  We study quasiparticle energy relaxation at sub-kelvin temperatures by
injecting hot electrons into an aluminium island and measuring the energy flux
from electrons into phonons both in the superconducting and in the normal
state. The data show strong reduction of the flux at low temperatures in the
superconducting state, in qualitative agreement with the presented
quasiclassical theory for clean superconductors. Quantitatively, the energy
flux exceeds that from the theory both in the superconducting and in the normal
state, possibly suggesting an enhanced or additional relaxation process.
