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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 existence of quasi-bound tetra-neutron resonances, formed as an ensemble of four interacting neutrons, was proposed on the basis of experimental results. This existence is in stark contrast with most of the state-of-the-art models including the most recent ones using ab-initio forces. The coupling of four neutrons can also play a significant role inside atomic nuclei, but this has not yet been studied experimentally.  If confirmed, tetra-neutron excitations would require 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 its tremendous importance, the real observation of the decay of paired or tetra nucleons, 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 A.MeV) 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, with astounding results on the potentiality of the method to derive information on pairing interaction. The PhD thesis will be composed of 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. In parallel and in a second step, the PhD Student will work on a new experiment, which is foreseen to be schedule in Spring 2019. This experiment will make use of an upgraded version of the neutron detector (neuLAND) that will allow for the detection of di- and tetra-neutron decays in an unprecedented manner. The student will be involved in the simulations related to the detection of neutrons with this new detector, in the mounting and in the calibration of the detectors. He will participate to other experiments related to this topic and performed with the same beam line in in other facilities. He will be in charge of the analysis and interpretation of data related to the search of di and tetra-neutron decay in close-to-drip line nuclei.


Expected skills good skill in software analysis, C++, enthusiasm

 


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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