  Consider a linear Boltzmann equation posed on the Euclidian plane with a
periodic system of circular holes and for particles moving at speed 1. Assuming
that the holes are absorbing -- i.e. that particles falling in a hole remain
trapped there forever, we discuss the homogenization limit of that equation in
the case where the reciprocal number of holes per unit surface and the length
of the circumference of each hole are asymptotically equivalent small
quantities. We show that the mass loss rate due to particles falling into the
holes is governed by a renewal equation that involves the distribution of
free-path lengths for the periodic Lorentz gas. In particular, it is proved
that the total mass of the particle system at time t decays exponentially fast
as t tends to infinity. This is at variance with the collisionless case
discussed in [Caglioti, E., Golse, F., Commun. Math. Phys. 236 (2003), pp.
199--221], where the total mass decays as Const./t as the time variable t tends
to infinity.
