The rp-process (rapid proton capture process) consists of consecutive proton captures onto seed nuclei to produce heavier elements^[1]. It is a nucleosynthesis process and, along with the s process and the r process, may be responsible for the generation of many of the heavy elements present in the universe. However, it is notably different from the other processes mentioned in that it occurs on the proton-rich side of stability as opposed to on the neutron-rich side of stability. The end point of the rp-process (the highest mass element it can create) is not yet well established, but recent research has indicated that in neutron stars it cannot progress beyond tellurium.^[2] The rp-process is inhibited by alpha decay, which puts an upper limit on the end point at ¹⁰⁵Te, the lightest observed alpha decaying nuclide^[3], though lighter isotopes of tellurium could potentially be proton-bound and alpha decaying.

[edit] Conditions

The process has to occur in very high temperature environments (above 1 x 10⁹ Kelvin) so that the protons can overcome the large coulomb barrier for charged particle reactions. A hydrogen rich environment is also a prerequisite due to the large proton flux needed. The seed nuclei needed for this process to occur are thought to be formed during breakout reactions from the hot CNO cycle. Typically proton capture in the rp-process will compete with (α,p) reactions, as most environments with a high flux of hydrogen are also rich in helium. The time-scale for the rp-process is set by β⁺ decays at or near the proton drip line, because the weak interaction is notoriously slower than the strong interaction and electromagnetic force.

[edit] Possible sites

Sites suggested for the rp-process are binary systems where one star is a neutron star. In these systems a star is feeding material onto the compact neutron star. The accreted material is rich in hydrogen and helium. Because neutron stars have high gravitational fields, the material falls with a high velocity towards the compact star, usually colliding with other accreted material en route, forming an accretion disk. As this material slowly builds up on the surface, it will have a high temperature, typically around 1 x 10⁸ K. Eventually, it is believed that thermonuclear instabilities arise in this hot atmosphere, allowing the temperature to continue to rise until it leads to a runaway thermonuclear explosion of the hydrogen and helium. During the flash, the temperature quickly rises, becoming high enough for the rp-process to occur. While the initial flash of hydrogen and helium lasts only a second, the rp-process typically takes up to 100 seconds. Therefore, the rp-process is observed as the tail of the resulting X-ray burst.

[edit] References

1. ^ Lars Bildsten, "Thermonuclear Burning on Rapidly Accreting Neutron Stars" in The Many Faces of Neutron Stars, ed. R. Buccheri, J. van Paradijs, & M. A. Alpar (Kluwer), 419 (1998)
2. ^ Schatz, H.; A. Aprahamian, V. Barnard, L. Bildsten, A. Cumming, M. Ouellette, T. Rauscher, F.-K. Thielemann, and M. Wiescher (April 2001). "End Point of the rp Process on Accreting Neutron Stars" (subscription required). Physical Review Letters 86 (16): 3471–3474. doi:10.1103/PhysRevLett.86.3471. http://link.aps.org/abstract/PRL/v86/p3471. Retrieved 2006-08-24. 
3. ^ Tuli, Jagdish K. (2005). Nuclear Wallet Cards (7th ed.). National Nuclear Data Center. http://www.nndc.bnl.gov/wallet/wc7.html. Retrieved 2007-08-16. 

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