Lifetime measurement for the21+state inSm140and the onset of collectivity in neutron-deficient Sm isotopes

Abstract

Background: The chain of Sm isotopes exhibits a wide range of nuclear shapes and collective behavior. While the onset of deformation for $Ng82$ has been well studied both experimentally and theoretically, fundamental data is lacking for some Sm isotopes with $Nl82$.Purpose: Electromagnetic transition rates represent a sensitive test of theoretical nuclear structure models. Lifetime measurements are furthermore complementary to Coulomb excitation experiments, and the two methods together can give access to spectroscopic quadrupole moments.Method: The lifetime of the ${2}_{1}^{+}$ state in $^{140}\mathrm{Sm}$ was measured with the recoil-distance Doppler shift technique using the reaction $^{124}\mathrm{Te}(^{20}\mathrm{Ne},4\mathrm{n})^{140}\mathrm{Sm}$ at 82 MeV. Theoretical calculations were performed based on a mapped collective Hamiltonian in five quadrupole coordinates (5DCH) and the Gogny D1S interaction.Results: The lifetime of the ${2}_{1}^{+}$ state in $^{140}\mathrm{Sm}$ was found to be 9.1(6) ps, corresponding to a $B(E2;{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})$ value of 51(4) Weisskopf units. The theoretical calculations are in very good agreement with the experimental result.Conclusions: The $B(E2;{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})$ value for $^{140}\mathrm{Sm}$ fits smoothly into the systematic trend for the chain of Sm isotopes. The new beyond-mean field calculations are able to correctly describe the onset of collectivity in the Sm isotopes below the $N=82$ shell closure for the first time.