Resonant Enhancement of Neutrinoless Double-Electron Capture in 152Gd

In the search for the nuclide with the largest probability for neutrinoless double-electron capture, we have determined the Q{sub {epsilon}{epsilon}} value between the ground states of {sup 152}Gd and {sup 152}Sm by Penning-trap mass-ratio measurements. The new Q{sub {epsilon}{epsilon}} value of 55.70(18) keV results in a half-life of 10{sup 26} yr for a 1 eV neutrino mass. With this smallest half-life among known 0{nu}{epsilon}{epsilon} transitions, {sup 152}Gd is a promising candidate for the search for neutrinoless double-electron capture.