Optical isotope shifts have been measured in the 5s[3/2${]}_2^0$-5p[3/2${]}_2$ transition in atomic krypton (λ=760 nm) using collinear-fast-beam laser spectroscopy. Isotope shifts were determined for the short-lived neutron-rich isotopes ${}_{}{}^{88}{}_{}{}^{}\mathrm{Kr}$ and ${}_{}{}^{90}{}_{}{}^{}\mathrm{Kr}$, as well as for all the stable isotopes between A=78 and 86, thus extending the data in this element, for the first time, beyond the N=50 shell closure. The deduced changes in the nuclear charge radii show a decrease of the charge radius with increasing neutron number below N=50, and a much more rapid increase above it. A preliminary interpretation of the results is given in terms of the droplet model.

• Received 3 April 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.1332