Interplay of resonant and l-window background amplitudes in 16O+16O.

Abstract

High resolution study of the $^{16}\mathrm{O}$${+}^{16}$O system at 15.5 \ensuremath{\le}${E}_{\mathrm{c}.\mathrm{m}.}$\ensuremath{\le}17.0 MeV is reported including excitation functions for elastic scattering, and angle integrated cross sections for ${\ensuremath{\alpha}}_{0}$ and ${\ensuremath{\alpha}}_{1}$ exit channels and several \ensuremath{\gamma}-ray channels. All show correlated resonant structure. Fifteen angular distributions for the ${\ensuremath{\alpha}}_{0}$ and ${\ensuremath{\alpha}}_{1}$ exit channels are measured in \ensuremath{\Delta}${E}_{\mathrm{c}.\mathrm{m}.}$\ensuremath{\simeq}50 keV steps for 15.5\ensuremath{\le}${E}_{\mathrm{c}.\mathrm{m}.}$\ensuremath{\le}16.4 MeV in the angular range 17\ifmmode^\circ\else\textdegree\fi{}\ensuremath{\le}${\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.}$\ensuremath{\le}93\ifmmode^\circ\else\textdegree\fi{} in steps of \ensuremath{\Delta}${\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.}$\ensuremath{\simeq}2.5\ifmmode^\circ\else\textdegree\fi{}. Phase shift analysis of the ${\ensuremath{\alpha}}_{0}$ angular distribution was carried out. Ambiguities in the extraction of the S matrix elements are removed using the usual techniques with an additional new constraint---the measurement of cross section at selected angles (zeros of ${P}_{l}$) arising from a single partial wave. Reaction amplitudes corresponding to a narrow l window around the grazing partial wave are shown to dominate the cross section around ${\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.}$=90\ifmmode^\circ\else\textdegree\fi{}. The grazing partial wave shows broad nonresonant structure; a specific parametrization of the energy dependence of the nonresonant (background) cross section corresponding to the l window is presented. The interplay of background amplitudes (l window) and resonant amplitudes is emphasized. Resonances in the ${\ensuremath{\alpha}}_{0}$ channel are located at ${E}_{\mathrm{c}.\mathrm{m}.}$=15.8, 15.9, and 16.1 MeV having ${J}^{\ensuremath{\pi}}$${=10}^{+}$, ${8}^{+}$, and ${8}^{+}$, respectively, and intermediate widths (\ensuremath{\Gamma}\ensuremath{\sim}70 keV). Spin assignments are obtained via a phase shift analysis and study of excitation functions measured at zeros of Legendre polynomials. Parametrization of the resonance plus background cross section yields the result \ensuremath{\surd}${\ensuremath{\Gamma}}_{0}$${\ensuremath{\Gamma}}_{\ensuremath{\alpha}}$ /\ensuremath{\Gamma}\ensuremath{\simeq}1.0%. The extracted partial widths are analyzed in terms of the Wigner limit and suggest resonances not of dinuclear structure, but of more complicated structure. Such states, having ${J}^{\ensuremath{\pi}}$${=8}^{+}$, are found to appear systematically at the empirically determined energies: ${E}_{\mathrm{c}.\mathrm{m}.}$=4\ifmmode\times\else\texttimes\fi{}2.8+N\ifmmode\times\else\texttimes\fi{}2.4 MeV (N=0,1,2 , . . .) in many different heavy ion resonant systems; this may suggest that the underlying alpha particle structure of the participant nuclei may play an ex- plicit role in these resonances.