Nuclear Seminar

Breakout from the CNO cycle and the ¹⁵O(a,g) reaction rate

**CANCELLED**

Dr. Wanpeng Tan
University of Notre Dame

Monday, February 13, 2006   4:00 p.m.   NSH 124
(Refreshments served prior to seminar in NSH 124)

 

The hot CNO cycles and the ensuing rp-process after the breakout play a principal role in energy production and nucleosynthesis of explosive hydrogen burning processes occurring in novae and accreting neutron stars. Recent studies have shown that the route via ¹⁵O(a,g), one of the most important breakout reactions, is critical for the burst amplitude and periodicity of X-ray bursters. There has been considerable effort in the past to investigate this reaction rate indirectly by obtaining gamma and alpha decay widths of the a-unbound states in ¹⁹Ne due to the lack of high intensity radioactive ¹⁵O beams for direct measurement. However, the reaction rate remains largely uncertain since previous experiments have only provided limits on the gamma widths and/or the alpha-decay branching ratios of the relevant resonances, particularly the critical level at 4.03 MeV. New experimental work conducted at the University of Notre Dame will be presented. Lifetimes of the 4.03 MeV state and other relevant states in ¹⁹Ne have been measured successfully using the ¹⁷O(³He,n-g) reaction. The results of our recent measurement on the alpha-decay branching ratios will also be discussed. Alpha-unbound states in ¹⁹Ne were populated via the reaction ¹⁹F(³He,³H-a) and triton-alpha coincidences were observed using a low energy particle detection Silicon array and the TWINSOL facility. The experimental results and the astrophysical implications will be discussed in the presentation.