Alpha-cluster study of Ca-40 and Ti-44 by the (Li-6,d) reaction

Alpha-cluster states in Ca-40, Ti-44 and Mo-94 have been investigated by the Ar-36(Li-6,d)Ca-40 at E = 50 MeV, the Ca-40(Li-6,d)Ti-44 reaction at E = 50 and 37 MeV and the Zr-90(Li-6,d)Mo-94 reaction at E = 37 MeV. Parity-doublet bands in Ca-40 and Ti-44 have been successfully established, while the trial to obtain the angular distributions of the Zr-90(Li-6,d)Mo-94 reaction was unsuccessful. This report reviews our experimental search for the alpha-clustering states in A greater than or equal to 40 nuclei. For the Ti-44 nucleus, the angular distributions' of positive-parity members of the ground-state band (the K-pi = 0(+) band) in Ti-44 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 K-pi = 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 alpha-clustering structure. The strongly excited state at E-x = 10.86 MeV by the 37 MeV experiment was found to be a candidate for the head of the higher-nodal K-pi = 0(+) band with N = 14. For the Ca-40 nucleus, the J(pi) = 8(+) state was found at 10.34 MeV, and the 6.93 MeV level was confirmed as a J(pi) = 6(+) state. These states are considered as the members of the K-pi = 0(+) band starting from the 0(+) state at E-x = 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 K-pi = 2(+) band with excited Ar-36 core (2(+)) and alpha-cluster, were also observed. The 0(+), 2(+), 4(+) and 6(+) members of a higher-nodal K-pi = 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 alpha-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 alpha-cluster models for nuclei heavier than A = 40.