Precise Calibration of Laser Frequency for determination of Ni Charge Radius*

Collinear laser spectroscopy is commonly used to determine charge radii, and requires accurate and precise knowledge of the laser frequency to a level of roughly 1 MHz. A Doppler-free laser spectroscopic system has been implemented at the BECOLA facility to calibrate the laser frequency against precisely-known transitions in molecular iodine. After calibration, the laser system will be used to determine charge radius of $^{\mathrm{54}}$Ni, which is important to address the soft nature [1] of doubly-magic $^{\mathrm{56}}$Ni as well as to deduce the slope parameter, $L$, in the symmetry energy of the nuclear equation of state [2]. The performance characteristics of the laser frequency calibration system will be presented and the results will be discussed. [1] M. Honma et al., Phys. Rev. C 65, 061301 (2002). [2] B. A. Brown et al., Phys. Rev. Research 2, 022035 (R) (2020).