Return of Collective Rotation inEr157andEr158at Ultrahigh Spin

Abstract

A new frontier of discrete-line gamma-ray spectroscopy at ultrahigh spin has been opened in the rare-earth nuclei (157,158) Er. Four rotational structures, displaying high moments of inertia, have been identified, which extend up to spin approximately 65 variant Planck's over 2pi and bypass the band-terminating states in these nuclei which occur at approximately 45 variant Planck's over 2pi. Cranked Nilsson-Strutinsky calculations suggest that these structures arise from well-deformed triaxial configurations that lie in a valley of favored shell energy which also includes the triaxial strongly deformed bands in (161-167) Lu.

Description of158Er at ultrahigh spin in nuclear density functional theory

“…If this configuration assignment is adopted, band 2 should be observed in spin range I = 44 − 66, which again is higher than experimental estimates of Ref. [11].…”

“…Similar to the case of TSD3(a), the increase of J (2) at low frequencies predicted for TSD3(b) is due to an unpaired band crossing with a strong interaction between the ν1/2[770](r = +i) and N = 5 (r = +i) orbitals. If this assignment is adopted for band 2, we must conclude that this structure is observed in a spin range I = 46−68, which again exceeds the experimental estimate [11] by 8h. According to CRMF calculations, an alternative configuration for band 1 can also be suggested.…”

“…Of particular interest are the bands recently observed at ultrahigh spins in the A ∼ 154 − 160 mass region. These are the bands seen in 154 Er [10], 157,158,159,160 Er [11][12][13][14][15], 157 Ho [12], and 160 Yb [16]. As discussed below, the interpretation of these structures is still under debate.…”

“…Note, however, that DFT calculations yield γ−deformations that are typically smaller in absolute value than the ones obtained in CNS.) The early interpretation of observed TSD bands have invoked configurations built either upon the TSD1 minimum [10,11] or TSD1 and/or TSD2 minima [13].…”

“…A high-fold analysis of the intensity profiles at the bottom of band 1 in 158 Er, compared to feeding intensities into the known yrast states, has allowed an estimation of the highest spin reached by this band to be ∼ 65h [11]. At present, the uncertainty of this procedure is not obvious, however, the experimental error on spin assignment can be larger than 4h [31].…”