Exploring the fission barrier of ${}^{235}\mathrm{U}$

In this work we have reexamined the shape isomer in ${}^{235}\mathrm{U}$. Reanalyzing data from a previous experiment gave a half-life of $T_{1/2}=(11\pm 3)\mathrm{ms}$ and a cross section for populating the isomer of ${\sigma }_{\mathrm{iso}}=(12\pm 1)\mu \mathrm{b}$. Combining these new results with measured properties of fission fragments from the reaction ${}^{234}\mathrm{U}(n,f)$ allowed extracting parameters describing the outer fission barrier. The deduced barrier parameters are $E_B=(5.7\pm 0.6)$ MeV and $\hslash {\omega }_B=(0.5\pm 0.1)$ MeV for the height and curvature, respectively, as well as the energy of the superdeformed ground state $E_{II}=(2.4\pm 0.6)$ MeV. Separate barrier parameters for the standard-1 and standard-2 fission modes have been estimated, too. Finally, the results obtained in this work have been successfully tested for their plausibility.

Figure 1

Decay time spectrum of the shape isomer in ${}^{235}\mathrm{U}$. The experimental data, together with statistical uncertainties, are compared to the result of a fit according to Eq. (1).

Figure 2

Post-neutron kinetic energy spectrum of fission fragments from ${}^{234}\mathrm{U}(n,f)$ at $E_n=0.95$ MeV, corresponding to prompt fission. In addition, the fit of two skew Gaussians is shown as dotted lines.

Figure 3

Two-dimensional presentation of isomeric fission events by their time and kinetic energy. The solid lines indicate the time bins in Fig. 1, while the dashed line separates the heavy from the light fragments by equal amount.

Figure 4

Overview of $\overline{\mathrm{TKE}}$ of fission fragments for different excitation energies in ${}^{235}\mathrm{U}$. Filled and open symbols indicate values “pre” and “post” neutron emission, respectively. Experimental data from Refs. [9, 15, 17, 18] in descending order are compared to results from gef calculations [20]. These points are fitted with third-order polynomials for “pre” and “post”; the latter is then shifted to match the results from this work, depicted as open circles.

Figure 5

Ratio of mode-weighted cross sections for S1 and S2 for the reaction ${}^{234}\mathrm{U}(n,f)$ in the energy range $E_n=0.2$–5.0 MeV (black dots) together with the result of a fit according to Eqs. (5) and (6) (line). The cross section values were taken from Ref. [22], while the mode weights were taken from Ref. [9].

Figure 6

Overview of possible parameters describing the outer fission barrier of ${}^{235}\mathrm{U}$. The solid line corresponds to the relation between $E_B$ and $\hslash {\omega }_B$ according to Eq. (4) with depicted uncertainties as shaded area. The open circle, emphasized with a dashed crosshair, denotes the combination for S2 obtained from the mode-weighted cross sections. In addition, published values from Refs. [3, 5, 6] are indicated by arrows.