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Abstract

Ever since the construction of the Experimental Storage Ring (ESR) at GSI, measurement of the bound-state beta decay of 205Tl81+ ions was one of the main physics cases to be performed. Bound-state beta decay (βb), is a particular weak interaction decay mode, in which the electron remains in a bound atomic state rather than being emitted into the continuum, which is favored in the case of highly-ionized atoms. In this thesis, I report on the first measurement of the bound-state beta decay of fully-ionized 205Tl81+ ions. For the experiment, 205Tl81+ ions were produced via the projectile fragmentation of a primary 206 Pb beam in the FRagment Separator (FRS), which were well separated from the 205Pb81+ contaminants by using the Bρ-∆E-Bρ technique. 205Tl81+ ions were then accumulated, stochastically and electron cooled, and stored for different storage times in the ESR in order to determine the half-life. The obtained value is 229±36 days which agrees within 3σ with the theoretically predicted value of 122 days. The measurement is essential for two physics cases. The first one is linked with the LOREX project (acronym of LORandite EXperiment), wherein, the measurement is needed to determine the nuclear matrix element of the solar pp neutrino capture by the ground state of 205Tl to the 2.3 keV excited state in 205Pb. With the longer measured half-life, the nuclear matrix element will become smaller which is pivotal for the solar pp neutrino studies. The second physics case is associated with the 205Pb/205Tl pair as an s-process cosmochronometer. In stellar medium, 205Tl can exist in the ionized form and βb decay to the first excited state of 205Pb can counter-balance the reduction of 205Pb ions due to the electron capture process. Smaller destruction of 205Tl is expected in the stellar plasmas due to the longer measured half-life of βb decay. This is crucial for the clarification of the fate of 205Pb in the early solar system.