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{{about|the neutrino detector|the ultraviolet space telescope|GALEX}}
{{confused|GALEX (disambiguation)}}
'''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. After brief interruption, the experiment was continued under a new name '''GNO''' ('''Gallium Neutrino Observatory''') from May 1998 to April 2003.

It was designed to detect solar [[neutrino]]s and prove theories related to the [[Sun]]'s energy creation mechanism. Before this experiment (and the [[SAGE (Soviet–American Gallium Experiment)|SAGE experiment]] that ran concurrently), there had been no observation of low energy solar neutrinos.

==Location==
The experiment's main components, the tank and the counters, were located in the underground astrophysical laboratory Laboratori Nazionali del Gran Sasso in the [[Italy|Italian]] [[Abruzzo]] province, near [[L'Aquila]], and situated inside the 2912-metre-high [[Gran Sasso]] mountain. Its place under a depth of rock equivalent of 3200 metres of water was important to shield from [[cosmic ray]]s. This laboratory is accessible by a highway A-24, which runs through the mountain.

==Detector==
The 54-m<sup>3</sup> 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:

: ν<sub>e</sub> + <sup>71</sup>Ga → <sup>71</sup>Ge + e<sup>−</sup>.

The threshold for neutrino detection by this reaction is very low (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.  
In fact, the low energy threshold makes the reaction with gallium suitable to the detection of neutrinos emitted in the initial proton fusion reaction of the [[proton-proton chain reaction]], which have a maximum energy of 420 keV.

The produced germanium-71 was chemically extracted from the detector, converted to [[germane]] (<sup>71</sup>GeH<sub>4</sub>). Its decay, with a [[half-life]] of 11.43 days, was detected by counters. Each detected decay corresponded to one detected neutrino.

==Results==
During the period 1991-1997, the detector measured capture rate of 73.1 [[Solar neutrino unit|SNU]] ([[Solar neutrino unit]]s). The follow-up GNO experiment found the capture rate 62.9. <ref>{{Cite journal | doi=10.3367/UFNe.0181.201109g.0975|title = The Russian-American gallium experiment SAGE| journal=Physics-Uspekhi| volume=54| issue=9| pages=941–949|year = 2011|last1 = Gavrin|first1 = Vladimir N.| bibcode=2011PhyU...54..941G}}</ref>

The rate of neutrinos detected by this experiment disagreed 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 has since been explained: such radiochemical neutrino detectors are sensitive only to electron neutrinos, and not to [[muon neutrino|muon neutrinos]] or [[tau neutrino|tau neutrinos]], hence the [[neutrino oscillation]] of electron neutrinos emitted from the sun and travelling to the earth accounts for the discrepancy.

Results from GALLEX together with [[Soviet–American Gallium Experiment|SAGE]] and later confirmed by the [[Soviet–American Gallium Experiment#Baksan Experiment on Sterile Transitions (BEST)|BEST]] experiment have reported a deficit in the expected decay of <math> {}^{71}\text{Ga}+\nu_e \rightarrow e^{-}+{}^{71}\text{Ge} 
</math> that has been named [[Gallium anomaly|the gallium anomaly]].

==Other experiments==
The first solar neutrino detection, the [[Homestake Experiment]], 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.<ref>[http://www.infn.it/csn2/schede_2000/GNO.htm GNO (Gallium Neutrino Observatory)] {{Webarchive|url=https://web.archive.org/web/20181001182147/http://www.infn.it/csn2/schede_2000/GNO.htm |date=2018-10-01 }}, October 2000. Retrieved 1 Oct. 2018.</ref> The project continued until 2003.

A similar experiment detecting solar neutrinos using liquid gallium-71 was the Russian-American Gallium Experiment [[SAGE (ruSsian American Gallium Experiment)|SAGE]].

==References==
{{reflist}}

==External links==
*[https://www.mpi-hd.mpg.de/lin/research_history.de.html The GALLEX experiment]

{{Neutrino detectors}}

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[[Category:Neutrino observatories]]