Search for Superscreening Effects in a Superconductor.

The decay of O-19(beta(-)) and Ne-19(beta(+)) implanted in niobium in its superconducting and metallic phases was measured using purified radioactive beams produced by the SPIRAL GANIL facility. Half-lives and branching ratios measured in the two phases are consistent within a 1 sigma error bar. This measurement casts strong doubts on the predicted strong electron screening in a superconductor, the so-called superscreening. The measured difference in screening potential energy is 110(90) eV for Ne-19 and 400(320) eV for O-19. Precise determinations of the half-lives were obtained for O-19, 26.476(9) s, and for Ne-19, 17.254(5) s.