TheC12(O16,γSi28)radiative capture reaction at sub-barrier energies

Abstract

The heavy-ion radiative capture $^{12}\mathrm{C}$$(^{16}\mathrm{O}$$,\ensuremath{\gamma}^{28}\mathrm{Si})$ was measured at the sub-Coulomb barrier bombarding energy ${E}_{\mathrm{lab}}=15.7$ MeV, which corresponds to the lowest important resonance observed in the $^{12}\mathrm{C}+$ $^{16}\mathrm{O}$ fusion excitation function. Thanks to combination of the bismuth germanate (BGO) $\ensuremath{\gamma}$-ray array and the ${0}^{\ensuremath{\circ}}$ DRAGON electromagnetic spectrometer at TRIUMF, the $\ensuremath{\gamma}$-decay spectrum from the entrance channel down to the ground state of ${}^{28}$Si was measured. Comparisons of the experimental spectrum to $\ensuremath{\gamma}$ spectrum extracted from Monte Carlo simulations of the complete setup suggest a ${J}^{\ensuremath{\pi}}={2}^{+}$ spin-parity assignment to the entrance channel and yield the radiative capture cross section ${\ensuremath{\sigma}}_{RC}=0.22\ifmmode\pm\else\textpm\fi{}0.04\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}$b. Combining this present spin assignment with previous data on radiative capture, a $J(J+1)$ systematics was constructed, and it indicated a moment of inertia commensurate with the $^{12}\mathrm{C}+$$^{16}\mathrm{O}$ grazing angular momentum. Strong dipole transitions are observed from the entrance channel to $T=1$ states around 11.5 MeV and are found to result from enhanced $M{1}_{IV}$ transitions to states exhausting a large part of the $M1$ sum rule built on the ground state of ${}^{28}$Si. This specific decay was also reported at bombarding energies close to the Coulomb barrier in our previous study of the $^{12}\mathrm{C}$$(^{12}\mathrm{C}$$,{\ensuremath{\gamma}}^{24}$Mg) heavy-ion radiative capture reaction. Similarities between both systems are investigated.