GALLEX
GALLEX or Gallium Experiment was a radiochemical experiment to detect neutrinos . It ran from 1991 to 1997 at Laboratori Nazionali del Gran Sasso (LNGS). The project was carried out by an international collaboration of American, German, French, Italian, Israeli and Polish scientists under the direction of the Max Planck Institute for Nuclear Physics Heidelberg (project leader was Till Kirsten ).
The aim of the experiment was to prove the solar neutrinos and thus to test theories on the energy production mechanisms of the sun . Previously there had been no observations of low energy solar neutrinos.
Installation site
The main components of the experiment, the tank and the counters, were located in the underground astrophysical laboratories Laboratori Nazionali del Gran Sasso in Italy , Abruzzo region , near L'Aquila within the 2912 meter high Gran Sasso massif. The installation site corresponds to a depth of 3200 meters of water. This is necessary to shield the detector from cosmic rays . The laboratory can be reached via the A-24 motorway, which runs through the mountain.
detector
The 54 m 3 detector tank was filled with 101 tons of a gallium trichloride - hydrochloric acid solution. This contained 30.3 tons of gallium , at that time almost a world annual production and, according to the SAGE detector, probably the largest amount of gallium ever used. The gallium in the solution served as a target for a neutrino-induced nuclear reaction (neutrino capture or inverse beta decay). This converts the gallium into germanium through the following reaction:
The threshold value for the neutrino detection for this reaction is 233.2 keV . So only neutrinos with an energy greater than 233.2 keV can be detected. The relatively low threshold is one reason why gallium was used as the detector material. Other detector reactions have higher threshold values, for example the Homestake experiment with 813 keV through the detection of argon -37. With the detection of 71 Ge, neutrinos from the primary proton-proton reaction of the sun with a maximum energy of 420 keV can also be detected.
The resulting 71 Ge was chemically extracted from the detector and converted into the gas [71] Monogerman GeH 4 . The decay of the 71 Ge atoms with a half-life of 11.43 days was demonstrated with proportional counters . Every detected decay corresponds to a captured neutrino.
Results
The GALLEX experiment was the first to detect neutrinos from the pp reaction, from which the sun generates most of its energy. Between 1991 and 1997 the detector measured a rate of 77.5 SNU ( solar neutrino units ), which corresponds to about 0.75 decays per day. This rate can only be explained by a contribution from pp neutrinos or a deficit due to neutrino oscillation .
The most important result was the statistically significant evidence of a lower number of neutrinos than the solar standard model predicted, which, depending on the author, gives values from 125 to 136 SNU. The Homestake experiment had already found a similar deficit for the higher-energy neutrinos from the sun: the solar neutrino problem that has been known for many years .
This discrepancy is now explained by the fact that neutrinos, in contrast to the previously valid standard theory, “oscillate”, ie can transform themselves from one type of neutrino to another. Radiochemical neutrino detectors only react to the electron neutrino type , not to the second and third neutrino flavor . The neutrino oscillation of the electron neutrinos produced in the sun on the way to earth is responsible for the discrepancy.
Other experiments
The follow-up experiment to GALLEX was the Gallium Neutrino Observatory or GNO, which started by LNGS in April 1998 and continued until 2003.
A similar experiment that used metallic gallium is the Russian-American gallium experiment SAGE .
Web links
Individual evidence
- ↑ a b c Wolfgang Hampel: The contribution of the GALLEX-GNO experiment to the solution of the solar neutrino problem. (pdf) In: MPG Activity Report 2004. mpg, November 10, 2004, accessed on February 26, 2010 (German).
Coordinates: 42 ° 25 ′ 14 " N , 13 ° 30 ′ 59.1" E