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Ion Chamber Internal Temperature Measurements and Simulations for Thermal Equilibration Times After Ambient Temperature Changes

D Saenz

D Saenz*, N Kirby, A Gutierrez, University of Texas HSC SA, San Antonio, TX


PO-BPC-Exhibit Hall-8 (Saturday, March 5, 2016)  Room: Exhibit Hall

Purpose: Temperature and pressure corrections are necessary to account for varying air mass in the sensitive volume of a vented ionization chamber (IC). Locations used to measure temperature (in room, in phantom) may not accurately represent true IC air temperature, especially with a temperature change from storage to irradiation environment. The purpose of the study was to characterize the IC air temperature dependence upon changes in ambient temperature and understand where to take temperature measurements.

Methods: Thermal conduction properties were investigated by modifying a PTW 0.3cc 31013 Semi-flex IC with a thermocouple replacing the central electrode. The IC temperature was recorded in three phantom geometries characteristic of common output measurements. The phantoms were kept at ~15°C before measurement in a treatment vault (~21°C). The chamber was at the vault prior to measurement. Finally, simulations were conducted to simulate the thermal conductivity properties of other common ICs.

Results: Two thermal equilibria were recorded on different time scales. The chamber temperature initially dropped to that of the cool phantom in the first equilibrium but increased as the phantom itself equilibrated with room temperature. In a 25.5x25.5x23.4 cm³ cube phantom, IC equilibration with the phantom required 3 minutes, while final equilibrium with the room required >24 hours. In a 7.5 cm slab phantom, 2 minutes were required for a chamber at dmax to reach within 0.5° of the minimum temperature. In this geometry, the subsequent increase in temperature was observed over 2 hours as the phantom reached room temperature. Over 3 hours were required for final equilibration with a 2 cm slab phantom.

Conclusion: In-phantom temperature recording was far more accurate than measurement in the air. Wait times of 3 and 2 minutes are needed for a cube and 7.5 cm slab phantom, respectively, to achieve 0.2% dosimetric accuracy.

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