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Shapes and Fission in Heavy Nuclei

Laser-assisted nuclear decay studies at ISOLDE: charge radii, electromagnetic moments and spin measurements with isotopically and isomerically-pure beams in the lead region

As stated in the recent review by K.Heyde and J.Wood (Review of Modern Physics) “Understanding the occurrence of shape coexistence in atomic nuclei is one of the greatest challenges faced by theories  of nuclear structure. We suggest that a major revolution is under way”. This topic of our research strongly contributes to the statement.

The overarching question addressed by this program is: What are the mechanisms behind the sharp shape evolution (for example, shape staggering and strong deformation changes) in atomic nuclei?  In particular, we focus on the influence of crossing the neutron mid-shell at N=104 and closed shell at N=126,  and on a search for signs of octupole deformation  in the charge radii in the vicinity of N=133.

We study  long isotopic chains of the elements of Au-At, both on the neutron-deficient and neutron-rich sides. These exotic nuclei have to be produced at accelerators, are often only available in very small amounts and have in many cases a very short half life. Therefore, very selective methods must be used to extract very scarcely-produced nuclei from the much-more abundantly produced species. In the recent years, we performed successful hyperfine splitting abd isotope shift measurements for the chains of Au, Hg, Tl, Pb, Bi, Po and At isotopes. The unique systematics of charge radii are being produced from these measurements.

The relevant experiments are performed at the mass-separator ISOLDE, see Figure, whereby we employ a range of different detection techniques to identify the nuclei of interest – silicon and germanium detectors for particles decays, MR-ToF mass spectrometer for an additional mass separation and also for ion counting, and a Faraday cup for long-lived/stable isotopes. The laser ionization provides a unique sensitivity for our experiments, via the use of the Resonance Ionization Laser Ion Source (RILIS).

The schematic view (above) of our experiments to study hyperfine splitting (HFS), isomer and isotope shifts and charge radii at ISOLDE. The atoms of the element under study are produced via spallation reactions by 1.5 GeV protons on a thick (50 g/cm2) UC target. After selective ionization by the Resonance Ionization Laser Ion Source (RILIS), the ions are extracted from the source, mass separated and further are counted by one the three methods: Windmill (consisting of Silicon and Ge detectors) – for measurements of nuclear decays; MR-ToF mass spectrometer, and Faraday Cup.