Page 309 - Radiochemistry and nuclear chemistry

292 Radiochemistry and Nuclear Chemistry

hyperons and nuclei), (ii) leptons are the "light"ones (the electron, the neutrino and the
muon); (iii) mesons have "intermediate" masses; these include the ~r-meson, the K-meson,
etc. The baryons and the mesons have also been considered as hadrons, "hard" or "strong"
particles as they take part in the strong nuclear force. Such properties were used to develop
a table of elementary particles, Table 10.1.
All the particles in Table 10.1 have spin 1. Quantum mechanical calculations and
experimental observations have shown that each particle has a fixed spin energy which is
determined by the spin quantum number s (s = tA for leptons and nucleons). Particles of
non-integral spin are called fermions because they obey the statistical rules devised by
Fermi and Dirac, which state that two such particles cannot exist in the same closed system
(nucleus or electron shell) having all quantum numbers the same (referred to as the Pauli
principle). Fermions can be created and destroyed only in conjunction with an anti-particle
of the same class. For example if an electron is emitted in//-decay it must be accompanied
by the creation of an anti-neutrino. Conversely, if a positron - which is an anti-electron -
is emitted in the/3-decay, it is accompanied by the creation of a neutrino.
Fermions are the building blocks of nature. There is another group of "particles" called
bosons, to which the photon and mesons belong. The bosons are the carriers of forces.
When two fermions interact they continually emit and absorb bosons. The bosons have an
even spin (0, 1, etc), they do not obey the Pauli principle, and they do not require the
formation of anti-particles in their reactions.
All the particles mentioned have their anti-particles (designated by a bar above the particle
symbol), except the photon and the mesons, who are their own antiparticles. We may think
about antimatter as consisting of antiprotons and antineutrons in an antinucleus surrounded
by antielectrons (i.e. positrons). Superficially, there would be no way to distinguish such
antimatter from our matter (sometimes called koino matter). It has been proposed that the
universe is made up of matter and antimatter as a requirement of the principle of symmetry.
In that case some galaxies, which perhaps can be observed, should be made up of
antimatter. When such antimatter galaxies (or material expelled from them) collide with
koino matter galaxies, both types of matter are annihilated and tremendous amounts of
energy released.
In order to reach the goal of a comprehensive yet simple theory of the composition of all
matter, the properties of the neutrinos and the quark theory must be considered.

10.6. The neutrino

The neutrino plays an essential role in the models of elementary particles and in the
theory of the formation and development of the universe. The existence of the neutrino was
predicted by Pauli in 1927 but it was not proven until 1956 when Reines and Cowan
detected them in experiments at the Savannah River (USA) nuclear reactor. Since neutrinos
are emitted in the//-decays following fission, nuclear reactors are the most intense neutrino
sources on earth. The detector in the discovery experiments consisted of a scintillating
solution containing cadmium surrounded by photomultipliers to observe the scintillations
which occurred as a consequence of the following reactions:

I Spin is an intrinsic property of elementary particles, sometimes thought of as a rotation.