Tracking neutrinos and solving an enigma
Editorial review 2026
Nuclear fusion reactions in the Sun produce a large number of neutrinos. When these neutrinos can be detected, they provide evidence of the Sun’s activity and of these fusion reactions.
These neutrinos take 2 seconds to cross the Sun and about 8 minutes to reach us. Their number is phenomenal. To give an idea, every square centimeter of the Earth’s surface is crossed each second by 65 billion solar neutrinos. But these neutrinos are extraordinarily difficult to detect. Out of 100000 billion solar neutrinos striking the Earth’s surface, there will be less than one interaction with an atom of our planet.
Three sources of solar neutrinos
This diagram of the nucleosyntheses taking place in the Sun shows the three main reactions producing neutrinos. The first is the fundamental pp reaction of fusion between two protons forming a deuterium nucleus. It is the principal source of energy and neutrinos. Following this primordial nucleosynthesis, other reactions generate neutrinos, particularly the one involving beryllium-7 and the one involving the decay of boron-8. The percentages indicate the proportion of the possible pathways. The energies of the neutrinos are also indicated © IN2P3
The principal source of neutrinos is the fusion reaction of two protons leading to the nucleosynthesis of deuterium and which physicists call the pp reaction. The pp reaction is the first nucleosynthesis process in the Sun, the one providing most of its energy: 92% of solar neutrinos (called “primordial”) are produced in this reaction.
The deuterium formed is composed of one proton and one neutron. It is rapidly burned to produce helium-3 by fusing with a proton. Then, in 83% of cases, a new fusion reaction leads to helium-4, the most stable of the light nuclei, which is none other than an alpha particle.
In 17% of cases, however, helium-3 fuses with helium-4 to produce a beryllium-7 nucleus which de-excites into lithium-7 through electron capture while emitting a neutrino. Finally, very rarely, beryllium-7 fuses with a proton to form boron-8, which emits a neutrino through beta decay. These two fusions are two sources of neutrinos in addition to those from the pp reaction.
These are the principal sources of solar neutrinos. The energy of the most abundant primordial neutrinos does not exceed 0.420 MeV and differs little from that of neutrinos from radioactivity. These low-energy neutrinos are difficult to detect. It required all the sensitivity of the Gallex detector and similar instruments to record them. On the other hand, the more energetic beryllium-7 neutrinos – mainly 0.84 MeV – are easier to identify. Boron neutrinos, although rarer, are the easiest to detect and identify because their energy can reach up to 15 MeV.
Flux and detection ranges
Energy distribution of solar neutrinos: primordial neutrinos from the pp reaction, from beryllium-7 and from boron-8. Boron neutrinos are by far the least numerous, but their much higher energy facilitates their detection. The detection ranges of the various observatories are indicated by green arrows. The first observatories such as Super-Kamiokande could only detect boron neutrinos of a few MeV, whereas more recent observatories such as Gallex were capable of going down to 0.233 MeV and detecting primordial neutrinos.
© Source Clefs CEA/M. Cribier
A deficit of detected neutrinos
At the beginning of the 1970s, American physicist Raymond Davis (1914-2006) installed in the Homestake gold mine in South Dakota a tank containing 400,000 liters of tetrachloroethylene at a depth of 1,500 meters. Neutrinos can transmute chlorine-37 nuclei (an isotope of chlorine) into the radioactive isotope argon-37. Ingeniously, Davis succeeded in extracting, purifying and counting the argon-37 atoms formed. Patiently, around 2,000 argon atoms were thus detected over more than twenty years, only 30% of what was expected from the Sun.
Starting in 1988, the Super-Kamiokande observatory in Japan confirmed this deficit. The target consisted of 2,100 tons of very pure water. Some neutrinos set electrons in motion. These emitted a flash of Cherenkov light that was detected. But only the energetic boron neutrinos generated enough light for this to occur.
At the end of the 1980s, two observatories, SAGE and GALLEX, were built beneath the Caucasus Mountains and beneath the Gran Sasso in Italy. GALLEX, standing for “Gallium Experiment,” is the name given to an experiment detecting neutrinos emitted by the Sun and carried out in the underground laboratory of Gran Sasso adjacent to the Gran Sasso tunnel in the Abruzzo massif in Italy. This experiment recorded data between 1991 and 1997 as part of an international collaboration.
They used gallium, which low-energy neutrinos (above 233 keV) can transmute into radioactive germanium. In 1992, GALLEX announced the first observation of primordial neutrinos resulting from fusion reactions between two protons at the core of the Sun. The primordial neutrino flux was once again measured to be about one-third lower than astrophysicists’ predictions. The neutrino deficit therefore did not concern only the energetic boron neutrinos!
Neutrino event
Neutrino event recorded by the Super-Kamiokande observatory in Japan in 1998. The observatory consists of a cylinder 40 meters high and 40 meters in diameter located in a mine beneath a mountain and filled with very pure water. Its walls are lined with photomultipliers. An energetic neutrino of several MeV propelled forward an electron from the target. The electron, which travels faster than light in water, emits a cone of Cherenkov light. This cone is detected by the photomultipliers. The trace of the cone on the walls makes it possible to determine the direction of the neutrino, which here points toward the Sun. © Super Kamiokande
The key to the enigma: neutrino oscillations
To explain this deficit, the hypothesis of neutrino oscillations was proposed, notably during the work carried out in 1999 by the Kamiokande experiment near Tōkyō led by Masatoshi Koshiba: solar neutrinos would transform during their journey into other neutrinos escaping detection…
CONTINUED: Neutrino oscillations
Source: Solar neutrinos, by Michel Cribier: CLEFS CEA n°49 (pdf)