The r-Process (2) ...
Because the permitted
decay paths depend on the detailed nuclear physics, some isotopes can be
produced only
in the s-process. Likewise, some isotopes (for example, the transuranium elements) can be produced
only in the r-process. However, most heavy elements can be produced by both the r-process and the s-process,
so their abundance in nature receives a contribution from both.
The preceding diagram illustrates a part of
the Segrè
chart for the region near the stability valley around mass
180. Some of these isotopes are
produced entirely by the s-process (for example, the mass 186 isotope of osmium, Os-186); some are produced
only by the r-process (for example, the mass-187 isotope of tungsten, W-187); others are produced by both
processes (for example, the mass-182 isotope of tantalum, Ta-182).
Relative r-Process and s-Process Contributions
By careful theoretical
analysis utilizing the observed abundances of the isotopes that can be produced only in one process
or the other, it is possible to disentangle the contributions of the s-process and r-process to the abundances
of heavy nuclei. The result of such an analysis is shown in the adjacent figure, which plots relative
abundances from the two sources as a function of mass number for mass numbers between 70 and
210. Notice the logarithmic scale. Clearly in some
mass regions the s-process dominates, in others the r-process dominates, and in still others they are of
comparable importance. This figure only extends up to mass 210. Essentially everything above
mass 210 can only be made in the r-process.
Supernova Light Curves
The light curves from supernovae provide confirmation of our basic understanding of
nucleosynthesis in stars. There is strong evidence in Type Ia
and Type II supernovae that at late times the light curve is powered by radioactive
decay of isotopes produced in the explosion. In both of these cases, the characteristics of the light
curve at some time
after initial peak (the shape and rate of decline) become what would be expected if the energy were being
supplied by beta decay of nickel-56 to cobalt-56 (half-life of 5.5 days) followed by beta
decay of the cobalt-56 produced in the previous step
to iron-56 (half-life of 77 days).
The
radioactive decay emits energetic gamma rays that are
absorbed in the expanding nebula and re-emitted as light at visible and other longer wavelengths.
This interpretation has been strengthened by the direct detection of gamma
rays emitted by these decays in supernova explosions,
which have characteristic energies that identify them uniquely.
Typical estimates are that a little over 1/2
solar mass of nickel-56 must be produced in the explosion to power late-time
Type Ia supernova light curves, first
by decays to cobalt-56 and then by decay of the cobalt-56 to iron-56.
For Type II supernovae, much less nickel-56 is produced because the mechanism
causing the explosion is
different. It is estimated that about 0.075 solar masses of nickel-56 were produced by
SN 1987A, along with much smaller quantities of lighter radioactive isotopes.
