The volume of the capture region to the right of the muon (bounded by the z = 0 plane) is proportional to E-3/2.
The calculation assumes viscous flow - i.e. the electron never accumulates any momentum, because it is constantly losing it through collision with the medium; thus at any instant the electron moves in the opposite direction from the vector sum of the electric field Eµ due to the muon and the applied Eext. In some materials where free electrons have very long mean free paths, this assumption may be invalid; in such cases one must calculate electron trajectories including accumulated inertia - a much more difficult job.