We report the first application of an ultrasensitive detection scheme in on-line collinear-laser spectroscopy. It is based on radioactive detection of optical depopulation pumping, using state-selective charge exchange as an intermediate step. This extends the calcium isotope shift measurements beyond the ${\mathit{f}}_{7/2}$ shell closure. The extracted ${}_{}{}^{50}{}_{}{}^{}\mathrm{Ca}$ mean-square charge radius constitutes the first experimentally determined one of a short-lived neutron-rich isotope beyond the N=28 shell closure. The steep increase in radius after the doubly magic ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$ nucleus indicates a strong coupling between the ${\mathit{p}}_{3/2}$ neutrons and the core protons.

• Received 25 November 1991

DOI:https://doi.org/10.1103/PhysRevLett.68.1679