Neutron-proton pairing in the N=Z radioactive fp-shell nuclei $^{56}$Ni and $^{52}$Fe probed by pair transfer

The isovector and isoscalar components of neutron-proton pairing are investigated in the N=Z unstable nuclei of the fp-shell through the two-nucleon transfer reaction (p, 3 He) in inverse kinematics. The combination of particle and gamma-ray detection with radioactive beams of 56 Ni and 52 Fe, produced by fragmentation at the GANIL/LISE facility, made it possible to carry out this study for the first time in a closed and an open-shell nucleus in the fp-shell. The transfer cross-sections for ground-state to ground-state (J=0 + , T=1) and to the first (J=1 + , T=0) state were extracted for both cases together with the transfer cross-section ratios σ ( 0 + ,T=1 ) / σ ( 1 + ,T=0 ) . They are compared with second-order distorted-wave born approximation (DWBA) calculations. The enhancement of the ground-state to ground-state pair transfer cross-section close to mid-shell, in 52 Fe, points towards a superfluid phase in the isovector channel. For the “deuteron-like” transfer, very low cross-sections to the first (J=1 + , T=0) state were observed both for 56 Ni(p, 3 He) and 52 Fe(p, 3 He) and are related to a strong hindrance of this channel due to spin-orbit effect. No evidence for an isoscalar deuteron-like condensate is observed.