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Pairing correlations and nuclear forces towards the drip line

Experimental nuclear physics and quantum physics

 

Pairing correlations 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. 

Despite its tremendous importance, the real observation of the decay of paired, is still lacking or very scarce as difficult to evidence for several reasons. First, neutrons are bound in a potential well, and any mean used so far to take them out of the potential well was subject to complex reaction mechanisms. Second, the observation of four neutrons requires a neutron array of extremely high efficiency, which is not easy to find. Third, when studying the evolution of pairing towards the drip line, we need to produce and study unstable nuclei, with large proton-to-neutron imbalance, which is only possible in few accelerator facilities worldwide.

We have recently discovered an innovating route to study nuclear pairing. It consists of suddenly promoting neutrons of the studied nuclei into the continuum (out of the range of the nuclear force) by using proton knockout reactions at the GSI facility (Germany). The high energy beams provided by the GSI facility (about 500 MeV/A) makes it a unique tool to ensure a quasi-free mechanism that, in the case of proton knockout, somehow leaves the neutrons as they were prior to the reaction. When the protons are removed from deeply bound states, the neutrons are artificially promoted to the corresponding excitation energy, which can be above multi-neutron emission thresholds.

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 existing worldwide LAND. Several nuclei were produced during this experiment, and so far only two cases were studied (and being submitted to publication), with astounding results on the potentiality of the method to derive information on pairing interaction.

In this Master 2 project, the student will analyse (with the help of the PhD student in his third year) other nuclei produced during the same experiment to get familiar with a complex set of detector systems, and to characterize the pairing correlations 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. In this analysis, the study of the spectroscopy of unbound states in the 14B nucleus is also foreseen, with the aim of characterizing for the first time the coupling between bound and unbound states and their role in modifying the effective nuclear forces.

In parallel to this analysis, the student will participate in an experiment at GANIL, either proposed by the group or by another (depending on the agenda), on the study of new phenomena at the nuclear drip line.

It is also possible to work on a theoretical interpretation of the previously obtained results, in collaboration of a theoretician of the team.

 

Expected skills: Ideally good skills in software analysis root, C++, enthusiasm


This internship leads to a PhD thesis.

 


Contact: Olivier SORLIN

GANIL, BP 5027, F-14 076 Caen cedex 05


Phone: +33 (0)2 31 45 45 25

Fax: +33 (0)2 31 45 44 21

e-mail: sorlin_at_ganil.fr


 

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