Nobel Lecture: Birth of neutrino astrophysics

Abstract

In giving this talk I am very much helped by the preceding talk because I can skip some of the topics. If you want further information, please refer to my review article, ‘‘Observational Neutrino Astrophysics’’ (Koshiba, 1992). I am to talk about the birth of neutrino astrophysics, but before the birth, there was a very important event, which was just described by Professor Davis (Nobel Lecture, Davis, 2003). It was the radiochemical work using the reaction ne1 37Cl going to e137Ar. He found that the observed neutrino flux was only one-third of what was theoretically expected. This could be considered as the conception of neutrino astrophysics and was in fact the impetus for us to begin seriously working on solar neutrinos. I will talk about two experiments. The first was carried out at the original KamiokaNDE, which might be called an Imaging Water Cerenkov detector with 20% of its surface covered by photomultipliers. The total mass of the water inside this detector was 3000 tons. Its cost was about 3 million U.S. dollars. This, mind you, was meant to be the feasibility experiment on the astrophysical detection of solar neutrinos. The second experiment was carried out at Super-KamiokaNDE, the same type of detector but with a better light sensitivity, that is, 40% of the entire surface was covered by photocathodes and the total mass of the water was 50 000 tons. Its cost was about 100 million U.S. dollars. This was considered to be the full-scale solar neutrino observatory. Both the facilities are situated about 1000 meters underground in the Kamioka Mine. The capital letters NDE at the end of the two names originally stood for Nucleon Decay Experiment. However, because of the detection of various neutrinos by these detectors, people started calling them Neutrino Detection Experiments. Figure 1 shows the interior of KamiokaNDE. You can see arrays of photomultipliers on the sidewalls as well as on the top and at the bottom. When we were preparing for this KamiokaNDE experiment, we heard that a much bigger experiment, but of similar type, was being planned in the United States (Bionta et al., 1983). We had to think very seriously about the competition with this bigger detector. Both experiments aimed at the de-