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Triaxial-rotor-plus-particle model calculations strongly support a pure axially-symmetric shape with large quadrupole deformation in Y isotopes.

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High spin band structures of neutron-rich 152−158 Pm isotopes have been obtained from the measurement of prompt γ-rays of isotopically identified fragments produced in fission of 238 U+ 9 Be and detected using the VAMOS++ magnetic spectrometer and EXOGAM segmented Clover array at GANIL and also from the high statistics γ-γ-γ and γ-γ-γ-γ data from the spontaneous fission of 252 Cf using Gammasphere. The excited states in 157 Pm and those above the isomers in even-A Pm isotopes 152,154,156,158 Pm have been identified for the first time. The spectroscopic information on the rotational band structures in odd-A Pm isotopes has been extended considerably to higher spins and the possibility of the presence of reflection asymmetric shapes is explored. The configuration assignments are based on the results of Cranked Relativistic Hartree-Bogoliubov calculations. From the systematics of bands in odd-A Pm isotopes and weak population of opposite parity bands, octupole deformed shapes in neutron rich Pm isotopes beyond N = 90 seem unlikely to be present.

From an experiment with Gammasphere and a 252 Cf spontaneous fission source, a new negative-parity band in 154 Nd and new negative-parity levels in 152 Nd were identified and the yrast bands were extended to 18 ϩ in 154 Nd and 20 ϩ in 152 Nd in a triple gamma coincidence study. These new negative-parity bands are consistent with octupole vibrational mode. There is a constant difference as a function of spin between the J 1 values for the negative-parity band in 152 Nd and J 1 for the similar negative-parity band in 154 Nd, however, their J 2 values are essentially identical. These bands indicate a new kind of identical band. ͓S0556-2813͑98͒01004-8͔PACS number͑s͒: 23.20. Lv, 21.10.Re, 25.85.Ca, 27.70.ϩq The neutron-rich neodymium (Zϭ60) nuclei with Aу142 are situated at the important intersection of two transitional regions: the transition from spherical to prolate quadrupole deformation and the transition from octupole vibrational excitations to the static octupole deformation ͓1-6͔. So far little is known about octupole excitation states in the neutron-rich nuclei in contrast to the considerable data on quadrupole collectivity ͓5,7͔. Studies on octupole vibrational excitations in neodymium nuclei can provide tests of relevant nuclear models.Our previous studies ͓5,8͔ in neutron-rich nuclei in the Aϳ155 region revealed both identical moments of inertia ͑kinetic and dynamic͒ and identical gamma-ray energies in some cases. Therefore it is very interesting to identify possible sidebands in neodymium nuclei, looking for phenomena connected to identical bands as well as octupole deformation. In 144,146 Ba identical octupole bands associated with stable octupole deformation were observed ͓5,9͔.The new levels in 152,154 Nd were obtained from the analysis of ␥-ray spectra produced in the spontaneous fission of 252 Cf. A detailed description of the experimental procedures and analysis methods can be found in Ref. ͓5͔ and Ref. ͓10͔.The new transitions assigned to 154 Nd were identified by setting double gates on the known yrast transitions in 154 Nd from Ref. ͓5͔ and making sure that whenever a double gate was set on the new transitions, one not only observes the transitions corresponding to the partners ( 94,95,96 Sr) of 154 Nd with 4, 3, and 2 neutrons emitted, respectively, but also obtains the same yield ratios for their partners as obtained from double gating on the well known yrast transitions in 154 Nd. Figure 1 shows two partial coincidence spectra obtained by FIG. 1. ͑a͒ Coincidence spectrum obtained by double gating on the 162.8 and 895.0 keV transitions in 154 Nd. ͑b͒ Coincidence spectrum obtained by double gating on the 268.5 and 338.3 keV transitions in 154 Nd.