High Spin Physics 2001, Warsaw, February 6-10, 2001



Abstract No: 023
Submitted on: 30 Dec 2000, 16:22 GMT
Title: Fast nuclear rotation and octupole deformation
Author(s): W. Urban1, W.R. Phillips2, M.K. Khan2, J.C Bacelar3, J. Jongman3, N. Schulz4, I. Ahmad5 and J. Nyberg6
Affiliation(s): 1Institute of Experimental Physics, Warsaw University, Warsaw, Poland 2Dep. of Physics and Astronomy, Manchester University, Manchester, UK 3KVI, University of Groningen, Groningen, The Netherlands

4Inst. de Recherches Subatomiques, Univ. Louis Pasteur, Strasbourg, France 5Argonne National Laboratory, Argonne, USA

6 NBI Riso, Roskilde, Denmark



Enhanced octupole interactions, which may cause octupole deformation, are expected in nuclei, in which opposite-parity orbitals, satisfying ${\Delta}j$=${\Delta}l$=3 relation, are placed close to the Fermi level. One member of the ${\Delta}j$=${\Delta}l$=3 pair is the high-j intruder orbital. Such orbital plays an important role in generating spin of a fast rotating nucleus and is a source of a characteristic backbending phenomenon. An octupole interaction provides an extra force binding the intruder orbital to the so called normal-parity orbitals, changing thus its response to fast nuclear rotation. Observation of spin alignment as a function of rotational frequency in nuclei with strong octupole interactions provides, therefore, a convenient tool to study octupole correlations in nuclei[1].

Standard example of such behaviour is that reported for the 222Th nucleus [2], where instead of a sharp backbanding, expected for the proton i13/2 intruder orbital, a gradual increase of alignment is observed. Calculations assuming the presence of octupole deformation in 222Th are in excellent agreement with the experiment [3] up to the highest observed spin, I=22. The same calculations predict that at still higher rotational frequency, the neutron j15/2 intruder orbital will allign, displaying a sharp backbending, since the alignment of two intruder orbitals should be enough to restore reflection symmetry in a nucleus.

Rotational frequencies observed in 222Th are too low to observe the predicted transition from octupole to reflection-symmetric shape. Higher rotational frequencies are observed in another region of strong octupole correlations, in the neutron-rich Lanthanides [4]. The 150Sm nucleus [5] displays a behaviour similar to that observed for 222Th, yet here seen to higher rotational frequencies. Studies of the neighbouring odd-A nuclei provide a unique identification of the observed alignment process in 150Sm, where after the second, sharp backbanding a reflection-symmetric shape is restored.

Bibliography

1
P.A. Butler and W. Nazarewicz, Rev. Mod. Phys. 68, 349 (1996)

2
M. Dahlinger et al, Nucl. Phys. A 484, 337 (1988)

3
W. Nazarewicz, Nuclear Structure Through Static and Dynamic Moments, ed. H.H. Bolotin (Conference Proceedings Press, Melbourne, 1987), p. 180.

4
W.R. Phillips et al, Phys. Rev. Lett. 57, 3257 (1986)

5
W. Urban et al, Phys. Lett. B 185, 331 (1987)

\fbox{ Invited talk. }

High Spin Conference
2000-12-31