The correct and incorrect generation of a cosine distribution of scattered particles for Monte-Carlo modelling of vacuum systems

Computer modelling of vacuum systems is often performed using the Monte-Carlo technique. One of the assumptions frequently made is that molecules originating at the boundaries of the system have a "cosine distribution". The nature of this distribution is occasionally misinterpreted and consequently, the molecular distribution is physically incorrect. This article describes the main point of confusion by firstly stating the cosine law and describing one possible method of "generating" the correct distribution. It goes on to describe the distribution that results from the most common misinterpretation of the cosine law. This is illustrated by quantifying the effect on the modelled conductance for circular cylinders of various lengths. (C) 2002 Elsevier Science Ltd. All rights reserved.