Developments for multiple-reflection time-of-flight mass spectrometers and their application to high-resolution accurate mass measurements of short-lived exotic nuclei

Masses are a key property of atomic nuclei, their accurate measurement helps to understand the nuclear structure and the stellar nucleosynthesis. Driven by the need to access nuclear masses of very exotic nuclei, where production rates are low (few per hour or day) and half-lives are short (few ms), a measurement technique, Multiple-Reflection Time-of-Flight Mass-Spectrometry (MR-TOF-MS), has been developed at different rare isotope beam facilities around the world. In the context of this work, improvements of the system at FRS/GSI have been done, enabling mass measurements of the most exotic and short-lived nuclei with the highest and unprecedented accuracy. The kinetic energy of the ions and the repetition rate of the spectrometer were increased, obtaining resolving powers in excess of 600.000 in less than 20 ms of measurement time and repetition rates exceeding 1000 Hz. In addition, an increase of the reliability and stability of the system, lowering the temperature coefficient of the spectrometer to 8 ppm/°C, has been shown. The system is universal and can measure elements independent of their chemical properties, demonstrated by measuring noble gases and very reactive elements like uranium. Moreover, the data analysis procedure to determine the mass value and the uncertainty was further improved. The method was specially developed to precisely extract the masses of very low statistics peaks and of overlapping peaks when only a change in peak shape can be observed. With this data analysis procedure, the effective mass resolving power is increased by up to a factor of 3 for overlapping peaks, e.g. the resolution of low-lying isomers. During three experiments at GSI (Germany) in 2014 and 2016, mass measurements of thermalized fission fragments of ${}^{238}$U as well as thermalized projectile fragments of ${}^{238}$U and ${}^{124}$Xe, produced and separated in the FRS, were performed with the MR-TOF-MS of the FRS-Ion Catcher with resolving powers up to 450.000. Following the improved data analysis procedure presented in this work, a total of 25 masses of shortlived nuclei were measured, of which 7 correspond to isomeric states with excitation energies down to about 280 keV. Isotopes of 8 different elements with half-lives of less than 300 ms have been measured. The reported masses obtained in this thesis with the MR-TOF-MS of the FRS-Ion Catcher have a minimum total relative uncertainty of 1.1 $\cdot$ 10${}^{-7}$, corresponding to an absolute value of 13 keV. The lowest uncertainty of a mass measurement performed with at the FRS-Ion Catcher with the improvements presented in this thesis is 6$\cdot$10${}^{-8}$ [Hornung, 2018]. The average deviation from the literature of all the measured masses in the FRS-Ion Catcher is of 4.3 $\pm$ 2.5 keV, showing no systematic deviation. Besides direct mass measurements, the broadband capabilities of the system in combination with a high resolving power were used for the identification via mass measurements of uranium ions and molecules in different charge states. This feature was applied to the study of the cleanliness of the system. The FRS-Ion Catcher is now fully operational and offers a superior combination of performance characteristics for measurements with exotic nuclei in unknown territory. In an experiment at TRIUMF (Canada) in 2017, mass measurements of neutron-rich gallium isotopes from A = 80 to A = 85 were performed with the newly installed TITAN's MR-TOF-MS, developed, built and commissioned at the Justus-Liebig University. Recently the electronics have been improved in the same way as for the MRTOF- MS at FRS/GSI. The masses of ${}^{84}$Ga and ${}^{85}$Ga, with half-lives of 85 ms and 92 ms, respectively, were measured for the first time in the context of this thesis. A minimum relative uncertainty of 3.3 $\cdot$ 10${}^{-7}$ was obtained. The mass measurements of ${}^{84}$Ga and ${}^{85}$Ga have a high impact on the understanding of the nucleosynthesis processes in the neutron star merger event observed by LIGO/VIRGO and followed by a blue kilonova transient, as they pin down the nuclear physics input for ${}^{82}$Se which can only be produced in the r-process. A summary of the nuclides of which the mass of ground, isomer or both states was presented in this thesis is shown in Fig. 4.1. On the way towards the new rare isotope beam (RIB) facilities such as the low energy branch (LEB) of the Super-FRS at FAIR or the advanced rare isotope laboratory (ARIEL) in TRIUMF, several experiments at GSI and TRIUMF are planed in the shortterm horizon for the spectrometers presented in this thesis, where high-accuracy mass measurements of exotic nuclei, enabled by the developments presented in the thesis, will be performed. The near-future experiments are: - In the FRS-Ion Catcher at GSI, new experiments will be carried out during the beamtime periods allocated in FAIR Phase-0 in 2018 and 2019. The MR-TOFMS will be used for direct mass measurements of exotic nuclei: around the N = Z region, below ${}^{100}$Sn [Plaß et al., 2018] and around the neutron-rich isotopes below ${}^{208}$Pb with N=126 (between Z = 65 to Z = 75) [Pietri et al., 2018]. The MR-TOF-MS will be used to perform the identification and counting of reaction products of nuclei undergoing a $\beta$-delayed neutron emission to obtain the $\beta$-delayed neutron emission probabilities [Mardor et al., 2018], which besides masses and half-lives, is the next important nuclear quantity for modelling the r-process abundances. Another experiment aims to perform multi-nucleon transfer (MNT) reaction studies and identify the reaction products via high precision mass measurements [Dickel et al., 2018], filling the need of experimental data for the models describing such reactions. - In TRIUMF, during the beamtime period of 2018, the MR-TOF-MS has scheduled a beamtime where mass measurements of neutron-deficient lanthanides will be performed. These high precision mass measurements will allow to identify the possible existence of a quenching effect of the N = 82 shell closure in neutron-deficient nuclei. The combination of the next generation RIB facilities with increased production rates of exotic nuclei, such as the LEB at FAIR, and a further developed MR-TOF-MS as part of the MATS (precision Measurements of very short-lived nuclei using an Advanced Trapping System for highly-charged ions) collaboration, with mass resolving powers exceeding one million and relative mass uncertainties in the low 10${}^{-8}$ level routinely achieved, will extend the mass measurements landscape over the neutron-rich area, gaining key information for understanding the structure of the nuclei and the synthesis of heavy elements in the universe.