CERN Accelerating science

Thesis
Report number	CERN-THESIS-2013-235
Title	Study of Neutron-Deficient $^{202-205}$Fr Isotopes with Collinear Resonance Ionization Spectroscopy
Author(s)	De Schepper, Stijn (Leuven U.)
Publication	66.
Thesis note	Master : Leuven U. : 2013-06-31
Thesis supervisor(s)	Neyens, Gerda ; Cocolios, Thomas ; Budincevic, Ivan
Note	Presented 31 Jun 2013
Subject category	Nuclear Physics - Experiment
Accelerator/Facility, Experiment	CERN ISOLDE
Abstract	The scope of this master’s thesis is the study of neutron-deficient $^{202−205}$Fr isotopes. These isotopes are inside the neutron-deficient lead region, a region that has shown evidence of shape coexistence. For this thesis, this discussion is limited to the phenomenon where a low lying excited state has a different shape than the ground state. Shape coexistence is caused by intruder states. These are single-particle Shell Model states that are perturbed in energy due to the interaction with a deformed core. In the neutron-deficient lead region the main proton intruder orbit is the 3s$_{1/2}$orbit. When going towards more neutron-deficient isotopes, deformation increases. The $\pi3s_{1/2}$orbit will rise in energy and will eventually become the ground state in odd- A bismuth (Z=83) isotopes. It is also observed in odd-A astatine (Z=85) isotopes, already in less neutron-deficient nuclei. The same phenomenon is expected to be present francium (Z=87) isotopes already at $^{199}$Fr. Although it is currently impos- sible to study the moments of such an exotic nucleus, it is important to characterize the evolution of nuclear structure in the francium chain towards more neutron- deficient isotopes. The main neutron intruder orbit is the 1i$_{13/2}$orbit. Deformation drives this orbit to raise in energy. Due to the large spin-gap and the difference in parity with the other Shell Model orbits in this shell, this orbit is cause for long-lived excited states. The nuclear observable of interest to study Shell Model states is the magnetic mo- ment and characterization of the evolution of the nuclear shape can be done us- ing charge radii. Using the newly developed Collinear Resonance Ionization Spec- troscopy beam line these model-independent observables are extracted using the hyperfine interactions between atomic electrons and the nucleus. The observables are used to extract information on occupation of single-particle Shell Model orbits and on the deformation of the nuclei. Using CRIS it is also possible to selectively ionize pure isomeric beams. This way the decay characteristics are used to disen- tangle the hyperfine structure spectra. Spins and configurations are already proposed in literature for $^{202−205}$Fr and their isomers, mainly based on decay systematics. In this work the results from literature are confirmed for most isotopes and isomers. $^{203}$Fr and $^{205}$Fr both have spin 9/2 and configuration $\pi1h_{9/2}$as expected. The high-spin isomeric states in $^{202}$Fr and $^{204}$Fr have been confirmed to have a pure single-particle $[\pi1h_{9/2}\otimes \nu1i_{13/2}]_{10-}$−configuration. The second isomeric state in $^{204}$Fr was confirmed to have a pure single-particle $[\pi1h_{9/2}\otimes \nu2f_{5/2}]_{7+}$ configuration. In contrary to what was proposed for the ground states of $^{202}$Fr and $^{204}$Fr we find evidence that suggests that the configuration is an admixture of $[\pi1h_{9/2}\otimes \nu3p_{3/2}]_{3+}$ and $[\pi1h_{9/2}\otimes \nu2f_{5/2}]_{3+}$, where the former is the dominant contribution. For the charge radii a departure from the spherical trend in Pb is observed earlier than the departure in Po. This could prove that adding protons to the nucleus polarizes the nucleus more strongly. From the charge radii it can also be concluded that in $^{204}$Fr the (3+) ground state and the (10−) have a different shape and are thus an example of shape coexistence
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