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Study of 69Br Proton Emission and the rp-Process 68Se Waiting Point

Andrew Rogers, Argonne National Laboratory
Vendredi 12 février 2010 à 10h30 - Salle des Séminaires

A complete understanding of the synthesis of the elements is one of the main outstanding questions in nuclear astrophysics. A number of nucleosynthesis processes are known which can account for much of the observed solar abundances of nuclei. However, there are certain elements that can only be produced via processes involving proton rich nuclei. The rapid proton capture or rp-process, is one such mechanism that occurs along the proton drip-line whereby a competition between fast proton captures on seed nuclei and subsequent beta-decay allow for the production of elements possibly as heavy as Te. Type I X-ray bursts are thought to be key sites for this process. To realistically model the rp-process in these systems experimental data such as masses, lifetimes, and proton capture rates along the proton drip-line are required. Such data are currently lacking for many of these nuclei.

The 68Se waiting point is of particular interest, where a long beta-decay half-life coupled with inhibited proton capture restricts the amount of material that is processed beyond mass 68 in the rp-process. However, sequential 2p-capture reactions through 69Br may significantly bypass this waiting point. This process depends exponentially on the 68Se proton capture Q-value which is currently poorly constrained. We have performed an experiment to measure Q-values of proton unbound states of exotic nuclei at the National Superconducting Cyclotron Laboratory (NSCL) Coupled Cyclotron Facility. The experiment was designed to reconstruct the decays of proton unbound nuclei, specifically 69Br, by detecting the decay protons using the MSU High Resolution Array (HiRA) in coincidence with a heavy residue, e.g. 68Se, which is measured in the large acceptance S800 magnetic spectrograph. The first direct measurement of ground state proton emission from 69Br and general implications for the rp-process will be discussed.