Direct Simulation of Low-Pressure Supersonic Gas Expansions and its   Experimental Verification
  The use of gas expansions to generate atomic or molecular beams has become a standard technique in nuclear and hadron physics for the production of polarized ion beams and gas targets. A direct simulation Monte Carlo method was used to understand the processes occurring in an expansion of highly dissociated hydrogen or deuterium gas at low densities. The results were verified in several measurements including time-of-flight and beam-profile determinations which showed that the supersonic gas expansions can properly be described by the Monte Carlo calculations. Additionally a new method of beam formation, the hollow carrier jet, was tested under the conditions of the atomic beam source operation. 
Alexander Nass and Erhard Steffens