  We consider a simple model of a cloud chamber consisting of a test particle
(the alpha-particle) interacting with two other particles (the atoms of the
vapour) subject to attractive potentials centered in $a_1, a_2 \in
\mathbb{R}^3$. At time zero the alpha-particle is described by an outgoing
spherical wave centered in the origin and the atoms are in their ground state.
We show that, under suitable assumptions on the physical parameters of the
system and up to second order in perturbation theory, the probability that both
atoms are ionized is negligible unless $a_2$ lies on the line joining the
origin with $a_1$. The work is a fully time-dependent version of the original
analysis proposed by Mott in 1929.
