High-spin spectroscopy of ^159,160Er

James Ollier and John Simpson

We have continued our experimental programme to study the highest possible angular momentum states by discrete line gamma-ray spectroscopy [1, 2, 3].

Our recent work in the mass 160 region has unearthed a series of collective structures in a range of Er [2], Yb [4] and Tm [5] nuclei.

These studies heralded entry to the regime of ultra-high-spin spectroscopy.

Our most spectacular work was the discovery of rotational bands that bypass the classic band-terminating states [2] and herald a return to collectivity in ^157,158Er.

These rotational structures have characteristics of triaxially strongly-deformed (TSD) bands.

In order to confirm this interpretation and gather an understanding of the nature of highly collective structures in the wider region

(from superdeformation near ¹⁵²Dy, TSD structures in the Lu's and enhanced deformation or triaxial deformation in the mass~170 Hf's)

we recently performed and experiment to study high spin states in ^159,160Er.

This experiment was performed using the Gammasphere spectrometer at Argonne National Laboratory (ANL) in the USA.

This work is a collaboration with the University of Liverpool, Florida State University, ANL, the U.S naval Academy, the University of Tennessee and Lund University.

Background subtracted g-ray spectra showing 3 new bands associated with the decays of ^159,160Er and their Dynamic moments-of-inertia (J^(2)) with bands found in other Er isotopes.

The analysis of this experiment has so far revealed three new bands, one in ¹⁵⁹Er and two in ¹⁶⁰Er.

These bands show similar characteristics in their moments-of-inertia to those observed previously in ^157,158Er [2] which suggest a common underlying structure.

Cranked Nilsson-Strutinsky calculations indicate that they may arise from well-deformed triaxial configurations, extending discrete-line spectroscopy to 60ћ.

[1] J Simpson et al., Phys. Lett. B327, 187 (1994)

[2] E. S. Paul et al., Phys. Rev. Lett. 98, 012501 (2007).

[3] A. O. Evans et al., Phys. Rev. Lett. 92, 252502 (2004).

[4] A. Aguilar et al., Phys. Rev C 77, 021302 (2008)

[5] C. Teal et al., Phys. Rev. C 78, 017305 (2008)