Solar neutrino detection can probe the state of the solar interior. The flux of pp-neutrinos (from hydrogen fusion: p+p->d+e++ve) is firmly predicted from the solar luminosity, any shortage would indicate restmass-mediated ve-disappearance during transit between the solar core and the detector.
The expected fluxes of the less abundant but higher energy neutrinos from 8B and 7Be are more sensitive to the details of the solar model. For them the observation of a deficit may indicate either incomplete understanding of the stellar interior or new physics through massive neutrinos. The low-threshold gallium detector operated by the GALLEX collaboration1 in the Gran Sasso undergound laboratory (Italy) is sensitive to pp-neutrinos. It succeeded in their detection.
For this, techniques were developed to routinely extract and detect a few radioactive 71Ge atoms from a 100 ton target. This first observation of hydrogen fusion inside a start transfers solar models since Eddington from the realm of theory into the sphere of observational facts.
The GALLEX result can accommodate the expected pp-neutrinos at full strength. Hence, massive neutrinos are not enforced. At the same time, GALLEX confirms a shortage of the higher energy neutrinos, consistent with the results of the Homestake and Kamiokande experiments. 1MPI Heidelberg; KFK Karlsruhe; LNGS L'Aquila (Gran Sasso); Università di Milano; TUM München; Observatoire de Nice; WIS Rehovot; Università di Roma; CE Saclay; BNL, Upton, N.Y.