Measurements of quasiparticle thermalization in a normal metal

We have probed the inelastic relaxation of gap-edge quasiparticles in a sup erconductor to conduction electrons in an adjoining normal metal. Quasipart icles are injected into a superconducting Al film by a normal-insulator-sup erconductor tunnel junction at a temperature of about 100 mK. These quasipa rticles diffuse throughout the superconductor and those that do not recombi ne are trapped in an adjoining Ag film. We find that trapped quasiparticles transfer more than 80% of their excitation energy to conduction electrons in the Ag over a broad range of electron and phonon temperatures and explai n this result with a model that incorporates electron-electron electron-pho non, and phonon-electron interactions.