2.2 Detector Design and Construction
2.2.3 Water Purification and Circulation System
All materials used in detector construction are extensively tested to avoid introdu-cing sources of radioactivity or water-soluble impurities into the detector. Despite these efforts however, radon emanating from materials in the tank, particularly the glass of photosensors and the support structure, and from the rock surrounding the detector is a major source of low-energy backgrounds. In addition, light scattering and absorption due to impurities in the water are a major source of uncertainty in event reconstruction. To reduce these effects, the water in the detector is constantly recirculated and purified.
In Super-Kamiokande, after continuous improvements the water system is now able to purify the water inside the detector to reach a water transparency of over 100 m and a radon concentration in the ID of less than 1 mBq/m3. In Hyper-Kamiokande, where the diagonal size of the detector increases to nearly 100 m, a similar or better water quality will be required. To achieve this goal, the design of the water system will be similar to that employed in Super-Kamiokande but scaled up to account for the larger detector mass.
The system consists of two separate stages (see figure 2.4), one for initial filling of the tank and one for ongoing recirculation of the water during operations. The water to fill the detector will come from the storage well of the snow-melting system of the nearby Kamioka town. During filling, 105 t/h of source water will be needed to fill the detector with purified water at a rate of 78 t/h, with approximately half a year needed to completely fill the detector. The water will be recirculated at a rate of 310 t/h, such that the total detector mass is recirculated approximately once per month, at the same rate as in Super-Kamiokande.
In the first stage, the raw water is passed through a 10 µm filter to eliminate dust and larger particles from the raw water, before going through reverse osmosis (RO) and additional filters (MB) to remove smaller particulates. A vacuum degasifier (VD) removes dissolved oxygen (which encourages growth of bacteria in the water) and radon from the water before the pre-cleaned water is fed into the second stage of the system.
In the second stage, water coming from the Hyper-Kamiokande tank is sterilized with UV light and filtered before going through a multi-stage process including RO, further UV irradiation (UV TOC), a cartridge polisher (CP) that removes heavy ions and more advanced filtration (MF, UF). The purified water is then cooled down in a heat exchanger (HE) to remove heat produced by photosensors and electronics in
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2.2 Detector Design and Construction
Rn/CO2/H2O/bacteria free air Rn/CO2/H2O/bacteria free air
60t/d
Figure 2.4: Water system design for Hyper-Kamiokande. See text for an explanation of the individual steps. Figure from reference [169].
the water as well as the water system itself. Finally, the cooled water is degasified in a VD and supplied back into the tank.
Radon-free air with a concentration of less than 1 mBq/m3 is used as a cover gas for the Hyper-Kamiokande tank as well as for buffer tanks which are part of the water system. To produce enough radon-free air, the system employed in Super-Kamiokande will be scaled up to the larger detector size.
To avoid radon from the surrounding rock entering the inner detector, there is no water exchange between the inner and outer detector. Radioactive impurities in the inner detector mostly originate from the photomultipliers and their support structure. Controlling the water flow in the detector is essential to limit the spread of these impurities in the inner detector and reduce their impact on the physics performance of the detector. Computer simulations of the water flow (see figure 2.5) show that supplying cold water at the bottom of the tank and draining water at the top leads to laminar flow and ensures effective water replacement. Supplying cold water at the top of the tank and draining water from the bottom would instead lead to convection in the tank, which leads to uniform water quality throughout the tank
Chapter 2 The Hyper-Kamiokande Detector
(a) Supply to the bottom and drain from the top
(b) Supply to the top and drain from the bottom 10days
10days
20days 30days 40days
20days 30days 40days
Figure 2.5: Simulation of water replacement efficiency in Hyper-Kamiokande. The tank is filled with old water (blue) at the start of the simulation and fresh water (red) is then supplied: (a) at the bottom of the tank while draining from the top; (b) at the top of the tank while draining from the bottom. Supplying fresh water from the bottom leads to a higher replacement efficiency, displayed as a more reddish colour, while supplying it from the top leads to large-scale convection in the tank and a more uniform water quality. Figure from reference [169].
and decreases the efficiency of water replacement. These simulations agree with observations in Super-Kamiokande.
Adding gadolinium to a water Cherenkov detector to detect neutron captures and thus better identify events was originally suggested by Beacom and Vagins in 2003 [211]. After extensive testing, gadolinium will be added to Super-Kamiokande in the near future [212] and is being explored as an option for Hyper-Kamiokande.
This would require changes to the water system to remove the gadolinium from drained water using molecular bandpass filters. Both components would then be cleaned separately and recombined before supplying the water back into the tank.
The necessary technologies have been developed for Super-Kamiokande and use a modular design, ensuring they can be scaled up for use in Hyper-Kamiokande.
Since gadolinium loading is not part of the Hyper-Kamiokande baseline design and may instead be added in a later upgrade, I am not considering its benefits in this thesis.
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2.2 Detector Design and Construction