The Determination of Attachment and Ionization Coefficients in Air

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Primary ionization (?) and attachment (at) coefficients are determined from measurements of the steady state spatial growth of ionization currents in air for values of E/p (E the electric field, p the gas pressure) in the range 30 to 38 v cm-1 mmHg-1 and for 100 < p < 300 mmHg. The values of the apparent primary ionization coefficient (?-at) obtained are in general agreement with previously published data. On the other hand, the mean values obtained for the attachment coefficient (at/p approximately equal to 3 ? 10-3 cm-1 mmHg-1) are found to be lower (about half) than the published values from previous similar investigations. It is shown experimentally that the mean values obtained for the attachment coefficient are not significantly dependent either on the treatment of the gas sample (provided adequate cold traps are used), or on the gas pressure within the range 100 to 300 mmHg. An analysis of the experimental and computing procedure is made and this shows that considerable care is necessary in the method used to obtain the coefficients from the actual experimental data. For example, if an estimated or extrapolated value of the initial current I0 is used in the derivation of the coefficients, a change of 15% in the value of I0 assumed can give rise to a difference by a factor of 20 in the value of the attachment coefficient deduced from the experimental data. It is shown that under typical experimental conditions, when ionization currents can be measured with an experimental error of 1% over a range of electrode separation from 0.3 to 3 cm, there is a considerable inherent spread (similar 50 to 100% about the mean) in the possible values of the attachment coefficient which will give agreement with the basic experimental data. Comparison of the values of the apparent primary ionization coefficient ?' obtained in the present investigation with those obtained at higher gas pressures (similar 400 to 1000 mmHg) in previous work in this Department confirms that the similarity relationship ?'/p = f(E/p) is not obeyed for values of E/p approximately equal to 35 v cm-1 mmHg-1 in air at pressures from 100 to 1000 mmHg.