Fission fragment mass distributions in reactions populating ${}^{200}\mathrm{Pb}$

The fission fragment mass distributions have been measured in the reactions ${}^{16}\mathrm{O}+{}^{184}\mathrm{W}$ and ${}^{19}\mathrm{F}+{}^{181}\mathrm{Ta}$ populating the same compound nucleus ${{}^{200}\mathrm{Pb}}^{*}$ at similar excitation energies. It is found that the widths of the mass distributions increase monotonically with excitation energy, indicating the absence of quasifission for both reactions. This is contrary to two recent claims of the presence of quasifission in the above-mentioned reactions.

Figure 1

Folding angle distributions of complementary fission fragments for the system ${}^{19}\mathrm{F}+{}^{181}\mathrm{Ta}$ at an excitation energy of 65 MeV. The rectangle indicates the gate used to select the fusion fission events for mass determination.

Figure 2

Experimental mass distributions of fission fragments for the reactions ${}^{19}\mathrm{F}+{}^{181}\mathrm{Ta}$ [(a)–(d)] and ${}^{16}\mathrm{O}+{}^{184}\mathrm{W}$ [(e)–(h)] at different excitation energies. They were fitted by a single Gaussian, shown by full (red) lines.

Figure 3

Variation of the standard deviation ${\sigma }_m$(u) of the fitted symmetric mass distribution with excitation energy. The calculated widths are shown in (blue) dashed line, the shaded region indicates uncertainties in calculation (see text). The predicted [8] variation of the sum of the fast fission and quasifission cross sections (normalized with respect to the fusion cross sections) with excitation energy are presented in the inset.

Figure 4

Variation of the ratio of the experimental (measured) and theoretically calculated standard deviation ${\sigma }_m$(u) of the fitted symmetric mass distribution with c.m. energy. The Coulomb barriers for the ${}^{19}\mathrm{F}+{}^{181}\mathrm{Ta}$ (77.8 MeV) and ${}^{16}\mathrm{O}+{}^{184}\mathrm{W}$ (71 MeV) reactions are shown by arrows. The dashed line (constant value = 1) is a guide to the eye.

Figure 5

Measured mass angle distributions of the fission fragments in the reaction ${}^{19}\mathrm{F}+{}^{181}\mathrm{Ta}$ at an excitation energy of 75.9 MeV.