p-process

The p-process is a nucleosynthesis process occurring in core-collapse supernovae (see also supernova nucleosynthesis) responsible for the creation of some proton-rich atomic nuclei heavier than iron.

History

When the p-process was originally proposed in the famous B²FH paper in 1957,^[1] the physics of the process itself was not understood. The authors believed that most atomic nuclei heavier than iron, which are generally neutron-rich, were created by the s-process and r-process, both mechanisms for creating neutron-rich nuclei through neutron capture. However, it was observed that some proton-rich nuclei cannot be produced by the s- or r-process (e.g. ¹⁹⁰Pt or ¹⁶⁸Yb). This simple observation suggested that there must be a process for creating certain heavy proton-rich nuclei, and so it was called simply the proton-process, or p-process for short. It is interesting to note that the process as it is now understood actually has nothing to do with proton capture as the name might suggest, and thus it should not be confused with the rp-process, which does involve proton capture. Sometimes the rp-process is mistakenly referred to as the p-process, because the nomenclature, for historical reasons, is slightly misleading.

Nuclear physics

If one considers a stable atomic nucleus, there are two ways one may increase the ratio of protons to neutrons — one may add protons or subtract neutrons. The rp-process works by adding protons. The p-process occurs through the mechanism of photodisintegration, which occurs when a gamma-ray, or energetic photon, knocks particles out of an atomic nucleus. This is why the p-process is sometimes referrred to as the gamma-process. By examining the chart of nuclides, one can see that from hydrogen to calcium ( 1 < A < 40 ) stable nuclei have an approximately equal number of protons and neutrons. However, for increasingly heavier stable nuclei, more than one neutron is required for each proton because of Coulomb repulsion. The p-process is responsible for the nucleosynthesis of some atomic nuclei with more than 100 nucleons ( A > 100 ), and so if one starts with a heavy, stable nucleus, removing either neutrons or an equal number of protons and neutrons will increase the proton ratio of the resulting nucleus. There are two important nuclear reactions which accomplish this task, neutron-photodisintegration and alpha-photodisintegration, written (γ,n) and (γ,α), respectively.

Astrophysics and Abundances

The temperature during a core-collapse supernova explosion reaches up to 2×10⁹ to 3×10⁹ Kelvin. The resulting black-body radiation produces a photon bath that can disintegrate the seed nuclei created by the s-process and r-process. Under these conditions it is believed that photodisintegration reactions are responsible for the production of some proton-rich atomic nuclei with more than 100 nucleons (A > 100). It has been recently proposed that neutron star mergers (collisions between two neutron stars in a binary star system) will have similar conditions and may also play a role in the production of p-process nuclei (nuclei created only by the p-process, not to be confused with P-isotopes), but this has yet to be observationally confirmed. Because the p-process operates for only a short time by knocking a few neutrons or alpha particles out of heavy nuclei synthesized through another route, p-process nuclei are less abundant than neighbouring isotopes and isotones. The p-process can also contribute to the production of the atomic nuclei produced by the s-process, r-process, or rp-process, but typically this contribution is very small.

References

1. ^ E. M. Burbidge, G. R. Burbidge, W. A. Fowler, F. Hoyle (1957). "Synthesis of the Elements in Stars". Reviews of Modern Physics 29 (4): 547-650. DOI:10.1103/RevModPhys.29.547. 

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