| © CAMBRIDGE UNIVERSITY PRESS 1997
| |
1.2 Some Basic Facts of Nuclear Physics
(1) | An atomic nucleus consists of Z protons and N neutrons, where Z is the atomic number defining the charge of the nucleus, the number of electrons in the neutral atom and hence the chemical element, and Z + N = A, the mass number of the nuclear species. Protons and neutrons are referred to collectively as nucleons. Different values of A or N for a given element lead to different isotopes, while nuclei with the same A and different Z are referred to as isobars. A given nuclear species is usually symbolised by the chemical symbol with Z as an (optional) lower and A as an upper prefix, e.g. 5626Fe.
Stable nuclei occupy a ``-stability valley'' in the Z, N plane (see Fig. 1.2), where one can imagine energy (or mass) being plotted along a third axis perpendicular to the paper. Various processes, some of which are shown in the figure, transform one nucleus into another. Thus, under normal conditions, a nucleus outside the valley undergoes spontaneous decays, while in accelerators, stars and the early universe nuclei are transformed into one another by various reactions.
| ||
(2) | The binding energy per nucleon varies with A along the stability valley as shown in Fig. 1.3, and this has the following consequences:
Nuclear reactions involving charged particles
(p, etc.)
require them to have
enough kinetic energy to get through in spite of the
electrostatic repulsion of the
target nucleus (the ``Coulomb barrier''); the greater the charges,
the greater the
energy required. In the laboratory, the energy is supplied by
accelerators, and
analogous processes are believed to occur in reactions induced in
the ISM by
cosmic rays (see Chapter 9). In the interiors of stars, the
kinetic energy exists by
virtue of high temperatures (leading to thermonuclear reactions)
and when one
fuel (e.g. hydrogen) runs out, the star contracts and becomes
hotter, eventually
allowing a more highly charged fuel such as helium to ``burn''.
There is no Coulomb barrier for neutrons, but free neutrons are
unstable so
that they have to be generated in situ, which again demands high
temperatures.
|