Alpha-particle doorway states inSi28

Excitation functions for the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}_{\mathrm{g}.\mathrm{s}.\mathrm{}}^{}{}_{}{}^{}$)${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ reaction feeding the $0^{+}$, 6.049-, $3^{-}$, 6.131-, $2^{+}$, 6.919-, $1^{-}$, 7.117-, $4^{+}$, 10.353-, and $4^{+}$, 11.095-MeV states of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ have been measured at ${\theta }_{\mathrm{lab}}=7.0\mathrm{}°$ over the energy range $22.7\le E_{\mathrm{c}.\mathrm{m}.\mathrm{}}\le 37.7$ MeV in 86- or 173-keV steps. The Ericson-fluctuation type analysis was performed and nonfluctuating enhancements with a width of 700 keV are observed at $E_{\mathrm{c}.\mathrm{m}.\mathrm{}}=23.6, 24.6, 28.4, \mathrm{and} 32.2$ MeV for the $0^{+}$, 6.049- MeV states. The most probable total spins for the lowest three states are assigned to be $J^{\pi }={14}^{+}, {14}^{+}, \mathrm{and} {15}^{-}$, respectively. These enhancements are strongly correlated to those for the $2^{+}$, 6.919- MeV state, but anticorrelated to those for the $3^{-}$, 6.131-MeV state. Model calculations based on Hauser-Feshbach and band-crossing theories were performed and none of them have succeeded to reproduce the data in a consistent way. Alpha-particle doorway states embedded in the high excitation of ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ (39-54 MeV) are indicative to describe these intermediate-structure resonances.