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 f7/2 shell closure. The extracted Ca-50 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 Ca-48 nucleus indicates a strong coupling between the P3/2 neutrons and the core protons.