The Segré chart is a schema for viewing isotopic data. Regions of stability and radioactive decay sequences can be mapped onto the schema.
Introduction
The Segré chart arranges the products of nucleosynthesis – the individual isotopes – by neutron number vs proton number. The 254 stable isotopes present as a beautiful but fragile arc of stability in Segré space:
- It is thought that some 6000 combinations of protons and neutrons can exist, albeit fleetingly in many cases.
- Less than 300 isotopes are stable enough to exist naturally on earth.
- So far, about 3000 have been observed in various nuclear experiments.
- Some particularly stable – and therefore common isotopes – have "magic numbers" of protons and/or neutrons. These arrangements can be explained by the shell model of nuclear structure, see here.
- Isotopes have a very precisely known mass. But most chemical elements consist of a mixture of isotopes, and there is usually a slight variation in isotopic abundance from different sources. Thus, elements with more than one isotope can only have an average atomic mass.
- Go to a fully interactive Segré chart, here.
Nuclear stability and the various modes of radioactivity are associated with distinct regions of the Segré chart.
- There is a, N vs. Z backbone of stability.
- All nuclei with Z > 82 are alpha emitters.
- To the right of the stable N vs. Z backbone (as drawn), nuclei have an excess neutrons nuclei undergo beta decay, emission of an electron. Free neutrons have a half-life of 617 seconds.
- To the left of the stable N vs. Z backbone (as drawn), nuclei have too few neutrons and either undergo electron capture or positron emission.
- There are many other rarer modes of decay.
- A particular nucleus may decay by more than one mode.
- A nucleus can be induced to undergo fission by capturing a neutron. The nucleus splits into two halves, the excess free neutrons and a burst of gamma radiation.
There are many Segré chart resorces in the web, including:
Isotope (Wikipedia)
Segré chart (Wikipedia)
Segré chart (Brookhaven)
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© Mark R. Leach 1999-2008
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