High precision experiments and decay spectroscopy of exotic nuclei are of great interest for nuclear structure and nuclear astro-physics. They allow for studies of the nuclear structure far from stability, test of fundamental interactions and symmetries and give important input for the understanding of the nuclear synthesis in the universe. In the context of this work a second generation stopping cell for the low energy branch of the Super-FRS was commissioned at the FRS at GSI and significant improvements were made to the device. The prototype stopping cell is designed as a cryogenic stopping cell (CSC), featuring enhanced cleanliness and high area density. The CSC was brought into full operation and its performance characteristics were investigated including the maximal area density, extraction times, cleanliness and extraction efficiencies. In three commissioning experiments at the current GSI FRS facility in 2011, 2012 and 2014 up to 22 isotopes from 14 elements produced by in-flight projectile fragmentation and fission of 238-U could be thermalized and extracted with high efficiency. For the first time projectile and fission fragmentation produced at 1000 MeV/u could be thermalized in a stopping cell and provided as a low-energy beam of high brilliance for high precision experiments. The technical improvements of the CSC, such as an improved RF carpet, new cryocooler-based cooling system, a monitoring system of the cleanliness and the high density operation, made it possible to thermalize heavy 238-U projectile fragments with total efficiencies of about 20% in the 2014 experiment. In addition the improvements lead to an increase in the stability and reliability of the CSC and the performance of the CSC during online experiments at the FRS Ion Catcher showed that the utilized techniques are ready for the final CSC for the low-energy branch of the Super-FRS at FAIR.The CSC was operated with an area density of up to 6.3 mg/cm^2 helium during online experiments, which is about three times larger than any stopping cell, using RF structures for the extraction of ions, has demonstrated. The area density and therefore the stopping power of the CSC is limited by the differential pumping. To overcome this limitation the CSC was tested with neon as a stopping gas with area densities of up to 11.3 mg/cm^2 helium equivalent, demonstrating a unprecedented area density for stopping cells based on RF structures. The RF carpet performed reliably and its potential for the future FAIR stopping cell was shown. During the experiments at GSI the mean extraction time of 221-Ac ions from the CSC to a silicon surface detector was measured, it amounts to 24 ms. This value is well in agreement with offline measurements using a pulsed 223-Ra recoil ion source. The combination of a high density stopping cell with high total efficiencies and a non-scanning high-resolution mass spectrometer can be used as an independent identification detector for exotic nuclei by their mass, allowing a recalibration of the in-flight detectors of any fragment separator. As a proof-of-principal experiment the CSC and a MR-TOF-MS have been used as a mass tagger for the FRS at GSI. 134-I ions were produced by in-flight fission from an 238-U primary beam at 1000 MeV/u and identified by the mass tagger. The new method does not rely on specific decay properties and therefore allows a recalibration of the fragment separator independent of the fragment and can also be used with stable nuclides. The usage of the CSC and a MR-TOF-MS will allow fast recalibration and a more effective usage of the limited amount of beam time for all experiments with exotic nuclei even in the case the nuclide of interest is not clearly identified by the in-flight detection scheme.With the CSC low energy experiments such as high-precision mass measurements and decay spectroscopy were made possible, the half lifes of 221-Ac and 223-Th have been measured, alpha spectroscopy of short lived nuclides (220-Ra, 17.9 ms) were performer. Due to the selective stopping of only one nuclide in the stopping cell the characteristic alphas of 24 nuclides were measured with almost zero background and their Q-alpha-values could be confirmed.Following a new approach, data from gamma spectroscopy, alpha spectroscopy and high resolution mass spectrometry of 211-Po were combined to study the angular momentum distribution arising from in-flight projectile fragmentation. This was possible by measuring the isomer ratio of 211-Po and comparing it to current predictions from the two-step abrasion-ablation model. It was shown that current models can not describe the angular momentum distribution of in-flight projectile fragmentation and that new measurements of isomer ratios are required in order to understand the angular momentum distribution.
Verknüpfung zu Publikationen oder weiteren Datensätzen
