Results from the GALLEX solar neutrino experiment: GALLEX I

The GALLEX collaboration has measured the rate of production of 71Ge from 71Ga by solar neutrinos in the Gran Sasso Underground Laboratory (Italy). In nearly one year of operation, 15 measurements of the production rate of 71Ge were carried out to give, after corrections for side reactions and other backgrounds, an average value of 82+17-16 (stat.)±8 (syst.) SNU (1σ) due to solar neutrinos. This conclusion constitutes the first observation of solar pp neutrinos. Our result is consistent with the presence of the full pp neutrino flux expected according to the 'standard solar model' together with a reduced flux of 8B+7B neutrinos observed in the Homestake and Kamiokande experiments. Astrophysical reasons remain as a possible explanation of the solar neutrino problem. On the other hand, if the result is to be interpreted in terms of the MSW effect, it would fix neutrino masses and mixing angles within a very restricted range. (author)