The use of elastic scattering to constraint the stellar reaction rate of ('12)C(alpha, gamma)('16)O

Details The use of elastic scattering to constraint the stellar reaction rate of ('12)C(alpha, gamma)('16)O The use of elastic scattering to constraint the stellar reaction rate of ('12)C(alpha, gamma)('16)O

Oct 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt.........9t&link_type=abstract

Thesis (PhD). UNIVERSITY OF NOTRE DAME, Source DAI-B 61/04, p. 2015, Oct 2000, 107 pages.

Astronomy and Astrophysics

Astrophysics

3

Scientific paper

The stellar reaction rate of 12C(α,γ) 16O is the most crucial unknown reaction in stellar nucleosynthetic reaction networks. At helium burning temperatures the rate depends on S(300), the astrophysical S-factor at Ecm = 300 keV. Following α-capture, electric dipole (E1) and quadrupole (E2) transitions both contribute to the S(300), but the cross section at 300 keV is too small to be measured directly. However, SE1 (300) and SE2 (300) are roughly proportional to the dimensionless reduced widths q2a of two subthreshold states at Ex = 7.12(1 -) and 6.92(2+) MeV. Elastic scattering 12C(α,α)12C should be sensitive to the widths of these states. Angular distributions of 12C(α,α)12 C were measured at 32 angles (24.0° <= θ lab <= 165.9°) at 354 energies (1.95 MeV <= E cm <= 4.95 MeV). The yields were normalized to a reference detector at 58.9° and subjected to an R-Matrix analysis. The reduced width amplitudes (γλl) obtained for an interaction radius of a = 5.5 fm for the subthreshold 2+ and 1- states were γ12 = 0.33 +/- 0.06 MeV1/2 and γ11 = 0.045 +/- 0.045 MeV1/2, respectively. The resulting dimensionless reduced widths q2a,l=2ma2 g2a,l/3ℎ2 were q2a (6.92) = 0.16 +/- 0.06 and q2a (7.12) = 0.006 +/- 0.006. The d-wave (l = 2) parameters extracted from the best fit to the elastic scattering data were subsequently used in an R-Matrix fit of α-capture and 16N β-delayed α-decay data. From this fit the quadrupole contribution was determined to be SE2 (300) = 44+12- 18 keV b. Including the previously established value of SE1 (300) = 79 +/- 21 keV b and allowing for cascade transitions, the total S-factor is estimated to be S(300) = 137 +/- 33 keV b. The calculated reaction rate for 12C(α,γ)16 O at 300 keV then becomes 1.3 +/- 0.3 times the standard (CF88) rate.

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