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, 220 Th, using 16 O, 40 Ar, 48 Ca, 82 Se and 124 Sn-induced reactions [1] , [2] , [3] , [4] revealed [1] that fusion was severely suppressed for the more symmetric reactions relative to the 16 O-induced reaction. This work presents ER measurements of two new reactions forming 220 Th using 28 Si, 34 S 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.