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(epsilon epsilon) value between the ground states of Gd-152 and Sm-152 by Penning-trap mass-ratio measurements. The new Q(epsilon epsilon) value of 55.70(18) keV results in a half-life of 10(26) yr for a 1 eV neutrino mass. With this smallest half-life among known 0v epsilon epsilon transitions, Gd-152 is a promising candidate for the search for neutrinoless double-electron capture.