The Segre Chart

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:

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 Mayer shell model of nuclear structure, as explained in the Nobel prize lecture, here. The magic numbers are: 2, 8, 20, 28, 50, 82 & 126, and they are the same for neutrons and protons. The ⁴He, ¹⁶O and ⁴⁰Ca isotopes are double magic:
Isotopes have a very precisely known mass, list of isotope masses here.

      ¹²C  =  12.000 000 0(0)
      ¹³C  =  13.003 354 8378(10)
      ¹⁴C  =  14.003 241 988(4)
The carbon-12 isotope, ¹²C, is defined as having a mass of 12.0 exactly. All quoted atomic masses are relative to the mass of ¹²C.
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 relative atomic mass.

Fluorine has only one stable isotope, ¹⁹F, so its mass is known to very high precision, 18.99840320(7).

Lead has four stable isotopes: ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb & ²⁰⁸Pb, but the relative abundances vary for interesting radionuclear and geological reasons. As a result the average relative mass of lead is only accurate to 1 decimal place: 207.2(1)

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Radioactivity

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.

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Resources

There are many Segré chart resources in the web, including, an excellent interactive Segré chart from the National Nuclear Data Center showing many features such as half-life and decay mode (click to access):