Anomalous saturation characteristics of a parallel plate ionization chamber

With a parallel plate ionization chamber having a guard plate, the saturation curve shows a maximum (convexity), instead of a monotonous increase, when a constant low voltage of the same polarity as that of the high voltage plate is applied to the guard plate. However, when the two plates are different in polarity of voltage, the saturation curve shows a monotonous increase, although remarkabe polarity effect, persists. The height and position of the maximum in the saturation curve depend on the kind of gas and the guard potential. Experiments were carried out with air, carbon dioxide, ethylene and other gases. Among the gases examined, argon exhibits remarkable polarity effect and a conspicuous maximum. The anomalous saturation characteristics are explained by considering variations in the ionizing volume and the ion recombination with increase of the voltage applied to the high voltage plate. The difference in mobility between positive and negative ions (or electrons) appears to play an important role in giving the polarity effect. (author)