Parity-doublet structure in the ${}_{57}{}^{147}\mathrm{La}_{90}$ nucleus

Excited states in ${}^{147}\mathrm{La}$, populated in spontaneous fission of ${}^{252}\mathrm{Cf}$, have been reinvestigated by means of $\gamma$-ray spectroscopy, using high-fold $\gamma$-ray coincidences measured with the Gammasphere array of Ge detectors. The 229.5-keV level, which has been assigned spin parity $9/2^{-}$ in a recent evaluation, is shown to have spin parity $11/2^{-}$. Consequently, the ground state has spin parity $5/2^{+}$. Excited levels in ${}^{147}\mathrm{La}$ have been arranged into a parity-doublet structure, showing that at medium excitation energy the ${}^{147}\mathrm{La}$ nucleus may have octupole deformation. The $B\left(E1\right)$ rates in ${}^{147}\mathrm{La}$, which are factor of four lower than in ${}^{145}\mathrm{La}$, suggest that the electric dipole moment in ${}^{147}\mathrm{La}$ is depressed by an extra mechanism, probably connected with the population of particular neutron orbitals.

Figure 1

Scheme of excited levels in ${}^{147}\mathrm{La}$, populated in spontaneous fission of ${}^{252}\mathrm{Cf}$, as observed in the present work. The thickness of each arrow representing transitions is proportional to its relative $\gamma$-ray intensity. The 15.4-, 120.8-, and 211.7-keV levels and their decays are drawn after Refs. [24, 25] to assist the discussion.

Figure 2

Sum of $\gamma$-ray spectra doubly gated in a triple-$\gamma$ histogram on pairs of lines from the 167.6-212.0-346.1-454.4-528.7-keV, yrast cascade in ${}^{147}\mathrm{La}$, as observed in this work.

Figure 3

A $\gamma$-ray spectrum doubly gated on the 167.6- and 212.0-keV lines in the ${}^{252}\mathrm{Cf}$ fission data. Energies are given in keV. Newly observed lines are marked with an asterisk.

Figure 4

A $\gamma$-ray spectrum doubly gated on 212.0- and 512.7-keV lines in the ${}^{252}\mathrm{Cf}$ fission data. Energies are given in keV. Newly observed lines are marked with asterisks.

Figure 5

A $\gamma$-ray spectrum doubly gated on the 212.0- and 603.1-keV lines in the ${}^{252}\mathrm{Cf}$ fission data. Energies are given in keV. The newly observed line is marked with an asterisk.

Figure 6

A $\gamma$-ray spectrum doubly gated on the 297.4- and 381.5-keV newly observed lines of ${}^{147}\mathrm{La}$ in the ${}^{252}\mathrm{Cf}$ fission data. Energies are given in keV. Newly observed lines are marked with asterisks.

Figure 7

An example of angular correlations in ${}^{147}\mathrm{La}$ for quadrupole-quadrupole (Q-Q) and dipole-quadrupole (D-Q) cascades, as measured in this work. See text for further information.

Figure 8

Systematics of excitations in odd-$A$ La isotopes. The data are taken from the present work and from Refs. [14, 15, 24, 25, 33, 35, 36, 37, 38]. Dashed lines are drawn to guide the eye. Parentheses indicate tentative spin-parity assignments. See text for more comments.

Figure 9

Total aligned angular momenta in bands of ${}^{147}\mathrm{La}$ and in the g.s. band of ${}^{146}\mathrm{Ba}$. (a) Positive parity bands; (b) negative parity bands. See text for the description of $\hslash {\omega }_1$ and $\hslash {\omega }_2$.

Figure 10

Energies of $\gamma$-ray transitions in bands of ${}^{147}\mathrm{La}$ vs. spin of the initial level. Lower abscissa corresponds to the npb and higher to the ppb Dashed lines are drawn to guide the eye. See text for more comments.