From a survey of the heavy-ion systems measured until now two conditions can be deduced empirically which must be fulfilled in order to find correlated resonances of the type measured in the low-energy excitation functions of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ reaction. With both conditions one is able to explain simultaneously the lack and the existence of correlated resonances in all measured reactions. The conditions are: (1) the level density of the corresponding compound nuclei and the number of open channels must be small, and (2) both nuclei in the entrance channel must be $\alpha$-particle nuclei. The latter condition seems reasonable because experimental evidence exists for the assumption that these resonances are due to $\alpha$-particle doorway states.

NUCLEAR REACTIONS Several heavy-ion reactions, energies in the vicinity of the Coulomb barrier, measured $\sigma (E,\theta )$, deduced explanation explanation for the occurrence of correlated resonances.

• Received 15 April 1974

DOI:https://doi.org/10.1103/PhysRevC.10.1331