Supernovae short-lived isotopes

Many CAIs, together with some chondrules and samples of differentiated asteroids, contained short-lived radioactive isotopes at the time they formed. This is deduced from the abundances of the daughter isotopes seen in modern meteorites. The short-lived isotopes include " Ca, A1, °Be, e, Mn, and ° Pd, with half-lives (in units of Myr) 0.13, 0.7, 1.5, 1.5, 3.7, and 6.5, respectively. Many of these isotopes could have been produced from stable ones by absorption of neutrons in a supernova or the outer layers of a giant star. In particular, °Fe can only be produced efficiently by stellar nucleosynthesis and so must have come from an external source (Shukolyukov and Lugmair, 1993). Conversely, some isotopes such as Be almost certainly formed in the protoplanetary nebula when material was bombarded by solar cosmic rays (McKeegan et al., 2000). Multiple sources are possible for some short-lived isotopes. The abundances of the decay products of... 

The s-process cannot explain the formation of the elements heavier than bismuth as the trans-bismuth elements have a number of short-lived isotopes which prevait the formation of thorium and uranium in the amoimts observed in nature. The heaviest elements are believed to be formed in supernova explosions. 

The supernova stage is very short-lived with extremely intense neutron production. It provides a method whereby the barrier of the short-lived isotopes between polonium and francium is overcome and the heaviest elements synthesized. This mode of element formation is known as the rapid or r-process see Figure 17.9. 

Although additional work clearly is required to sort out the details, it seems likely that a supernova was involved in the formation of the solar system and that the Sun originated in a region of cluster star formation. One avenue of future work will be to investigate the potential contributions to stable isotopes from a late supernova. It is difficult to establish the level of such a contribution from observations because there are no labels on the atoms of stable isotopes produced in the last supernova as there are for the short-lived radioactive isotopes. It might be possible to infer the contribution by comparing solar system... 

The release of so many neutrons all at once provides another mechanism for the production of new elements. Now the density of neutrons is so great that n,y reactions of the type seen for the s process can occur with isotopes having very short half-lives. Suppose, for example, that a nucleus of the only stable isotope of bismuth, bismuth-209, is projected from the center of a star during a supernova explosion. Then, an s-like n,y reaction can occur, as follows ... 

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