Direct evidence of “washing out” of nuclear shell effects

Constraining the excitation energy at which the nuclear shell effect washes out has important implications on the production of superheavy elements and many other fields of nuclear physics research. We report the fission fragment mass distribution in $\alpha$ induced reaction on an actinide target for wide excitation range in close energy interval and show direct evidence that the nuclear shell effect washes out at excitation energy $\sim 40$ MeV. The calculation shows that the second peak of the fission barrier also vanishes around similar excitation energy.

Figure 1

The excitation function of fission for ${}^4\mathrm{He}+{}^{232}\mathrm{Th}$ reaction. The solid (black) circles are the present measurement. Measurement of Ralarosy et al. [14] is shown by solid (blue) squares.

Figure 2

Measured distributions of folding angles of the fissioning nuclei formed in the reaction ${}^4\mathrm{He}+{}^{232}\mathrm{Th}$ at an excitation energy of 23 MeV.

Figure 3

FFMD at different excitation energies. Fitting by three Gaussians for $E^{*}=21$ MeV to 40.5 MeV are shown by dash blue (symmetric component) and violet dash-dotted (asymmetric components) lines. The overall fitting is shown by full (red) lines. The mass distributions at higher excitation energies $(\ge 43.6$ MeV) are best fitted by a single Gaussian.

Figure 4

FFMD at excitation energy 23 MeV fitted by two Gaussian (a) and three Gaussian (b) distributions. The asymmetric components are shown by (violet) dash-dot line and symmetric component is shown by (blue) dash line. The overall fitting is shown by solid (red) lines.

Figure 5

The variation of the ratio (relative unit) of the symmetric fission yield to the total fission yield at different excitation energies.

Figure 6

Variation of the width of the fitted symmetric mass distribution with excitation energy.

Figure 7

(a) Variation of the free energy as a function of deformation $(c$, see text) for ${}^{236}\mathrm{U}$ for ground state and excitation energies 21, 24.9, 30.8, 37.7, 40.6, 43.6, 49.5, 55.3, and 64.2 MeV. (b) Variation of fission barrier as a function of excitation energy. The shaded (yellow) region represents the uncertainty in fission barrier calculation.