High-spin states in 157,158Er and 158,159Tm.

Abstract

High-spin states in /sup 157,158/Er and /sup 158,159/Tm have been studied via the /sup 122/Sn(/sup 40/Ar,5,4n) and /sup 141/Pr(/sup 22/Ne,5,4n) reactions, respectively, using in-beam gamma-ray spectroscopy techniques which included gamma - gamma coincidence, angular correlation, angular distribution, excitation function, and multiplicity measurements. The favored ( pi =+, alpha =+1/2) /b i//sub 13/2/ neutron band in /sup 157/Er has been established up to the 53/2/sup +/ level ( h omega =0.417 MeV), and the favored ( pi =-, alpha =-1/2) and unfavored ( pi =-, alpha =+/sup 1///sub 2/)/b h//sub 11/2 / proton bands in /sup 159/Tm, up to the 59/2/sup -/ ( h omega =0.454 MeV) and 61/2/sup -/( h omega =0.480 MeV) levels, respectively, i.e. well above the range of rotational frequency h omega where a second backbending has been observed in the neighboring even-even nuclei. In /sup 158/Er, the yrast levels have been observed up to 38/sup +/, and in /sup 158/Tm, up to 25/sup -/. The favored /b i//sub 13/2/ band of the odd-/b N/ nucleus /sup 157/Er displays a strong upbending at h omega =0.40 MeV, whereas, in the two branches of the /b h/ /sub 11/2/ yrast band of the odd-/b Z/ nucleus /sup 159/Tm, no alignment effect has been observed between h omega =0.27 and 0.48 MeV. The experimental results have been analyzed in the framework of the cranked shell model. On the basis of blocking arguments and cranked shell model calculations, the obtained results show that the second backbending in the yrast band of even-even nuclei near /b A/=158 is due to an /b h//sub 11/2/ proton alignment. The signature splitting of the intrinsic excitation energies (Routhians) and aligned angular momenta in /sup 159/Tm has been reproduced in cranked shell model calculations by introducing a triaxial deformation. The observed steep rise of the /b B/(/b M/ 1)//b Q//sub 0/ /sup 2/ values in the favored and unfavored bands of /sup 159/Tm are consistent with an increase of |/b g//sub K/-/b g//sub R/| at the firsts backbending due to the alignment of a neutron pair around h omega =0.27 MeV. The evolution of the kinematical and dynamical moments of inertia at very high rotational frequencies ( h omega >0.50 MeV) yields information on the nuclear pairing correlations in this frequency range.