Carbon-12

History

Before 1959, both the IUPAP and IUPAC used oxygen to define the mole; the chemists defining the mole as the number of atoms of oxygen which had mass 16 g, the physicists using a similar definition but with the oxygen-16 isotope only. The two organizations agreed in 1959–60 to define the mole as follows.

This was adopted by the CIPM (International Committee for Weights and Measures) in 1967, and in 1971, it was adopted by the 14th CGPM (General Conference on Weights and Measures).

In 1961, the isotope carbon-12 was selected to replace oxygen as the standard relative to which the atomic weights of all the other elements are measured, consistently with the above definition of the mole.

In 1980, the CIPM clarified the above definition, defining that the carbon-12 atoms are unbound and in their ground state.

In 2018, IUPAC specified the mole as exactly "elementary entities". The number of moles in 12 grams of carbon-12 became a matter of experimental determination.

Hoyle state

The Hoyle state is an excited, spin-0, resonant state of carbon-12. It is produced via the triple-alpha process and was predicted to exist by Fred Hoyle in 1954. The existence of this 7.7 MeV resonance is essential for the nucleosynthesis of carbon in helium-burning stars and predicts an amount of carbon production which matches observations. The existence of the Hoyle state has been confirmed experimentally, but its precise properties are still being investigated.

The Hoyle state is populated when a helium-4 nucleus fuses with a beryllium-8 nucleus in a high-temperature (108 K) environment with densely concentrated (105 g/cm3) helium. As a consequence of the short half-life of 8Be, two helium nuclei fusing into it must be followed within ~10−16 seconds by a third, forming carbon. The Hoyle state also is a short-lived resonance with a half-life of ; it primarily decays back into its three constituent alpha particles, though 0.0413% of decays (or 1 in 2421.3) occur by emission of gamma rays into the ground state of 12C.

In 2011, an ab initio calculation of the low-lying states of carbon-12 found (in addition to the ground and excited spin-2 state) a resonance with all of the properties of the Hoyle state.

References

References

1. [http://www.ciaaw.org/carbon.htm "Standard Atomic Weights: Carbon"]. [[CIAAW]]. 2011.
2. (2004-01-26). "Atomic Weights and the International Committee — A Historical Review".
3. Hoyle, F.. (1954). "On Nuclear Reactions Occurring in Very Hot Stars. I. the Synthesis of Elements from Carbon to Nickel". The Astrophysical Journal Supplement Series.
4. (2014). "The Hoyle state in ¹²C". Progress in Particle and Nuclear Physics.
5. (2013). "Measurement of the radiative branching ratio for the Hoyle state using cascade gamma decays". EPJ Web of Conferences.
6. (2011). "Ab Initio Calculation of the Hoyle State". Physical Review Letters.
7. (2011). "Viewpoint: The carbon challenge".