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Insight in nuclear superfluidity from di-neutron and tetra-neutron structures

PhD in experimental nuclear physics

Pairing interactions play crucial roles in atomic nuclei and in quantum many-body physics in general. Restricting first to finite nuclei, two-neutrons and or two-protons pairing are responsible for the odd-even staggering observed in the binding energy of atomic masses for the fact that all even nuclei have a J=0+ ground state, and for their small moment of inertia as compared to a rigid body. More generally, pairing correlations imply a smoothing of the level occupancy around the Fermi energy surface, an enhancement of pair transfer probabilities as well as a superfluid behaviour in the nuclear rotation and vibration. Transition from BCS (Bardeen Cooper-Schrieffer) to BEC (Bose-Einstein Condensation) pairing correlations has been evoked from the modelling of the interior to the surface, respectively, of the neutron-rich nuclei 11Li , 6He and 18C. Enlarging to nuclear matter, pairing plays a major role in the modelling of the rotation, the magnetization and the cooling of the neutron stars.  Recently, the formation of tetra-neutron resonances, either from an ensemble of four interacting neutrons or from the coupling of four neutrons inside atomic nuclei was proposed on the basis of experimental results. If confirmed, tetra-neutron excitations would require as a higher range of (four-body) nucleon interactions with expected important consequences in the description of finite nuclei, of neutron stars and in the determination of neutron-captures in the big-bang and in neutron-star mergers. Despite of its tremendous importance, the real observation of the decay of paired or tetra nucleons (four) is still lacking or very scarce as difficult to evidence.

We recently discovered an innovating route to study nuclear pairing by promoting suddenly neutrons to the continuum in using quasi-free knockout reaction at the GSI facility (Germany). Once in the continuum, we deduce their relative distance and correlation inside the nucleus, from the study of their 3-body decay.  This is obtained from the measurement of the quadri-vectors of the incident and the residual nuclei in a spectrometer, as well as of the neutrons, detected in the most efficient detector worldwide LAND. Several nuclei were produced during this experiment, and so far only two cases were studied, with astounding results on the potentiality of the method to derive information on pairing interaction.

The PhD thesis will be composed in two steps. In the first, the student will analyse other nuclei produced during the same experiment to get familiar with a complex detector system, and to characterize the pairing interaction in atomic nuclei from the study of several cases, i.e. as a function of the binding energy of the system, the orbitals, and the production method. It is planned to propose in 2017 a new experiment, to be scheduled in 2018, using an upgraded version of the neutron detector (neuLAND) that will allow for the detection of tetra-neutron decays in an unprecedented manner. In the second step, the student will participate in the simulations related to the detection of neutrons with this new detector, in the development of a method to derive 4-neutrons correlations, and in the analysis of the experiment.


Expected skills good skill in software analysis, enthusiasm

 

Contact :  Olivier SORLIN

GANIL, BP 5027, 14 076 Caen cedex 05


Tél. 02 31 45 45 25

 Fax : 02 31 45 44 21 

e-mail : sorlin_at_ganil.fr



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