Experimental Status of Two-Neutron Transfer Reactions in Medium and Heavy Nuclei

The systematics of the strong L=0 transitions observed in two-nucleon transfer reactions in the mass regions near N=126 and N=28 is discussed within the framework of the pairing vibrational model of Bohr. Near 208Pb, the evidence consists of (t, p) and (p, t) data for the even-even Pb nuclei, and for 209Pb and 207T1. Near N=28, the pairing vibrational spectrum is expected to have a characteristic isospin structure. The evidence for this structure is reviewed on the basis of recent (p, t), (t, p) and (3He, p) data, with particular emphasis on the mass-56, mass-52 and mass-48 nuclei, for which the data are most abundant. Tentative identifications of pairing vibrational 0+ states are made for the nuclei 56Ni, 56Co, 52Mn, 52Cr, 50Ti, 48V, 48Ti and 48Ca. In the lighter Ca isotopes, the pairing vibrational strength seems to be fragmentated and the simple zero-order picture breaks.down. It is suggested that the model might be extended to comprise also 2+ excitations built upon the pairing vibrational states. Possible candidates for such pairing-quadrupole states are indicated by (t, p) data for 48Ca, 50Ti, 52Cr, and 209Pb. The energies and intensities of these L=2 transitions are, however, in poor agreement with a zero-order picture and indicate the presence of considerable mixing effects. The success of the model for the classification of strong L=0 and L=2 transitions in two-nucleon transfer reactions near closed shells cannot be assessed in detail from the present scanty data material. Further experiments, which emphasize the measurement of relative transition probabilities in two-nucleon transfer reactions, are strongly indicated. Evidence concerning pairing vibrational transitions near N=50 and N=82 would also be highly desirable. (author)