S-process - The S-process in Stars

The S-process in Stars

The S-process is believed to occur mostly in asymptotic giant branch stars. In contrast to the R-process which is believed to occur over time scales of seconds in explosive environments, the S-process is believed to occur over time scales of thousands of years, passing decades between neutron captures. The extent to which the s-process moves up the elements in the chart of isotopes to higher mass numbers is essentially determined by the degree to which the star in question is able to produce neutrons. The quantitative yield is also proportional by the amount of iron in the star's initial abundance distribution. Iron is the "starting material" (or seed) for this neutron capture – beta-minus decay sequence of synthesizing new elements.

The main neutron source reactions are:

13 6C	+	4 2He	→	16 8O	+	n
22 10Ne	+	4 2He	→	25 12Mg	+	n

One distinguishes the main and the weak s-process component. The main component produces heavy elements beyond Sr and Y, and up to Pb in the lowest metallicity stars. The production sites of the main component are low-mass Asymptotic Giant Branch stars. The main component relies on the 13C neutron source above. The weak component of the S-process, on the other hand, synthesizes S-process isotopes of elements from iron group seed nuclei to 58Fe on up to Sr and Y, and takes place at the end of helium- and Carbon-burning in massive stars. It employs primarily the 22Ne neutron source. These stars will become supernovae at their demise and spew those s isotopes into interstellar gas.

The S-process is sometimes approximated over a small mass region using the so-called "local approximation", by which the ratio of abundances is inversely proportional to the ratio of neutron-capture cross-sections for nearby isotopes on the s-process path. This approximation is – as the name indicates – only valid locally, meaning for isotopes of nearby mass numbers, but it is invalid at magic numbers where the ledge-precipice structure dominates.

Because of the relatively low neutron fluxes expected to occur during the S-process (on the order of 105 to 1011 neutrons per cm2 per second), this process does not have the ability to produce any of the heavy radioactive isotopes such as thorium or uranium. The cycle that terminates the S-process is:

209Bi captures a neutron, producing 210Bi, which decays to 210Po by β- decay. 210Po in turn decays to 206Pb by α decay:

209 83Bi	+	n	→	210 83Bi	+	γ
210 83Bi			→	210 84Po	+	e−	+	ν e
210 84Po			→	206 82Pb	+	4 2He

206Pb then captures three neutrons, producing 209Pb, which decays to 209Bi by β- decay, restarting the cycle:

206 82Pb	+	3 n	→	209 82Pb
209 82Pb			→	209 83Bi	+	e−	+	ν e

The net result of this cycle therefore is that 4 neutrons are converted into one alpha particle, two electrons, two anti-electron neutrinos and gamma radiation:

4 n

→

4 2He

+

2 e−

+

2 ν e

+

γ

The process thus terminates in bismuth, the heaviest "stable" element. (Bismuth is actually slightly radioactive, but with a half-life so long—a billion times the present age of the universe—that it is effectively stable over the lifetime of any existing star.)