  We address the problem that the cross section for the collisions of unstable
particles diverges, if calculated by standard methods. This problem is
considered for beams much smaller than the decay length of the unstable
particle, much larger than the decay length and finally also for pancake-
shaped beams. We find that in all cases this problem can be solved by taking
into account the production/propagation of the unstable particle and/or the
width of the incoming wave packets in momentum space.
