Nuclear drip line
The nuclear drip line is the boundary beyond which atomic nuclei are unbound with respect to the emission of a proton or neutron.
As such, the drip line may be defined as the boundary beyond which proton or neutron separation energy becomes negative, favoring the emission of a particle from a newly formed unbound system.
For example, to determine if 12C, the most common isotope of carbon, can undergo proton emission to 11B, one finds that about 16 MeV must be added to the system for this process to be allowed.
[4] Although the location of the drip lines is well defined as the boundary beyond which particle separation energy becomes negative, the definition of what constitutes a nucleus or an unbound resonance is unclear.
[4] For more massive nuclei, particle emission half-lives may be significantly longer due to a stronger Coulomb barrier and enable other transitions such as alpha and beta decay to instead occur.
This renders unambiguous determination of the drip lines difficult, as nuclei with lifetimes long enough to be observed exist far longer than the timescale of particle emission and are most probably bound.
[5] However, the next even nuclide outside the one-particle drip line may still be particle stable if its two-particle separation energy is non-negative.
[5] The one- and two-neutron drip lines have been experimentally determined up to neon, though unbound odd-N isotopes are known or deduced through non-observance for every element up to magnesium.
The other proton and neutron decays occurred much earlier in the life of the atomic species and before the earth was formed.
[8][9] Explosive astrophysical environments often have very large fluxes of high-energy nucleons that can be captured on seed nuclei.
While the neutron drip line is very poorly determined experimentally, and the exact reaction flow is not precisely known, various models predict that nuclei along the r-process path have a two-neutron separation energy (S2n) of approximately 2 MeV.
Beyond this point, stability is thought to rapidly decrease in the vicinity of the drip line, with beta decay occurring before further neutron capture.
[13] In fact, the nuclear physics of extremely neutron-rich matter is a fairly new subject, and already has led to the discovery of the island of inversion and halo nuclei such as 11Li, which has a very diffuse neutron skin leading to a total radius comparable to that of 208Pb.
Although there are nuclear uncertainties, compared to other explosive nucleosynthesis processes, the rp-process is quite well experimentally constrained, as, for example, all the above waiting point nuclei have at the least been observed in the laboratory.
While the rp-process in X-ray bursts may have difficulty bypassing the 64Ge waiting point,[15] certainly in X-ray pulsars where the rp-process is stable, instability toward alpha decay places an upper limit near A = 100 on the mass that can be reached through continuous burning.
[6] Even before the limit near A = 100 is reached, the proton flux is thought to considerably decrease and thus slow down the rp-process, before low capture rate and a cycle of transmutations between isotopes of tin, antimony, and tellurium upon further proton capture terminate it altogether.
[17] However, it has been shown that if there are episodes of cooling or mixing of previous ashes into the burning zone, material as heavy as 126Xe can be created.
Up to germanium, the location of the drip line for many elements with an even number of protons is known, but none past that point are listed in the evaluated nuclear data.
There are a few exceptional cases where, due to nuclear pairing, there are some particle-bound species outside the drip line, such as 8B and 178Au.
