F. Barranco

The halo of the exotic nucleus 11Li: a single Cooper pair

Abstract:If neutrons are progressively added to a normal nucleus, the Pauli principle forces them into states of higher momentum. When the core becomes neutron saturated, the nucleus expels most of the wave function of the last neutrons outside to form a halo, which, because of its large size, can have a lower momentum. It is an open question how nature stabilizes such a fragile system and provides the glue needed to bind the halo neutrons to the core. Here, we show that this problem is similar to that of the instability of the normal state of an electron system at zero temperature solved by Cooper, a solution which is at the basis of BCS theory of superconductivity. By mimicking this approach using, aside from the bare nucleon-nucleon interaction, the long wavelength vibrations of the nucleus 11Li, the paradigm of halo nuclei, as tailored glues of the least bound neutrons, we are able to obtain a unified and quantitative picture of the observed properties of 11Li.

Evidence for Phonon Mediated Pairing Interaction in the Halo of the Nucleus 11Li

With the help of a unified nuclear structure–direct reaction heory we analyze the reaction 1H(11Li,9Li) 3H. The two halo neutrons are correlated through the bare and the induced (medium polarization) pairing interaction. By considering all dominant reaction processes leading to t he population of the 1 /2 (2.69 MeV) first excited state of9Li, namely multistep transfer (successive, simultaneous a nd non–orthogonality), breakup and inelastic channels, it is possible to show that the experiment provide s, for the first time in nuclear physics, direct evidence of phonon mediated pairing. PACS: 25.40.Hs, 25.70.Hi, 74.20.Fg, 74.50. +r

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.

Role of finite nuclei on the pairing gap of the inner crust of neutron stars.

The state dependent pairing gap is calculated for a Wigner cell lying in the inner crust of a neutron star, allowing neutrons to interact through the Argonne v14 potential. Although the values of the pairing gap will be renormalized by the induced interaction, the result found, that the pairing gap is sensitive to the presence of the finite atomic nucleus in the sea of free neutrons, is expected to be quite general.

Nuclear effects in the break-up of 11Li

Abstract We study the nuclear processes leading to the break-up of the neutron halo in 11Li on different targets. The reaction mechanisms at work are nuclear diffraction and stripping. It is supposed that they act on a single neutron of the halo, leading to the unstable nucleus 10Li, which then decays in flight. Adding the contribution from Coulomb break-up yields angular distributions in overall agreement with the experimental findings.

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.

Quantum calculation of vortices in the inner crust of neutron stars

The self-consistent mean-field quantum mechanical solution of a vortex and a nucleus immersed in a sea of free neutrons, a scenario representative of the inner crust of neutron stars, is presented for the first time. Because of quantal size effects the phase space for vortices inside the nucleus is essentially zero, so that the vortex core opens up and surrounds the nucleus. As a consequence, pinned configurations (in which a vortex becomes anchored to the nucleus) are favored at low and high densities in the inner crust. This result is qualitatively different from that obtained in all previous models, which predict pinning at intermediate densities.

Vortex–nucleus interaction in the inner crust of neutron stars

Abstract The structure of a vortex in the inner crust of neutron stars is calculated up to density equal to one fourth of the nuclear saturation density, within the framework of quantum mean field theory, taking into account the interaction with the nuclei composing the Coulomb lattice. Vortices are associated with Coopers pairs formed out of single-particle levels of opposite parity and, due to (quantal-size) shell effects, their formation is hindered within the nuclear volume, by an amount that depends on the Fermi energy and on the effective mass associated with the adopted nuclear two-body interaction. When the vortex axis goes through the center of a nucleus, the typical linear rise of the pairing gap away from the vortex axis is delayed by about 8 fm, as compared to the case of a vortex in uniform matter. Also the velocity field is suppressed in a large region close to the interface between the nucleus and the neutron gas. As a consequence, pinning of a vortex on a nucleus leads to a loss of condensation energy, contrary to the predictions of all previous models. This result strongly influences the density dependence of the pinning energy, relevant in the study of glitches. We find that pinning of vortices on nuclei is favoured at low density.

Correlation between mean square radii and zero-point motions of the surface in the Ca isotopes

Abstract The charge and mass mean square radii of the Ca isotopes display an anomalous A-dependence. It is found that it can be correlated with the zero-point fluctuations associated with the low-lying collective states of the different isotopes.

Many-body effects in nuclear structure

Abstract.We calculate, for the first time, the state-dependent pairing gap of a finite nucleus (120Sn) diagonalizing the bare nucleon-nucleon potential (Argonne v14) in a Hartree-Fock basis (with effective k-mass