Accurate nuclear charge radii measurements of neutron-deficient nickel isotopes*

Charge radii of neutron deficient $^{\mathrm{54,55,56}}$Ni isotopes have been determined at the BEam COoler and LAser spectroscopy facility at NSCL to address the soft nature of doubly-magic $^{\mathrm{56}}$Ni. A novel approach was taken to precisely calibrate the Ni beam energy, which allowed us to accurately measure the isotope shifts relative to stable $^{\mathrm{60}}$Ni, from which the charge radii were deduced. Details of the calibration procedure and the obtained charge radii will be discussed.