Charge radii of neutron-deficient $^{36, 37}$K*

Neutron and proton shell closures have been identified by discontinuities in the chain of charge radii. The shell-closure signature can be seen, e.g. at the $N$ = 28 and 50 neutron-shell closures for the isotopic chains of K through Mn. The signature at $N$ = 20, however is absent in the Ca region. The charge radii of neutron-deficient $^{36, 37}$K isotopes were determined to aid in understanding this abnormal behavior. The experiment was performed at the BEam COoling and LAser spectroscopy (BECOLA) facility at NSCL/MSU. The charge radii were obtained from isotope shifts determined from the hyperfine structure measurements by the optical pumping and subsequent b-decay asymmetry detection. The absence of an unambiguous signature of the shell effect is attributed to a balance of monopole and quadrupole proton-core polarizations above and below $N$ = 20, respectively. The global behavior of the isotopic chain of K charge radii will be discussed.