INTRODUCTION

Certain nuclides with a specific combination of protons and neutrons may transform spontaneously or be transformed by external agents to another nuclide with a different combination of neutrons and protons. We will consider such transformations as relevant to radiometric dating in the present chapter.

SPONTANEOUS NUCLEAR TRANSFORMATIONS

Radioactivity

The spontaneous transformation or decay of a potentially unstable nuclide to a more stable nuclide is called radioactivity. The energy released by the decay is carried mainly by particles and radiation. The decaying nuclide and its product nuclide are customarily labelled parent (p) and daughter (d), respectively. If d is radioactive, it decays to another nuclide until a stable d is produced. The nuclear decay process obeys the following conservation laws of Physics: (1) Mass/energy, (2) Electric charge, (3) Linear momentum, (4) Angular momentum, and (5) Nucleon number (Kaplan, 1955; Beiser, 1973; Leighton, 1959).

It was noted in the first chapter that stable nuclides define a narrow band in a Z vs N plot (Figure 1.3) corresponding to the greatest stability of Z/N ratio as a function of N. Unstable nuclei that deviate from the path of stability eventually transform into stable nuclei by different decay modes and rates. A diagonal section across the path of the stability valley will contain isobaric nuclides with the most stable isobar in or close to the path. Neutron-rich isobars will fall below the path and proton-rich isobars above it. Unstable nuclides heavier than 209Bi along the path are also neutron-rich. Although nuclides decay in many modes, the modes most common and relevant to radiogenic isotope geochronometry are few (Dalrymple, 1991), and are shown in Figure 2.1.

Beta (β-) decay

Neutron-rich nuclei below the stability zone reach stability by converting one of their neutrons into a proton, an electron (β-), and an antineutrino (v), and then emitting the latter two. The electron created in the nucleus just before its emission is called a beta or b particle to distinguish it from an orbital or extra-nuclear electron.