Continuous Observation of Solar Neutrinos and Detection of Supernova Neutrinos

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Kamiokande is a water Cherenkov detector which is located at the Kamioka Underground Laboratory of Institute for Cosmic Ray Research, University of Tokyo. It is the only realtime detector out of the four solar-neutrino detectors currently at work in the world. It can also measure the energy of recoil electrons. Because of these characteristic features, its observation results have been highly valued. In 1990, the Kamiokande detector was upgraded by, e.g., replacing electronics with a new high-performance system. In order to fully exploit the new electronics, however, power supplies and data-acquisition sytem had also to be upgraded ; this was achieved in 1991 supported by the present Grant-in-Aid.Main results of the Kamiokande's solar-neutrino observation, which is one of the two subjects of this research project, obtained from the published data up to July, 1993 (corresponding to 1,667 live detector days), are summarized as follows.(1) The observed solar neutrino flux from ^8B decay i … More s about 50% of theoretical expectations.(2) The recoil-electron energy spectrum is explained by the ^8B-decay neutrino spectrum.(3) There is no difference between the day-time and nighttime solar-neutrino fluxes within experimental uncertainties.(4) Together with the Kamiokande-II observation, a total of 6 years and a half Kamiokande's observation shows no appreciable time variation of the solar-neutrino flux associated with the variation of the solar activity.Including the data which are not yet published, the total Kamiokande's solar-neutrino observation amounts to 2,140 live detector days up to the end of February, 1995.The other subject of this research project is detection of neutrino bursts from supernova explosions due to gravitational collapse of massive stars. Though no neutrino burst was detected during the term of this research project, a world best upper limit of the stellar collapse rate in our Galaxy, 0.44 yr^<-1> as of November, 1993, has been obtained. It should be mentioned that an on-line, semi-realtime neutrino-burst watch (in every -15 minutes) has been installed in the Kamiokande's monitoring system during the term of this research project. This enables us to discover a Galactic collapse-driven supernova explosion before it is observed optically. Less