GALLEX and the solar neutrino problem

This paper gives a sinthetic overview of the present situation on the 'solar neutrino problem'; the experimental discussion is specifically concerned with the GALLEX experiment. GALLEX is a radiochemical experiment sensitive to neutrinos coming from the primary pp fusion reaction in the Sun. It is performed in the Gran Sasso underground laboratory (Italy) by a mostly European collaboration, and is based on the neutrino capture reaction on Gallium 71 with an energy threshold of 233 KeV. After three years monitoring the solar neutrino flux, the signal measured is 79+-10 (stat)+/-6 (sys.)10(-36) neutrino captures per second and per target atom. This result is more than two sigma lower than what is predicted by solar models. Recently a man made neutrino source has been prepared (51Cr, 1.67 MCi initial activity), and a full check of the experiment has been performed with success.