Low-spin Levels in Sm140 : Five 0+ States and the Question of Softness Against Nonaxial Deformation

Background: Investigation of the $_{62}^{140}\mathrm{Sm}_{78}$ nucleus, situated in the area close to the magic $N=82$ neutron shell, offers the opportunity to find and study interesting phenomena resulting from the interplay of collective and other degrees of freedom.Purpose: Experimental identification of low-spin low-energy levels, particularly ${0}^{+}$, in $^{140}\mathrm{Sm}$ and theoretical interpretation within the collective general Bohr Hamiltonian (GBH) model.Method: The $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ angular correlation technique for $\ensuremath{\gamma}$ radiation after the $\ensuremath{\beta}$/EC decay of $^{140}\mathrm{Eu}\ensuremath{\rightarrow}^{140}\mathrm{Sm}$ and $^{140}\mathrm{Gd}\ensuremath{\rightarrow}^{140}\mathrm{Eu}\ensuremath{\rightarrow}^{140}\mathrm{Sm}$ was used to determine spins of excited states of $^{140}\mathrm{Sm}$. The $^{140}\mathrm{Gd}$ and $^{140}\mathrm{Eu}$ nuclei were produced in the $^{104}\mathrm{Pd}+^{40}\mathrm{Ar}$ reaction at the HIL UW cyclotron. In the theoretical part the full five-dimensional GBH model was applied in two variants: the simple phenomenological Warsaw model and the microscopic version with six inertial functions and a potential calculated from mean-field theory.Results: The spin and parity of six low spin (0,1,2) low lying excited levels of $^{140}\mathrm{Sm}$ were measured. Two new states at around 2 MeV were identified. A analysis of the consequences of possible admixtures on the determination of the spin of a level was performed. The theoretical models applied successfully describe most of the spectrum of $^{140}\mathrm{Sm}$ giving hints on the origin of the states observed in the experiment.Conclusions: Significant softness against nonaxial deformation seems to be essential to interpret the properties of $^{140}\mathrm{Sm}$. Further experimental studies are needed to check if some low-energy excitations are not deformation driven.