Triaxility and chiral bands in nuclei

Chirality in triaxial nuclei is a topic of current interest in nuclear structure, being intensively investigated both theoretically and experimentally. It is characterized by the presence of three angular-momentum vectors, which are noncoplanar. The observation of two almost degenerate DeltaI=1 rotational bands with the same parity, of which the first example was the doublet bands in 134Pr, has been so far taken as a sign of chiral bands. Calculations in the framework of the 3D tilted axis cranking (TAC) model explain the doublet bands as result of the existence of chirality in the intrinsic frame of the nucleus. However, a critical and detailed analysis of the experimental information on the observed nearly degenerate bands in the N=75 isotones, in particular 134Pr and 136Pm, which are often considered as the best candidates for chiral bands, reveal features which are in clear disagreement with the interpretation of the two bands as chiral bands. For I = 14-18 in 134Pr, where the observed energies are almost degenerate, we have obtained a value of 2.0(4) for the ratio of the transition quadrupole moments of the two bands. It is therefore questionable if and to what extent a reminiscence of the chiral geometry is present in the 134Pr data, considering the substantial shape difference documented between the bands.