Effect of nuclear structure and fissility on quasifission

In order to investigate the interplay between nuclear structure (shell closure) and dynamics, fission fragment mass distributions have been measured for reactions ${}^{35,37}\mathrm{Cl}+{}^{206,208}\mathrm{Pb}$. These reactions have similar entrance channel parameters except for the number of closed shell configuration in the entrance channel. Fission fragment mass distributions have been measured for these reactions in the energy region close to the Coulomb barrier. No mass-angle correlation was observed in any of the four reactions, indicating the absence of fast quasifission (QF), but mass distributions were wider than that of compound nucleus (CN) fission, which indicates the presence of slow QF. With comparable fissility and deformation values, extra stability against QF was observed for reactions which have more closed shell configurations in the entrance channel. Mass-angle distributions of these reactions do not follow the systematics with respect to mean fissility. A new systematics has been obtained by considering quadrupole deformation of the heavy reaction partner along with the fissility, which explains the absence of mass-angle correlation in the presently measured reactions.

Figure 1

Center-of-mass angle ${\theta }_{\mathrm{c}.\mathrm{m}.}$ vs mass ratio for ${}^{35}\mathrm{Cl}+{}^{208}\mathrm{Pb}$ at $E/V_B=1.04$.

Figure 2

Experimental mass ratio distributions for the reactions ${}^{35,37}\mathrm{Cl}+{}^{206,208}\mathrm{Pb}$. Symbols represent experimental data. Dashed green lines show the single Gaussian fits. Solid red lines show fits using three Gaussian peaks. Dotted blue and short-dashed black lines show symmetric and asymmetric components, respectively.

Figure 3

(a) Experimental and estimated mass widths of the mass distributions as a function of excitation energy (Ex) for reactions ${}^{35,37}\mathrm{Cl}+{}^{206,208}\mathrm{Pb}, {}^{34}\mathrm{S}+{}^{208}\mathrm{Pb}$, and ${}^{30}\mathrm{Si}+{}^{208}\mathrm{Pb}$.

Figure 4

(a) Ratio of experimental width ($\sigma$) to estimated width (${\sigma }_{\mathrm{cal}})$ as a function of excitation energy for reactions ${}^{35,37}\mathrm{Cl}+{}^{206,208}\mathrm{Pb}$. (b) $\sigma /{\sigma }_{\mathrm{cal}}$ at $E/V_B\approx 1.0$ as a function of $N_M$ (number of closed shell configurations).

Figure 5

Reaction numbers are given in Table 1. Reactions taken from results by du Rietz et al. are numbered as 1–42. ${}^{35,37}\mathrm{Cl}+{}^{206,208}\mathrm{Pb}$ reactions are labeled by numbers 43–46. Reactions taken from other references [20, 22, 23, 24, 25, 26, 27] are marked by numbers 47–61. (a) $\sigma$ for reactions as a function of ${\chi }_m=0.75{\chi }_{\mathrm{eff}}+0.25{\chi }_{\mathrm{CN}}$ at $E/V_B\approx 1.0$. (b) ${\chi }_{\mathrm{CN}}$ as a function of ${\chi }_m$ for various MAD types. Open diamond, circle, and square symbols represent MAD1, MAD2, and MAD3 type reactions. Transitions from MAD3 to MAD2 and MAD2 to MAD1 are shown by dashed and and dash-dotted lines. (c) Same as (a), but with respect to ${\chi }_m^n=0.6({\chi }_{\mathrm{eff}}+\beta /3)+0.4{\chi }_{\mathrm{CN}}$. The horizontal line shows $\sigma =0.07$, which represents the value of $\sigma$ for CNF. A vertical line is drawn to guide the eye where the value of $\sigma$ increases rapidly with respect to ${\chi }_m^n$. A linear fit in the intermediate region is shown by a solid line. (d) Same as (b), with respect to $0.6({\chi }_{\mathrm{eff}}+\beta /3)+0.4{\chi }_{\mathrm{CN}}$. A solid line separates the $\sigma \approx 0.07$ region from the higher $\sigma$ region.