  Energetic particles spectra at interplanetary shocks often exhibit a power
law within a narrow momentum range softening at higher energy. We introduce a
transport equation accounting for particle acceleration and escape with
diffusion contributed by self-generated turbulence close to the shock and by
pre-existing turbulence far upstream. The upstream particle intensity steepens
within one diffusion length from the shock as compared with diffusive shock
acceleration rollover. The momentum spectrum, controlled by macroscopic
parameters such as shock compression, speed, far upstream diffusion coefficient
and escape time at the shock, can be reduced to a log-parabola and also to a
broken power law. In the case of upstream uniform diffusion coefficient, the
largely used power law/exponential cut off solution is retrieved.
