Discovery of the neutrino

The neutrino is very reluctant to react with its environment. A very intensive neutrino source and a very heavy target are needed to capture even a few neutrinos.
    With the advent of the first nuclear reactors in the 1940's it was realised that they could serve as intensive neutrino sources with a flow of approximately 10¹² -10¹³ per second per cm². This was many orders of magnitude greater than what was obtained from naturally radioactive substances.
    Frederick Reines and his colleage Clyde L. Cowan, Jr. proposed in 1953 a reactor experiment to capture neutrinos through the reaction:

(anti)neutrino + proton –› neutron + positron.

Reines and Cowan realised the importance of detecting both the neutron and the positron to reduce the risk of incorrect interpretation. Despite the large flow of neutrinos from the reactor, a low counting rate was expected. But Reines and Cowan succeeded finally in recording a few events per hour.
    The observation of neutrinos was a pioneering contribution that paved the way for the "impossible" neutrino experiments. These attempt to capture neutrinos from cosmic radiation. The neutrinos may originate in the sun or in supernovas (exploding stars). Their reluctance to react with atomic nuclei or electrons, thus allowing themselves to be captured, necessitates very large detector volumes: many thousand cubic metres of liquid or large regions of sea or ice.