The study of chiral bands in triaxially deformed nuclei [1] within the 3d-Tilted Axis cranking Model (TAC) has shown that the aplanar geometry of the angular momentum vector < [I\vec] > exists only in a relatively short spin and frequency interval. This is because the energetical minimum stabilizing such an aplanar orientation is rather shallow and it can be easily driven towards a principal plane. Thus, for higher frequencies the favored a.m. direction returns to the normal (planar) tilt although the triaxial shape may still stay closely at maximal triaxiality g = 30^o. Therefore, the question arises how sensitive is the aplanar spin orientation of the TAC model against the possible quantum interference (tunneling) in between the symmetry-related minima of the potential energy surface. This question goes beyond the TAC mean field model but can be investigated by symmetry restoration. A simple approach to it is the rotor plus particle model (RPM) in which a triaxial rotor core is coupled to two external high-j quasiparticles. The RPM is empirical with regard to the moments of inertia and the coupling to the deformed field which enter in terms of adjustable parameters. However, the above mentioned symmetry restoration of the RPM is by construction exactly carried out since the angular momentum is an exact quantum number of the model. In addition, the calculation of the important transition matrix elements is easily done and it automatically implies good a.m. states too.
Already Meng and Frauendorf [2] demonstrated in a schematic single j-shell RPM that the resulting bands have the chiral doublet pattern as predicted by the TAC model. Below we present the results from a similar calculation obtained with a more solid RPM description [3] in which both the pair and deformation field can be properly treated. This version allows one to study the energetical splitting of the chiral doublets and to calculate the inband and interband transition strength in a realistic manner. In this sense the RPM can be considered as a complementary approach to the TAC model. In Fig. 1 we show the lowest three bands calculated for the coupling of two quasiparticles (ph_11/2nh_11/2) to a rotor (g = 0 and 30^o) for ¹³⁴Pr. Obviously, the chiral structures can not be formed with an axially deformed rotor in agreement with the prediction of TAC.

Fig. 1 Calculated energy E vs. spin I for the three lowest bands of a rotor (left: g=30^o, right: g=0^o ) two quasiparticle system. Only the triaxial rotor displays the chiral doublet structure.

¹ Department of Physics and Astronomy, University of Tennessee, TN 37996, USA
² Physics Department, Tennessee Technological University, Cookville, TN 38505, USA

References

[1]	V.I.Dimitrov, S. Frauendorf and F. Dönau, Phys. Rev. Lett. 84, 5732 (2000)
[2]	S. Frauendorf and J. Meng, Nucl. Phys. A 617, 131 (1997)
[3]	S. E. Larsson, G. A. Leander and I. Ragnarsson, Nucl Phys A 307 189 (1978)
	I. Ragnarsson and P. B. Semmes, Hyperfine Interactions 43, 425 (1988)

 IKH 06/20/01 © F. Dönau