The development of high sensitive N[lc]aI detector for dark matter search

Abstract

The dark matter problem is one of the most important topics of astrophysics and particles physics in several decades. Many observations show that the most of the mass in the universe is composed of non-barons. Many candidates for dark matter have been proposed and studied by theoretically and experimentally. WIMPs (Weakly Interacting Massive Particles) is one of the promising candidate of dark matter. The aim is to detect the WIMPs by WIMPs-nucleus direct interaction. A Nal(Tl) scintillator for the WIMPS detector has been applied. It is suitable detector for dark matter search, because both I-127 and Na-22 have the large cross-section for WIMPs, and the I-127 nucleus is expected to emit a 57.6 keV gamma ray by inelastic scattering. Comparing with a germanium detector and bolometer, it is easy to operate because it works in the normal temperature. It is the great advantage to obtain the good statistical. accuracy because the detector volume can be enlarged with low cost. To reach the goal of the sensitivity, the concentration in the detector was required below a few ppt. So the method has been developed to measure accurately the concentration of U-chain in the detector, and to trace the origin of the U-chain contaminants. The best way to determine the concentration of U-chain in the beta-alpha delayed coincidence method. The beta rays from B-214 are observed as a prompt signal and the alpha rays from Po-214 are observed as a delayed signal by performing the delayed coincidence method. The concentration of Bi and Po isotopes of the order of 1 ppt was successfully measured, which is difficult by the chemical analysis. The ultra-low concentration of U-chain in the detector has been measured by only the delayed coincidence method. However the U-chain is not always in the secular radioactive equilibrium. Because the half-life of Pb-210 is long (T(sub 1/2) = 22 y), the radioactivity of the progeny of Pb-210 is often stronger than that of Pb-214. In this work, the concentration of Po-210 was successfully determined by a surprisingly simple method. The required sensitivity for WIMPS and recent development of highly-pure NaI(Tl) detectors will be discussed.