Optimum strategy for energy degraders and ionization cooling
  Methodology for calculating the profile and emittance of a particle beam as it is slowed down in matter, including the effects of multiple scattering, axial magnetic field and lithium lens. Strategies are determined for minimum final emittance. For ionization cooling, boron carbide is superior to liquid hydrogen while a beryllium lens has merit. 
Francis J.M. Farley