Solar Neutrino

Electron neutrinos are produced in the Sun as a product of nuclear fusion. By far the largest fraction of neutrinos passing through the Earth are Solar neutrinos.

The main contribution comes from the so-called proton-proton chain reaction. The net reaction is:

,

or in words:

4 protons + 2 electrons = Helium + 2 electron neutrinos.

The highest flux of solar neutrinos come directly from the proton-proton interaction, and have a low energy, up to 400 keV. There are also several other significant production mechanisms, with energies up to 18 MeV.

The number of neutrinos can be predicted by the Standard Solar Model. The detected number of electron neutrinos was only 1/3 of the predicted number, and this was known as the solar neutrino problem. It led to the idea of neutrino oscillation and the fact that neutrinos can change flavour. This was confirmed when the total flux of solar neutrinos of all types was measured and it agreed with the earlier predictions of expected electron neutrino flux, as seen by Sudbury Neutrino Observatory, and thus confirmed that neutrinos have mass.

The energy spectrum of solar neutrinos is also predicted by solar models. It is essential to know this energy spectrum because different neutrino detection experiments are sensitive to different neutrino energy ranges. The Homestake Experiment used chlorine and was most sensitive to solar neutrinos produced by the decay of the beryllium isotope 7Be. The Sudbury Neutrino Observatory is most sensitive to solar neutrinos produced by 8B. The detectors that use gallium are most sensitive to the solar neutrinos produced by the proton-proton chain reaction process. In 2012 the collaboration known as Borexino reported detecting low-energy neutrinos for the proton-electron-proton (pep reaction) that produces 1 in 400 deuterium nuclei in the sun. The detector contained 100 metric tons of liquid and saw on average 3 events each day (due to carbon 11 production) from this relatively uncommon thermonuclear reaction.

Neutrinos can trigger nuclear reactions. By looking at ancient ores of various ages that have been exposed to solar neutrinos over geologic time, it may be possible to interrogate the luminosity of the Sun over time, which, according to the Standard Solar Model, has changed with time.