Fission fragment angular distributions for the systems ${}^{14}\mathrm{N}+{}^{232}\mathrm{Th}$ and ${}^{11}\mathrm{B}+{}^{235}\mathrm{U}$ at near and sub-barrier energies

Fission fragment angular distributions have been measured for the systems ${}^{11}\mathrm{B}+{}^{235}\mathrm{U}$ and ${}^{14}\mathrm{N}+{}^{232}\mathrm{Th}$, at sub and near-barrier energies. Complete and incomplete momentum transfer fission components have been separated around $90°\text{c.m.}$ for ${}^{14}\mathrm{N}+{}^{232}\mathrm{Th}$. The contribution of incomplete momentum transfer events is found to be about $8\%$ over the present energy range of investigation. In the ${}^{14}\mathrm{N}+{}^{232}\mathrm{Th}$ case, anisotropy data are found to be anomalous throughout the energy range, where as corresponding data for ${}^{11}\mathrm{B}+{}^{235}\mathrm{U}$ are found to be normal, with respect to the saddle-point statistical model predictions. The large ground state spin values of ${}^{11}\mathrm{B}$ and ${}^{235}\mathrm{U}$ are found to play a major role at the sub-barrier region in influencing the anisotropy values.

Figure 1

Two-dimensional scatter plot of energy of primary fragments ($\Delta E$ detector) vs energy of the complementary fragments (strip detector). The events within the gate are the coincident fission events.

Figure 2

Position spectrum obtained for $87.74\text{MeV}$ ${}^{14}\mathrm{N}+{}^{232}\mathrm{Th}$ folding angle measurements. Each peak corresponds to a particular correlation angle of the fission fragments.

Figure 3

Crossover angle plot for ${}^{11}\mathrm{B}+{}^{235}\mathrm{U}$ at three typical energies.

Figure 4

(a) Fission fragment folding angle data measured for ${}^{14}\mathrm{N}+{}^{232}\mathrm{Th}$ at $E_{\mathrm{lab}}=87.74\text{MeV}$. (b) Same as (a) with $E_{\mathrm{lab}}=76.64\text{MeV}$.

Figure 5

$\mathrm{B}+\mathrm{U}$ fission cross sections ${\sigma }_{\text{fission}}$ as a function of $E_{\text{c.m.}}∕V_B$. ${}^{11}\mathrm{B}+{}^{235}\mathrm{U}$: filled circles; ${}^{11}\mathrm{B}+{}^{238}\mathrm{U}$: filled squares [39] and open triangles [12]. The solid curve is the Wong model calculation as mentioned in text.

Figure 6

$\mathrm{X}+\text{Th}$ fission cross sections ${\sigma }_{\text{fission}}$ as a function of $E_{\text{c.m.}}∕V_B$. ${}^{14}\mathrm{N}+{}^{232}\mathrm{Th}$: filled circles; ${}^{16}\mathrm{O}+{}^{232}\mathrm{Th}$: filled down triangles [11]; and ${}^{12}\mathrm{C}+{}^{232}\mathrm{Th}$: open triangles [13].

Figure 7

The measured fission fragment anisotropies (A) for the ${}^{11}\mathrm{B}+{}^{235}\mathrm{U}$ system are compared with the different model calculations. The dashed line is the calculation with the ECD-$K$ state model with $\text{spin}=0$, the dotted line is the same calculation involving target spin, the solid line is a calculation involving both target and projectile spins, and the dot-dashed line is the SSPM calculation.

Figure 8

The measured fission fragment anisotropies (A) for the ${}^{14}\mathrm{N}+{}^{232}\mathrm{Th}$ system are compared with the different model calculations. The dotted line is the calculation with the ECD-$K$ state model with $\text{spin}=0$, the solid line is the same calculation involving the projectile spin, and the dot-dashed line is the SSPM calculation.

Figure 9

Fission fragment anisotropy data for the ${}^{10}\mathrm{B}$, ${}^{12}\mathrm{C}$, ${}^{14}\mathrm{N}$, and ${}^{16}\mathrm{O}+{}^{232}\mathrm{Th}$ system are plotted as a function of $E_{\text{c.m.}}∕V_B$ and compared with the SSPM calculations. The solid line is the calculation with presaddle neutron correction. The dotted line is the calculation without neutron correction.

Figure 10

Fission fragment anisotropies for the ${}^{14}\mathrm{N}+{}^{232}\mathrm{Th}$ system. The solid line represents the quasifission model calculation with ${\theta }_{\text{crit}}=27°$, the effect of $\pm 5°$ change in angle are indicated by the dashed line. The dotted line is the assumed quasifission anisotropy while the dot-dashed line is the SSPM calculations.

Figure 11

(a) Preequilibrium fission model predictions for the ${}^{11}\mathrm{B}+{}^{235}\mathrm{U}$ system. The dotted line is the calculation with $\text{spin}=0$, the solid line is the calculation involving both target and projectile spin, and the dot-dashed line is the SSPM prediction. (b) Similar calculation for the system ${}^{14}\mathrm{N}+{}^{232}\mathrm{Th}$. The dotted line is the calculation with $\text{spin}=0$, the solid line is the calculation involving projectile spin, and the dot-dashed line is the SSPM prediction.