1  Nucleosynthesis and nuclear decay

 

1.1       The chart of the nuclides

 

In the field of isotope geology, neutrons, protons and electrons can be regarded as the fundamental building blocks of the atom. The composition of a given type of atom, called a nuclide, is described by specifying the number of protons (atomic number, Z) and the number of neutrons (N) in the nucleus. The sum of these is the mass number (A). By plotting Z against N for all of the nuclides that have been known to exist (at least momentarily), the chart of the nuclides is obtained (Fig. 1.1). In this chart, horizontal rows of nuclides represent the same element (constant Z) with a variable number of neutrons (N). These are isotopes.

 


Fig. 1.1. Chart of the nuclides in coordinates of proton number, Z, against neutron number, N. ( O ) = stable nuclides;  ( Q ) = unstable nuclides;  ( 3 ) = naturally occurring long-lived unstable nuclides; ( u ) = naturally occurring short-lived unstable nuclides. Some geologically useful radionuclides are marked. Smooth envelope = theoretical nuclide stability limits. For a more detailed nuclide chart, see the links page.

 

            264 stable nuclides are known, which have not been observed to decay (with available detection equipment). These define a central ‘path of stability’, coloured black in Fig. 1.1. On either side of this path, the zig-zag outline defines the limits of experimentally known unstable nuclides (Hansen, 1987). These tend to undergo increasingly rapid decay as one moves out on either side of the path of stability. The smooth outer envelopes are the theoretical limits of nuclide stability, beyond which ‘prompt’ decay occurs. In that case the synthesis and decay of an unstable nuclide occurs in a single particle interaction, giving it a zero effective lifetime. As work progresses, the domain of experimentally known nuclides should approach the theoretical envelope, as has already occurred for nuclides with Z < 22 (Hansen, 1987).

 

            Recent experiments on the Darmstadt heavy ion accelerator, reviewed by Normile (1996), have helped to expand the field of experimentally known unstable nuclides on the neutron-rich side of the path of stability (Fig. 1.1). These new isotopes were manufactured by bombarding target material with 238U, triggering fission reactions which produce a few very neutron-rich nuclei. The products were separated by mass spectrometry and their properties examined using high-energy detectors (e.g. Fig. 14.41). Knowledge about these unstable nuclei will improve our understanding of the nucleosynthetic r-process, which operates within this part of the nuclide chart (Fig. 1.7).

 

            A small number of unstable nuclides have sufficiently long half-lives that they have not entirely decayed to extinction since the formation of the solar system. A few other short-lived nuclides are either continuously generated in the decay series of uranium and thorium, or produced by cosmic ray bombardment of stable nuclides. These nuclides, and one or two extinct short-lived isotopes, plus their daughter products, are the realm of radiogenic isotope geology. Most of those with half-lives over 0.5 Myr are marked in Fig. 1.2. Nuclides with half-lives over 1012 yr decay too slowly to be geologically useful. Observation shows that all of the other long-lived isotopes either have been or are being applied in geology.


Fig. 1.2. Unstable nuclides with half-lives (t1/2) over 0.5 Myr, in order of decreasing stability. Geologically useful parent nuclides are marked. Some very long-lived radionuclides with no geological application are also marked, in brackets.

 

 

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