very delayed blog on beta-delayed proton emission studies for nuclear astrophysics

Livius Trache told us some stories on measurements of reactions relevant to Hydrogen burning in novae and X-ray bursts. As mentioned in previous talks we need good nuclear physics data in order to link astronomical observations and nucleosynthesis models. His research group studies radiative capture reactions, such as (p,γ), (α,γ) and (n,γ). The interest lies on the determination of astrophysical S factors (remember that we measure cross sections, but we plot S).

We're mostly concerned with resonant reactions, which are two step processes: (1) formation of metastable state, (2) decay. The cross section given by Breit-Wigner and varies rapidly with energy and its contribution to the reaction rate depends on two factors: (1) location in energy, and (2) probability of occurrence. There are two big problems in this kind of studies first, that the reactions in stars involve radioactive nuclei, i.e. we need RIBs to produce them, and second, that the energies involved are low and the cross sections very small, therefore we need to use indirect methods.

Some indirect methods with RIBs introduced: Coulomb Dissociation, Transfer Reactions, Breakup of loosely bound nuclei, Spectroscopy of resonances, Decay spectroscopy, Trojan horse method.

One case of interest given as example is the 23Al decay to 23Mg (compound system). Also here comes the 22NA puzzle again: how does depletion of 22Na in novae happen? We can contibute by studying 22Mg(p,g) and 22Na(p,g) reactions. Livius et at conduct experiments at MARS, at Texas A&M University using inverse kinematics reactions such as (p,n), (p,2n), (p,3n), among others. We are shown technical details of the facility and experimental details, then some results: they found out that the 22Mg(p,g) reaction is not important for depletion of 22Na in novae since its cross section is too small and photodissociation happens quickly. They are also able to determine half-lives, branching ratios, a new value for the mass of 23Al, resonances in 23Mg, etc. It was very interesting to learn that very close lying resonances can be disentangled by using a very thin (~40 micrometer-thick) Si detector to measure beta-proton decays.