Penning-Trap Mass Measurements of the Neutron-Rich K and Ca Isotopes: Resurgence of the N = 28 Shell Strength

6 pagesg, 5 figures, to submit to PRCWe present Penning-trap mass measurements of neutron-rich 44,47-50K and 49,50Ca isotopes carried out at the TITAN facility at TRIUMF-ISAC. The 44K mass measurement was performed with a charge-bred 4+ ion utilizing the TITAN EBIT, and agrees with the literature. The mass-excess values obtained for 47K and 49,50Ca are more precise and agree with the 2003 Atomic Mass Evaluation (AME'03). The 48,49,50K mass excesses are more precise than the AME'03 values by more than one order of magnitude. For 48,49K, we find deviations by 7 sigma and 10 sigma, respectively. The new 49K mass excess lowers significantly the two-neutron separation energy at the neutron number N = 30 compared to the separation energy calculated from AME'03 values, and thus, strengthens the N = 28 shell gap energy at Z = 19 by approximately 1 MeV