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Study of 34Ca: an unbound, doubly-magic, bubble nucleus?

PhD in experimental nuclear physics and quantum physics

The 34Ca nucleus exhibits several interesting properties. It is expected to be bound with respect to one-proton emission, but unbound with respect to two-proton (2p) emission, making it one of the few 2p emitter candidates in the chart of nuclides. With 20 protons and 14 neutrons, 34Ca can be viewed as one of the few existing doubly closed-shell nuclei. The 34Ca nucleus is the mirror nucleus of 34Si, between which protons and neutrons are interchanged. In 34Si, a proton central density depletion (a bubble) was discovered, which is a unique feature in atomic nuclei. Owing to its mirror property, 34Ca would have a neutron central depletion.

The nuclear force is basically charge-independent. Therefore, the spectrum and the binding energy of an unknown nucleus can usually be derived from the corresponding values in the mirror nucleus, by applying Coulomb corrections. Exceptions exist and break this mirror symmetry, in particular when one of the nuclei of the mirror system is unbound. In such a case, its coupling to unbound states can modify drastically its binding energy and level scheme. The presence of a bubble structure can be an additional (so far unexplored) reason for the breaking in mirror symmetry.

The first motivation of this study is therefore the exploration of the mirror symmetry in this peculiar nucleus. A large contribution of the coupling to the continuum and of the Coulomb field difference could induce a large change in binding energy and possibly the breaking of the magicity in 34Ca. The other motivation relates to the way protons are emitted, if 34Ca is confirmed to be a 2p emitter (one of the purpose of the PhD work). Do they decay as paired protons and hereby witness the pairing configuration inside the precursor nucleus, or do they decay independently from each others?

The proposed route to study 34Ca, e. g. to determine its mass, its spectroscopy and to measure its 2p correlation, is to use the transfer reaction 36Ca(p,t)34Ca at 50 MeV/A in inverse kinematics with a cryogenic H target. Eight 10 x 10cm2 highly-segmented Si and CsI detectors will be placed at forward angles to detect the tritons, while a combination of micro-strip detectors, ionization chamber and plastic scintillator will be used around zero degree to identify the final reaction products and the two protons.

Together with (p,t) reaction, the (p,d) reaction, which has a 20 times higher cross section, can be studied as well, as deuterons are also emitted at forward angles. It follows that besides 34Ca; other unknown nuclei in its vicinity will be studied such as 35,36Ca, 33,34,35K and 35K. Some of them will also encounter a possible breaking of mirror symmetry, which would have important consequence in the determination of proton capture rates occurring during explosive conditions of X-ray bursts (the accretion of a white dwarf on a neutron star).

The experimental work will be in principle done at GANIL in spring or summer 2018. The student will take part in the calibration of the detectors, in the experiment and other experiments using the same detector system. He (she) will then analyse and interpret the data. Other experiments are foreseen in other facilities during the 3 years of the PhD, to which the student will be invited to participate.

 

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