Neutrinos: 1995

The GALLium EXperiment - A Radiochemical Experiment to Detect Solar Neutrinos

Until the year 1990 there was no observation of the initial reaction in the nuclear fusion chain. This changed with the installation of the Gallium Experiments. Gallium as target allows neutrino interaction via νe + 71Ga → 71Ge + e. The threshold of this reaction is 233 keV (see the spectrum) , low enough also to detect neutrinos from the initial proton fusion reaction. The experimental process is explained in the following: the target is 30 tons of natural gallium in a 100 ton aqueous gallium chloride solution. 71Ge is chemically extracted from the target solution, converted to 71GeH4, and the radioactive decay is detected with tiny proportional counters.


The target tank and the counting device were located in the italian underground lab for astrophysical research LNGS (Laboratori Nazionali del Gran Sasso). The rock overburden shielded the detector against interfering cosmic radiation. The GALLEX was performed by an international collaboration of scientists from France, Germany, Italy, Israel, Poland and the US, headed by MPIK Heidelberg.

Gallex detected electron neutrinos in the order of magnitude predicted by the standard solar model. This was the very first observation of the solar pp neutrinos and thus an important confirmation of the nuclear fusion process inside the sun. The GALLEX also measured a neutrino signal that was somewhat smaller than predicted by the solar model, however the reduction factor was different than in the chlorine experiment done by Cowan and Reines in 1956. Two explanations were possible: either the standard solar model was not correct, or the neutrino has some features which were not yet taken into account in the standard model of elementary particles. Until the year 1990 there was no observation of the initial reaction in the nuclear fusion chain. This changed with the installation of the Gallium Experiments. Using gallium as a target allowed neutrino interactions via νe + 71Ga → 71Ge + e. The threshold of this reaction is 233 keV (see the spectrum), low enough to detect neutrinos from the initial proton fusion reaction. The experimental process can be explained as the following: 71Ge was chemically extracted from the a target solution of 30 tons of natural gallium in a 100 ton aqueous gallium chloride after having been converted to 71GeH4 by neutrino interactions and then detected by tiny proportional counters designed to measure radioactive decay.