Other
Scientific paper
Nov 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt........11c&link_type=abstract
Thesis (PhD). YALE UNIVERSITY, Source DAI-B 61/05, p. 2601, Nov 2000, 83 pages.
Other
1
Scientific paper
In current models of X-ray bursters and nova explosions, the energy generation and nucleosynthesis at temperatures of T ~ 0.4 GK are determined by the Hot-CNO cycles [Cha92). In X-ray bursters, the star may then break out of the Hot-CNO cycles to the rp-process, providing a way to enhance the rate of energy generation and trigger the subsequent explosion. While the initial breakout path is thought to be primarily through the 15O(α,γ) 19Ne reaction, at higher temperatures (T ~ 1 GK) another bridge is available through the 18Ne(α,p)21Na reaction [Wie99]. At temperatures characteristic of breakout to the rp-process, the rate is mainly dependent on the properties of individual resonances in the compound nucleus 22Mg. However, only two states in 22Mg in the excitation energy region of interest had been previously measured in other studies, and their spectroscopic properties are unknown [End90]. Moreover, based on the structure of the isospin mirror 22 Ne, there are about 25 missing states in this region, pointing to the need for further spectroscopy studies of 22Mg in order to determine the locations of important resonances for the 18Ne(α,p) 21Na reaction. While breakout from the Hot-CNO cycle supplies the energy trigger for X-ray bursts, our present understanding of explosive nucleosynthesis in novae indicates that the temperature on the surface of the accreting white dwarf is too low for breakout to happen. For ONeMg novae, the sources of energy for the thermonuclear runaway are instead the NeNa and MgAl cycles. In the NeNa cycles, in particular, the reaction 21Na(p,γ) 22Mg is especially important because it bypasses the 21Na β-decay, resulting in greater energy generation during the explosion. Additionally, the reaction plays a key role in determining the final abundance of 22Na produced in the nova nucleosynthesis. In light of the above, we have measured the location of resonances in 22Mg using the 12C(16O,6He) 22Mg reaction. We have measured 18 new levels in 22Mg in the excitation energy region of importance to the 18Ne(α,p) 21Na reaction (Ex > 8.14 MeV). Our resulting reaction rate is up to two orders of magnitude faster than previous estimates. Moreover, we have found a new level at Ex( 22Mg) = 6.041(11) MeV that lies in the region of interest for the 21Na(p,γ)22Mg reaction, confirming one of the key results from [Bat99]. (Abstract shortened by UMI.)
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