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Solar neutrinos
They come along with the process of thermonuclear fusion inside the stars (our sun or any other star in the universe). Their energy is quite weak (some MeV) and they can travel in a long and quite way. They come from different nuclear reactions whose main reaction (85% of the solar neutrinos come from it) is:
p is a proton, H is a deuterium nucleus, e is an anti-electron and the last one is a neutrino.
Depending on the nuclear reaction concerned, the neutrino has not the same energy.
Neutrinos from mankind activity
These are high energy neutrinos produced by the particles accelerators and low energy neutrinos coming out of nuclear reactors. The first ones, whose energy can reach about 100 GeV, are produced to study the structure of the nucleons (protons and neutrons composing the atomic nuclei) and to study the weak interaction. The second ones are here although we did not ask for them. They are an abundant product made by the nuclear reactions inside the reactors cores (a standard nuclear plant radiate about 5 1020 neutrinos per second) and their energy is around 4 MeV. They have been the first to be detected and the first to be used to put some limits on the neutrino oscillation.
Neutrinos from the earth
Our great old planet has kept since its birth many radioactive atomic nuclei. This is what we call "natural radioactivity". This radioactivity is quite important, badly known and its main contribution could be to keep in fusion the matter under the crust of the earth. The power coming from this natural radioactivity is estimated at about 20.000 Giga Watts (about 20.000 nuclear plants!) and the neutrinos coming from this radioactivity are numerous: about 6 millions per second and per cm2. But those neutrinos, despite of their quantity, are often locally drowned in the oceans of neutrinos coming from the nuclear plants.
Neutrinos from cosmic rays
When a cosmic ray (proton coming from somewhere in space) penetrates the atmosphere, it interacts with an atomic nucleus and this generates a particles shower. Under the same principle which guides the neutrinos production at CERN, some neutrinos are created: they are called "atmospheric neutrinos". Some experiments like Kamiokande and Super-Kamiomande in Japan have tried to see the oscillations of the neutrinos inside those particle showers. The results in 1998 seem positive.
Neutrinos from the Big-Bang
The "standard" model of the Big-Bang predicts, like for the photons, a cosmic background of neutrinos. Those neutrinos, nobody has never seen them. They are yet very numerous: about 330 neutrinos per cm3. But their energy is theoretically so little (about 0.0004 eV), that no experiment, even very huge, has been able to detect them.
Some other neutrinos could come from very cataclysmic phemomena like explosions of supernovae or neutron stars coalescences. They are not only pure theoretical speculations since in 1987 a supernovae exploded in the Magellan clouds, 150.000 light-years from our earth and... its neutrinos were detected!!
This picture shows the rest of the star which exploded in 1987.
Didier Verkindt