  We investigate single quasiparticle excitation dynamics on a small
superconducting aluminum island connected to normal metallic leads by tunnel
junctions. We find the island to be free of excitations within the measurement
resolution allowing us to determine Cooper pair breaking rate to be less than 3
kHz. By tuning the Coulomb energy of the island to have an odd number of
electrons, one of them remains unpaired. We detect it by measuring its
relaxation rate via tunneling. By injecting electrons with a periodic gate
voltage, we probe electron-phonon interaction and relaxation down to a single
quasiparticle excitation pair, with a measured recombination rate of 8 kHz. Our
experiment yields a strong test of BCS-theory in aluminum as the results are
consistent with it without free parameters.
