The Segré chart is a schema for viewing isotopic data. Regions of stability and radioactive decay sequences can be mapped onto the schema.
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:
- Neutrons need to be present so that positively charged protons can bind together using the strong nuclear force. Slightly more than one neutron is required per proton and as the mass increases so does the neutron ratio. Carbon 12 has six protons and six neutrons. Iron 55 has 26 protons and 29 neutrons. Uranium 238 has 92 protons and 146 neutrons.
- Less than 300 isotopes are stable enough to exist naturally on Earth.
- So far, about 3000 have been observed in various nuclear experiments.
- It is thought that some 6000 combinations of protons and neutrons can exist, albeit fleetingly in many cases.
- 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 discussed in her Nobel prize lecture, here. The magic numbers are: 2, 8, 20, 28, 50, 82 & 126, and they are the same for protons and neutrons.
Three isotopes, 4He, 16O & 40Ca, 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)
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.