Fission mass widths in ${}^{19}\mathrm{F}+{}^{232}\mathrm{Th},{}^{16}\mathrm{O}+{}^{235,238}\mathrm{U}$ reactions at near-barrier energies

The fission fragment mass ratio distributions of fusion-fission events have been measured for the reactions of ${}^{19}\mathrm{F}+{}^{232}\mathrm{Th},{}^{16}\mathrm{O}+{}^{235}\mathrm{U}$, and ${}^{16}\mathrm{O}+{}^{238}\mathrm{U}$ at energies near and below the fusion barrier. It is found that the mass ratio widths follow a decreasing trend with decreasing energy, contrary to recent claims of anomalous mass widths attributed to a quasifission mechanism.

Figure 1

(Color online) The folding angle ${\theta }_{\mathrm{AB}}$ as a function of ${\theta }_B$ and the coplanarity angle ${\varphi }_{\mathrm{AB}}$ as a function of ${\theta }_{\mathrm{AB}}$ for the reaction ${}^{19}\mathrm{F}+{}^{232}\mathrm{Th}$ at $E_{\mathrm{lab}}=97\hspace{1em}\mathrm{MeV}$, using the thin $50\hspace{1em}\mu g/{\mathrm{cm}}^2$ target (a and b) and the thick $2.1\mathrm{mg}/{\mathrm{cm}}^2$ target (c and d).

Figure 2

(Color online) The perpendicular velocity [4] is a measure of off-beam components of the linear momentum transferred to the fissioning system. This quantity is used to separate FMT fission events from TR leading to the fission.

Figure 3

Fission fragment mass ratio distributions following FMT events in the ${}^{19}\mathrm{F}+{}^{232}\mathrm{Th}$ reaction (thin Th target). Gaussian fits are shown as solid lines.

Figure 4

Fission fragment mass ratio widths following FMT events as a function of $E_{c.m.}/V_C$ for the three systems studied in this work using thin targets.

Figure 5

Fission fragment mass widths following FMT events as a function of $E_{c.m.}$ for the ${}^{19}\mathrm{F}+{}^{232}\mathrm{Th}$ reaction using the thin and thick targets. The stars joined by dashed lines are the results reported recently by Ghosh et al. [1].