| Co-Investigator(Kenkyū-buntansha) |
KOSHIO Yusuke The University of Tokyo, Institute for Cosmic Ray Research, Assistant Professor, 宇宙線研究所, 助手 (80292960)
FUKUDA Yoshiyuki Miyagi University of Education, Department of Physics, Associate Professor, 教育学部, 助教授 (40272520)
竹内 康雄 東京大学, 宇宙線研究所, 助教授 (60272522)
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| Budget Amount *help |
¥16,400,000 (Direct Cost: ¥16,400,000)
Fiscal Year 2006: ¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 2005: ¥7,100,000 (Direct Cost: ¥7,100,000)
Fiscal Year 2004: ¥7,300,000 (Direct Cost: ¥7,300,000)
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| Research Abstract |
It is possible to investigate properties of neutrinos by using neutrinoless double beta decay. Discrimination of Majorana and Dirac property of neutrinos is one of the most important tasks in particle physics. If we observe neutrinoless double beta decay, absolute mass of neutrinos can be also studied. The object of this research is to develop new technique for neutrinoless double beta decay experiment. In this research, xenon is used for calorimeter (scintillation yield is comparable to that of NaI(Tl)) as well as double beta decay nuclide. To achieve high sensitivity, we need to reduce background. Since the most of background originates from ambient gamma rays and gamma rays from photomultiplier tubes, we devised a special design. The detector consists of xenon enclosed in an acrylic vessel, water shield in an elliptic tank, and photomultiplier tubes. By putting an acrylic vessel with xenon at one focus and photomultiplier tubes at another focus, water works a radiation shield both f
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or ambient gamma rays and gamma rays from PMTs. In addition to that, scintillation photons from the vessel are collected to photomultiplier tubes efficiently since the inner surface of the elliptic water is made of mirror. The research involves developments of 1.acrylic vessel, 2.wavelength shifter, 3.scintillation light yield of high pressure liquid xenon at room temperature, 4.elliptic tank, 5.estimation of background, and 6.vessel made of plastic scintillator.
The most important results are obtained for 2. and 3. As for 2, we obtained a conversion efficiency of 49+/-4% with 4% w/w TPB (tetra-phenyl butadiene) in polystyrene. This number is the probability of emitting a visible photon when we inject one photon with 175nm which is the wavelength of scintillation. The time constant of this conversion was faster than that of scintillation light (45ns). The conversion efficiency went down to 20% when we put this material into liquid xenon directly. Since this indicated that the liquid xenon affected the shifter, we need to devise protection for the shifter. As for 3, we firstly measured scintillation light yield at high pressure, room temperature liquid xenon. We measured it using radioactive source under high pressure (5.7MPa), room temperature (3 C). It was normalized with scintillation light yield under -100C, 0.06MPaG and found that it is 0.85. After evaluating difference of light collection efficiencies among two measurements, we are going to publish these results. As for 4, a prototype of elliptic tank was produced but we found bad condition of inner surface. We are going to improve the surface condition. As for 5 and 6, we continue their study. Less
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