Sangmook Kang

Polarity effect of the thimble-type ionization chamber at a low dose rate

It is known that the current collected from an ionization chamber exposed to a constant radiation intensity changes in magnitude when the polarity of the collecting potential is reversed. It is called the polarity effect of the ionization chamber. There are many possible causes that induce the polarity effect and one of them can be a field distortion due to a potential difference between the guard electrode and the collector. We studied how much the polarity effect depends on the design of the electrodes in the thimble-type ionization chamber. Two thimble-type ionization chambers, which had different electrode structures, were designed and fabricated at KAERI. We calculated the field distortions due to the potential difference between the guard electrode and the collector for the two ionization chambers. MAXWELL and Garfield were employed to calculate the electron drift lines inside the chamber. The polarity effects of the two ionization chambers were measured, and they were consistent with the field calculation. We could conclude that the polarity effect is mostly induced from the field distortion due to the potential difference between the guard electrode and the collector in our experiment and it depends significantly on the design of the electrodes.

Fabrication of CZT Planar-Type Detectors and Comparison of their Performance

CZT is one of the most promising materials for X-ray and γ-ray radiation detectors operated at room temperature. We have studied the fabrication process of the CZT planar-type detector, which includes the physical and chemical surface treatment, the fabrication of metal contact, and the surface passivation. CZT planar-type detectors with various metal-semiconductor contacts were made. The dark current and the γ-rays from 241Am were measured with each detector, and they were compared. From the above studies, the effective fabrication method of CZT detector could be deduced. Our work could be helpful to fabricate the high resolution CZT detector, which could be used in various application areas such as nuclear medicine, radiation safety, X-ray and γ-ray astronomy and homeland security

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.