Dipole responses in Nd and Sm isotopes with shape transitions

Photoabsorption cross sections of Nd and Sm isotopes from spherical to deformed even nuclei are systematically investigated by means of the quasiparticle-random-phase approximation based on the Hartree-Fock-Bogoliubov ground states using the Skyrme energy density functional. The gradual onset of deformation in the ground states as the neutron number increases leads to characteristic features of the shape phase transition. The calculations well reproduce the isotopic dependence of broadening and the emergence of a double-peak structure in the cross sections without any adjustable parameter. We also find that the deformation plays a significant role for low-energy dipole strengths. The $E1$ strengths are fragmented and considerably lowered in energy. The summed $E1$ strength up to 10 MeV is enhanced by a factor of 5 or more.

Figure 1

Intrinsic electric quadrupole moments of Nd and Sm isotopes. For spherical nuclei, values extracted from calculated $B(E2)$ are shown by triangles. The experimental values are taken from Ref. [29].

Figure 2

Photoabsorption cross sections in (a) Nd and (b) Sm isotopes as functions of photon energy. The experimental data [7, 8] are denoted by filled squares.

Figure 3

Photoabsorption cross sections of ${}^{144}\mathrm{Sm}$ and ${}^{154}\mathrm{Sm}$ obtained by employing the SkM*, SLy4, and SkP functionals. The $K^{\pi }=0^{-}$ and $K^{\pi }=1^{-}$ components are shown by dotted and thin solid lines, respectively, in ${}^{154}\mathrm{Sm}$.

Figure 4

Calculated low-energy $E1$ strengths $B(E1;0^{+}\to 1^{-})$ as functions of energy in ${}^{142,150,152}\mathrm{Nd}$. The arrow indicates the neutron emission threshold energy. (d) shows the unperturbed strengths multiplied by 1/15 in ${}^{142}\mathrm{Nd}$. The neutron threshold of ${}^{142}\mathrm{Nd}$ is 11.6 MeV.

Figure 5

As Fig. 4 but in ${}^{144,152,154}\mathrm{Sm}$. (d) shows the unperturbed strengths multiplied by 1/15 in ${}^{144}\mathrm{Sm}$. The neutron threshold of ${}^{144}\mathrm{Sm}$ is 11.9 MeV.

Figure 6

Calculated transition densities of the $K^{\pi }=0^{-}$ state at 7.49 MeV and of the $K^{\pi }=1^{-}$ state at 7.33 MeV in ${}^{154}\mathrm{Sm}$.