On the origin of the heavy elements.

The inverse correlation between the metal abundances and the ages of stars suggests that the elements have been formed in stellar interiors. An analysis of the cosmic abundances of nuclear isobars, and calculations relating to the growth of nuclide abundances by neutron capture, show that the following three mechanisms are necessary, and probably sufficient, to produce the observed cosmic abundances of the nuclides with mass number greater than 70. (i) Neutron capture on a slow time-scale. When helium thermonuclear reactions commence in a hydrogen-exhausted stellar core, explosive instability forces an expansion of the core, probably accompanied by mixing between the core and envelope (Hoyle and Schwarzschild 1955). Further helium reactions then produce neutrons. These are initially captured predominantly by the iron abundance peak and eventually large abundances of certain heavy nuclides are produced. The mean time between neutron captures in a typical heavy nucleus is of the order of 10 to 100 years. The observed cosmic abundances correspond to the production and capture by heavy nuclides of about i8 neutrons per silicon nucleus initially present in the core. (2) Neutron capture on a fast time-scale. It has been suggested (Hoyle 1946) that in supernovae a rapid implosion of the core is accompanied by a thermonuclear explosion in the envelope. If the hydrogen abundance in the envelope is comparable to the abundance of the products of advanced thermonuclear reactions, the explosion will rapidly exhaust the hydrogen and then produce very high fluxes of neutrons by exothermic (a, n) reactions. It has been suggested (G. R. Burbidge, F. Hoyle, E. M. Burbidge, R. F. Christy and W. A. Fowler, unpublished) that this is the mechanism involved in Type I supernova explosions. The mean time between neutron captures in a typical heavy nucleus will be of the order of a second, and the effects of closed neutron shells are reflected in the broad cosmic abundance peaks centered at mass numbers 8o, 130 and 195. Trans-bismuth elements are also formed in large abundances by this mechanism. (3) Photonuclear reactions on the products of the slow neutron capture time-scale. If in supernova explosions, probably of Type II, the hydrogen is not exhausted in the exploding layers, the production of neutrons by (a, n) reactions is negligible. However, if the temperature reaches ~2 X i~~ 0K for a few seconds, then a few neutrons and a smaller number of protons can be ejected by photonuclear reactions from heavy nuclides previously produced in the layer by neutron capture on the slow time-scale. This produces nuclides on the neutron deficient side of the nuclear mass valley but leaves the general abundance trend unchanged from that produced by the neutron capture on the slow time-scale. Hoyle, F. and Schwarzschild, M. 1955, Ap. J. Suppl. Ser. 2, 1. Hoyle, F. 1946, M. N. io6, 343. Atomic Energy of Canada, Limited.