The β-delayed α-spectrum of ^(16)N and the astrophysical aspects of the ^(12)C(α, y) ^(16)O reaction

Abstract

Radiative alpha-capture by ^(12)C is a key process occurring during the helium-burning phase in red giant stars, and its rate remains one of the most significant uncertainties in the nucleo-synthetic calculations for massive stars. This is largely due to the lack of precise experimental information concerning the values of the reduced α-particle widths of the J^π = 1^- and 2^+ sub-threshold states in ^(16)O to which the higher-energy radiative capture data are only weakly sensitive. Of these two states, the reduced a-width of the E_x=7.12 MeV J^π=l^- level has been predicted to have a considerable effect on the structure of the hitherto unmeasured low-energy region of the β-delayed α-particle spectrum of ^(16)N. Experiments using the TRIUMF isotope separator TISOL have been performed to measure this a-spectrum down to an energy of E_α=600 keV, utilizing a coincidence technique which also accounts completely for the detector response function. The α-spectrum, containing 10^6 counts, has been incorporated into both R- and K-matrix analyses along with the previously measured ^(12)C(α,y)^(16)O cross section and the ^(12)C+α elastic phase shifts to yield a much improved value for Sm(300) keV. In light of this new determination of S_(EI)(300), the available radiative capture data and elastic scattering phase shifts are re-analyzed, along with the β-delayed α-spectrum of ^(16)N in an attempt also to place improved limits on the S_(E2)(300) contribution to the ^(12)C(α,y)^(16)O cross section.