TITLE:
Thermal Equilibration in the Cavity Volume of a Farmer Ion Chamber for Routine Dosimetry
AUTHORS:
Yuichi Kato, Hiraku Fuse, Kazuya Shinoda, Katsumi Miyamoto, Tatsuya Fujisaki
KEYWORDS:
Farmer Ion Chamber, Thermal Equilibrium Time, Temperature Characteristic, Routine Dosimetry
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.4 No.4,
November
4,
2015
ABSTRACT: A Farmer ion chamber with an air cavity volume is the most widely used dosimeter for accurate dose determinations in radiotherapy. The quantity of ionization in the cavity volume occurred a given radiation dose has to be corrected to the cavity air temperature according to a dosimetry protocol because the mass of air in the cavity volume is subject to atmospheric variations. In the present study, we aim to measure the thermal equilibration time in the cavity volume of a Farmer ion chamber for the routine dosimetry. The Farmer ion chamber’s electrode was replaced by a thin thermocouple and coated by the PMMA for a waterproofing so that the measurement of the temperature in the cavity performed in water. As a result of the measurement, A Farmer ion chamber in thermal equilibrium with waterproofing equilibrates rapidly, followed by an exponential fall-off. In water, equilibration to less than 10% of the initial temperature difference required only a few minutes. Thermal equilibrium time is hardly affected by the room temperature change.