FIG. 7. B ( E 1) /B ( E 2) ratios as a function of the initial angular momentum for transitions in the yrast and first negative parity bands. Experimental values (filled circles) are taken from [12]. 2 4 6 8 10 12 14 16 0 2 4 6 8 I B(E1)/B(E2)  ( 10 $_{-6}$fm-2 ) angular momentum (transitions from the state of the ground state band). This is in agree- ment with the experimental data with the exception of two data points at 13 − and 14 + . As it is mentioned in [12] the large value of B ( E 1) /B ( E 2) ratio at 13 − can be attributed to the loss of E 2 strength in the backbending. The rather small B ( E 1) /B ( E 2) value for the 14 + attributed to the spread of E 1 strength due to the presence of two 13 − final states. We suggest a cluster interpretation of the multiple negative parity bands in $^{220}$Th assum- ing collective oscillations of nucleus in mass-asymmetry degree of freedom. This collective motion leads to the admixture of the very asymmetric cluster configurations to the intrinsic nucleus wave function and creates deformations with even- and odd-multipolarities. To take care of reflection asymmetric modes with nonzero values of K , the harmonic quadrupole oscillations of the heavy cluster is considered. The resulting energy spectrum consists of the ground state band and several negative parity bands which exhibit nearly equidistant behavior. The angular momentum dependence of the parity splitting is described. The possibility for the negative values of the parity splitting is related in the model to the inter- IV. CONCLUSION 16