Abstract
The GALLEX detector at the Gran Sasso Underground Laboratory has observed since May 1991 a flux of solar neutrinos in sufficient quantity to account for the solar luminosity as reflected in the flux of low-energy pp-neutrinos from the primary hydrogen fusion reaction in the solar core. However, in assigning the measured signal of 76 ± 8 SNU to pp-neutrinos, little or no signal is left to account for the other neutrino fluxes also expected from the Standard Solar Model (SSM), in particular the 7Be neutrinos ( 67Be-neutrino problem'). The significance of this observation has developed with our continuing reduction of the statistical and systematic errors and with the demonstration of reliable detector performance in the 51Cr neutrino source experiment and in recent spiking tests with 71As. Together with the results from Homestake and Superkamiokande, the data are best explained by matter mediated neutrino oscillations (MSW effect), pointing towards a muon-neutrino mass of about 2.5 meV/c 2.
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