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Access to stored and cooled highly-charged radionuclides offers unprecedented opportunities to perform high-precision investigations of their decays. Since the few-electron ions, e.g. hydrogen- or helium-like ions, are quantum mechanical systems with clear electronic ground state configurations, the decay studies of such ions are performed under well-defined conditions and allow for addressing fundamental aspects of the decay process. Presented here is a compact review of the relevant experiments conducted at the Experimental Storage Ring ESR of GSI. A particular emphasis is given to the investigations of the two-body beta decay, namely the bound-state β-decay and its time-mirrored counterpart, orbital electron-capture.

In the doubly magic nucleus 208Pb , five states are observed at 5648, 5658, 5686, 5694, 5836keV by 208Pb ( p, p′) via isobaric analog resonances in 209Bi . The 5686, 5694, 5836 states with spins 6-, 7-, 8- are shown to contain almost the full strength of the particle-hole configuration g9/2 f7/2 . The 5658 5- state contains a major g9/2 f7/2 fragment with admixtures of d5/2 f5/2 , d5/2 f7/2 . The 5648, 5649keV states observed in a quintuplet at Ex = 5.63 - 5.65 MeV are preliminarily identified to have spins (4)- and 6+ to 9+. The 5648keV (4)- state contains almost half of the g9/2 f7/2 strength and half of the h9/2 d5/2 strength with admixtures of d5/2 f5/2 , d5/2 p3/2 , d5/2 f7/2 . [ABSTRACT FROM AUTHOR]

The reaction 54Fe( $$ \vec d $$, p)55Fe was studied at the Munich Q3D spectrograph with a 14MeV polarized deuteron beam. Excitation energies, angular distributions and analyzing powers were measured for 39 states up to 4.5MeV excitation energy. Spin and parity assignments were made and spectroscopic factors deduced by comparison to DWBA calculations. The results were compared to predictions by large-scale shell model calculations in the full pf -shell and it was found that reasonable agreement for energies and spectroscopic factors below 2.5MeV could only be obtained if up to 6 particles were allowed to be excited from the f7/2 orbital into p3/2 , f5/2 , and p1/2 orbitals across the N = 28 gap. For levels above 2.5MeV the experimental strength distribution was found to be significantly more fragmented than predicted by the shell model calculations. [ABSTRACT FROM AUTHOR]

An experiment has been carried out using the 9Be(³He,t)9B* reaction at a beam energy of 33 MeV. A large acceptance silicon-strip array was used to detect the 9B* break-up in coincidence with the triton ejectiles in the high-resolution Munich-Q3D spectrograph. The excitation energy regime <3 MeV has been explored and the spectrum resulting from proton decaying states, isolated and characterized. Additional resonance strength is observed at 1.86 MeV ±70 keV(stat) ±35 keV(syst), in agreement with two other recent measurements at higher energies and different angles. The consequences for the "missing"1/2+ first excited state are discussed. Additionally, the branching ratios for the 2.36 MeV 5/2- state have been measured as Γα0/Γ = 0.98±0.12 and Γp0/Γ = 0.016 ± 0.008, in close agreement with earlier work. [ABSTRACT FROM AUTHOR]

The 9Be(6Li, d)13C* reaction at a beam energy of 42 MeV has been investigated using a large-acceptance silicon-strip detector array and the high-resolution Q3D magnetic spectrograph. The Q3D facilitated the unambiguous determination of the reaction channel via identification of the deuteron ejectile, thereby providing the spectrum of excited states in 13C in the range from 10.7 to 15.0 MeV. The silicon array was used to detect and identify the 13C recoil-breakup products with efficiencies of up to 49%. The results obtained for the absolute partial branching ratios represent the first complete measurements for states in this energy region and allow the extraction of reduced widths. The quantities measured for Γn0/ Γtot and Γn1/ Γtot are 0.91 ± 0.11 and ≤0.13 (10.753 MeV), 0.51 ± 0.04 and 0.51 ± 0.04 ( 10.818 MeV), 0.68 ± 0.03 and 0.42 ± 0.02 (10.996 MeV), 0.49 ± 0.08 and 0.71 ± 0.11 (11.848 MeV), and 0.49 ± 0.08 and 0.53 ± 0.08 (12.130 MeV), respectively. For the two observed higher-lying energy levels, Γα0/Γtot and Γn1/Γtot have been measured as 0.54 ± 0.02 and 0.45 ± 0.02 (13.760 MeV) and 0.94 ± 0.03 and 0.13 ± 0.02 (14.582 MeV), respectively. The consequences for the proposed molecular structures in 13C are explored following the extraction of reduced widths. [ABSTRACT FROM AUTHOR]

By using a large-acceptance position-sensitive silicon detector array in coincidence with the high-resolution Munich Q3D spectrograph, unambiguous measurements have been made of the absolute α-particle decay widths from excited states in 16O* in the energy range 13.85 to 15.87 MeV. Carbon targets have been bombarded with 42-MeV 6Li beams to induce 612C(36Li, d)816O* reactions. The deuteron ejectiles were measured in the Q3D and the results gated by 4He + 12C breakup products detected in the silicon array, the efficiency of which was modeled using Monte Carlo simulations. By comparing total population and breakup-gated spectra, the following absolute α-decay widths have been measured with high resolution: Γα0/Γtot = 0.87 ± 0.11 (13.980 MeV), 1.04 ± 0.15 (14.302 MeV), 0.92 ± 0.10 (14.399 MeV), 0.59 ± 0.04 (14.815 MeV), 0.88 ± 0.18 (15.785 MeV), and Γα1/Γtot = 1.14 ± 0.08 (14.660 MeV), 0.46 ± 0.06 (14.815 MeV). [ABSTRACT FROM AUTHOR]