Recent progress in the study of heavy-ion fusion hindrance at extreme sub-barrier energies.

A new phenomenon, heavy-ion fusion hindrance at extreme sub-barrier energies, has been discovered at ANL several years ago [1]. It was first observed in medium-mass systems, but later measurements and analyses showed that this might be a general behavior of heavy-ion fusion at extreme sub-barrier energies. As a result, it could also have an effect on fusion reactions of importance in nuclear astrophysics, such as $^{12}$C + $^{12}$C etc. [1]. The main difference between the systems studied earlier and astrophysically important fusion reactions is in the reaction Q-values, which are positive for lighter nuclei and negative for medium-mass systems. We have, therefore, recently studied fusion in the system $^{28}$Si + $^{30}$Si with a Q-value of 14.3 MeV, which is comparable to that of $^{12}$C + $^{12}$C (Q=13.9 MeV). Results from this experiment as well as from other studies will be discussed. [1] C.L. Jiang \textit{et al}., Phys. Rev. Lett. 87, 052701 (2002); Phys. Rev. C73, 014603 (2006)~; Phys. Rev. C75, 015803 (2007).