S & R Process

1. S & R Process Matt Penrice Astronomy 501 University of Victoria

2. Outline • Overview • S-Process • R-Process • Conclusion

3. Overview • Neucleosythiesis beyond iron is driven primarily by neutron capture reactions • If the time between neutron capture reactions is long compared to the time for beta decay to occur it is known as the s-process • If the time between neutron capture reactions is short compared to the time for beta decay to occur it is known as the r-process

4. S-Process • Because the neutron capture rate for the s-process is long compared to the beta decay rate, the s-process follows the valley of beta-stability • Neutron capture continues along the stable isotopes of a given element until it reaches an unstable isotope where is subsequently beta decays into a new element

5. Abundance Change • We can assume a constant Temp during a given neutron irradiation • Can derive the time integrated neutron flux • Using this we arrive at the rate of change of the abundance of a given nuclei

6. D.D. Clayton, WA. Fowler, TE. Hull & B.A. Zimmerman, Ann. Phys., 12,

7. Possible sites for s-process • One candidate is the helium burning shell of a red giant • One possible source for the neutrons is the reaction • Helium shell flashes can also cause mixing which leads to the creation of carbon 13 and subsequently the neutron source reaction

8. R-Process • Works similar to the s-process, but the neutron density is much higher so the neutron capture rates are much higher than beta decay rates • Rapid accumulation of neutrons occurs until the neutron binding energy approaches zero. This occurs when the photodisintegration rate equals the neutron capture process • The nuclei must wait until a beta decay occurs before continuing with neutron capture

9. Time dependence of abundances • The abundances are characterized by nuclear charge, below is an equation for a given waiting point • Lamb = decay rate of isotope (charge) at time t • The equation reaches a point of equilibrium at • Therefore Nz abundances correlate with the beta-decay lifetimes at the waiting points

10. Alan P. Dickin Radiogenic Isotope Geology

11. Highlights of figure • Magic neutron numbers N=50,82,126 • These neucli have higher then average beta decay time and build up to relatively high abundances • The r-process is terminated by neutron induced fission • The fission reaction cycles material back into the process

12. R-Process • The r-process occurs in unstable nuclei which are extremely difficult to work with in the laboratory. This gives rise to large uncertainty in calculations • Possible sites for the r-process are supernova and other extreme events where the condition for high neutron flux can be met

13. Conclusion • For the s-process the beta decay rate is longer then the neutron capture rate, where as for the r-process the situation is reversed • A possible site for the s-process is the helium shell of a red giant star • A possible site for the r-process is a supernova • Differences between s and r-proceces can be investigated by looking at isotopes which are produced in isolation from each other