In the 37Cl– 37Ar case, the transition probabilities to all relevant excited states can be evaluated, so that the cross section for capture can be calculated accurately over the range of neutrino energies expected from the sun.

The excited states decay rapidly to the ground state by emitting gamma-rays. In complex nuclei such as 37Cl, the neutrino absorption also produces 37Ar in various excited states. Again, the ν e capture cross section can be calculated, knowing the half-life, the threshold energy (0.814 MeV), the spins of the 37Cl and 37Ar nuclei, the weak interaction coupling constant, and the overlap of the wave functions of orbital electrons in the nucleus. It is the inverse of the electron capture of 37Ar, a decay process with a half-life of 35 days. Reaction (5) is used for detecting neutrinos from the sun. A beam of 5-MeV neutrinos would be reduced in intensity by 50% in traveling through 45 light years of water! With this cross-section value, we can calculate the distance that an antineutrino or neutron will penetrate through ordinary water. For example, a 5-MeV antineutrino would have a cross section of 2.4 × 10 −43 cm 2/H atom. Given the half-life of the neutron (∼15 min), the threshold energy, the spin change (none in this case), and the weak-interaction coupling constant, the theory of weak interactions may be used to calculate the cross section for reaction (4) as a function of the antineutrino energy. The minimum energy needed to carry out the reaction is called the threshold energy, E(thresh). All other fundamental fermions (leptons, quarks) are charged Dirac particles.Īnd requires that the antineutrino have sufficient energy to provide the mass difference between the neutron and the proton and to create the positron, a minimum total energy of 1.804 MeV. If the explanation is correct in this case, the massive neutrinos must be truly neutral Majorana particles. The most plausible explanation of this smallness is connected with the physics beyond the Standard Model, namely, with a violation of the lepton number at a mass scale M that is much larger than the mass scale of the violation of electroweak symmetry ∼10 2 GeV, which determines the values of masses of the leptons, quarks, W ±- and Z 0-bosons. The exceptional smallness of the neutrino masses requires special explanation. The investigation of the neutrino oscillations, the neutrinoless double β-decay and β-spectrum of 3 H-decay, will allow one to obtain important information on the neutrino masses, neutrino mixture, and neutrino nature (Dirac or Majorana?).

The investigation of the neutrino oscillations that is going on all over the world is a new field of research. The evidence for this phenomenon, predicted many years ago, was obtained recently. Because of the smallness of neutrino masses, new physical phenomenon, neutrino oscillations, the periodical transitions between different types of neutrinos in vacuum or in matter, become possible. The neutrino masses are many orders of magnitude smaller than the masses of their family partners (electron, muon, tau). The neutrinos are also exceptional particles with respect to their internal properties. They have enormous penetration properties and give us the unique possibility to investigate the internal structure of the nucleon, the internal invisible region of the sun where solar energy is produced. The neutrinos play a special role in the fields of particle physics and astrophysics. Bilenky, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 XVIII Conclusion

Neutrino definition