The nuclei 218,220,222Th were investigated by conversion electron and y-ray spectroscopy after compound nucleus reactions of a4N with 2~ and 26'180 with 2~ The intense y-background from fission was suppressed by spectroscopy of conversion electrons and y-rays in coincidence with the evaporation residues or their e-decay. Level schemes were determined for 218Th up to U=10 + and for 22~ and 222Th up to U =15-. The observed structure of 218Th may be explained in the spherical shell model with residual interaction by two neutron excitation. For N>130 completely different level schemes are observed showing very intense, collective electric dipole transitions (B(E1)_~ 10 -2 W.u.) with energies of typically 200 keV. Interpretation in terms of stable octupole deformations of the ground state as well as in an cz-cluster picture are discussed.
The groundstate and a negative parity band of 222Th have been observed in the 2OBpb (l~o, 4n) 222Th reaction by conversion electron and Tray spectroscopy identifying the evaporation residues in a recoil separator. The de-excitation pattern is characterized by strong El transitions interconnecting the levels of alternating parity. The yrast levels may be interpreted as a molecularcluster band.For the light actinides (Rn, Ra and Th) a steady transition is observed in the ground state band from single particle to strongly deformed character when adding neutrons to the closed shell of N=126. Rather low lying bands of negative parity also are identified in these nuclei. Many contradicting pictures, e.g. static octupole deformationi), octupole vibrations 2) or molecular-cluster models 3) have been developed to describe the negative parity states. In this short contribution we report on the investigation of 222Th, where the expectations of themolecular cluster model can be verified. For 222Th no excited levels have been known until now.The direct observation of excited states of evaporation residues (ER) after the 208pb (180,4n) 222Th reaction is difficult due to the large background from prompt fission and heavy ion transfer reactions. As shown in fig.l the ER are separated in an electrostatic field of 55 kV from fission fragments and projectiles and are detected in a 58x34 mm 2 surface barrier counter. The good separation results from the high charge states > 4-. .
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