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Register a new userImproved method for measuring low concentration radium and its application to the Super-Kamiokande Gadolinium project
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Chemical extraction using a molecular recognition resin named Empore Radium Rad Disk was developed to improve sensitivity for the low concentration of radium (Ra). Compared with the previous method, the extraction process speed was improved by a factor of three and the recovery rate for $^{226}$Ra was also improved from 81$pm$4% to $>$99.9%. The sensitivity on the 10$^{-1}$ mBq level was achieved using a high purity germanium detector. This improved method was applied to determine $^{226}$Ra in Gd$_2$(SO$_4$)$_3{cdot}$8H$_2$O which will be used in the Super-Kamiokande Gadolinium project. The improvement and measurement results are reported in this paper.
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In order to improve Super-Kamiokandes neutron detection efficiency and to thereby increase its sensitivity to the diffuse supernova neutrino background flux, 13 tons of Gd2(SO4)3*8H2O(gadolinium sulfate octahydrate) was dissolved into the detectors otherwise ultrapure water from July 14 to August 17, 2020, marking the start of the SK-Gd phase of operations. During the loading, water was continuously recirculated at a rate of 60 m3/h, extracting water from the top of the detector and mixing it with concentrated Gd2(SO4)3*8H2O solution to create a 0.02% solution of the Gd compound before injecting it into the bottom of the detector. A clear boundary between the Gd-loaded and pure water was maintained through the loading, enabling monitoring of the loading itself and the spatial uniformity of the Gd concentration over the 35 days it took to reach the top of the detector.During the subsequent commissioning the recirculation rate was increased to 120 m3/h, resulting in a constant and uniform distribution of Gd throughout the detector and water transparency equivalent to that of previous pure-water operation periods. Using an Am-Be neutron calibration source the mean neutron capture time was measured to be $115.6pm0.6$ $mu$s, which corresponds to a Gd concentration of $110.9pm1.4$ (stat.only) ppm, as expected for this level of doping. This paper describes changes made to the water circulation system for this detector upgrade, the Gd loading procedure, detector commissioning, and the first neutron calibration measurements in SK-Gd.
Procedures and results on hardware level detector calibration in Super-Kamiokande (SK) are presented in this paper. In particular, we report improvements made in our calibration methods for the experimental phase IV in which new readout electronics have been operating since 2008. The topics are separated into two parts. The first part describes the determination of constants needed to interpret the digitized output of our electronics so that we can obtain physical numbers such as photon counts and their arrival times for each photomultiplier tube (PMT). In this context, we developed an in-situ procedure to determine high-voltage settings for PMTs in large detectors like SK, as well as a new method for measuring PMT quantum efficiency and gain in such a detector. The second part describes the modeling of the detector in our Monte Carlo simulation, including in particular the optical properties of its water target and their variability over time. Detailed studies on the water quality are also presented. As a result of this work, we achieved a precision sufficient for physics analysis over a wide energy range (from a few MeV to above a TeV). For example, the charge determination was understood at the 1% level, and the timing resolution was 2.1 nsec at the one-photoelectron charge level and 0.5 nsec at the 100-photoelectron charge level.
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 $2.63pm0.22~mathrm{mBq/m^{3}}$.
Krypton-85 is an anthropogenic beta-decaying isotope which produces low energy backgrounds in dark matter and neutrino experiments, especially those based upon liquid xenon. Several technologies have been developed to reduce the Kr concentration in such experiments. We propose to augment those separation technologies by first adding to the xenon an 85Kr-free sample of krypton in an amount much larger than the natural krypton that is already present. After the purification system reduces the total Kr concentration to the same level, the final 85Kr concentration will have been reduced even further by the dilution factor. A test cell for measurement of the activity of various Kr samples has been assembled, and the activity of 25-year-old Krypton has been measured. The measured activity agrees well with the expected activity accounting for the 85Kr abundance of the earth atmosphere in 1990 and the half-life of the isotope. Additional tests with a Kr sample produced in the year 1944 (before the atomic era) have been done in order to demonstrate the sensitivity of the test cell.
Photomultiplier tubes (PMTs) are often used in low-background particle physics experiments, which rely on an excellent response to single-photon signals and stable long-term operation. In particular, the Hamamatsu R11410 model is the light sensor of choice for liquid xenon dark matter experiments, including XENONnT. The same PMT model was also used for the predecessor, XENON1T, where issues affecting its long-term operation were observed. Here, we report on an improved PMT testing procedure which ensures optimal performance in XENONnT. Using both new and upgraded facilities, we tested 368 new PMTs in a cryogenic xenon environment. We developed new tests targeted at the detection of light emission and the degradation of the PMT vacuum through small leaks, which can lead to spurious signals known as afterpulses, both of which were observed in XENON1T. We exclude the use of 26 of the 368 tested PMTs and categorise the remainder according to their performance. Given that we have improved the testing procedure, yet we rejected fewer PMTs, we expect significantly better PMT performance in XENONnT.
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