Monte Carlo simulation of a Knudsen effusion mass spectrometer sampling system

Abstract

This code allows the geometry (aperture spacing and diameters) of the sampling system to be optimized for maximum transmission. The calculated effusate distributions and low average number of orifice wall collisions illustrated the advantages of restricted collimation. Calculated transmission factors are also compared to literature values calculated via the analytical method of Chatillon and colleagues. Published in 2017. This article is a U.S. Government work and is in the public domain in the USA.

Erratum to: “Knudsen cell mass spectrometry using restricted molecular beam collimation. I. Optimization of the beam from the vaporizing surface”

“…The original results presented in the paper by Nuta and Chatillon [1] showed that the transmission yield varied with the location of the emission plane relative to the effusion orifice entrance. Recently, however, Radke et al [3] calculated, using the Monte Carlo method, the transmission probability for the Knudsen cell collimation device that they used in their mass spectrometric method and their results agreed with the formulae proposed by Morland et al [2] Thus, we have returned to our calculations, checking first that our published equations [1] remained correct, and we have found an error resulting from the transcription of Eqn. (12) from this paper to the computer, specifically for the parameter H 1 .…”

Knudsen effusion mass spectrometry: Current and future approaches

“…Molecular flow can be modeled analytically 55,56 or simulated using a Monte Carlo technique. 39,[57][58][59][60][61] Today, Monte Carlo is probably the most useful technique. A valid simulation can be run on a desktop computer, and a number of factors can be easily explored.…”

“…This is shown in Figure 7. According to the nomenclature of Chatillon, the aperture in the copper plate is the “field aperture,” and the aperture adjacent to the ionizing region is the “source aperture.” The geometry of the NASA Glenn system is shown in Figure 8A, and modeling parameters (aperture spacing and orifice diameters) are shown in Figure 8B 61 . Chatillon et al have derived equations for the flux through these different apertures based on the analogy of light transmission between two disks, which decreases inversely with the square of the distance between the two disks 68,69 .…”

“…(C) Results of Monte Carlo simulation and analytical formulation for optimizing dimensions. Source : Reprinted with permission from Radke et al, 61 John Wiley and Sons. [Color figure can be viewed at wileyonlinelibrary.com]…”

“…This means the vapor species strikes the orifice wall and is reemitted in a completely random direction. Molecular flow can be modeled analytically 55,56 or simulated using a Monte Carlo technique 39,57–61 . Today, Monte Carlo is probably the most useful technique.…”