| Project/Area Number |
19H00675
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| Research Category |
Grant-in-Aid for Scientific Research (A)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Review Section |
Medium-sized Section 15:Particle-, nuclear-, astro-physics, and related fields
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| Research Institution | The University of Tokyo |
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Principal Investigator |
MARTENS Kai 東京大学, カブリ数物連携宇宙研究機構, 准教授 (20535025)
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| Co-Investigator(Kenkyū-buntansha) |
竹田 敦 東京大学, 宇宙線研究所, 准教授 (40401286)
伊藤 好孝 名古屋大学, 宇宙地球環境研究所, 教授 (50272521)
身内 賢太朗 神戸大学, 理学研究科, 准教授 (80362440)
平出 克樹 東京大学, 宇宙線研究所, 特任助教 (10584261)
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| Project Period (FY) |
2019-04-01 – 2023-03-31
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| Project Status |
Granted (Fiscal Year 2022)
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| Budget Amount *help |
¥40,690,000 (Direct Cost: ¥31,300,000、Indirect Cost: ¥9,390,000)
Fiscal Year 2022: ¥9,360,000 (Direct Cost: ¥7,200,000、Indirect Cost: ¥2,160,000)
Fiscal Year 2021: ¥9,230,000 (Direct Cost: ¥7,100,000、Indirect Cost: ¥2,130,000)
Fiscal Year 2020: ¥8,580,000 (Direct Cost: ¥6,600,000、Indirect Cost: ¥1,980,000)
Fiscal Year 2019: ¥13,520,000 (Direct Cost: ¥10,400,000、Indirect Cost: ¥3,120,000)
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| Keywords | neutron veto / dark matter / direct detection / gadolinium / water Cherenkov / neutron tagging / gadolinium sulfate / WIMP / solar axion / 暗黒物質 / 水チェレンコフ型検出器 / 中性子検出器 |
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| Outline of Research at the Start |
暗黒物質(DM)の正体の解明は現代の物理学における最重要課題の一つである。有効質量約4tの液体キセノン検出器を用いたXENONnT実験は世界最高感度でのDM探索を行う。この実験ではDMによるキセノン原子核の反跳現象を探索するが、検出器内部の放射性不純物起因の中性子でも同様の現象が起こるため、中性子を同定する中性子検出器がDMの証拠を掴む鍵を握る。本研究ではスーパーカミオカンデで培われた中性子検出技術の初の転用例として、中性子検出器の建設や較正作業、安定な運転を行い、中性子による事象の80％以上を排除する。これで感度を一桁向上し、DM－核子散乱断面積2x10-48cm2の探索感度で発見を目指す。
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| Outline of Annual Research Achievements |
Three major neutron veto milestones were reached in FY2022:
1.) The gadolinium water purification system (GdWPS) was first commissioned with pure water in closed loop circulation (CLC), separate from the 700 tonne water shield that contains also the neutron veto. We thus verified the basic functions of all its components.
2.) Maintaining water quality in the full 700 tonne XENONnT water shield - which comprises the detector's muon and neutron veto - our GdWPS proved that it improved the quality of the shields pure water.
3.) After returning to CLC operation we finally added gadolinium sulfate (GdS) to the ~3 tons of water in the closed loop, bringing it up to the ultimate target concentration of 0.5 % of gadolinium sulfate in January. We optimize operating parameters for coming neutron veto operation with GdS dissolved in the whole water shield.
Together these three milestones mean that now the neutron veto systems are all ready to be loaded with GdS and thus reach our design neutron tagging efficiency. Unfortunately the rest of the experiment is not yet in a state that we can proceed at this point in time, but we are ready and all our systems have been shown to work as expected. We can dissolve GdS at any time and reach full efficiency as soon as the GdS is evenly distributed in the 700 tonne water shield.
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| Current Status of Research Progress |
Current Status of Research Progress
4: Progress in research has been delayed.
Reason
If the XENONnT liquid xenon time projection chamber (TPC) gets upgraded, all the GdS that is dissolved in the 700 tonne water shield will be irretrievably lost - and the neutron veto reduced to using the hydrogen capture signal rather than much larger gadolinium capture signal. As the XENON collaboration is still deliberating if and when to access the TPC, and the GdS we have can only be used once, we have to wait for this collaboration decision to be taken before we decide when to introduce GdS into the water shield and thus significantly enhance the veto's neutron tagging efficiency. Achieving this remains the final goal to be achieved with this research grant, and our recent discovery during science run zero nuclear recoil analysis that our XENONnT detector's neutron background is significantly larger than expected adds new urgency to achieving this goal. The collaboration expects us to deliver, but we all have to wait until the next collaboration meeting decides on how to proceed. In the meantime we will continue to improve the GdWPS's performance while tuning it in CLC operation for optimal performance when treating gadolinium water for the whole 700 tonne water shield.
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| Strategy for Future Research Activity |
(Gadolinium) water transparency is the most important parameter determining a water Cherenkov neutron veto's efficiency. In CLC we can change the flow patterns and speed through the GdWPS and systematically study how to optimize water transparency while minimizing gadolinium removal in the de-ionization resin that removes all ions that pass through it from the water and which is necessary to remove the non-Gd and non-sulfate ions that pure water leaches out of all detector materials it touches. These dissolved ions are a main source of absorption for light in the wavelength range that is determined by the neutron veto's photosensor sensitivity and the Cherenkov light's emission spectrum. We have a UV spectrometer on-site at LNGS with which we can regularly check the water transparency in the relevant wavelength range and thus monitor the effects of our exploration of the GdWPS operating parameter range, taking into account the loss of GdS that those operating parameters allow to reach the de-ionization resin. This program is ongoing, and is progressing smoothly: we now have a good parameters at hand, but will continue to try to improve on them until XENONnT is ready for GdS.
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