Reduced quasifission competition in fusion reactions forming neutron-rich heavy elements

Measurements of mass-angle distributions (MADs) for Cr + W reactions, providing a wide range in the neutron-to-proton ratio of the compound system, ${(N/Z)}_{\mathrm{CN}}$, have allowed for the dependence of quasifission on the ${(N/Z)}_{\mathrm{CN}}$ to be determined in a model-independent way. Previous experimental and theoretical studies had produced conflicting conclusions. The experimental MADs reveal an increase in contact time and mass evolution of the quasifission fragments with increasing ${(N/Z)}_{\mathrm{CN}}$, which is indicative of an increase in the fusion probability. The experimental results are in agreement with microscopic time-dependent Hartree-Fock calculations of the quasifission process. The experimental and theoretical results favor the use of the most neutron-rich projectiles and targets for the production of heavy and superheavy nuclei.

Figure 1

Measured MADs for systems ${}^{50}\mathrm{Cr}+{}^{180}\mathrm{W}, {}^{54}\mathrm{Cr}{+}^{180}\mathrm{W}, {}^{52}\mathrm{Cr}{+}^{184}\mathrm{W}$, and ${}^{54}\mathrm{Cr}{+}^{186}\mathrm{W}$ are shown in (a)–(d), respectively. The contour scale ($z$ axis) of each MAD is scaled to match the maximum fission fragment yield. The black crosses in (a) and (d) are the results of TDHF calculations at various impact parameters from $b=2.5$–5 fm. Panels (f)–(i) show the projected mass ratio distributions. The solid red line represents a Gaussian fit over the region from $0.35\le M_{\mathrm{R}}\le 0.65$. The dashed blue line is an estimate for a pure fusion-fission mass distribution. The mass resolution can be seen from the width of the elastic scattering peaks which are presented (scaled by 0.09) in (f). Panels (e) and (j) show the MAD and mass distribution for ${}^{12}\mathrm{C}+{}^{208}\mathrm{Pb}$, where no quasifission is present.

Figure 2

Upper limits of $P_{\mathrm{CN}}$ extracted from the measured mass distributions of each system as a function of (a) $[{(N/Z)}_{\mathrm{CN}}$, bottom axis; $x_{\mathrm{CN}}$, top axis], and (b) mass asymmetry, $\alpha$.

Figure 3

TDHF results for the (a) mass ratio, charge ratio, and (b) contact time as a function of impact parameter for the ${}^{50}\mathrm{Cr}+{}^{180}\mathrm{W}$ and ${}^{54}\mathrm{Cr}+{}^{186}\mathrm{W}$ systems. In the upper right corners of each panel, characteristic density contours are shown depicting quasifission (top, ${}^{54}\mathrm{Cr}+{}^{186}\mathrm{W}$ at $b=3$ fm) and fusion (bottom, ${}^{50}\mathrm{Cr}+{}^{180}\mathrm{W}$ at $b=0$ fm) events from the TDHF calculations.

Figure 4

Scaled mass quadrupole moment ${\beta }_2$, as a function of time from the TDHF calculations of the ${}^{54}\mathrm{Cr}+{}^{186}\mathrm{W}$ and ${}^{50}\mathrm{Cr}+{}^{180}\mathrm{W}$ systems at $b=2.5$ fm. Select density contours are shown along each trajectory.