SOLAR NEUTRINOS OBSERVED BY GALLEX AT GRAN-SASSO

We have measured the rate of production of Ge-71 from Ga-71 by solar n eutrinos. The target consists of 30.3 t of gallium in the form of 8.13 M aqueous gallium chloride solution (101 t), shielded by almost-equal -to 3300 m water equivalent of standard rock in the Gran Sasso Undergr ound Laboratory (Italy). In nearly one year of operation, 14 measureme nts of the production rate of Ge-71 were carried out to give, after co rrections for side reactions and other backgrounds, an average value o f 83+/-19 (stat.)+/-8 (syst.) SNU (1sigma) due to solar neutrinos. Thi s conclusion constitutes the first observation of solar pp neutrinos. Our result is consistent with the presence of the full pp neutrino flu x expected according to the ''standard solar model'' together with a r educed flux of B-8+Be-7 neutrinos as observed in the Homestake and Kam iokande experiments. Astrophysical reasons remain as a possible explan ation 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.