The r-Process
The s-process can build up heavier nuclei slowly. However,
it is restricted to the stability valley because, if
enough neutrons are captured to move out of it, one or more beta decays will occur to
return to the stability valley. Thus, the s-process cannot account for the observed
existence of nuclei out of the
stability valley. Also, because there is a gap in the stability valley beyond 209-Bi (the mass-209 isotope of
the element bismuth) the s-process cannot cross the gap to build the heaviest nuclei like uranium, thorium,
or plutonium (these are termed the transuranium elements).
Rapid Neutron Capture
But suppose the density of free neutrons
were so high that instead of capturing one or two neutrons before a beta decay
there was a very high probablility for capturing 5 or 10 or
even
20 neutrons before a beta decay could take place, as illustrated in the following figure.
Then the rapid capture of neutrons would take us far to the
right in the Segrè chart and
out of the stability valley. This could do two things: First, it could produce a whole
series of "neutron-rich" isotopes (isotopes having more neutrons than those in the stability valley) when the
population beta decays back toward the stability valley after the flux of neutrons
decreases. Second,
if the capture is rapid enough the gap noted
above can be bridged, and beta decay back to the stability valley can produce the stable isotopes of the
transuranium elements that cannot be produced in the s-process.
The r-Process
This is exactly the idea of the rapid neutron capture or r-process.
The difference between the s-process and the r-process is that in the r-process the flux of neutrons is large
and the capture rate is very high; thus it is a rapid capture. The s-process is thought to
take place primarily in red giant stars, but the large flux of neutrons required for the r-process cannot be
produced in a normal star. The most likely site for the r-process is in a Type II supernova
(see the right panel).
The r-Process Path
The path followed in the r-process during a supernova is illustrated by the red area in the following diagram.
Notice that the r-process path runs very close to the neutron drip line. That is, it runs through extremely
neutron-rich isotopes. When the neutrons moderate in intensity as the supernova explosion begins to wane, the
resulting population then beta decays back toward the stability valley.
This
animation illustrates the r-process path in the chart of the nuclides.
