Question: Where do neutrinos come from?

Susan Cartwright answered on 16 Jun 2015:

Many places. Stars like the Sun emit huge numbers, because hydrogen fusion makes them (to convert hydrogen – protons – to helium – two protons plus two neutrons – you have to convert protons into neutrons, which results in the emission of a neutrino). There are 65 billion solar neutrinos per square centimetre per second everywhere on the Earth.

They were made in the early universe. On average, there are about 400 neutrinos per cubic centimetre everywhere in the entire universe that date back to a few seconds after the Big Bang.

Cosmic rays make them. When protons from outer space strike the Earth’s atmosphere, they make lots of unstable particles called pions, which decay into muons and neutrinos.

Certain types of radioactive decay make them. For example, carbon 14, the isotope used in carbon dating, decays into nitrogen-14 plus an electron plus a neutrino (an antineutrino, actually).

We can make them in the lab, by smashing protons into a target – this is the same process as cosmic rays.

That’s not a complete list, but it gives you an idea.