GALLEX
Encyclopedia
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 neutrino
s and prove theories related to the Sun
's energy creation mechanism. Before this experiment, there had been no observation of low energy solar neutrinos.
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 ray
s. This laboratory is accessible by a highway A-24, which runs through the mountain.
-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:
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.
(Solar neutrino unit
s), 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.
, 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.
Laboratori Nazionali del Gran Sasso
Laboratori Nazionali del Gran Sasso is a particle physics laboratory of the INFN, situated near the Gran Sasso mountain in Italy, between the towns of L'Aquila and Teramo, about 120 km from Rome. In addition to a surface portion of the laboratory, there are extensive underground facilities...
(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 neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
s and prove theories related to the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
's energy creation mechanism. Before this experiment, 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 ItalianItaly
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...
Abruzzo
Abruzzo
Abruzzo is a region in Italy, its western border lying less than due east of Rome. Abruzzo borders the region of Marche to the north, Lazio to the west and south-west, Molise to the south-east, and the Adriatic Sea to the east...
province, near L'Aquila
L'Aquila
L'Aquila is a city and comune in central Italy, both the capital city of the Abruzzo region and of the Province of L'Aquila. , it has a population of 73,150 inhabitants, but has a daily presence in the territory of 100,000 people for study, tertiary activities, jobs and tourism...
, and situated inside the 2912 metres high Gran Sasso
Gran Sasso
Gran Sasso d'Italia is a mountain located in the Abruzzo region of central Italy. The Gran Sasso forms the centerpiece of the Gran Sasso e Monti della Laga National Park which was established in 1993 and holds the highest mountains in continental Italy south of the Alps and is part of the...
mountain. Its place under a depth of rock equivalent of 3200 metres of water was important to shield from cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
s. This laboratory is accessible by a highway A-24, which runs through the mountain.
Detector
The 54-m3 detector tank was filled with 101 tons of gallium trichlorideGallium trichloride
Gallium trichloride is the chemical compound with the formula GaCl3. Solid gallium trichloride exists as a dimer with the formula Ga2Cl6. It is colourless and soluble in virtually all solvents, which is unusual for a metal halide. It is the main precursor to most derivatives of gallium and a...
-hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
solution, which contained 30.3 tons of gallium. The gallium in this solution acted as the target for a neutrino-induced nuclear reaction
Nuclear reaction
In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce products different from the initial particles...
, which transmuted it into germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
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
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
-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
Proton-proton chain reaction
The proton–proton chain reaction is one of several fusion reactions by which stars convert hydrogen to helium, the primary alternative being the CNO cycle. The proton–proton chain dominates in stars the size of the Sun or smaller....
, with an upper energy limit of 420 keV.
The produced germanium-71 was chemically extracted from the detector, converted to germane
Germane
Germane is the chemical compound with the formula GeH4, and the germanium analogue of methane. It is the simplest germanium hydride and one of the most useful compounds of germanium. Like the related compounds silane and methane, germane is tetrahedral. It burns in air to produce GeO2 and...
(71GeH4). 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 an overall rate of 77.5 SNUSolar neutrino unit
Solar neutrino unit is a new unit defined for convenience, which is more suitable for very low event rates.In principle there are two ways of detecting solar neutrinos: radiochemical and real time experiments...
(Solar neutrino unit
Solar neutrino unit
Solar neutrino unit is a new unit defined for convenience, which is more suitable for very low event rates.In principle there are two ways of detecting solar neutrinos: radiochemical and real time experiments...
s), roughly 0.75 decays a day.
The rate of neutrinos detected by this experiment agreed with standard solar model
Standard Solar Model
The Standard Solar Model refers to a mathematical treatment of the Sun as a spherical ball of gas...
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
Standard Model
The Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
predicted (the solar neutrino problem
Solar neutrino problem
The solar neutrino problem was a major discrepancy between measurements of the numbers of neutrinos flowing through the Earth and theoretical models of the solar interior, lasting from the mid-1960s to about 2002...
). After detector calibration the amount did not change. This discrepancy - an example of the solar neutrino problem
Solar neutrino problem
The solar neutrino problem was a major discrepancy between measurements of the numbers of neutrinos flowing through the Earth and theoretical models of the solar interior, lasting from the mid-1960s to about 2002...
- 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
Neutrino oscillation
Neutrino oscillation is a quantum mechanical phenomenon predicted by Bruno Pontecorvowhereby a neutrino created with a specific lepton flavor can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates...
of electron neutrinos emitted from the sun, between the earth and the sun, accounts for the discrepancy.
Other experiments
The first solar neutrino detection experiment, the HomestakeHomestake Experiment
The Homestake experiment was an experiment headed by astrophysicists Raymond Davis, Jr. and John N. Bahcall in the late 1960s. Its purpose was to collect and count neutrinos emitted by nuclear fusion taking place in the Sun. Bahcall did the theoretical calculations and Davis designed the experiment...
, used chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
-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.