Transverse ionization cooling of muons is modeled as a Brownian motion
of the muon beam as it traverses a Li or Be rod. A Langevin-like equa
tion is written for the free particle case (no external transverse mag
netic field) and for the case of a harmonically bound beam in the pres
ence of a focusing magnetic field. We demonstrate that the well-knows
muon cooling equations for short absorbers can be extrapolated to the
useful case of a long absorber rod with a focusing magnetic field pres
ent.