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Experimental Study of Humidity Effect On Charge Measurement of An Ionization Chamber in Clinical High-Energy Photon Beams

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Y Sato

Y Sato1,2*, M Shimizu1 , Y Morishita1 , M Sato2 , M Hoshina2 , (1) National Metrology Insiture of Japan, Tsukuba, Ibaraki, (2) Graduate school of Health Science, Komazawa University, Setagaya, Tokyo


SU-I-GPD-T-550 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Humidity effect on measured charges of an ionization chamber is one of important corrections in radiation dosimetry. In the present code of practice, the humidity correction is supposed independent of the radiation qualities and treated to be unity with an uncertainty of 0.1 %. However, this is based on the data measured for 50 kV-X rays, Cs-137 and Co-60 gamma rays by Niatel about 40 years ago. Here we plan to re-examine the Niatel’s results and to extend the measurements to any of clinical radiation beams (i.e. high-energy photon, electron and ion beams). In the presentation, we show the humidity corrections measured with high-energy photon beams.

Methods: We developed a sealed box with a controller to change and keep the humidity. It takes about 1 minutes to change the ambient humidity in the box to the target value. An ionization chamber (PTW 30013) with an aluminum build-up cap (thick: 3.9 mm) was placed in the box, and irradiated with high-energy photon beams from a clinical linac (6, 10 and 15 MV) after the ambient humidity has reached the target.

Results: The signal current changed rapidly at the start of the irradiation and became stable within 3 hours. This indicates that the humidity inside the ionization chamber changes with this time scale. We evaluated the humidity correction using the converged data and found it similar to those in TG-21 Appendix B. A ratio of W values of humid air was also evaluated by combining the measured results with simulations by the Monte Carlo code (EGS5), and the results were in good agreement with the Niatel’s result.

Conclusion: The humidity effects and the ratio of W values for high-energy photon beams were measured. The results are in agreement with those in TG-21 and by Niatel for Co-60 gamma rays.

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