Study of neutron deficient iridium isotopes by using laser spectroscopy

Resonance ionization spectroscopy was performed on neutron deficient iridium isotopes ^182-189Ir, ¹⁸⁶Ir^m and stable isotopes ^191,193Ir. Hyperfine spectra were recorded from the optical transition at 351,7 nm between the 5d⁷6s^{2 4}F_9/2 ground state and the 5d⁷6s6p ⁶F_11/2 excited state. Radioactive iridium isotopes were obtained from β⁺/EC decay of radioactive mercury nuclei deposited on a graphite substrate. The radioactive mercury nuclei were produced at the ISOLDE facility at CERN through spallation reactions, by bombarding a molten lead target with the 1 GeV proton beam delivered by the PS-Booster. Magnetic dipole moments and spectroscopic quadrupole moments were extracted from the hyperfine spectra. The mean square charge radius variations, as deduced from the measured isotopic shift, show a sharp change between ¹⁸⁷Ir and ¹⁸⁶Ir^g, accompanied by a sudden increase in deformation: from β2 ∼ 0,16 to β2 > 0,2. These results were analysed in the framework of an axial rotor plus one or two quasiparticles. The wave functions of the osmium and platinum cores which are used in order to describe the iridium nuclei were calculated from the HF+BCS method with the Skyrme SIII effective interaction. The cores were constrained to take the deformation parameters extracted from the isotopic shift measurements. One shows then that this sudden deformation change corresponds also to a change in the proton state that describes the odd nuclei ground state or that participates in the coupling with the neutron in odd-odd nuclei. This state is identified with the π3/2⁺[402] orbital for the smaller deformations nuclei and with the π1/2^-[541] orbital stemming from the h_9/2 subshell for bigger deformations nuclei. (author)