The Cowan-Reines Neutrino Experiment was the first experiment to prove the existence of a particle known as a neutrino (the experiment actually proved the existence of its antimatter counterpart, the anti-neutrino).
This particle was first proposed by Wolfgang Pauli. Investigations into the Beta decay after the discovery of radioactivity revealed that the electron emitted during Beta decay possessed a wide range of energy (a continuous energy spectra). Certainly when the energy of the electron is below the maximum, due to the laws of energy conservation, the remaining energy must be transferred somewhere else rather than disappear. Thus the neutrino is proposed as an additional particle released in Beta decay which possesses a certain amount of energy as well. This would be the source of the remaining energy released in the Beta decay when the electron's energy is not at the maximum.
The set up of the experiment was simple. Two tanks of water was set up with 40 kilograms of Cadmium Chloride dissolved into the water. The entire set up was then placed in close proximity to a nuclear reactor, where the most reliable large source of neutrino flux could be found.
Should the experiment react in accordance to their prediction of the neutrino, in accordance to the predicted expectation of inverse Beta decay (obviously obtained by the knowledge of Beta decay), the anti-neutrino produced from the nuclear reactor should interact with a proton to create a neutron and a position. The position would quickly annihilate with an electron to create a burst of Gamma radiation, which serves as an indicator that the reactions of the experiment had worked in accordance to their expectations. As an added precaution, Cadmium Chloride was added into the tank as a neutron absorber to absorb the neutron produced as part of the reaction. This absorption by the Cadmium (108) atom would create another isotope of Cadmium, Cadmium (109) with a release of Gamma radiation as well. It was mathematically calculated that the positron-electron annihilation would occur approximate 5 microseconds before the absorption of the neutron as it takes more time for the neutron to encounter a Cadmium atom. Hence if the experiment produces two Gamma ray bursts spaced 5 microseconds apart, it would be the unique signature that reaction occurred in accordance to the theories and thus the theories are correct.
SUMMARY
Via the inverse beta decay, the predicted anti-neutrino should interact with a proton to produce a neutron and positron. The position annihilates with an electron to create an initial burst of γ radiation. Cadmium Chloride is added into the tank as a neutron absorber. Mathematical calculations implied that it would take approximately 5 microseconds for the product neutron to encounter and react with the Cadmium (108) to produce Cd (109) isotope and γ rays. The two bursts of γ radiation is a conclusive signature for the presence of neutrinos.
Sources:
1. Cowan-Reines Neutrino Experiment: https://en.wikipedia.org/wiki/Cowan%E2%80%93Reines_neutrino_experiment
2. ''Neutrino'' by Frank Close
3. Beta decay: https://en.wikipedia.org/wiki/Beta_decay
