Abstract
The (α, 3nγ) reaction was used to study the N = 87 isotones 149Sm, 151Gd and 153Dy with 35 to 45 MeV α-beams. The measurements included γ-ray singles excitation function, angular distribution, and linear polarization studies as well as γγ coincidence measurements. The high-spin level patterns are remarkably similar for the three isotones. The predominant γ-decay proceeds via E2 transitions through several parallel ΔI = 2 bands associated with the i 3 2 , h 9 2 and f 7 2 intrinsic configurations. Also observed in each nucleus is a weakly populated ΔI = 1 band, dominated by Ml radiation and built upon the h 11 2 intrinsic state. From the observed band structures we conclude that the h 11 2 configurations are strongly deformed, in contrast to the other intrinsic configurations. A simple explanation for the strongly deformed h 11 2 configurations in these otherwise weakly deformed nuclei is based upon the Nilsson diagram and shown to be related to the well known ground-state shape change occurring between 88 and 90 neutrons. Other cases, recently observed, where the same mechanism is applicable are surveyed.
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