Precise Calibration of Laser Frequency for determination Sc Charge Radii*

A kink structure is observed at the magic numbers in chains of charge radii but is missing at the neutron number $N \quad =$ 20 for $_{\mathrm{18}}$Ar, $_{\mathrm{19}}$K and $_{\mathrm{20}}$Ca. Determination of the charge radii of proton-rich $_{\mathrm{21}}$Sc isotopes across $N \quad =$ 20 are planned to address the disappearance of shell-closure signature. Collinear laser spectroscopy, which requires accurate and precise knowledge of laser frequency, will be used to deduce the charge radii. To calibrate the laser frequency, a Doppler-free spectroscopic measurement of molecular iodine is being implemented to measure precisely-known transitions in the visible to near-infrared wavelength region. The status of development and test results will be discussed.