Nonequilibrium quasiparticle dynamics in Ta are studied using a superconducting Ta film with an Al tunnel junction connected at each end. The quasiparticle system is driven out of the equilibrium by absorption of an x-ray photon. Millions of quasiparticles, created by each photon, diffuse in the Ta film. When the quasiparticles reach the Al junctions they lose energy by emitting phonons and are trapped in the Al film. By measuring the tunneling current, the number of excess quasiparticles can be calculated. In Ta, the diffusion constant of 8.2Ϯ0.2 cm 2 /s and quasiparticle lifetime of 83 Ϯ5 s at 0.21 K are derived from fitting the measured current pulses, and are compared with theoretical predictions.

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