ESTRO 2024 - Abstract Book

S3369

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

"effective pulse width" fit parameter. From the solutions of the HPM, an analytical expression for free electron fraction f e , charge collection efficiency f and the ion recombination correction factor k s = 1/f is obtained. With these expressions in hand, an extrapolation procedure is employed. The results from the extrapolation procedure are compared to data from Figures 6a and 6d from Kranzer et al. [2]

Results:

Several interesting features are present in the expression for the free electron fraction and charge collection efficiency (1) even and odd powers of the chamber voltage are present, (2) coefficient terms in the form of exponentials are present due to the attachment rate expression, (3) the conventional Boag expression for the free electron fraction is recovered in the high voltage limit of f e , (4) the expression is an asymptotic series and (5) the leading order voltage dependence goes like 1/V 2 , in contrast to the 1/V dependence from pulsed Boag theory. Due to being an asymptotic series, the optimal truncation of the series for the collection efficiency and consequently the recombination correction factor are determined. The expression for the recombination correction factor is then expanded to order 1/V 2 , in accordance with observation (5). A general equation, consisting of a constant term and a 1/V 2 term is also proposed. The derived expression (to order 1/V 2 ) and the general expression for the recombination correction are fit to the data (above 200-300V, depending on DPP value) from Figures 5a and 5b of Kranzer et al. [2]. These figures present the ratio Q(300V)/Q(V) against voltage for the Advanced Markus (plate separation of 1 mm) and EWC05 chamber (plate separation of 0.5 mm), respectively. This ratio is equivalent to f(300V)k s (V) . An extrapolation of the fits to 1/V=0 , yields an estimate for f(300V) and are plotted against measured values from Figures 6a and 6d from Kranzer et al. [2], shown in Figures 1 and 2 below, respectively.