$ \ce{^{A}_{Z}X} + p \longrightarrow \ce{^{(A+1)}_{(Z+1)}X'} + \gamma $ Proton capture is a process in which an atomic nucleus ($\ce{^{A}_{Z}X}$) and a proton ($p$) collide and merge to form an isotope with one higher atomic mass ($A+1$) and atomic number ($Z+1$). The new element is then can be neutron-deficient, such that multiple successive proton captures prove to be energy inefficient and unlikely. This is due to the increased Coulomb barrier and associated charge repulsion. Successive proton captures at extremely high proton densities are called [[#Rapid Proton Capture]], and they form very short-lived radionuclides before they use other decay method to more stable configurations. **Important Notes:** - If the temperature could be increased arbitrarily to overcome the Coulomb barrier for successive proton captures, then protons would undergo [[Photodisintegration]] before they could be captured. - The p-nuclides (proton-rich isotopes not produced by the [[Neutron Capture#s-process|s-process]] or [[Neutron Capture#r-process|r-process]]) were originally through to be produced this way, but their origins are still unknown. **P-Process Sites:** - Originally thought to have occurred in core-collapse supernovae, but the proper conditions have not been found there - Type II supernova explosions, accretion onto neutron star surfaces resulting in Type I X-ray bursts? ## Rapid Proton Capture The rapid proton capture processes occur when an atomic nucleus ($\ce{^{A}_{Z}X}$) undergoes successive proton captures, where the proton flux is high enough that other decay methods **cannot** occur between captures. *(hence, a "fast"/"rapid" process)* This allows the nuclei to become proton-rich and highly unstable. This process can continue until the nuclei becomes proton-saturated, and then further captures will result in prompt [[Proton Emission]] or [[Alpha Decay|Alpha Emission]]. ### rp-process The so-called rp-process is the purest form of the rapid proton capture process described above. It is predicted that the rp-process is responsible for many of the proton-rich, heavy elements in the universe, but the end point (the highest-mass element) is not yet well established. The highest element observed to be made through this process is $A = 104$ ($\ce{^{104}Te}$) - At proton densities near $\sim \mathcal{O}(10^{23} \; {\rm /cm^{3}})$ at $T \approx 2 \; {\rm GK}$, the reaction path is close to the proton drip line. - The minimum time between proton captures is restricted to $10–600 \; {\rm s}$, where the nuclides that are more proton-saturated have shorter interaction windows the further way from the valley of stability. ### pn-process The **neutron-rich rapid proton capture** occurs by using a supply of free neutrons to initiate [[(n,p) Reaction|(n,p) reactions]] (faster than proton captures or beta decays) to quickly generate protons and achieve the same result as the [[#rp-process]]. Since free neutrons tend to not be present in proton-rich plasma, they can be obtained through other simultaneous reactions. ### $\nu$p-process Another way to obtain the neutrons required for the [[(n,p) Reaction|(n,p) reactions]] (used in the [[#pn-process]]) in proton-rich environments is to use the anti-neutrino capture with protons, transforming a proton and an anti-neutrino into a positron and a neutron. $ \bar{\nu}_{e} + p \longrightarrow n + e^{+} $ Since (anti-)neutrinos interact very weakly with protons, a high flux of anti-neutrinos has to act on a plasma with high proton density to generate the necessary neutrons for the [[#pn-process]]. ### p-alpha process