D

The age t [a] can be derived, depending whether N0, N, or D is known, according to one of the following equations:

The use of a radioactive system for age determination presupposes that neither the parent nor the daughter nuclides are lost or gained except through the decay process itself—a condition that is known as closed system.

There are several types of radioactive decay: a-decay takes place under, the emission of an a-particle, which is a 4He nucleus. During b-decay, the nucleus emits a b-particle, which is an electron whereby a neutron is converted into a proton. Electron capture occurs when the nucleus captures an extranuclear, orbiting electron from the innermost atomic shell (K-shell) whereby a nuclear proton in converted into a neutron. During spontaneous fission, the atomic nucleus splits into two heavy fragments and two or three neutrons. Some nuclides exhibit a dual decay mechanism, as a-decay and spontaneous fission for 238U. Radioactive decay is generally accompanied by the emission of energy discrete g-rays—a type of electromagnetic radiation coming from the exited nucleus.

Instead of being stable, the daughter nuclide may be radioactive and disintegrate itself. Several such radioactive daughter nuclides following each other form a decay chain until finally a stable end product is reached. Most prominent for chronometric dating is the decay chain starting from 238U and ending at 206Pb involving several steps of a- or b-disintegration. If the decay chain stays undisturbed, i.e., under closed-system conditions, a balance between production and decay of the interim members is gradually established. At this stage, which is called radioactive or secular equilibrium, all radioactive members N1, N2, N3, etc.

assume equal radioactivity dN/dt

In nature, various kinds of radioactive nuclides occur and many of them can be used for chronometry. Due to their origin, these nuclides can be divided into various groups: Primordial nuclides are still left over from the time of nucleosynthesis and, thus, are older than the formation of the earth (e.g., 238U). Radiogenic nuclides are produced by radioactive decay (e.g., 230Th). Cosmogenic nuclides are formed by the interaction of cosmic rays with the atmosphere and the earth's surface (e.g., 14C). Anthropogenic nuclides are produced in nuclear plants and explosions (e.g., 3H). For the Quaternary period, there is a wide spectrum of dating methods available (Wagner 1998). O Figure 10.1 presents an outline of the

O Figure 10.1

Important radiometric dating methods for the Plio-Pleistocene period

O Figure 10.1

Important radiometric dating methods for the Plio-Pleistocene period

radiometric dating methods with sound paleoanthropologic potential.

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