Lifetime measurements in ${}^{138}\mathrm{Nd}$

Lifetimes of several short-lived excited states in ${}^{138}\mathrm{Nd}$ were measured with the ROSPHERE array at IFIN-HH, Bucharest, using the recoil-distance Doppler shift technique following the ${}^{123}\mathrm{Sb}({}^{19}\mathrm{F},4n$) reaction. The resulting electromagnetic transition probabilities are compared to large-scale shell model calculations and to constrained Hartree-Fock-Bogoliubov calculations with the Gogny D1S interaction, configuration mixing, and a five-dimensional collective Hamiltonian formalism. The onset of collectivity in Nd isotopes below the $N=82$ shell closure and the deformation induced by the alignment of protons and neutron holes in the $h_{11/2}$ orbitals are discussed.

Figure 1

Background-subtracted spectra showing the Doppler-shifted and unshifted components of the $2_1^{+}\to 0_1^{+}$ (a), $4_1^{+}\to 2_1^{+}$ (b), and ${10}_2^{+}\to 9_2^{-}$ (c) transitions, measured with the backward detectors for different distances. The transitions were detected in coincidence with the shifted components of $\gamma$ rays feeding the $2_1^{+}$, $4_1^{+}$, and ${10}_2^{+}$ states, respectively (see Table 1).

Figure 2

Normalized values of the intensities of the shifted (b) and unshifted (c) components of the $2_1^{+}\to 0_1^{+}$ transition, measured for different distances using the backward detectors. The lifetimes obtained for the different distances are shown in (a), where the horizontal lines indicate the weighted average and its uncertainty.

Figure 3

Partial level scheme of ${}^{138}\mathrm{Nd}$, based on Ref. [31]. The lifetimes of the states measured in the present work are shown together with the previously known lifetimes of the ${10}_1^{+}$ state [8, 9] and the $7_1^{-}$ state [25], which are marked with an asterisk ($★$). The widths of the arrows indicate the relative intensities of the transitions.

Figure 4

Experimental $B(E2;\downarrow )$ values for transitions in the ground-state band of ${}^{132}\mathrm{Nd}$ [37, 38], ${}^{134}\mathrm{Nd}$ [39], ${}^{136}\mathrm{Nd}$ [40], ${}^{138}\mathrm{Nd}$ (present work), and ${}^{140}\mathrm{Nd}$ [41] compared to theoretical calculations based on the 5DCH approach with the Gogny D1S interaction.

Figure 5

Potential energy surface (PES) for the ground-state of ${}^{138}\mathrm{Nd}$ based on constrained Hartree-Fock-Bogoliubov calculations with the Gogny D1S interaction. The lines and numbers indicate the potential energy in MeV.

Figure 6

Comparison of $B\left(E2\right)$ values for transitions in the quasi-rotational bands on top of the ${10}_1^{+}$ and ${10}_2^{+}$ states in ${}^{138}\mathrm{Nd}$ (a) and ${}^{140}\mathrm{Sm}$ (b) with $B\left(E2\right)$ values of transitions in the ground-state bands of the respective $(Z-2)$ and $(N+2)$ cores. The $B\left(E2\right)$ values for ${}^{136}\mathrm{Ce}$ [33], ${}^{138}\mathrm{Nd}$ (present work), ${}^{140}\mathrm{Nd}$ [41], ${}^{140}\mathrm{Sm}$ [15], and ${}^{142}\mathrm{Sm}$ [43] are given in Weisskopf units, and the widths of the transitions are proportional to the corresponding $B\left(E2\right)$ values.