Andrea Idini

Cooper pair transfer in nuclei

The second-order distorted wave Born approximation implementation of two-particle transfer direct reactions which includes simultaneous and successive transfer, properly corrected by non-orthogonality effects, is tested with the help of controlled nuclear structure and reaction inputs against data spanning the whole mass table, and showed to constitute a quantitative probe of nuclear pairing correlations.

Calculation of the transition from pairing vibrational to pairing rotational regimes between magic nuclei ¹⁰⁰Sn and ¹³²Sn via two-nucleon transfer reactions.

Absolute values of two-particle transfer cross sections along the Sn-isotopic chain are calculated. They agree with measurements within errors and without free parameters. Within this scenario, the predictions concerning the absolute value of the two-particle transfer cross sections associated with the excitation of the pairing vibrational spectrum expected around the recently discovered closed shell nucleus(50)(132)Sn(82) and the very exotic nucleus (50)(100)Sn(50) can be considered quantitative, opening new perspectives in the study of pairing in nuclei.

Nuclear field theory predictions for 11Li and 12Be: Shedding light on the origin of pairing in nuclei

Recent data resulting from studies of two-nucleon transfer reaction on 11Li, analyzed through a unified nuclear-structure-direct-reaction theory have provided strong direct as well as indirect confirmation, through the population of the first excited state of 9Li and of the observation of a strongly quenched ground state transition, of the prediction that phonon-mediated pairing interaction is the main mechanism binding the neutron halo of the 8.5-ms-lived 11Li nucleus. In other words, the ground state of 11Li can be viewed as a neutron Cooper pair bound to the 9Li core, mainly through the exchange of collective vibration of the core and of the pigmy resonance arizing from the sloshing back and forth of the neutron halo against the protons of the core, the mean field leading to unbound two-particle states, a situation essentially not altered by the bare nucleon-nucleon interaction acting between the halo neutrons. Two-neutron pick-up data, together with (t, p) data on 7Li, suggest the existence of a pairing vibrational band based on 9Li, whose members can be excited with the help of inverse kinematic experiments as was done in the case of 11Li(p, t)9Li reaction. The deviation from harmonicity can provide insight into the workings of medium polarization effects on Cooper-pair nuclear pairing, let alone specific information concering the “rigidity” of the N = 6 shell closure. Further information concerning these questions is provided by the predicted absolute differential cross sections σabs associated with the reactions 12Be(p, t)10Be(g.s.) and 12Be(p, t)10Be(pv) (≈10Be(p, t)8Be(g.s.)). In particular, concerning this last reaction, predictions of σabs can change by an order of magnitude depending on whether the halo properties associated with the d5/2 orbital are treated selfconsistently in calculating the ground state correlations of the (pair removal) mode, or not.

Pairing Interaction and Two-Nucleon Transfer Reactions

Soon after the formulation of BCS theory [1], it was recognized by Bohr, Mottelson, and Pines that the existence of an energy gap in the intrinsic excitation spectrum of deformed nuclei displayed a suggestive analogy with that observed in the electronic spectra of metallic superconductors and could, like this one, be described at profit in terms of correlated pairs [2]. Their paper represented the starting point of more than fifty years of experimental and theoretical BCS flavored studies of pairing in nuclei [3].

Core polarization and neutron halos

The effects of core polarization on the structure of Li and Be halos and their consequences on one-and two-neutron transfer reactions are discussed.

Dynamical processes in the structure of halo nuclei and their experimental evidence

High polarizability and halo phenomena are strongly correlated, as evidenced by the qualitative changes of the nuclear spectrum which arise taking into account the dynamical coupling of the valence neutrons with the collective vibrations of the system. Direct experimental evidence of their role in the induced pairing interaction binding the halo neutrons of 11Li to the core 9Li is provided by the absolute cross section of the reaction 1H(11Li,9Li)3H to the first excited state of 9Li.