| 研究課題/領域番号 |
19H00675
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| 研究機関 | 東京大学 |
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研究代表者 |
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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| 研究分担者 |
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)
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| 研究期間 (年度) |
2019-04-01 – 2023-03-31
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| キーワード | neutron veto / water Cherenkov / gadolinium |
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| 研究実績の概要 |
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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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
4: 遅れている
理由
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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| 今後の研究の推進方策 |
(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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