Description of even-even triaxial nuclei within the Coherent State and the Triaxial Rotation Vibration Models

The Coherent State Model (CSM) and the Triaxial Rotation-Vibration Model (TRVM) are alternatively used to describe the ground, γ and β bands of 228 Th. CSM is also applied to the nuclei 126 Xe and 130 Ba, which were recently considered in TRVM. The two models are compared with respect to their predicted results for energy levels and E2 branching ratios. The TRVM was applied to the nuclei 126 Xe and 130 Ba and the obtained results were in equally good agreement with available data as in Interacting Boson Approximation (IBA). A short while ago, new data for 228 Th were obtained, according to which, 228 Th behaves as a nucleus without axial symmetry. Since the first successful applications of CSM referred to the Pt region, which corresponds to the O (6) symmetry in the IBA interpretation, it is expected that triaxial nuclei like 126 Xe, 130 Ba and 228 Th can be realistically described by CSM. We furthermore apply the TRVM for the new data of 228 Th. We are not only interested in the predictions of the two models but we would also like to point out resemblances and differences of their theoretical ingredients. The two models yield similarly good results concerning the excitation energies and transition probabilities for 228 Th. Moreover, they are at par concerning the interpretation of the 03+ band as the beta band. For the remaining two nuclei the considered models are at variance with respect to the interpretation of the first excited 0+ band. Indeed, the TRVM describes the first excited Kπ = 0+ band as a gamma band, i.e. build upon a gamma vibration state, while within the CSM this is a beta band. This difference in interpretation has an echo in the predicted branching ratio characterising the states of this band. One suggests that by including anharmonicities in the TRVM Hamiltonian, the discrepancies for branching ratios might be removed. (authors)