P-process
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The p process was believed to be a proton capture process which occurs during supernovae explosions. Then it was realized that the proton densities are too small and the observed isotopic abundances could not be explained by this kind of reaction.
Today its believed that photodisintegration reactions are responsible for the production of the proton-rich p-isotopes with masses A>100. The relevant reactions are (gamma,n)- and (gamma,alpha)-reactions. The temperature during a supernova explosion reaches up to <math>2-3*10^9 K<math>. The black body radiation produces a photon bath that can disintegrate the seed nuclei from the s-process and r-process. This is the reason why the p-process is sometimes called gamma-process.
The p-process contribution to isotopic abundances of elements that can also be produced in the s-process or r-process is usually very small. However there are p-only isotopes that cannot be produced in the s- or r-process (e.g. <math>{}^{190}<math>Pt or <math>{}^{168}<math>Yb). These isotopes have very small abundances compared to neighbour nuclei.
Sometimes the term p-process includes also the rp-process (rapid proton capture process). The astrophysical scenario for this process is still not firmly established but it is believed that a neutron star in a binary system which is accrediting mass from a main sequence star could be one possible scenario. During X-ray bursts the temperature and the proton density is high enough to start proton capture reactions and proton rich elements up to mass A=100 can be produced.