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. Following capture, if the two nuclei reseparate before forming an equilibrated compound nucleus, fusion cannot proceed. This outcome is called quasifission.

In most cases, quasifission and fusion-fission distributions have considerable overlap. To obtain a direct experimental signature of fusion, evaporation residue (ER) measurements are therefore the most reliable signature of compound nucleus formation. ER cross section measurements forming the same compound nucleus, ²²⁰Th, using ¹⁶O, ⁴⁰Ar, ⁴⁸Ca, ⁸²Se and ¹²⁴Sn-induced reactions [1], [2], [3], [4] revealed [1] that fusion was severely suppressed for the more symmetric reactions relative to the ¹⁶O-induced reaction.

This work presents ER measurements of two new reactions forming ²²⁰Th using ²⁸Si, ³⁴S projectiles. These measurements conclusively show an exponential suppression of the ER cross section as a function of $Z_p{Z}_t$. The fission characteristics show no mass-angle correlation, demonstrating that here ER cross sections are suppressed by slow quasifission.