Nuclear Seminar

^{Probing Explosive Stellar Environments with Gammasphere}

Dr. Dariusz Seweryniak
Argonne National Laboratory

Monday, January 31, 2005   4:00 p.m.   NSH 124
(Refreshments served prior to seminar in NSH 124)

The light nuclei adjacent to the proton dripline are situated along the path of the rp-process, which takes place in explosive stellar environments. Properties of excited states above the proton threshold in these nuclei determine the rate of resonant proton capture reactions. However, available information about these states is often ambiguous and incomplete.

In a series of experiments the ²⁰Na, ²²Mg, ²⁶Si, and ²⁷Si nuclei, among others, were produced using heavy-ion fusion-evaporation reactions. The ²⁰Na nucleus controls the reaction sequence ¹⁵O(a,g)¹⁹Ne(p,g)²⁰Na, which connects the CNO cycles with the rp-process. The properties of the ²²Mg, ²⁶Si, and ²⁷Si nuclei determine the production rate of the long-lived g emitters ²²Na and ²⁶Al, which are accessible for detection by orbiting g telescopes.

Excited states in the nuclei of interest were studied using in-beam g-ray spectroscopic methods. Prompt g rays were detected using the GAMMASPHERE array of Compton suppressed Ge detectors. They were assigned to individual reaction channels on the basis of the A/Q and DE-E measurement of the recoiling nuclei at the focal plane of the Argonne Fragment Mass Analyzer.

New states were identified and the properties of the known sates were determined more accurately in all studied nuclei. The results obtained so far, which will be discussed during the talk, indicate that large high-resolution g-ray detector arrays, coupled with auxiliary detectors for reaction channel selection, are a powerful tool to study astrophysically important nuclei.