  The inelastic scattering rates of quasiparticles in a two-dimensional d-wave
superconductor, which arise from interactions with either acoustic phonons or
other quasiparticles, are calculated within second order perturbation theory.
We discover a strong enhancement of scattering with collinear momenta, brouth
about by the special kinematics of the two-dimensional fermions with Dirac-like
spectrum near the nodes of the d-wave order parameter. In the case of a local
instantaneous interparticle potential we find that either an RPA-type
resummation of the perturbation series or an inclusion of non-linear
corrections to the Dirac spectrum is called for in order to obtain a finite
scattering rate in the limit $\omega/T\to 0$. In either way, we find drastic
changes in the scattering rate, as compared to the naively expected cubic
temperature dependence.
