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1.2 Some Basic Facts of Nuclear Physics


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.

Figure 1.2

Fig. 1.2. Chart of the nuclides, in which Z is plotted against N. Stable nuclei are shown in dark shading and known radioactive nuclei in light shading. Arrows indicate directions of some simple nuclear transformations. After K.S. Krane, Introductory Nuclear Physics, ©1988 by John Wiley & Sons. Reproduced by permission of John Wiley & Sons, Inc.

Stable nuclei occupy a ``beta-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.

Figure 1.3

Fig. 1.3. Binding energy per nucleon as a function of mass number. Adapted from Rolfs & Rodney (1988).


The binding energy per nucleon varies with A along the stability valley as shown in Fig. 1.3, and this has the following consequences:

(a) Since the maximum binding energy per nucleon is possessed by 62Ni, followed closely by 56Fe, energy is released by either fission of heavier or fusion of lighter nuclei. The latter process is the main source of stellar energy, with the biggest contribution (7 MeV per nucleon) coming from the conversion of hydrogen into helium (H-burning).

(b) Some nuclei are more stable than others, e.g. the alpha-particle nuclei 4He, 12C, 16O, 20Ne, 24Mg, 28Si, 32S, 36Ar, 40Ca. Nuclei with a couple of A-values (5 and 8) are violently unstable, owing to the nearby helium peak. Others are stable but only just: examples are D, 6,7Li, 9Be and 10,11B, which are destroyed by thermonuclear reactions at relatively low temperatures.


Nuclear reactions involving charged particles (p, alpha 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.

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