11th Week: The s,r and p-Process B

Nuclei that have gained protons due to nucleosynthesis make it harder for other charged particles to react due to the coulomb repulsion. This is however different if for neutrons which are neutral react with these nuclei. If the half life of beta decay is short when we look at the timescale for neutron capture we say that this is a s-process. On the other hand if the half life of beta decay is big when compared to the timescale for neutron capture it is called the r-process.

S Process

This process is likely seen in phases of stellar evolution. The s-process elements are not produced in normal ways. Up until this point in nucleosynthesis stars burn to produce energy. The s-process now goes beyond the iron/nickel limit of binding energy. These elements are produced mainly by a large neutron burst. These neutrons will slowly be captured by nuclei such as nickel which will then immediately beta decay to copper. This process then continues all the way to bismuth then stops as bismuth is alpha unstable.

R Process

This process allows for neutron-rich nuclei. These r-process reactions are seen in supernovae where there are large amounts of neutrinos. In this case the neutron capture is much faster than the beta decay (the reverse of the s process). This creates large numbers of neutron rich nuclei which will decay to the line of stability after the neutron burst ends. Certain mass numbers will have a much higher abundance due to the r process. These mass numbers are known as waiting points. At these isotopes the beta decay rate becomes comparable to the neutron capture timescale and this causes multiple new branches to appear in the r process.