Dipole strength distributions of the stable odd-mass $N=82$ isotones ${}^{139}\mathrm{La}$ and ${}^{141}\mathrm{Pr}$

The low-lying dipole strength distributions of the odd-mass isotopes ${}^{139}\mathrm{La}$ and ${}^{141}\mathrm{Pr}$ were studied in nuclear resonance fluorescence experiments using bremsstrahlung beams. Excited states were observed at excitation energies up to 4 MeV. Spectroscopic information was obtained on excitation energies, decay widths, decay branching ratios, and transition probabilities. The states belonging to the $[[2^{+}\otimes 3^{-}]\otimes \mathit{particle}/\mathit{hole}]$ coupling were a special focus, and the results are compared with other stable odd-mass nuclei at or near the $N=82$ shell closure.

Figure 1

Excitation energies of the first 2${}^{+}$ quadrupole vibration (square), first 3${}^{-}$ octupole vibration (diamond), the sum of both (hexagon), and the 1${}^{-}$ two-phonon state (cross) in some stable even-even $N=82$ nuclei.

Figure 2

Spectra of photons scattered from ${}^{139}\mathrm{La}$ and ${}^{141}\mathrm{Pr}$ measured at a scattering angle of 127° with bremsstrahlung beams with end-point energies of 4.1 MeV. Calibration lines (${}^{27}\mathrm{Al}$) and background lines (Bg) are indicated. A portion of the ${}^{141}\mathrm{Pr}$ spectrum is ploted with a linear scale to demonstrate the quality of the NRF data.

Figure 3

Versions of the assignments of transitions as ground-state and inelastic decays for ${}^{141}\mathrm{Pr}$. In Table , the A versions are listed. The B versions can be found in Table .

Figure 4

Comparision of the experimentally observed strength distributions of ${}^{139}\mathrm{La}$ and its even-even core nuclei, ${}^{138}\mathrm{Ba}$ and ${}^{140}\mathrm{Ce}$. Note that for the odd-mass nucleus the product $g·{\Gamma }_0$ is plotted and for the even-even core nuclei simply ${\Gamma }_0$ is given. The excitation energies of the 3${}^{-}$ octupole phonons (not observed in NRF experiments) in the even-even nuclei are marked with the dotted lines.

Figure 5

Comparision of the experimentally observed strength distributions of ${}^{141}\mathrm{Pr}$ and its even-even core nuclei, ${}^{140}\mathrm{Ce}$ and ${}^{142}\mathrm{Nd}$. Note that for the odd-mass nucleus the product $g·{\Gamma }_0$ is plotted and for the even-even core nuclei simply ${\Gamma }_0$ is given. The excitation energies in the 3${}^{-}$ octupole phonon (not observed in NRF experiments) of the even-even nuclei are marked with the dotted lines.

Figure 6

Plot of the strength-weighted average excitation energies $\langle E\rangle$ of the $[[2^{+}\otimes 3^{-}]\otimes \mathrm{particle}]$ multiplets in the odd-mass nuclei and the excitation energies $E_{1^{-}}$ of the 1${}^{-}$ states in their even-even core nuclei. The region within the dashed lines corresponds to the $N=82$ isotones. The regions below and above these lines correspond to the barium and neodynium isotopes, respectively.

Figure 7

Comparision of the summed $E1$ strengths, $\Sigma B(E1)\uparrow$, of the $[[2^{+}\otimes 3^{-}]\otimes \mathrm{particle}]$ coupling for odd-mass nuclei to the $E1$ strengths, $B(E1)\uparrow$, of the two-phonon 1${}^{-}$ states in their even-even core nuclei in the region of the $N=82$ shell closure.