Development of cooled scintillators for dark matter detection

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We confirmed that an undoped cesium iodide crystal is a very efficient scintillator if it is cooled down to the liquid nitrogen temerature.We newly found that a cooled monocrystal of lead molybdate also functions as a scintillator at temperature below 100K.Since it contains molybdenum-100, a double beta decay nuclide, it may open new perspective of the double beta decay experiments. We performed a short term experiment with this crystal for the evaluation, and obtained 8.9*10^<17> for the lower limit of the half life of ^<100>Mo double beta decay. We also found that lead-210 impurity in the crystal is the limitting factor of the experiment.A lead tungstate, similar material to the lead molybdate, is found to be also a scintillator below 100K.