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Determining effective collecting volumes of ionization chambers by means of PTB’s microbeam

23.12.2020

Air‑filled ionization chambers are used to measure the radiation dose in radiation therapy. For this purpose, the conventional method consists in assuming that the entire air volume located between the central electrode and the outer electrode of the ionization chamber is the sensitive collecting volume of the detector. Due to the distribution of the electric field generated by the high voltage applied between the electrodes, it is, however, possible that non‑sensitive areas occur inside this air volume. In such non‑sensitive volumes, charge carriers caused by the incident radiation do not contribute to the chamber signal. Knowledge of the effective collecting volume is essential for the exact modeling of ionization chambers for purposes such as Monte Carlo calculations of detector‑specific correction factors or dose response.

The objective of this research work is to measure the effective collecting volume of new types of ionization chambers of different manufacturers at PTB’s microbeam in Braunschweig. Protons are accelerated up to an energy of 15 MeV in the cyclotron and then focused to a beam diameter of less than 10 µm at the microbeam facility. The ionization chambers are scanned in 2D in the focus plane of the beam by means of a motorized microscope stage. The signal generated by the protons in the chamber is measured at each point. The number of incident protons is monitored at the beam exit window by means of a scintillator.

It is the first time that non‑sensitive regions within the air volume of ionization chambers have been measured so close to the chamber’s stem. This is a point where the electric field lines do not run from the outer electrode to the central electrode as in the effective collecting volume, but from the outer electrode to the guard electrode in the stem of the chamber. Charge carriers generated by the radiation in these non‑sensitive regions do not therefore contribute to the chamber’s signal. This new measurement method allows current developments in ionization chamber dosimetry to be verified experimentally.

 

Fig. 1: Ionization chamber in the focus plane of PTB’s microbeam. The proton beam hits the chamber from the top, and the chamber’s signals are measured at every point in a grid with a spacing of 0.1 mm.

Fig. 2: Image of 2D relative signal distribution, measured for 1352 points across half of the chamber’s volume shown using the example of PTW’s Semiflex 3D 31021. The charge measured for every point decreases from yellow at the central electrode (bottom) to blue at the outer electrode (top). The non‑sensitive region is located at the top right.

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Opens local program for sending emailU. Giesen, Department 6.4, Working Group 6.45

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