Neutron transfer reactions at large internuclear distances studied with the PRISMA spectrometer and the AGATA demonstrator, D. Montanari, Università degli Studi di Padova - INFN Padova, Italy − Two-nucleon transfer reactions play a key role in investigating correlations between nucleons in nuclei.  With heavy ion reactions, multiple transfer of nucleons become available, giving the possibility to compare the relative role of single particle and pair transfer modes [1].  Below the Coulomb barrier, nucleons are transferred in a restricted excitation energy window and, at large internuclear distances, the interacting nuclei are slightly influenced by the nuclear potential.  These conditions allow to diminish the complexity of theoretical calculations and thus to extract more quantitative information on pair correlations [2,3].

Using the large solid angle magnetic spectrometer PRISMA, at the Laboratori Nazionali di Legnaro, a first reaction at sub-barrier energies has been performed in inverse kinematics for the closed shell system ⁹⁶Zr+⁴⁰Ca [4]. An excitation function ranging from above to well below the Coulomb barrier has been measured and transfer probabilities [5] have been extracted for the neutron transfer channels.  The comparison between data and microscopic calculations shows the importance played by transitions to 0+ excited states and of states with high multipolarity and non natural parity.

As a next step, we recently measured an angular distribution of the transfer channels in the reaction ⁶⁰Ni+¹¹⁶Sn, using PRISMA coupled to the AGATA demonstrator gamma array.  In this superfluid system the ground state Q-values for the neutron transfer channels are close to their optimum Q-values.  It is therefore interesting to study the behavior of the transfer probabilities compared with the previously measured closed shell system and to compare the results with the same kind of theoretical calculations.

On the basis of an event-by-event reconstruction of the ion trajectories inside the PRISMA spectrometer [6], the projectile-like fragments have been fully identified in atomic number Z, mass A and Q-value, leading to the determination of the transfer cross sections. The AGATA demonstrator, whose detected gamma rays are Doppler corrected taking into account the vector velocity measured with PRISMA, has been employed to extract the transfer strength to excited states.

In this talk the results of these recent measurements will be presented, and a discussion will be made on the possibilities offered in the field by exploiting large solid angle spectrometers.

[1] R.A. Broglia and A. Winther, "Heavy Ion Reactions", Addison-Wesley Pub. Co., Redwood City CA, 1991.

[2] B. F. Bayman and J. Chen,  Phys. Rev. C26, 1509 (1982).

[3] G. Potel, F. Barranco, E. Vigezzi and R. A. Broglia, Phys. Rev. Lett. 105, 172502 (2010).

[4] L. Corradi et al., Phys Rev. C 84, 034604 (2011).

[5] W. von Oertzen and A.Vitturi, Rep. Prog. Phys. 64, 1247 (2001).