S & R Process

Matt PenriceAstronomy 501

University of Victoria

Outline

Overview

S-Process

R-Process

Conclusion

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

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

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€

τ =v t Nn (t)dt0

t

∫

€

dNAdτ

=σ A −1NA −1 −σ ANA

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

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

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613C+2

4He→816O+ n

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1022Ne+2

4He→1225Mg+ n

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

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

€

dNz(t)

dt= λ Z −1(t)NZ −1(t) − λ Z (t)NZ (t)

€

NZ ∝1

λ Z

Alan P. DickinRadiogenic Isotope Geology

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

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A ≈ Amax /2

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

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