Mathematics – Logic
Scientific paper
Jun 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008phdt........20c&link_type=abstract
Proquest Dissertations And Theses 2008. Section 0052, Part 0606 [Ph.D. dissertation].United States -- Colorado: Colorado Schoo
Mathematics
Logic
1
Cross Section, Fluorine-17, Neon-18
Scientific paper
The 17 F(p,g) 18 Ne reaction is of significant astrophysical importance in such cosmological events as novae and x-ray bursts. The decay of 17 F (T 1/2 =
64.49 seconds) is thought to drive the late expansion of the nova envelope, and the 17 F(p,g) 18 Ne reaction affects the production of 18 F, a target of gamma-ray astronomy, as well as being an important link in the (a,p) reaction chain during the ignition phase of x-ray bursts. However, many reaction rates used in astrophysical calculations are highly uncertain due to a lack in experimental data. As part of an ongoing campaign to provide this data, for the first time the 17 F(p,g) 18 Ne reaction cross section has been directly measured. The measurement was performed in inverse kinematics, using a mixed beam of ~35-70% radioactive 17 F and the remainder stable 17 O from the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory, at beam intensities of typically >5x10^6 17 F particles per second. The radioactive beam bombarded a differentially-pumped windowless hydrogen gas target, and the Daresbury Recoil Separator was used to separate 18 Ne recoils from unreacted primary beam. Identification of recoils was achieved in an ionization chamber at the focal plane. A strength og = 115 ± 42 (stat) ± 94 (sys) meV was determined for the proton capture resonance at 1178 keV. The direct capture astrophysical S ( E ) factor, measured at an energy of 800 keV in the center of mass, has been assigned an experimental upper limit of 65 keV b. The astrophysically important 599.8 keV resonance was determined to have a strength og = 33 ± 14 (stat) ± 17 (sys) meV, stronger than previously predicted by theory. This experiment thereby led to a reaction rate in some temperature regimes faster than expected by theory, and reduced the uncertainty in the resonant contribution to the reaction rate significantly.
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