High-Resolution Experiments with the Multiple-Reflection Time-Of-Flight Mass Spectrometer at the Fragment Separator FRS

Abstract

The study of exotic nuclei far from the valley of stability provides basic information for a better understanding of nuclear structure and the synthesis of the elements in the universe. Nuclear structure and reaction experiments with exotic nuclei have top priorities in research at GSI-FAIR and other modern accelerator facilities.The work of this thesis covered quite different fields of activity. For the present and future performance of the FRS Ion Catcher new experimental developments for the efficient transport and diagnosis of the ions between the Cryogenic Stopping Cell (CSC) and the Multiple-Reflection-Time-Of-Flight Mass Spectrometer (MR-TOF-MS) have been constructed and successfully commissioned. The new diagnosis unit consists of resistive RFQs, an RFQ switch yard, a calibration source and a dedicated RFQ mass filter. A versatile laser ablation calibration ion source has been constructed and commissioned. A novel method of unambiguous isotope identification via atomic range and high-resolution mass measurement (R-m method) has been successfully applied with the combination of the FRS Ion Catcher and the in-flight separator FRS. The R-m method is fast, sensitive and universal and can also be favourably applied for exotic nuclei at low energies when multiple ionic charges states make an unambiguous application of the standard Broh-DeltaE*-TOF method difficult. The advantages of the R-m method have been demonstrated with 155Yb ions. This novel, universal experimental method has a unique potential for the discovery and study of exotic nuclei produced at low kinetic energies. Of course, this statement holds not only for fragmentation products, but is valid for fusion and nucleon-transfer reactions as well. It is obvious that the R-m method is ideal for a combination with in-flight separators.Another powerful experimental method, the spatial separation of pure isomeric and pure isobaric beams with an MR-TOF-MS, was pioneered in measurements with 211g,mPo ions. The high mass resolving power of the MR-TOF-MS combined with the Bradbury-Nielsen Gate (BNG) were the key parts in this application. The conversion from the time coordinates to the spatial coordinates at the MR-TOF-MS was performed with BNG which let the ions of interest pass and deflects the other ions. Based on the isomer-resolved time-of-flight spectrum of 211g,mPo ions, the pulsing of the deflecting voltages on the BNG was adjusted.In the present experiments, the MR-TOF-MS has reached a maximum accuracy of 6E-8 in the measurement with 213Fr ions. The most short-lived nuclide in the present study was 213Rn with a half-life of 19.5 ms. New data analysis methods have been refined and applied to obtain the achieved characteristics for mass spectrometry with an MR-TOF-MS. The MR-TOF-MS installed with the FRS Ion Catcher has presently in the performance a leading role worldwide.Additional to these important experimental-technical developments performed in the frame work of this thesis, exotic nuclei, produced in high-energy fragmentation reactions with 124Xe and 238U projectiles at the entrance of the in-flight separator FRS, were investigated via mass spectrometry. The FRS was operated with different energy-degrader systems placed at the central and final focal planes. Energy bunching and thus a range compression is essential to have an high efficiency for stopping in the gas-filled CSC. The energy bunching is possible with a mono-energetic degrader and the high ion-optical resolving power of the FRS. The experiments gave access to exotic nuclei in the region above the doubly-magic 208Pb nucleus and to neutron-deficient nuclei below the doubly-magic 100Sn nucleus. The measured nuclei are at, or close to the neutron shells of N=50 and N=126.The masses of 15 short-lived nuclei have been measured with MR-TOF-MS. For seven nuclides the ground-state masses were determined in the past only via decay spectroscopy. The analysis and evaluation of the measured masses are confirmed by a comparison with well-known masses from the literature. From these promising results, one can clearly conclude that the FRS Ion Catcher is now well suited to measure new masses over the full range of atomic elements in the coming experiments.The accessible isomeric states were investigated via high-resolution, accurate mass spectrometry. The isomer-to-ground state ratios and the excitation energies have been measured. A new isomeric state has been observed in 97Ag nuclei. The clear identification of the ground states and the isomeric states and the measured excitation energies substantially helps to understand the level schemes of nuclei.The combination of the in-flight fragment separator FRS and the FRS Ion Catcher has opened new fields for the research with exotic nuclei. The new technical developments constructed and commissioned in the framework of this thesis will be applied in future FRS experiments planned for FAIR phase 0.