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GALLEX or Gallium Experiment was a radiochemical neutrino detection experiment that ran between 1991 and 1997 at the Laboratori Nazionali del Gran Sasso (LNGS). This project was performed by an international collaboration of French, German, Italian, Israeli, Polish and American scientists led by the Max-Planck-Institut für Kernphysik Heidelberg.
It was designed to detect solar neutrinos and prove theories related to the Sun's energy creation mechanism. Before this experiment, there had been no observation of low energy solar neutrinos.
The experiment's main components, the tank and the counters, were located in the underground astrophysical laboratory Laboratori Nazionali del Gran Sasso in the Italian Abruzzo province, near L'Aquila, and situated inside the 2912 metres high Gran Sasso mountain. Its place under a depth of rock equivalent of 3200 metres of water was important to shield from cosmic rays. This laboratory is accessible by a highway A-24, which runs through the mountain.
The 54-m3 detector tank was filled with 101 tons of gallium trichloride-hydrochloric acid solution, which contained 30.3 tons of gallium. The gallium in this solution acted as the target for a neutrino-induced nuclear reaction, which transmuted it into germanium through the following reaction:
νe + 71Ga → 71Ge + e-.
The threshold for neutrino detection by this reaction is 233.2 keV, and this is also the reason why gallium was chosen: other reactions (as with chlorine-37) have higher thresholds and are thus unable to detect low-energy neutrinos. This reaction was also able to detect neutrinos from the initial proton fusion reaction of the proton-proton chain reaction, with an upper energy limit of 420 keV.
The produced germanium-71 was chemically extracted from the detector, converted to germane 71GeH4. Its decay, with a half life of 11.43 days, was detected by counters. Each detected decay corresponded to one detected neutrino.
During the period 1991-1997, the detector measured an overall rate of 77.5 SNU (Solar neutrino units), roughly 0.75 decays a day.
The rate of neutrinos detected by this experiment agreed with standard solar model predictions. Thanks to the use of gallium, it was the first experiment to observe solar initial pp neutrinos. Another important result was the detection of a smaller number of neutrinos than the standard model predicted (the solar neutrino problem). After detector calibration the amount did not change. This discrepancy - an example of the solar neutrino problem - has since been explained. Such radiochemical neutrino detectors are sensitive only to electron neutrinos, and not to the second and third generation flavours of neutrinos - the neutrino oscillation of electron neutrinos emitted from the sun, between the earth and the sun, accounts for the discrepancy.
The first solar neutrino detection experiment, the Homestake, used chlorine-37 to detect neutrinos with energies down to 814 keV.
After the end of GALLEX its successor project, the Gallium Neutrino Observatory or G.N.O., was started at LNGS in April 1998.
A similar experiment detecting solar neutrinos using liquid gallium-71 was the Russian-American Gallium Experiment SAGE.
The GALLEX detector