CERN Accelerating science

Thesis
Report number	CERN-THESIS-2018-289
Title	Laser spectroscopy of neutron-deficient mercury isotopes and commissioning of a gas-jet based RFQ ion guide
Author(s)	Sels, Simon Mark C (CERN)
Publication	180.
Thesis note	PhD : Leuven U. : 2018
Thesis supervisor(s)	Van Duppen, Piet
Note	Presented 2018
Subject category	Nuclear Physics - Experiment
Abstract	The neutron-deficient mercury isotopes are situated in the region of the Z = 82 proton shell closure, which is known for its manifestation of shape coexistence. Shape coexistence is the phenomenon in which different degrees or types of nuclear deformation are present at low excitation energy. The mercury isotopes are in fact one of the prime examples of shape coexistence, which is for example shown by the presence of ground-state shape staggering around the N = 104 neutron mid-shell. This staggering was observed by measuring the isotope shift along the chain of isotopes which extended towards the lighter isotopes until mass number A = 181. One of the key questions that remained unanswered was where this staggering phenomenon ends. Several isotopic chains in the lead region that cross the N = 126, show a strong deviation from the trend of the lighter isotopes at the neutron shell closure. No such measurements of this ‘kink’ were performed so far in the lead region below Z = 82. The first part of this thesis reports on an in-source laser-spectroscopy experiment performed at the ISOLDE radioactive ion-beam facility in CERN. These were the first measurements of the hyperfine structure and isotopes shifts in the mercury isotopes below A = 181 and also the first to go across N = 126 on the neutron-deficient and neutron-rich sides of the chain respectively. The mercury radioactive ion beams were produced by impinging a 1.4 GeV proton beam on a molten-lead target. The reaction products were extracted, resonantly ionized using a three-step laser-ionization scheme, mass separated and sent to one of three detection setups. Strongly-produced ions were measured on a Faraday Cup. The α and γ decay of short-lived and weakly-produced isotopes ( 177 − 182 Hg) was measured at the Windmill setup with silicon and germanium detectors. These decay modes were used to identify the isotopes of interest from the contaminated ion beam. Isotopes undergoing mainly β decay 182 − 185, 202, 202, 206 − 208 Hg were identified using their time-of-flight in a Multi-reflection Time-of-Flight mass spectrometer. v vi ABSTRACT From the results, both the end-point of the shape staggering at A = 180 and the observation of a charge-radius kink at N = 126 are confirmed. The results of the neutron-deficient isotopes were compared to state-of-the-art theoretical calculations using either Density Functional Theory (DFT) or the Monte-Carlo Shell Model (MCSM). The DFT calculations are able to qualitatively reproduce a ground-state staggering by tuning different functional parameters. By calculating the experimentally-observed spin states, the MCSM results reproduce the shape-staggering and electromagnetic moments of the isotopes. In the second part of this thesis, the design, simulation and commissioning of radio-frequency quadrupole (RFQ) ion guides are discussed. In the commissioning tests of the RFQ ion guides, the transport efficiency, longitudinal emittance, beam energy spread and time profiles were measured. These results were compared to simulations performed with SIMION and IonCool software packages. The ion guides form part of an experimental setup that is dedicated to the development of in-gas(-jet) laser-ionization spectroscopy. This technique makes use of the properties of a supersonic gas jet to produce an ensemble of atoms that is suited for high-resolution laser-spectroscopy studies. Besides this, the technique is also suited for the creation of exotic beams of radioactive ions, when it will be installed at facilities using in-flight production of isotopes.

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