Gamow-Teller beta decay of the very neutron-deficient N=50 nuclide⁹⁸Cd

For the first time, detailed decay-spectroscopic investigations were performed for the very neutron-deficient N=50 nuclide⁹⁸Cd. The⁹⁸Cd activity was produced in spallation reactions between 600 MeV protons and a natural tin target, yielding a⁹⁸Cd beam intensity of 10 to 60 atoms/s at the collector of the ISOLDE massseparator. By means of γ-ray and conversion-electron spectroscopy, 19 transitions were found to follow the β⁺/EC decay⁹⁸Cd→⁹⁸Ag. The transitions at 61 and 107 keV were shown to be M1(+E2) and E2, respectively, and the⁹⁸Cd half-life was measured as 9.2±0.3 s. The Q_EC value of⁹⁸Cd is determined semiempirically and is compared to model predictions together with the measured Q_EC values of the neighbouring cadmium isotopes^100,102Cd and the N=50 isotones⁹²Mo,⁹⁴Ru, and⁹⁶Pd, taken from the literature. The newly established decay scheme of⁹⁸Cd includes 9 excited states of⁹⁸Ag. Four states at 1691, 1861, 2164, and 2544 keV are directly fed by 0⁺ → 1⁺ Gamow-Teller beta transitions with a summed strength of 3.5_-0.7^+0.8. This value corresponds to 25±5% of the strength predicted for the GT transformation of a g_9/2 proton (in⁹⁸Cd) into a g_7/2 neutron (in⁹⁸Ag) by the extreme single-particle shell model. The GT-strength splitting and quenching, observed for⁹⁸Cd, are compared with the corresponding data for lighter even-even N=50 isotones, and are discussed with reference to the predictions of more sophisticated nuclear models. We find that only in some cases it is possible to explain qualitatively the observed GT strength distribution and its total magnitude without renormalizing the free-neutron value of the axial-vector coupling constant.