Studying (alpha,p) reactions important to astrophysical environments

Shigeru Kubono gave a review of (alpha,p) reactions important to explosive stellar environments, touching on sites, observation, some theory, and experimental methods. Though focusing on (alpha,p) reactions, the talk was actually a well composed introduction to the field of the nuclear astrophysics of proton-rich environments.

The proton-rich astrophysical sites covered where classical novae, where a white dwarf star accretes matter from it's main sequence companion; x-ray bursts, where a neutron star accretes matter from it's main sequence companion; and the proton-rich environment in type II supernovae. Observationally we observe the isotopes created in these environments via gamma ray astronomy. P-nuclei which of are particular interest, due to the seeming inability of type II supernovae to produce them in the large quantities they are observed, are 92Mo and 96Ru. At this point the talk turned its focus primarily to alpha-induced reactions with the statement "alpha-induced reactions play an extremely important role for high temperature nucleosynthesis."

A particularly interesting visual tool used to study alpha-induced reactions is the cluster nucleosynthesis diagram (CND). A description without a picture of the diagram would not do it justice, but at the moment I can't get a hold of one. Effectively the diagram shows the energetic favorability of adding alpha particles to nuclei, where simpler systems are at the upper left and more complex systems are at the bottom left of the diagram. An important quality of the diagram is that a lower position on the diagram indicates the particular system is more bound.

After reminding us that nucleosynthesis takes place at very low energy, the talk turned to experimental methods used at RIKEN to study low energy nuclear physics. As we were told, any low energy rare isotope beam (RIB) system consists of the following: 1)Ion Source 2)Accelerator 3)Beam Transport 4)Production Target 5)Separator. RIKEN's low E RIB set-up "CRIB" probes energies less than or equal to 10MeV/u with a beam intensity of 10^3-8 particles per second, a purity of 90-100%, and dE/E between 0.5 and 1$.

We were also given an overview of the thick target method for resonant scattering. This method uses a beam of a single energy, but relies on the fact that the beam will have different energy loss for given events, effectively giving data for a range of energies (clever!). The example target presented was H2 gas and the measurement was made by a Si detector, into which the proton was directed if a resonant state was reached in the beam-target interaction.

Finally an example of why experimental data is crucial, even in the face of sexy theories. I don't remember the reaction rate which was used as an example (a (p,gamma) reaction with something around sodium), but we were shown that the Hauser-Feshbach prediction given by the NACRE collaboration disagreed with data by many orders of magnitude.