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Pear shape in neutron deficient Xe isotopes

PhD thesis in experimental nuclear physics

The shape of an atomic nucleus is one of its fundamental properties. It results from a delicate balance between microscopically, macroscopically and symmetry effects. The existence of nuclei with stable deformed shapes was realized early in the history of nuclear physics. The observation of large quadrupole moments led to the suggestion that some nuclei might have spheroidal shapes, which was confirmed by the observation of rotational band structures and since such a shape is symmetric under space inversion, all members of the rotational band will have the same parity. Instead, nuclei that present a reflection-asymmetric shape, as for example octupole, will develop low-lying negative-parity states. The study of the octupole shape is of general interest in nuclear physics because it is related to the topics of parity breaking, dipole moment and reflection asymmetry in nuclear matter. The octupole degree of freedom is used in the description of the alpha clustering in light nuclei, asymmetric fission and cluster emission in heavy compound nuclei.

Systematic studies in theoretical models have shown that almost all even-even nuclei present a dynamic octupole correlation in their ground state with correlation energies of 1.5 MeV at most.  Only in very few cases, the contribution of the octupole correlation energy reaches 1 MeV at the mean field level: in the neutron deficient Ra, Rn and 80Zr, in the neutron rich Ba isotopes around A=144 and in the neutron deficient Ba close to 100Sn. The neutron deficient Xe isotopes belong to one of these areas which favor the octupole correlation. Pioneer experimental work in 114Xe have shown an unexpected enhancement of the octupole correlation which cannot be explained by the most advanced theoretical models and suggest a new mechanism inducing large octupole shapes.

In a recent accepted experiment at the GANIL facility, we will address specific aspects of the collectivity development when approaching the N=Z line in light Xe isotopes by performing a precise lifetime measurement of excited states in 112Xe. Through the lifetime of the 5- state and its branching ratios, we will address the question of the enhancement of the octupole correlation (or pear shape) closer to 100Sn.  In addition, the quadrupole degree of freedom will be investigated through the lifetime of the 2+ and 4+ measured for the first time.

The experiment will be performed in 2018 at the GANIL facility as a part of a larger experimental campaign using the advanced AGATA, NEDA and DIAMANT arrays. This large scale experimental setup will be installed in autumn 2017 at GANIL.

The successful candidate will participate in the installation and tests of the setup and will be in charge of the analysis of the experiment.

 

Expected skills: Master in fundamental nuclear physics; C/C++ programming

 

Contact :  Emmanuel CLEMENT

GANIL, BP 5027, 14 076 Caen cedex 05


Tél. 02 31 45 49 08

Fax : 02 31 45 44 21

e-mail : clement_at_ganil.fr



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