Alpha-Cluster Study of 40Ca and 44Ti by the (6Li,d) Reaction

Abstract

Alpha-cluster states in 40Ca, 44Ti and 94Mo have been investigated by the 36Ar(6Li,d)40Ca at E = 50 MeV, the 40Ca(6Li,d)44Ti reaction at E = 50 and 37 MeV and the 90Zr(6Li,d)94Mo reaction at E = 37 MeV. Parity-doublet bands in 40Ca and 44Ti have been successfully established, while the trial to obtain the angular distributions of the 90Zr(6Li,d)94Mo reaction was unsuccessful. This report reviews our experimental search for the α-clustering states in A ≥ 40 nuclei. For the 44Ti nucleus, the angular distributions of positive-parity members of the ground-state band (the Kπ = 0+ band) in 44Ti were well reproduced by the finite-range distorted-wave Born approximation calculations up to the transferred angular momentum of L = 8. The members of the negative-parity band (the Kπ = 0− band) with the principal quantum number N = 13 were fragmented into several levels. The summed spectroscopic factors for each spin give reasonable values as the α-clustering structure. The strongly excited state at Ex = 10.86 MeV by the 37 MeV experiment was found to be a candidate for the head of the higher-nodal Kπ = 0+ band with N = 14. For the 40Ca nucleus, the Jπ = 8+ state was found at 10.34 MeV, and the 6.93 MeV level was confirmed as a Jπ = 6+ state. These states are considered as the members of the Kπ = 0+ band starting from the 0+ state at Ex = 3.35 MeV. The members of the negative-parity counterpart in the parity doublet bands were observed, in some fragments, from the 1− state at 8.15 MeV to the 7− state at 13.83 MeV of excitation energy. Members of the Kπ = 2+ band with excited 36Ar core (2+) and α-cluster, were also observed. The 0+, 2+, 4+ and 6+ members of a higher-nodal Kπ = 0+ band with N = 14 were observed. They are also fragmented into strongly populated states in an excitation energy region from the 0+ state at 11 MeV to the 6+ state at 15 MeV. The large spectroscopic factors suggest a much stronger α-clustering character. The existence of a negative-parity band and the completion of the parity-doublet bands in 40Ca and 44Ti give considerable support to α-cluster models for nuclei heavier than A = 40.