Direct Observation of Bound-State β − Decay of Fully Ionized 205

J. Kurcewicz1, F. Bosch1, H. Geissel 1,2, Yu. A. Litvinov1,2, K. Beckert1, P. Beller1, D. Boutin1,2, C. Brandau1, L. Chen1,2, T. Faestermann 3, B. Franzke1, M. Hausmann 4, P. Kienle3,5, O. Klepper 1, R. Knöbel 1,2, C. Kozhuharov 1, S. A. Litvinov1,2, L. Maier3, M. Mazzocco1, F. Montes1, A. Musumarra6, C. Nociforo1, F. Nolden1, W. R. Plaß1,2, C. Scheidenberger 1,2, M. Steck1, B. Sun1, K. Takahashi 7, H. Weick1, N. Winckler1,2, and M. Winkler1 1GSI, Darmstadt, Germany; 2JLU, Gießen, Germany; 3TUM, München, Germany;4MSU, East Lansing, U.S.A.; 5SMIsP, Vienna, Austria;6LNS, Catania, Italy;7ULB, Brussels, Belgium Theβ− decay to an electronic bound state in the daughter atom, accompanied by the emission of a monoenergetic neutrino was first observed at GSI in 1992 [1], more than 40 years after its theoretical prediction [2]. This phenomeno n of minor importance for neutral atoms, might become a strong decay channel for highly ionized atoms in stellar environments. Thus the heavily altered nuclear half-lives might have a large impact on the path of the nucleosynthesis process. Here we report on the direct observation of the bound-stateβ− decay of fully ionizedHg. The experiment was performed at the SIS synchrotron of GSI Darmstadt, which delivered a 750 A GeV Pb beam with an intensity of the order of 10 ions/spill. The ions of interestHg were produced in the projectile fragmentation. The 4 g/cm thick beryllium target was placed at the entrance of the projectile Fragment Separator (FRS). The Hg were selected by passing the magnetic sections of the separator and injected into the ESR storage ring, where they could remain for extended periods of time circulating with a revolution frequency of the order of 2 MHz. Both, stochastic and electron cooling were applied in order to decrease the velocity spread of the circulating ions. The beamline of the ESR is equipped with a set of Schottky noise probes which collect the mirror charge of each passing ion, thus providing the information about its revolution frequency. The 30th harmonic of the noise signal generated in the Schottky pick-up electrodes was amplified and mixed down with a local oscillator providing a reference frequency. The difference signal in the frequency range of 0-300 kHz was digitized by using a 16-bit ADC and a Fast Fourier Transformation was applied to the data yielding revolution frequency spectra of the coasting ions. Analyzing the frequency peaks present in the subsequently accumulated Schottky spectra, a time distribution of the number of the bare Hg and the βb-decay daughter Tl ions stored in the ESR was constructed. In general the time evolution of the number of ions of mother activity can be described by the following expression