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Register a new userMeasurement of Radon Concentration in Super-Kamiokandes Buffer Gas
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To precisely measure radon concentrations in purified air supplied to the Super-Kamiokande detector as a buffer gas, we have developed a highly sensitive radon detector with an intrinsic background as low as 0.33$pm$0.07 mBq/m$^{3}$. In this article, we discuss the construction and calibration of this detector as well as results of its application to the measurement and monitoring of the buffer gas layer above Super-Kamiokande. In March 2013, the chilled activated charcoal system used to remove radon in the input buffer gas was upgraded. After this improvement, a dramatic reduction in the radon concentration of the supply gas down to 0.08 $pm$ 0.07 mBq/m$^{3}$. Additionally, the Rn concentration of the in-situ buffer gas has been measured 28.8$pm$1.7 mBq/m$^{3}$ using the new radon detector. Based on these measurements we have determined that the dominant source of Rn in the buffer gas arises from contamination from the Super-Kamiokande tank itself.
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The radioactive noble gas radon can be a serious background source in the underground particle physics experiments studying processes that deposit energy comparable to its decay products. Low energy solar neutrino measurements at Super-Kamiokande suffer from these backgrounds and therefore require precise characterization of the radon concentration in the detectors ultra-pure water. For this purpose, we have developed a measurement system consisting of a radon extraction column, a charcoal trap, and a radon detector. In this article we discuss the design, calibration, and performance of the radon extraction column. We also describe the design of the measurement system and evaluate its performance, including its background. Using this system we measured the radon concentration in Super-Kamiokandes water between May 2014 and October 2015. The measured radon concentration in the supply lines of the water circulation system was $1.74pm0.14~mathrm{mBq/m^{3}}$ and in the return line was $9.06pm0.58~mathrm{mBq/m^{3}}$. Water sampled from the center region of the detector itself had a concentration of $<0.23~mathrm{mBq/m^{3}}$ ($95%$ C.L.) and water sampled from the bottom region of the detector had a concentration of $2.63pm0.22~mathrm{mBq/m^{3}}$.
The measurement of the internal $^{222}$Rn activity in the NEXT-White detector during the so-called Run-II period with $^{136}$Xe-depleted xenon is discussed in detail, together with its implications for double beta decay searches in NEXT. The activity is measured through the alpha production rate induced in the fiducial volume by $^{222}$Rn and its alpha-emitting progeny. The specific activity is measured to be $(38.1pm 2.2~mathrm{(stat.)}pm 5.9~mathrm{(syst.)})$~mBq/m$^3$. Radon-induced electrons have also been characterized from the decay of the $^{214}$Bi daughter ions plating out on the cathode of the time projection chamber. From our studies, we conclude that radon-induced backgrounds are sufficiently low to enable a successful NEXT-100 physics program, as the projected rate contribution should not exceed 0.1~counts/yr in the neutrinoless double beta decay sample.
The Jiangmen Underground Neutrino Observatory (JUNO), a 20ktons multi-purpose underground liquid scintillator detector, was proposed with the determination of the neutrino mass hierarchy as a primary physics goal. Due to low background requirement of the experiment, a multi-veto system ,which consists of a water Cherenkov detector and a top tracker detector, is required. In order to keep the water quality good and remove the radon in the water, a ultra-pure water system, a radon removal system and radon concentration measurement system have been designed. In this paper, the radon removal equipments and its radon removal limit will be presented.
The Jiangmen Underground Neutrino Observatory will build the worlds largest liquid scintillator detector to study neutrinos from various sources. The 20 kt liquid scintillator will be stored in a $sim$600 t acrylic sphere with 35.4 m diameter due to the good light transparency, chemical compatibility and low radioactivity of acrylic. The concentration of U/Th in acrylic is required to be less than 1 ppt (10$^{-12}$ g/g) to achieve a low radioactive background in the fiducial volume of the JUNO detector. The mass production of acrylic has started, and the quality control requires a fast and reliable radioassay on U/Th in acrylic. We have developed a practical method of measuring U/Th in acrylic to sub-ppt level using the Inductively Coupled Plasma Mass Spectrometer (ICP-MS). The U/Th in acrylic can be concentrated by vaporizing acrylic in a class 100 environment, and the residue will be collected and sent to ICP-MS for measuring U/Th. All the other chemical operation is done in a class 100 clean room, and the ICP-MS measurement is done in a class 1000 clean room. The recovery efficiency is studied by adding the natural nonexistent nuclei $^{229}$Th and $^{233}$U as the tracers. The resulting method detection limit (MDL) with 99% confidence can reach 0.02/0.06 pg $^{238}$U/$^{232}$Th /g acrylic with $sim$75% recovery efficiency. This equipment and method can not only be used for the quality control of JUNO acrylic, but also be further optimized for the radioassay on other materials with extremely low radioactivity, such as ultra-pure water and liquid scintillator.
Type 5A molecular sieves (MS) have been demonstrated to remove radon from SF$_6$ gas. This is important for ultra-sensitive SF$_6$ gas-based directional dark matter and related rare-event physics experiments, as radon can provide a source of unwanted background events. Unfortunately, commercially available sieves intrinsically emanate radon at levels not suitable for ultra-sensitive physics experiments. A method to produce a low radioactive MS has been developed in Nihon University (NU). In this work, we explore the feasibility of the NU-developed 5A type MS for use in such experiments. A comparison with a commercially available Sigma-Aldrich 5A type MS was made. The comparison was done by calculating a parameter indicating the amount of radon intrinsically emanated by the MS per unit radon captured from SF$_6$ gas. The measurements were made using a specially adapted DURRIDGE RAD7 radon detector. The NU-developed 5A MS emanated radon up to 61$pm$9$%$ less per radon captured (2.1$pm$0.1)$times 10^{-3}$, compared to the commercial Sigma-Aldrich MS (5.4$pm$0.4)$times 10^{-3}$, making it a better candidate for use in a radon filtration setup for future ultra-sensitive SF$_6$ gas based experiments.
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