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The authors have measured the apparent cross section for excitation of 5016 AA radiation (3 1 P to 2 1 S) by impact of 100 eV electrons on helium as a function of pressure in cylindrical excitation chambers of various proportions. The analysis of Phelps (1958) appears to describe the data provided that his upper bound to the fraction of resonance photons which reach the walls is taken and that the distance from the observed axial region to the closest wall is taken as the effective radius.
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Heddle et al. (1970) studied this question.
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