  We examine energy relaxation of non-equilibrium quasiparticles in "dirty"
superconductors with the electron mean free path much shorter than the
superconducting coherence length. Relaxation of low-energy non-equilibrium
quasiparticles is dominated by phonon emission. We derive the corresponding
collision integral and find the quasiparticle relaxation rate. The latter is
sensitive to the breaking of time reversal symmetry (TRS) by a magnetic field
(or magnetic impurities). As a concrete application of the developed theory, we
address quasiparticle trapping by a vortex and a current-biased constriction.
We show that trapping of hot quasiparticles may predominantly occur at
distances from the vortex core, or the constriction, significantly exceeding
the superconducting coherence length.
