Observation of a fission mode with very short elongation for the neutron-rich ${}^{257}\mathrm{Md}$ nucleus at high excitation energy

Mass and total kinetic-energy (TKE) distributions of fission fragments from ${}^{257}\mathrm{Md}$ nucleus populated by complete fusion of ${}^{19}\mathrm{F}$ with ${}^{238}\mathrm{U}$ have been measured in the range of excitation energy $E^{*}\approx 37\text{−}48\mathrm{MeV}$. Due to the skewness of the measured TKE distributions, two Gaussian distributions were required to explain the data. The events with high TKE must originate from the supershort mode of fission, the presence of which has been confirmed by measuring two additional reactions: ${}^{19}\mathrm{F}+{}^{232}\mathrm{Th}$ and ${}^{18}\mathrm{O}+{}^{238}\mathrm{U}$ involving first a different target and then a different projectile leading to compound nuclei ${}^{251}\mathrm{Es}$ and ${}^{256}\mathrm{Fm}$, respectively, at similar excitation energies and comparing their mass-TKE correlation plots. The existence of considerable supershort mode of fission for the ${}^{257}\mathrm{Md}$ nucleus up to $E^{*}\approx 48$ MeV, observed for the first time, reveals that such rare fission modes for neutron-rich exotic nuclei are far off from the liquid drop model predictions, even at high excitation energies.

Figure 1

Typical plots of (a) correlation of time of flights (${\mathrm{TOF}}_1$ vs ${\mathrm{TOF}}_2$) of both fission fragments, cleanly identifying the binary fission events for both the beam pulses, and (b) $v_{\parallel }\text{−}{v}_{\mathrm{cn}}$ versus $v_{\perp }$, obtained for ${}^{19}\mathrm{F}+{}^{238}\mathrm{U}$ reaction at $E_{\mathrm{beam}}=98$ MeV. The area inside the red circle corresponds to the binary events following the complete fusion process.

Figure 2

Measured TKE distributions for (a)–(c) ${}^{257}\mathrm{Md}$, (d) ${}^{256}\mathrm{Fm}$, and (e), (f) ${}^{251}\mathrm{Es}$ nuclei. TKE distributions shown in panels (a) and (b) were fit using two Gaussian distributions, in contrast to the others that were fit using single-Gaussian distribution.

Figure 3

Mass-TKE correlations obtained from the fissioning nucleus (a)–(c) ${}^{257}\mathrm{Md}$, (d) ${}^{256}\mathrm{Fm}$, and (e), (f) ${}^{251}\mathrm{Es}$. The events within the dotted rectangles, with higher TKE, are likely to be due to the supershort mode of fission.

Figure 4

Comparison of the widths ${\sigma }_{\text{TKE}}$ of the measured TKE-distributions for the fissioning nuclei ${}^{257}\mathrm{Md}, {}^{256}\mathrm{Fm}$, and ${}^{251}\mathrm{Es}$ along with the gef predictions (lines).

Figure 5

Mass-distributions obtained from the fissioning nucleus (a)–(c) ${}^{257}\mathrm{Md}$, (d) ${}^{256}\mathrm{Fm}$, and (e), (f) ${}^{251}\mathrm{Es}$. The contributions from symmetric and asymmetric modes are shown by filled gray and cyan regions, respectively.

Figure 6

(a) Comparison of contributions from the supershort mode for the present nucleus ${}^{257}\mathrm{Md}$ with the available literature data (symbols) and gef calculations (solid line). (b) Comparison of determined contributions from the asymmetric mode (symbols) with the gef calculation (dashed lines) for each nucleus populated in the present experiment.