Finding the Antineutrino


As physicists were examining the nature of Beta decay there were a number problems that developed that lead to the discovery of a whole new sort of Lepton. When the nuclei undergo alpha decay the mass of the original nucleus is equal to the mass of the final nucleus, the mass of the alpha particle, and the energy of the alpha particle as it leaves the nucleus (found through E=mc˛). The problem is that in beta decay the sum of the mass of the final nucleus, the beta particle, and the energy of the beta particle is never quite equal to the original nucleus. While Niels Bohr suggested the abandonment of the conservation of energy, Wolfgang Pauli suggested an idea that we believe to be true now. In 1930, Pauli suggested that during beta decay another particle was given off to conserve the energy. This particle would have to be electrically neutral because the charge is already balanced with the beta particle. In 1934, Enrico Fermi picked up the search for this particle, calling it a Neutrino or little neutron.

Along with the violation of the conservation of energy there are a number of other violations in beta decay that had to be corrected. In the conservation of momentum the only way the decay could avoid a violation is if the beta particle moved in a direction opposite to the proton (an proportional speed) but it does not. Secondly, protons, neutrons, and electrons (and their antiparticles) have a spin of ±˝. If there were only protons and beta particles produced then Angular Momentum would not be conserved. Finally, comes the conservation of Lepton number. Protons and neutrons are not Leptons and therefore have a Lepton number of zero. Electrons have a Lepton number of one and positrons have a Lepton number of negative one. But when you examine the beta decay reaction without a third final particle Lepton number is not conserved.

Original Final
Neutron Proton Electron Final Net Number
Baryon # +1 +1 0 +1
Lepton # 0 0 +1 +1


Obviously the Lepton number is not conserved while the Baryon number may be. In contrast, if we add a Neutrino with a Lepton number of -1 and Baryon number of 0 the Lepton and Baryon numbers balance.

Original Final
Neutron Proton Electron Neutrino Final Net Number
Baryon # +1 +1 0 0 +1
Lepton # 0 0 +1 -1 0


The only problem with this idea of the neutrino was that there was not evidence of such a particle and it was no very reactive so it would be very hard to detect. These Physicists were looking for a mass less, neutrally charged particle, which was not inclined to react. Therefore, it was thought, to detect a neutrino they needed a whole lot of them so that the sheer number of neutrinos counteracts the very low probability of a reaction. It was found that in the process of fission there were a lot of unstable isotopes produced that promptly decayed producing billions and billions of antineutrinos in the process. Using this it was hypothesized that, if an electron hitting a proton is essentially the same as a proton emitting a positron then if a proton were hit by an antineutrino there would be a neutron and positron produced. Based on this idea an experiment was designed using a solution of cadmium chloride. The positron that was produced would react with an electron producing detectable radiation and the neutron would go into the nucleus of the Cadmium atom producing three or four more photons. Using this idea as the basis of an experement, it was announced that the antineutrino's distinct reactions had been observed in 1956 proving the existence of the antineutrino.

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