Structure of the fission transition nucleus 227Ra

The height of the fission barrier and the character of the low lying single particle levels have been determined for the fission transition nucleus ²²⁷Ra by fitting the energy variation of the previously reported fission cross section and fragment angular distributions in the neutron energy range $3.6<{E}_{n}<4.1$ MeV using a Hauser-Feshbach formalism. The best fit to the experimental data gave a fission barrier height E_f=8.2±0.1 MeV and a single particle state sequence at the barrier of 3/2, \textonehalf{}, 5/2, \textonehalf{}, which agrees very well with the theoretical predictions of Nix and M\"oller. An extension of the calculations to higher neutron energies ($4.7<{E}_{n}<9.0) allowed deduction ofK_{0}^{}{}_{}{}^{2}$ and the transition nucleus level density as a function of excitation energy. A discontinuity in the $K_0^{{}^2}$ values suggests a value of the pairing gap parameter (2Δ_f=2.7 MeV) that is considerably larger than the corresponding one at the equilibrium deformation (2Δ₀=1.7 MeV). The transition nucleus and residual nucleus level densities were compared with theoretical calculations and support the view that a collective rotational and vibrational enhancement of the intrinsic nuclear level density is necessary to account for the experimental data.NUCLEAR REACTIONS, FISSION ²²⁶Ra(n,f), E=3.6-9.0 MeV; ²²⁷Ra* deduced levels, K, π, $K_0^{{}^2}$, and level density.