Editing P-process

{{Short description|Processes in astrophysics}}
{{DISPLAYTITLE:''p''-process}}
{{nuclear physics}}
The term '''p-process''' (''p'' for [[proton]]) is used in two ways in the scientific literature concerning the [[astrophysics|astrophysical]] origin of the elements ([[nucleosynthesis]]). Originally it referred to a proton capture process which was proposed to be the source of certain, naturally occurring, neutron-deficient [[isotope]]s of the [[chemical element|elements]] from [[selenium]] to [[Mercury (element)|mercury]].<ref name="b2fh">{{cite journal | title = Synthesis of the Elements in Stars | author-link1 = Margaret Burbidge | author1 = Burbidge, E. M. | author-link2 = Geoffrey Burbidge | author2 = Burbidge, G. R. | author-link3 = William Alfred Fowler | author3 = Fowler, W. A. | author-link4 = Fred Hoyle | author4 = Hoyle, F. | journal = Reviews of Modern Physics | volume = 29 | issue = 4 | pages = 547–650 | date = 1957 | doi = 10.1103/RevModPhys.29.547 | bibcode = 1957RvMP...29..547B | doi-access = free }}</ref><ref name="cameron">{{cite journal | author-link = Alastair Cameron | author = Cameron, A. G. W. | title = Nuclear Reactions in Stars and Nucleogenesis | journal = Publications of the Astronomical Society of the Pacific | date = 1957 | volume = 69 | issue = 408 | pages = 201–222 | jstor = 40676435 | bibcode = 1957PASP...69..201C |doi = 10.1086/127051 | doi-access = free }}</ref> These [[nuclide]]s are called [[p-nuclei]] and their origin is still not completely understood. Although it was shown that the originally suggested process cannot produce the p-nuclei, later on the term p-process was sometimes used to generally refer to any [[nucleosynthesis]] process supposed to be responsible for the p-nuclei.<ref name="arnould">{{cite journal | author = Arnould, M. | author2 = Goriely, S. | title = The p-Process of Stellar Nucleosynthesis: Astrophysics and Nuclear Physics Status | journal = Physics Reports | date = 2003 | volume = 384 | issue = 1–2 | pages = 1–84 | bibcode = 2003PhR...384....1A | doi = 10.1016/S0370-1573(03)00242-4 }}</ref>

Often, the two meanings are confused. Recent scientific literature therefore suggests to use the term p-process only for the actual proton capture process, as it is customary with other nucleosynthesis processes in astrophysics.<ref name="posnic2010">{{ cite journal | author = Rauscher, T. | title = Origin of p-Nuclei in Explosive Nucleosynthesis | journal = Proceedings of Science | date = 2010 | volume = NIC XI | issue = 59 | arxiv = 1012.2213|bibcode = 2010arXiv1012.2213R }}</ref>

==The proton capture p-process==
Proton-rich nuclides can be produced by sequentially adding one or more protons to an [[atomic nucleus]]. Such a [[nuclear reaction]] of type (p,γ) is called ''proton capture reaction''. By adding a proton to a nucleus, the [[chemical element|element]] is changed because the chemical element is defined by the [[proton number]] of a nucleus. At the same time the ratio of protons to [[neutron]]s is changed, resulting in a more neutron-deficient isotope of the next element. This led to the original idea for the production of p-nuclei: free protons (the nuclei of [[hydrogen]] [[atom]]s are present in stellar [[Plasma (physics)|plasmas]]) should be captured on heavy nuclei (''seed nuclei'') also already present in the stellar plasma (previously produced in the [[s-process|''s''-process]] and/or [[r-process|''r''-process]]).<ref name="b2fh" /><ref name="cameron" />

Such proton captures on [[stable nuclide]]s (or nearly stable), however, are not very efficient in producing p-nuclei, especially the heavier ones, because the [[electric charge]] increases with each added proton, leading to an increased repulsion of the next proton to be added, according to [[Coulomb's law]]. In the context of nuclear reactions this is called a [[Coulomb barrier]]. The higher the Coulomb barrier, the more [[kinetic energy]] a proton requires to get close to a nucleus and be captured by it. The average energy of the available protons is given by the [[temperature]] of the stellar plasma. Even if this temperature could be increased arbitrarily (which is not the case in stellar environments), protons would be removed faster from a nucleus by [[photodisintegration]] than they could be captured at high temperature. A possible alternative would be to have a very large number of protons available to increase the effective number of proton captures per second without having to raise the temperature too much. Such conditions, however, are not found in [[Type II supernova|core-collapse supernovae]] which were supposed to be the site of the p-process.<ref name="arnould" /><ref name="posnic2010" />

Proton captures at extremely high proton densities are called [[P-nuclei#Rapid proton capture processes|rapid proton capture processes]]. They are distinct from the p-process not only by the required high proton density but also by the fact that very short-lived [[radionuclides]] are involved and the reaction path is located close to the [[proton drip line]]. Rapid proton capture processes are the [[rp-process]], the [[P-nuclei#The .CE.BDp-process|νp-process]], and the [[P-nuclei#The pn-process|pn-process]].

==History==
The term p-process was originally proposed in the famous [[B2FH paper|B<sup>2</sup>FH paper]] in 1957. The authors assumed that this process was solely responsible for the p-nuclei and proposed that it occurs in the hydrogen-shell (see also [[stellar evolution]]) of a [[star]] exploding as a [[type II supernova]].<ref name="b2fh" />  It was shown later that the required conditions are not found in such supernovae.<ref name="truran">{{ cite journal | author = Audouze, J. | author2 = Truran, J. W. | title = p-Process Nucleosynthesis in Postshock Supernova Envelope Environments | journal = The Astrophysical Journal | date = 1975 | volume = 202 | issue = 1 | pages = 204–213 | bibcode = 1975ApJ...202..204A | doi = 10.1086/153965 }}</ref>

At the same time as B<sup>2</sup>FH, [[Alastair Cameron]] independently realized the necessity to add another nucleosynthesis process to [[neutron capture nucleosynthesis]] but simply mentioned proton captures without assigning a special name to the process. He also thought about alternatives, for example photodisintegration (called the [[Gamma process (astrophysics)|γ-process]] today) or a combination of p-process and photodisintegration.<ref name="cameron" />

==See also==
{{Portal|Astronomy|Physics}}
*[[p-nuclei]]
*[[Nucleosynthesis]]
*[[rp-process]]
*[[r-process]]
*[[s-process]]

==References==
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{{Nuclear_processes}}
{{supernovae}}

[[Category:Nuclear physics]]
[[Category:Nucleosynthesis]]
[[Category:Supernovae]]
[[Category:Proton]]
[[Category:Concepts in stellar astronomy]]