  Reactive or elastic two-body collisions in an ultracold gas are affected by
quantum statistics. In this paper, we study ensemble-averaged collision rates
for a two-species fermionic gas. The two species may have different masses,
densities and temperatures. We investigate how averaged collision rates are
affected by the presence of Fermi spheres in the initial states; Pauli blocking
of final states is not considered. It is shown that, independently on the
details of the collision, Fermi-averaged collision rates deviate from
Boltzmann-averaged ones, particularly for a gas with strong imbalance of masses
or densities.
