The search for solar neutrinos at Kamioka

The Kamiokande II¹ collaboration upgraded the Kamioka nucleon decay experiment to allow a search for solar neutrinos. The ⁸B solar neutrinos are detected via neutrino‐electron scattering yielding electrons above the Cherenkov threshold in water. The current threshold of 10 MeV gives an expected signal of 0.1 events/day, about an order of magnitude below the background. This background is due to a combination of uranium dissolved in the water, photons generated in the rock, and beta decays from long‐lived isotopes caused by the breakup of ¹⁶O by cosmic ray muons traversing the detector. Each background can be reduced in the coming months and allow the threshold to be reduced to 7 MeV.