Design of guard ring geometry in thimble type ionization chamber

We designed and fabricated a thimble type ionization chamber with the guard-ring electrode. During the measurement, the potential between the guard electrode and the collecting electrode can be drifted, which will change the active volume of the ionization chamber. From MAXWELL and Garfield calculation, the variation of the active volume of the ionization chamber was obtained with changing the potential difference between the collector and guard electrode. We made ionization chambers with two designs of the guard ring. The polarity effect of two ion chamber was measured, and the result of it shows the consistency with the simulation.