MUTUAL-0(2)(+) Excitation in the OXYGEN-16 + Oxygen -16 System.

Heavy-ion elastic and inelastic reactions between alpha-nuclei with A <= 40 are known to display broad and narrow energy -dependent structure well above the Coulomb barrier. Such structure has been frequently interpreted as evidence for quasi-molecular states in the compound nuclear systems, a claim supported in particular systems by correlation of resonances and spin assignments across channels. In the present work, the cross section for mutual excitation of the ^{16}O(0_sp {2}{+}) state in the heavy-ion reaction ^{16}O + ^{16}O has been measured over the energy range rm E_ {cm} = 30-43.5 MeV. This excitation function was angle-averaged over theta_ {cm} = 75^circ-90 ^circ while broader angular distributions are displayed in full for the elastic scattering and single -0_sp{2}{+} excitation channels. Structure of gross (~2 MeV) and intermediate (~500 keV) width is reported in the mutual excitation reaction over the entire energy range and peaks are identified at rm E_{cm} = 31.75, 33.0, 37.0, and 40.5 MeV. A detailed angular distribution measurement was performed for the intermediate structure at rm E_{cm } = 37.0 MeV which, when fit with Legendre polynomials, was tentatively assigned dominant partial waves of l = (20,22)hbar.. Heavy-ion events were identified by recoil coincidence of two large silicon detectors, one of which was position -sensitive and served as the stop detector of an E- DeltaE gas telescope that allowed particle identification. Mutual-0_sp{2} {+} excitation was separated from exit channels involving the neighboring ^ {16}O(3_sp{1}{-}) state by observing products of their decays. The dominant e^+/- pair creation branch of the 0_sp{2}{+}to0 _sp{1}{+} (Delta J = 0) decay was recorded with a segmented electron detector array that was insensitive to the dominant gamma-emission (E_gamma = 6.13 MeV) of the 3_sp{1} {-} state. Array detection efficiencies for these two decays were directly measured with the ^{16}O(p,p^') reaction. Theoretical coupled-channels calculations of H. Sakuragi are presented that qualitatively reproduce gross features of the mutual-0_sp{2}{+} excitation function and the angular distribution at rm E_{cm} = 37.0 MeV. The channels included were single and mutual excitation of the lowest four positive parity states in ^ {16}O: rm0_sp{1 }{+}, 0_sp{2}{+ }, 2_sp{1}{+} and 2_sp{2}{+}. Internal wavefunctions were described by the orthogonality condition model developed with alpha-cluster wavefunctions.