Observation of suppression of heavy-ion fusion by slow quasifission

The formation of superheavy elements (SHEs) by nuclear fusion can be conceptually divided into two steps: capture, and compound nucleus formation. Once captured, the two nuclei may reseparate before fusing to form a compact compound nucleus. This outcome is called quasifission. Fusion-fission, in many cases, leads to reactions outcomes inseparable from quasifission. Evaporation residue (ER) measurements are therefore the most reliable, direct experimental signature of fusion. ER cross section measurements forming the same compound nucleus, ²²⁰Th, using ¹⁶O, ⁴⁰Ar, ⁴⁸Ca, ⁸²Se and ¹²⁴Sn-induced reactions [1-4] revealed [1] that fusion was severely suppressed for the more symmetric reactions relative to the ¹⁶O-induced reaction. Here two new reactions forming ²²⁰Th using ²⁸Si, ³⁴S projectiles provide conclusive evidence that the ER cross section is exponentially suppressed as a function of Z_pZ_t. The fission characteristics show no mass-angle correlation, demonstrating that here ER cross sections are suppressed by slow quasifission.