S-process
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The s-process or slow-neutron-capture-process is a nucleosynthesis process that occurs at lower neutron density, lower temperature conditions in stars. Under these conditions the rate of neutron capture by atomic nuclei is slow relative to the rate of radioactive beta-decay. This process produces stable isotopes by moving along the valley of beta stability in the chart of isotopes. The s-process produces approximately half of the elements heavier than iron, and therefore plays an important role in the galactic chemical evolution. The s-process differs from the more rapid r-process and the proton-capture or p-process both in terms of reaction pathways and astrophysical environments.
The s-process is believed to occur in stars more massive than Earth's sun, most notably 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. 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, and by the amount of iron in the star's initial abundance distribution. Iron is the "starting material" (or seed) for this neutron capture - beta decay sequence of synthesizing new elements.
The main neutron source reactions are:
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 site of the main component are low-mass Asymptotic Giant Branch stars. The weak component of the s-process comprises elements from the iron group up to Sr and Y, and takes place at the end of He- and C-burning in massive stars.
The s-process is often mathematically treated using the so-called local approximation, which gives a theoretical model of elemental abundances based on the assumption of constant neutron flux in a star, so that the ratio of abundances is inversely proportional to the ratio of neutron-capture cross-sections for different isotopes. This approximation is - as the name indicates - only valid locally, meaning for isotopes of similar mass number.
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:
210Bi → 210Po + β-
210Po → 206Pb + α
Pb-206 then captures three neutrons, producing Pb-209, which decays to Bi-209 by beta decay, restarting the cycle.de:S-Prozess fr:Processus S ko:S-과정
