Observation of Bound-State ¯ ¡ Decay of Fully Ionized 207Tl at the FRS-ESR

The bound state ¯ ¡ decay (¯¡b decay) is a particular decay mode of the weak interaction in which the decay electron remains in a bound atomic state rather than being emitted into the continuum. In the present work, we observed ¯¡b decay of bare 20781Tl 81+ in the FRS-ESR facility at GSI. Those ions were produced through the projec-tile fragmentation of primary 833 MeV/u 208Pb beam impinged on a 9Be target. It was the ¯rst time to use a secondary beam in this mass re-gion for the measurement of ¯¡b decay in the ex-perimental storage ring ESR. 207Tl81+ were suc-cessfully separated from its neighbor of H-like 207Pb81+ by the B-¢E-B method in the frag-ment separator FRS. We succeeded to reduce the number of 207Pb81+ ions down to roughly of »1 % as the contamination ratio. They are ¯-nally injected into the ESR and electron-cooled with stochastic pre-cooling. It was also the ¯rst time to apply stochastic pre-cooling to ¯¡b de-cay measurement in the ESR. We could reduce the beam cooling time by aobut 90 % compared to those at the previous experiments [1], where only the electron cooling was applied. As shown in Fig. 1, the numbers of both the 207Tl81+ parent ions and their ¯¡b decay daugh-ters were recorded as a function of the storage time by the Schottky mass spectrometry [2] in the ESR. The ¯¡b decay as well as the contin-uous ¯ ¡ decay probability (¸¯¡ b and ¸¯¡c) and, hence, the bound-to-continuum branching ratio has been analyzed (see Fig. 2). The obtained branching ratio is in good agreement with theo-retical calculations [3], within an, experimental uncertainty. The comparison of ¸¯¡ b and of ¸¯¡c with the theoretical values seems to be system-atically small. However the ratio agrees with the theoretical value within an error. This fact suggests that some theoretical assumptions are not good enough, to calculate individual valu