tried the same experimental method as XENON10 group because they used a two phase xenon detector similar to our detector and got the best result at present. The detector is operated as a three-dimensional position detector by using a signal of the proportional scintillation as trigger in substitution for a signal of the direct scintillation and restricted a sensitive region (self-shielding). For example, more than 80% reduction of background was achievable with the3cm thickness of liquid xenon shielding (active xenon~5kg). The signal from nuclear recoil was studied by irradiation of neutrons (241Am-Be) to the detector. The correlation between the ratio of the number of photons from direct scintillation and proportional scintillation and the energy was analyzed for electron recoil by gamma rays or δ decays and xenon nuclear recoil, respectively. As a result from about two days search, the event to be xenon nuclear recoil was only one.More than two months search is expected that the research sensitivity reaches to the same or better region which was obtained by XENON10 experiment by the present device.