Background: The systematic trend in mean-square charge radii as a function of proton or neutron number exhibits a discontinuity at the nucleon-shell closures. While the established N = 28 shell closure is evident in the 10 charge radii of the isotopic chains of K through Mn, a similar signature of the N = 20 shell-closure is absent in the Ca region.Purpose: The isotope shift between neutron-deficient 36 K and 37 K was determined to investigate the change of the mean-square charge radii across N = 20 in the K isotopic chain.Methods: The D1 atomic hyperfine spectra of 36 K and 37 K were measured using an optical pumping and 15 subsequent β-decay asymmetry detection technique. Atomic rate equations were solved to fit the resonant line shape. The result was compared to Skyrme energy-density functional and shell-model calculations.Results: The isotope shift was obtained as δν 37,36 = −139(4)(3) MHz. Using a re-evaluated isotope shift, δν 39,37 = −264(2)(3) MHz, the isotope shift relative to 39 K was determined to be δν 39,36 = −403(5)(4) MHz. The differential mean-square charge radius was then deduced as δ r 2 39,36 = −0.16 (5)(8)
Conclusions:The absence of the shell-closure signature at N = 20 in the K isotopic chain is understood as a balance between the monopole and the quadrupole proton-core polarizations below and above N = 20, respectively.