How can you prove it? The way in which Bahcall made it very plausible is by showing that probably the neutrinos which arrive on earth have different properties according to their energy. The high energy neutrinos which come from boron and which are observed in Kamiokande are converted halfway into µ-neutrinos. The medium energy neutrinos, however, don't get out of the sun at all, they're practically all converted into µ-neutrinos and therefore are not visible. And he concluded that by comparing the result of Davis in chlorine with the result of Kamiokande, the comparison of this... these two shows pretty definitely that the beryllium neutrinos do not come to the earth. The beryllium neutrinos should be seen by Davis. And then a third experiment was done using gallium, again I think suggested by Bahcall. In gallium you can observe the primary neutrinos which are made in the fundamental reaction making two protons coming together and forming deuterons. And... and now, this reaction has the great advantage that we know exactly how much there is of this reaction, namely this reaction after all feeds the energy of the sun, and therefore if we just look at the luminosity of the sun we can tell how intense this proton-proton reaction must be. And the proton-proton neutrinos can be observed with gallium. It was first done in Russia by a Russian-American group known as the SAGE Experiment, Soviet American Gallium Experiment, and then repeated in Italy by a European group.