S-process

The s - process ( s for engl. Slow, slow dt ) is one of the Nukleosyntheseprozesse.

Background

S is a process Neutroneneinfangprozess that occurs at low neutron densities and relatively low temperatures, in contrast to the fast r process. It causes the construction elements to a mass number A = 210, including particularly stable.

The s - process runs primarily from in stars that are located in the asymptotic giant of the Hertzsprung- Russell diagram. These are stars with diameters from thousands times the solar diameter, in their core hydrogen and helium burning have already come to a halt and is fused in a shell around the core helium into carbon, the so-called shell burning. In them, it also comes to fusion reactions that release neutrons.

Since neutrons ( symbol n ) as opposed to protons have no electric charge, they can freely penetrate to the nucleus and γ be deposited there under emission of gamma quanta. As a result, mass number A and neutron number N by 1 and a new isotope increase arises. The starting material of the s- process is primarily iron, which was present in the star from the beginning.

If an atomic nucleus by the addition due to relative neutron excess unstable, a neutron by radioactive β is - decay, that is converted by the emission of an electron e and an electron- antineutrino into a proton. This results in the atom of another element with the same mass number, but increased by 1 atomic number Z ( number of protons ), and decreased by 1 neutron number N; therefore the atom in the periodic table takes a different course. Due to the slow flow of the Neutronenanlagerung extending over millennia, it is characteristic of the s- process that the β - decay of unstable isotopes takes place before another neutron is attached. As a result, can be formed by him that not all stable heavy elements.

The s - process is often described mathematically by the local approximation so-called, which is a theoretical model of the elemental abundances, based on the assumption of a constant neutron flux in the star. Thus, the ratio of the abundances obtained as inversely proportional to the ratio of the effective neutron capture cross section for the different isotopes, because the greater this cross section, the higher the probability of neutron capture and the associated transformation into another isotope.

Because of the relatively low neutron flux (of the order 105-1011 neutrons per square centimeter per second), you would expect during the s-process, the heavy, neutron-rich isotopes such as thorium and uranium can not be formed, since the required output nuclei Neutronenanlagerung before re- the β - decay subject. These isotopes are formed in the r-process.

S- process ends with a cycle of ( bismuth ) back from the output of the core bismuth isotope 209Bi on this:

Due to the s- process are the stars of the asymptotic Riesenasts the suppliers of the half of the heavy elements beyond iron 56Fe. The synthesized elements are transported by convection to the outside up to the stellar surface, where they can be detected spectroscopically. 1952 for the first time the radioactive technetium was observed in red giants, which could be only a short time before due to its half-life of a few million years ago by the s- process and thus supported the theory.

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