Mass measurements at the N=Z and N=126 limits at the FRS Ion Catcher and development of the Cryogenic Stopping Cell for the Super-FRS

Abstract

Over the past century, nuclear physics has played a vital role in our understanding of the atomic nucleus, its structure and interactions. Most of this knowledge, however, originates from a few hundred nuclei that naturally occur on Earth. One of the ways of testing and improving our understanding is to study versions of nuclei with extreme ratios of neutrons to protons – the so-called exotic nuclei. They exhibit unusual phenomena, and their properties drive processes of creation of elements in the Universe. Exotic nuclei are created in stellar events and in radioactive ion beam (RIB) facilities. The research with exotic nuclei poses major challenges because these nuclei are unstable and can be produced in small quantities only. Furthermore, the more exotic the nucleus is, the larger is the difficulty to reach it. There exists a gap between the nuclei that the scientific community is interested in and the nuclei that are accessible. One prominent instance is the rapid neutron-capture process (r-process), responsible for the creation of approximately half of the nuclei heavier than iron. The nuclei around N=126 which lead to the formation of the third r-process abundance peak (at A≈195) still cannot be accessed in state-of-the art RIB facilities. Therefore, the description of the r-process relies on predictions of theoretical models. The models quite often deviate from true values, and thus require new data to be validated against. On the experimental side, this issue is approached from three perspectives: (i) building more powerful next-generation RIB facilities, (ii) pushing the limits of the existing RIB facilities by improving the instrumentation and detection methods, and (iii) exploring new techniques and reactions for producing the exotic nuclei. The example of the next-generation RIB facility is the Facility for Antiproton and Ion Research (FAIR), which is under construction at the GSI Helmholtz Center for Heavy Ion Research (Darmstadt, Germany). The superconducting fragment separator (Super-FRS) is the central instrument of FAIR’s research program on nuclear structure, astrophysics and reactions. This work contributes to our understanding of atomic nucleus by building an advanced and more powerful detection system, and demonstrating its potential to shrink the mentioned gap between “interesting” and “accessible”. It is centered on a novel cryogenic stopping cell (CSC) for the Super-FRS at FAIR. The CSC converts intense and fast beams of exotic nuclei of all elements produced at the Super-FRS into low-energy beams in a quick and efficient manner, to enable a variety of experiments e.g., mass, decay and laser spectroscopy. In this work, its concepts are developed in detail to ensure the unprecedented performance parameters and maximize the discovery potential of these experiments at the Super-FRS, FAIR. Furthermore, the CSC, as shown in this work, can be used for investigating reaction mechanisms. These include both conventional reactions like fission, projectile fragmentation and promising candidates like multi-nucleon transfer reactions, aimed to produce hard-to-reach very heavy neutron-rich exotic nuclei. The related developments are tested on a prototype of the CSC employed at the FRS Ion Catcher (FRC-IC) setup at GSI, and are part of this thesis. The importance and potential of the system to improve our understanding of nuclear structure and reaction mechanism have been demonstrated in experiments conducted at the FRS-IC. There, the high-accuracy measurements of masses, isomer excitation energies and isomer-to-ground-state ratios were performed at the neutron-deficient and neutron-rich limits of the nuclide chart by the means of a multiple-reflection time-of-flight mass-spectrometry (MR-TOF-MS). The studies carried out in this work include the heaviest N = Z nuclides as they provide an excellent opportunity to probe nuclear shell and mean-fi eld models, the discovery of an isomeric state, and the lightest isotope measured so far at N = 126 as a milestone towards the third abundance peak of the r-process.