TH-AB-201-06: Examining the Influence of Humidity On Reference Ion Chamber Performance
- McMaster University, Hamilton, ON (Canada)
- National Research Council, Ottawa, ON (Canada)
Purpose: International dosimetry protocols require measurements made with a vented ionization chamber to be corrected for the influence of air density by using the standard temperature-pressure correction factor. The effect of humidity, on the other hand, is generally ignored with the provision that the relative humidity is between certain limits (15% to 80%). However, there is little experimental data in the published literature as to the true effect of humidity on modern reference-class ion chambers. This investigation used two different radiation beams – a Co-60 irradiator and a Sr-90 check source – to examine the effect of humidity on several versions of the standard Farmer-type ion chamber. Methods: An environmental cabinet controlled the humidity. For the Co-60 beam, the irradiation was external, whereas for the Sr-90 measurements, the source itself was placed within the cabinet. Extensive measurements were carried out to ensure that the experimental setup provided reproducible readings. Four chamber types were investigated: IBA FC65-G (×2), IBA FC65-P, PTW30013 & Exradin A19. The different wall materials provided potentially different mechanical responses (i.e., in terms of expansion/contraction) to the water content in the air. The relative humidity was varied between 8 % and 97 % and measurements were made with increasing and decreasing humidity to investigate possible hysteresis effects. Results: Measurements in Co-60 were consistent with the published data obtained with primary standard cavity chambers in ICRU Report 31. Ionization currents with Sr-90 showed no dependence with the relative humidity, within the measurement uncertainties. Very good repeatability of the ionization current was obtained over successive wet/dry cycles, no hysteresis was observed, and there was no dependence on chamber type. Conclusion: This null result is very encouraging as it indicates that humidity has no significant effect on these particular types of ionization chambers, consistent with recommendations in current megavoltage dosimetry protocols.
- OSTI ID:
- 22679188
- Journal Information:
- Medical Physics, Vol. 43, Issue 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
- Country of Publication:
- United States
- Language:
- English
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