Y. Nakano

Measurement of radon concentration in super-Kamiokande’s buffer gas

Abstract 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 ± 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 ± 0 . 07 mBq ∕ m 3 . Additionally, the Rn concentration of the in-situ buffer gas has been measured 28 . 8 ± 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.

Development of the database for low-background studies in Kamioka

Needless to say, it is important to select ultra-low background materials for dark matter and 0νββ detectors. All materials and components must be evaluated before the detector construction. This presentation will focus on developing a database for such purpose based on the open source database, Persephone (radiopurity.org). The aim of this project is to share information on low-radioactivity materials measured by several techniques in Kamioka observatory among Japanese community. Improvements of the functions of the database and the typical contents will be shown. In addition, several running programs for screening low-radioactive materials at Kamioka will be discussed.

Radon background study in Super-Kamiokande

Super-Kamiokande (SK), a 50 kton water Cherenkov detector in Japan, observes 8B solar neutrinos with neutrino-electron elastic scattering. SK searches for distortions of the solar neutrino energy spectrum caused by the edge of the MSW resonance in the core of the Sun. The installation of new front-end electronics in 2008 marks the beginning of the 4th phase of SK (SK-IV). With the improvement of the water circulation system, calibration methods, reduction cuts, this phase achieved the lowest energy threshold thus far (3.5 MeV kinetic energy). To improve the sensitivity to the MSW effect, it is required to achieve lower energy threshold. For this purpose, understanding the origin of background events and reducing them are important. Currently, the main background is known as a beta decay of 214Bi in a Radon decay chain. So far, SK collaboration has developed several techniques for studying Radon contamination in the SK water. In this proceedings, a measurement system which can measure Radon concentration in the SK water with the accuracy of 0.1 mBq/m3 level is presented. In addition, an evaluation of Radon background events in SK injecting Radon rich water into the SK tank, as well as future prospects are also presented.

Radon assay and radioactivity database in Kamioka

A new cooperative program among underground experiments, theorists, and low-background researchers was started in 2014 in Japan. The title of this program is “Revealing the history of the universe with underground particle and nuclear research”, and the purpose of this program is to achieve technical and scientific synergies among underground researchers. More than 70 researchers are currently contributing to this program.

Solar neutrino results from Super-Kamiokande

Super-Kamiokande (SK), a 50 kton water Cherenkov detector in Japan, observes 8B solar neutrinos with neutrino-electron elastic scattering. SK searches for distortions of the solar neutrino energy spectrum caused by the edge of the MSW resonance in the core of the Sun. It also searches for a day/night solar neutrino flux asymmetry induced by the matter in the Earth. The installation of new front-end electronics in 2008 marks the beginning of the 4th phase of SK (SK-IV). With the improvement of the water circulation system, calibration methods, reduction cuts, this phase achieved the lowest energy threshold thus far (3.5 MeV kinetic energy). SK observed solar neutrinos for about 19 years, that is 5,200 days. This long operation covers more than 1.5 solar activity cycles. An analysis about a possible correlation between solar neutrino flux and 11 year activity cycle will be presented. To test the matter effect induced by the Sun and the Earth, the energy spectrum and the day/night solar neutrino flux asymmetry from SK-I to SK-IV is analyzed. A global oscillation analysis using SK-I,II,III, and SK-IV data and combined with the results of other solar neutrino experiments as well as KamLAND reactor experiment has been carried out. The results of this global analysis will be presented as well.

Development of a high sensitivity radon detector for purified gases

For underground particle physics experiments, noble gas radon from U-chain decay series could be a source of serious background events. We have developed a high sensitivity detector for radon using electrostatic collection and PIN photodiode. Recently, we have developed a new type of radon detector. We report basic performance of the new 80L radon detector.