GALLEX

GALLEX or Gallium Experiment was a radiochemical experiment to detect neutrinos. It ran from 1991 to 1997 at the Laboratori Nazionali del Gran Sasso ( LNGS ). The project was developed by an international collaboration of American, German, French, Italian, Israeli and Polish scientists led by the Max Planck Institute for Nuclear Physics in Heidelberg conducted (Project was Till Kirsten ).

The aim of the experiment is the detection of solar neutrinos and thus the testing of theories on the energy generation mechanisms of the sun. Previously, there were no observations of low-energy solar neutrinos.

Installation site

The main components of the experiment, the tank and the counters were in the underground Laboratori Nazionali del astrophysical laboratories of Gran Sasso in Italy, Abruzzo region, near L' Aquila within the 2912 -meter-high Gran Sasso massif. The installation location corresponds to a depth of 3200 meters of water. This is necessary to shield the detector from cosmic rays. The lab is accessible via the motorway A-24, which runs through the mountain.

Detector

The 54 m3 capacity detector tank was filled with 101 tons of gallium trichloride solution hydrochloric acid. It contained 30.3 tons of gallium, probably the largest amount of gallium that has ever been used. The gallium in the solution was used as a target for a neutrino - induced nuclear reaction ( Neutrinoeinfang or inverse beta decay ). Thus, the gallium is converted by the following reaction in germanium:

The threshold for the detection neutrino is this reaction at 233.2 keV. It can therefore only be detected neutrinos with an energy greater than 233.2 keV. The relatively low threshold value is one reason why gallium was used as detector material. Other detector responses have higher thresholds, such as the Homestake experiment with 813 keV by the detection of argon -37. With the proof of 71Ge neutrinos from the primary proton-proton reaction of the sun can be detected with a maximum energy of 420 keV.

The resulting 71Ge was chemically extracted from the detector and converted into the mono German GeH4 gas. The disintegration of 71Ge atoms with a half-life of 11.43 days was detected with a proportional counters. Each detected decay corresponds to a trapped neutrino.

Results

The GALLEX experiment was the first experiment, the neutrinos from the pp reaction, from which the sun produces most of its energy, proved. Between 1991 and 1997, the detector measured a rate of 77.5 SNU (solar neutrino units), which corresponds to about 0.75 counts per day. This rate can only be explained by a contribution of pp- neutrinos or deficit by neutrino oscillation.

The most important result was the statistically significant evidence of fewer neutrinos than predicted by the standard solar model, which yields values ​​125-136 SNU depending on the author. A similar deficit the Homestake experiment was for the higher-energy neutrinos from the sun even came: well known for many years, solar neutrino problem.

This discrepancy is now explained by the fact that neutrinos, unlike the previously applicable standard theory have a mass and therefore oscillate, ie be able to convert from one neutrino flavor to another. Radiochemical neutrino detectors respond only to a neutrino ( electron neutrinos ), not the second or third neutrino flavor. The neutrino oscillation of electron neutrinos produced in the sun on the way to earth is responsible for the discrepancy.

Other experiments

The follow-up experiment GALLEX Gallium Neutrino Observatory was or GNO, which began from the LNGS in April 1998 and continued until 2003.

A similar experiment that used metallic gallium, was the Russian-American Gallium Experiment SAGE.