Physics:Diffusion in gases

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During the drift in electric fields, charged particles diffuse according to a Gaussian distribution

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where

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It is convenient to define a reduced drift velocity, the mobility at atmospheric pressure

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with

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From classical arguments it can be shown that the diffusion coefficient is given by the Nernst-Einstein relation

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with

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The mobility depends on the energy distribution, the mean free path and the inelasticity 35x24px , i.e. the fraction of energy lost on each impact.

For positive ions, the following table gives some values for the mean free path Hepb img157.gif and the diffusion coefficients D for different molecules under normal conditions (from Schultz77 and Sauli91):

height12pt width0pt Gas Hepb img157.gif [cm] D [cm2/s] Hepb img194.gif [cm2 sec 53x28px
height12pt width0pt H2 70x28px 0.34 13.0
He 70x28px 0.26 10.2
Ar 70x28px 0.04 1.7
O2 70x28px 0.06 2.2
H2O 70x28px 0.02 0.7

For electrons, the neutralization by ions and the attachment by molecules with electron affinity must be considered. Except for very low fields the mobility of electrons is not a constant; the mean free path varies in some gases with the electric field (Ramsauer effect), all resulting in a diffusion coefficient dependent on the electric field.

Note that the limiting accuracy is not given by the standard deviation from ft(x), but depends on the number of electrons necessary to trigger the shift-line electronics. If n electrons are produced and k electrons are needed to overcome the electronics threshold, the following formula holds:

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For more details, Hepb img34.gif Piuz83, Breskin84, Charpak84, Peisert84, Amendolia86, Sauli91.