Pairing phase transition and two-nucleon transfer reactions in rotating nuclei

The reaction mechanism associated with two-nucleon transfer reactions induced in heavy-ion collisions and leading to high-spin states in deformed nuclei is discussed making use of realistic form factors. The theory is applied to the study of the pairing phase transition expected to take place as a function of spin in rapidly rotating nuclei. This transition is connected with the alignment of pairs of nucleons at progressively higher rotational frequency. The reaction process is controlled by the size of the matrix element V mixing the ground-state and the s-band. When |V| is smaller than about 200 keV, Coulomb excitation preferably populates the ground-state band. In this case, both the changes in the value of the pairing gap, and in the sign of the pairing-transfer matrix elements are difficult to detect and must be looked for in the cross sections of weakly populated states, which are affected by the largest theoretical uncertainties. However, for large values of the interaction strength, Coulomb excitation tends to populate the yrast band. In this case, one would expect a clearer signal in the cross sections, of the decrease of the pairing gap associated with the band crossing.