History of s-process nucleosynthesis and outline

In the middle of the twentieth century, Paul W. Merrill (1952) detected the beta-unstable element technetium in the spectra of red supergiants known as S stars, which have long been recognized as being AGB stars. 99Tc is the longest lived isotope of technetium, and its half life of 211 000 yr is several orders of magnitude shorter than the lifetime of a core helium-burning precursor of an AGB star. When observed mass-loss rates are taken into account, the lifetime of a star in the AGB phase is roughly an order of magnitude larger than the half life of 99Tc. It is therefore incontrovertible that the 99Tc observed has been formed in AGB stars. On the other hand, given the temperatures and densities in AGB models, it is clear that an element of atomic charge as large as that of Tc cannot be a consequence of reactions between charged particles and it has become accepted that neutron capture must be involved.

The earliest theoretical studies of neutron-capture nucleosynthesis were carried out by A. G.W. Cameron (1955) and by E. Margaret Burbidge, Geoffrey R. Burbidge, William A. Fowler, and Fred Hoyle (1957) and, over the years, numerous studies have been devoted to examining how the distribution of abundances in an initial set of heavy elements is altered when this set is placed in a bath of neutrons at various assumed number densities for various exposure times.