Thus in an ideal situation determinations may be made of 10–^12 g of an analyte. Furthermore, the energy
spectrum of the emissions may be used to identify and distinguish between a number of different
radionuclides. Currently much attention is directed towards the determination of radionuclides present
environmentally as pollutants.
10.1—
Nuclear Structure and Nuclear Reactions
For most purposes a simple picture of the atomic nucleus is adequate. It may be regarded as a collection
of neutrons and protons (collectively known as nucleons) arranged in a series of energy levels
similar to the arrangement of extranuclear electrons (Figure 10.1). The diameter of a nucleus is about
10 –^14 m. Transitions between nuclear levels may occur, accompanied by the emission or absorption of
energy which is frequently in the form of electromagnetic radiation (γ-rays). The levels are widely
spaced, so that the radiation will be of high photon energy and short wavelength (10–^11 to 10–^7 nm or
100 keV to 5 MeV). Nuclear transitions and their associated electromagnetic radiations are as
characteristic of an individual nucleus as electronic and vibrational transitions are of atoms and
molecules.
Figure 10.1
Representation of nuclear energy levels showing the energy levels
of nucleons and the 'magic numbers' corresponding to filled shells.
Shaded areas represent the gaps between the shells.Decay Reactions
The overall stability of a nucleus is closely related both to total and relative numbers of neutrons and
protons within it. If the mass number is large (A > 209) a major instability will exist and the nucleus
will adjust to a lower energy state, in part, by the emission of large particles, , α-particles. For