Sudden Change in the Nuclear Charge Distribution oC Very Light Gold Isotopes

On-line resonance-ionization mass spectrometry has been applied to determine the isotope shift and hyperfine structure of 185- 189Au and 189Aum in the 6s 2S 1 / 2- 6p 2Pl/2 (A =268 nm) transition. The Au atoms were obtained as daughters of mass-separated Hg isotopes produced at the ISOLDE facility at CERN ionized by a three-color, two-step resonant photoionization process, detected and n1ass selected by time of flight. A drastic change of the nuclear charge radius was observed between 187Au and 186Au, which is interpreted as an onset of strong deformation of f3 ~ 0.25 in 186Au and 185Au. PAes numbers: 21.10.Ft, 21.10.Ky, 27.70.+g The nuclei ncar Z = 80 and N = 104 have attracted considerable attention following the discovery by Bonn el al. 1 that there is a big change in the ground-state mean-square charge radius between 187Hg and 185Hg, and its interpretation as an onset of strong deformation at.,4:s 185. This result, obtained from a measurement of the optical isotope shift (IS), was completely unexpected since at that time the unanimous view was that nuclei near closed shells, such as the isotopes of Hg (Z = 80)