The Effect of Resolving Power on Measurements of the Absorption Coefficient of Electrons in Gases

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Abstract
The absorption coefficient of electrons has been studied as a function of the resolving power of the experimental apparatus. An electron beam of 0.4 mm radius was used in a Mayer type of apparatus in which the opening at the end of the scattering chamber was of variable aperture. Values of the absorption coefficient were obtained in helium and in mercury vapor for 20, 40, 80 and 135 volt electrons over an angular aperture range of from 2° to 11° as measured from the end of the electron gun. All curves showed a definite decrease in the absorption coefficient with increase in angular aperture. These curves were corrected for the positive ion current, from the efficiency of ionization data of Bleakney and Smith, to give curves for the total number of electrons collected as a function of the size of the opening. These showed also a definite decrease with increase in aperture, corresponding to a preference for scattered electrons to be deflected through small angles. In helium the curves were steeper the higher the electron velocity, indicating that this preference is more decided for fast electrons than for slower electrons. In mercury vapor the curves were more nearly alike. The results do not indicate as decided a preference for small angle scattering as is indicated by the angular distribution curves of Arnot in mercury vapor. This discrepancy is not explained.

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