Surface dose assessment using extrapolation chambers can achieve accuracy within 1% when small electrode separations are used and depth is appropriately defined. The results confirm that the measured % ionization ratios for gaps s ≤ 1 s 1 mm are constant because perturbation effects are minimized. Extrapolation from larger gaps is unreliable due to increasing changes in P repl P ₑ₄₋ and extrapolation is only needed to establish where % ionization ratios become constant for small gaps and are hence accurate. Surface measurements with fixed parallel-plate chambers are inaccurate if the gap size is too large.
Reis et al. (Wed,) studied this question.
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