QUADRUPOLE-MOMENTS AND NUCLEAR SHAPES OF NEUTRON-DEFICIENT GOLD ISOTOPES

The hyperfine structure splitting and the isotope shift of the neutron-deficient gold Au-194-191 in the optical transition 5d(9) 6s(2 2) D-3/2--> 5d(10) 6p P-2(1/2) (627.8 nm) have isotopes been determined by collinear laser spectroscopy at the on-line isotope separator ISOLDE. The nuclear magnetic moments, spectroscopic quadrupole moments and changes in the mean square charge radii are deduced. Experimental electromagnetic moments and deformation parameters are compared with results of a particle-triaxial-rotor model calculation for both odd-A and odd-odd gold nuclei. Very good overall agreement between calculation and experimental data has been achieved. We find, for the first time, consistent evidence for a relatively large contribution from hexadecapole deformation (epsilon(4) = 0.06, beta(4) = -0.05) to odd-A gold isotopes ground states in the region of 183 less than or equal to A less than or equal to 197. Calculated values of quadrupole deformation are consistent with the prolate-to-oblate shape change between ground states of Au-186 and Au-187. In addition, our results suggest another shape change between Au-190 (oblate) and Au-191 (triaxial). The former change is accompanied by a considerable change in quadrupole deformation, the latter occurs at almost constant deformation.