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.
Kim
Yong-Kyun
YK
Korea Atomic Energy Research Institute, Daejeon 305-600, Republic of Korea.
Park
Se-Hwan
SH
Kim
Han-Soo
HS
Kang
Sang-Mook
SM
Ha
Jang-Ho
JH
Chung
Chong-Eun
CE
Cho
Seung-Yeon
SY
Kim
J K
JK
D002138:Calibration; D004566:Electrodes; D004583:Electrons; D007477:Ions; D010315:Particle Accelerators / Q000295:instrumentation*; D011829:Radiation Dosage; D011839:Radiation, Ionizing; D011874:Radiometry; D011878:Radiotherapy / Q000295:instrumentation / Q000379:methods*; D011879:Radiotherapy Dosage
D007477:Ions