  The motion of superfluid vortices in a neutron star crust is at the heart of
most theories of pulsar glitches. Pinning of vortices to ions can decouple the
superfluid from the crust and create a reservoir of angular momentum. Sudden
large scale unpinning can lead to an observable glitch. In this paper we
investigate the scattering of a free vortex off a pinning potential and
calculate its mean free path, in order to assess whether unpinned vortices can
skip multiple pinning sites and come close enough to their neighbours to
trigger avalanches, or whether they simply hop from one pinning site to another
giving rise to a more gradual creep. We find that there is a significant range
of parameter space in which avalanches can be triggered, thus supporting the
hypothesis that they may lie at the origin of pulsar glitches. For realistic
values of the pinning force and superfluid drag parameters we find that
avalanches are more likely in the higher density regions of the crust where
pinning is stronger. Physical differences in stellar parameters, such as mass
and temperature, may lead to a switch between creep-like motion and avalanches,
explaining the different characteristics of glitching pulsars.
