Predominant Time Scales in Fission Processes in Reactions of S, Ti and Ni with W: Zeptosecond versus Attosecond

The inhibition of fusion by quasifission is crucial in limiting the formation of superheavy elements in collisions of heavy nuclei. Time scales of $\sim {10}^{-18}\mathrm{s}$ inferred for fissionlike events from recent crystal blocking measurements were interpreted to show either that quasifission itself is slower than previously believed, or that the fraction of slow fusion-fission is higher than expected. New measurements of mass-angle distributions for ${}^{48}\mathrm{Ti}$ and ${}^{64}\mathrm{Ni}$ bombarding W targets show that in these reactions quasifission is the dominant process, typically occurring before the system formed after contact has made a single rotation, corresponding to time scales of $\le {10}^{-20}\mathrm{s}$.

Figure 1

Schematic illustration of the evolution of a dinuclear system. (a) Three different quasifission outcomes (I, II, and III) depend on the sticking time ($t_S$) and rotation speed ($\omega$). (b) Corresponding MAD, illustrating the correlation between emission angle (${\theta }_{\mathrm{c}.\mathrm{m}.}$) and mass ratios ($M_R$). The solid (red) line shows the correlated mass and angle evolution of the projectilelike fragment and the dashed (blue) line that of the targetlike fragment.

Figure 2

The upper panels show the experimental MAD and corresponding projection onto $M_R$ for ${}^{64}\mathrm{Ni}+{}^{184}\mathrm{W}$, ${}^{48}\mathrm{Ti}+{}^{186}\mathrm{W}$, and ${}^{34}\mathrm{S}+{}^{186}\mathrm{W}$ (see text). The lower panels show simulated MAD for same reactions and energies, with the $M_R$ spectra at the bottom, and the sticking time distributions above. The capture mean angular momenta $⟨L⟩$ from CCFULL used in the simulations are also given. These result in good agreement between the simulations and the measurements (top panels).