The Segré Chart

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:


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  • 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)



Nucleosynthesis
Quantum Numbers to Periodic Tables

© Mark R. Leach 1999-2008


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