The Determination of Saturation Currents in Free-air Ionization Chambers by Extrapolation Methods

The usual method of determining saturation currents in free-air chambers is to plot the reciprocal of the current, j , against the reciprocal of the collecting electric field, E , and extrapolate linearly to 1/ E =0. It is shown that this method is valid only if the recombination taking place is initial recombination within ion clusters. Experimental and theoretical evidence is produced to show that in such chambers, at dose rates of 1 r/min or more, it is general (volume) rather than initial recombination which is taking place. Theory shows that under these circumstances one should plot 1/ j against 1/ E 2 , or more accurately against j / E 2 , and extrapolate to 1/ E 2 or j / E 2 =0.