Quasifission processes in ${}^{40,48}\mathrm{Ca}+{}^{144,154}\mathrm{Sm}$ reactions

Mass-energy and angular distributions of fission fragments for the ${}^{48}\mathrm{Ca}+{}^{144,154}\mathrm{Sm}\to {}^{192,202}\mathrm{Pb}$, ${}^{40}\mathrm{Ca}+{}^{154}\mathrm{Sm}\to {}^{194}\mathrm{Pb}$ reactions have been measured. Fusion suppression and the presence of quasifission at energies near and below the Coulomb barrier have been observed for the reactions with the deformed target ${}^{154}\mathrm{Sm}$. In the case of the spherical ${}^{144}\mathrm{Sm}$ target no evidence of quasifission has been found. Quasifission cross sections have been extracted from total fission-like events by analysis of their mass and angular distributions.

Figure 1

Sketch of the experimental set-up for measurements of the mass-energy and mass-angular distributions of binary reaction products. See explanations in the text.

Figure 2

Two-dimensional $\mathrm{TKE}\text{–}\mathrm{mass}$ matrices (upper panels), yields of fragments and their $\langle \mathrm{TKE}\rangle$ as a function of the fragment mass (middle and bottom panels, respectively) in the ${}^{16}\mathrm{O}$+${}^{186}\mathrm{W}$ reaction at the different CN excitation energies (designated above upper panels). Solid lines in the middle and bottom panels are Gaussian and parabola fits to the mass and $\langle \mathrm{TKE}\rangle$ distributions, respectively.

Figure 3

The same as in Fig. 2 but for the reaction ${}^{48}\mathrm{Ca}$+${}^{154}\mathrm{Sm}$. In the bottom panels parabola fits obtained for $\langle \mathrm{TKE}\rangle$ in ${}^{16}\mathrm{O}$+${}^{186}\mathrm{W}$ at the same excitation energies are shown for comparison (solid lines).

Figure 4

The same as in Fig. 2 but for the reaction ${}^{40}\mathrm{Ca}$+${}^{154}\mathrm{Sm}$.

Figure 5

The same as in Fig. 2 but for the reaction ${}^{48}\mathrm{Ca}$+${}^{144}\mathrm{Sm}$.

Figure 6

The experimental (symbols) and calculated (lines) variances for fission fragment mass distributions obtained in the reactions ${}^{16}\mathrm{O}$+${}^{186}\mathrm{W}$ and ${}^{48}\mathrm{Ca}$+${}^{154}\mathrm{Sm}$.

Figure 7

Angular distributions of all fission-like products obtained in the ${}^{48}\mathrm{Ca}$+${}^{144}\mathrm{Sm}$ and ${}^{48}\mathrm{Ca}$+${}^{154}\mathrm{Sm}$ reactions (upper and lower panel, respectively). Symbols are experimental data, solid lines correspond to the calculation with Eq. (6).

Figure 8

Differential cross sections for fission-like fragments in the reaction ${}^{48}\mathrm{Ca}$+${}^{144}\mathrm{Sm}$ for different fragment mass bins. The solid lines are best fits to the data using Eq. (7).

Figure 9

The same as Fig. 8 but for the reaction ${}^{48}\mathrm{Ca}$+${}^{154}\mathrm{Sm}$. Decompositions of distributions for light masses according to Eq. (7) are shown with dashed and dash-dotted lines (asymmetrical and symmetrical parts, respectively).

Figure 10

Mass distributions obtained for the ${}^{48}\mathrm{Ca}$+${}^{154}\mathrm{Sm}$ reaction, i.e., the same as in Fig. 3 (small solid circles). Solid lines are Gaussians with variances according to Eq. (5), large open circles correspond to the symmetric part of the mass-angular distributions obtained with Eq. (7), solid squares represent the QF component and dashed lines are the Gaussian fits to the QF component.

Figure 11

Location of the closed shell (designated by arrows connected with horizontal lines for complementary fragments) in the fragment mass distributions obtained at the lowest CN excitation energies for the three reactions ${}^{48}\mathrm{Ca}$+${}^{154}\mathrm{Sm}$ (upper panel), ${}^{40}\mathrm{Ca}$+${}^{154}\mathrm{Sm}$ (middle panel) and ${}^{48}\mathrm{Ca}$+${}^{144}\mathrm{Sm}$ (bottom panel).

Figure 12

Fusion cross sections (as the sum of CN-fission and ER [14]) in comparison with QF cross sections obtained for the ${}^{48}\mathrm{Ca}$+${}^{154}\mathrm{Sm}$ reaction.

Figure 13

The relative contribution of QF to the capture cross sections for ${}^{48}\mathrm{Ca}$+${}^{154}\mathrm{Sm}$ (filled triangles) and ${}^{40}\mathrm{Ca}$+${}^{154}\mathrm{Sm}$ (open circles) reactions.