| Nuclear physics |
|---|
 |
High-energy processes |
The termp-process (p forproton) is used in two ways in the scientific literature concerning theastrophysical 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-deficientisotopes of theelements fromselenium tomercury.[1][2] Thesenuclides are calledp-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 anynucleosynthesis process supposed to be responsible for the p-nuclei.[3]
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.[4]
Proton-rich nuclides can be produced by sequentially adding one or more protons to anatomic nucleus. Such anuclear reaction of type (p,γ) is calledproton capture reaction. By adding a proton to a nucleus, theelement is changed because the chemical element is defined by theproton number of a nucleus. At the same time the ratio of protons toneutrons 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 ofhydrogenatoms are present in stellarplasmas) should be captured on heavy nuclei (seed nuclei) also already present in the stellar plasma (previously produced in thes-process and/orr-process).[1][2]
Such proton captures onstable nuclides (or nearly stable), however, are not very efficient in producing p-nuclei, especially the heavier ones, because theelectric charge increases with each added proton, leading to an increased repulsion of the next proton to be added, according toCoulomb's law. In the context of nuclear reactions this is called aCoulomb barrier. The higher the Coulomb barrier, the morekinetic 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 thetemperature 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 byphotodisintegration 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 incore-collapse supernovae which were supposed to be the site of the p-process.[3][4]
Proton captures at extremely high proton densities are calledrapid 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-livedradionuclides are involved and the reaction path is located close to theproton drip line. Rapid proton capture processes are therp-process, theνp-process, and thepn-process.
The term p-process was originally proposed in the famousB2FH 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 alsostellar evolution) of astar exploding as atype II supernova.[1] It was shown later that the required conditions are not found in such supernovae.[5]
At the same time as B2FH,Alastair Cameron independently realized the necessity to add another nucleosynthesis process toneutron capture nucleosynthesis but simply mentioned proton captures without assigning a special name to the process. He also thought about alternatives, for example photodisintegration (called theγ-process today) or a combination of p-process and photodisintegration.[2]
