The influence of Penning ionization on the experimental determination of ionization coefficients in helium at low values of E/N

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Abstract
Using ultrahigh vacuum techniques and highly purified gas samples, measurements of the spatial growth of prebreakdown ionization in uniform fields E were made from helium in the range 4.4*101819 cm-3 and for 8.4*10-17-16 V cm2, where N is the gas number density. Values of the Townsend primary ionization coefficient alpha were determined from the measurements and alpha /N was found to increase with decreasing N at a given value of E/N. Comparison is made with previous theoretical and experimental data and it is shown that the results are quantitatively consistent with Penning ionization of impurities present in amounts which were assessed by comparison with theoretically computed values of alpha /N to be about 1 volume per million.