Near-yrast structure of odd-$\mathit{A}$, neutron-rich Pr isotopes

The neutron-rich praseodymium isotopes ${}^{151}$Pr and ${}^{153}$Pr have been studied by prompt $\gamma$-ray spectroscopy using ${}^{248}$Cm and ${}^{252}$Cf spontaneous-fission sources placed inside the EUROGAM-II and Gammasphere germanium arrays, respectively. Rotational bands based on $3/2^{-}$[541] states, with similar structures, have been assigned to these nuclei. These bands decay by intraband $E2$ transitions. Interband $E1$ transitions, reported in other works, were not observed. Delayed conversion-electron and $\gamma$-ray spectroscopy of an $A=151$ nucleus has been performed at the Lohengrin mass spectrometer. A previously reported 35.1-keV isomer of ${}^{151}$Pr has been determined to decay by an $E1$ transition, and its half-life of 50(8) $\mu$s has been measured. Calculations performed using a reflection-symmetric quasi-particle-rotor model successfully reproduce the energies of the excited states of these nuclei and their decay patterns. The spin of the isomer has been assigned to be (1/2${}^{+}$, 3/2${}^{+}$) from a comparison with the calculations. The long half-life of this isomer and the lack of intraband $E1$ transitions show an absence of strong octupole correlations in the observed states of ${}^{151,153}$Pr. This is explained in terms of increasing quadrupole deformation reducing the number of Nilsson orbitals close to the Fermi surface available to form octupole collectivity.

Figure 1

Background-subtracted spectrum of delayed $\gamma$ rays, detected by the Ge detectors at Lohengrin up to 15 $\mu$s after the arrival of an $A$ $=$ 151 ion.

Figure 2

Background-subtracted delayed conversion-electrons and $\gamma$ rays observed up to 15 $\mu$s after the arrival of an $A$ $=$ 151 ion in the Si(Li) detectors.

Figure 3

Time spectrum of 35.1-keV $\gamma$ rays in coincidence with $A$ $=$ 151 ions.

Figure 4

A $\gamma$-ray spectrum obtained by double gating on transitions of ${}^{93}$Rb. New lines are observed at 141.9, 221.6, 291.8, and 296.0 keV.

Figure 5

A $\gamma$-ray spectrum double-gated on the 913-keV line of ${}^{93}$Rb and the new 221.6-keV line.

Figure 6

Two $\gamma$-ray spectra obtained by double-gating on (a) the 221.8- and 296.0-keV lines and (b) the 221.0- and 291.8-keV lines of the new cascades.

Figure 7

Angular correlation measurements of $\gamma$ rays in the new cascades. The lines show the theoretical values of $E2$–$E2$ (QQ) and $E2$–$E1$ (QD) correlations.

Figure 8

Level scheme of ${}^{151}$Pr, as obtained in this work.

Figure 9

Level scheme of ${}^{153}$Pr, as obtained in this work.

Figure 10

Mass correlation plot for odd-$A$ Pr isotopes, as obtained in this work from the ${}^{248}$Cm fission data (empty circles and empty squares) and from ${}^{252}$Cf fission data (full circles and full squares). Empty and full squares represent the average mass of Rb and Y isotopes obtained by gating on cascades assigned in Ref. [8] to ${}^{151}$Pr and ${}^{153}$Pr. These cascades are reassigned in this work to ${}^{152}$Pr and ${}^{154}$Pr, respectively. See the text for further explanations.

Figure 11

Two $\gamma$-ray spectra obtained from the ${}^{252}$Cf data set by double-gating on (a) the 221.8- and 296.0-keV lines and (b) the 221.0- and 291.8-keV lines of the new cascades.

Figure 12

Two $\gamma$-ray spectra obtained from the ${}^{252}$Cf data set by double-gating on (a) the 142.1- and 296.0-keV lines and (b) the 141.6- and 291.8-keV lines of the new cascades

Figure 13

Mass population diagrams for (a) odd-$A$ Pr and even-$A$ Ce isotopes and (b) for even-$A$ Ce and Nd isotopes from the ${}^{248}$Cm (empty symbols) and ${}^{252}$Cf fission data (full symbols). Cerium, praseodymium, and neodymium isotopes are shown as circles, squares, and diamonds, respectively. See text for further explanation.

Figure 14

Comparison of experimentally measured and calculated partial decay schemes of ${}^{151}$Pr. See text for more details.

Figure 15

Comparison of experimentally measured and calculated partial decay schemes of ${}^{153}$Pr.

Figure 16

Aligned angular momenta of bands of ${}^{151}$Pr and ${}^{148,150}$Ce.