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British Journal of Healthcare and Medical Research - Vol. 9, No. 2

Publication Date: April, 25, 2022

DOI:10.14738/jbemi.92.12183. Bangassi, T. N. K., Samba, O. N., & Raynaert, N. (2022). Correction Factors for the LSDG Free-Air Ionization Chamber for Low and

Medium-Energy X-Rays with Monte Carlo Calculations. British Journal of Healthcare and Medical Research, 9(2). 196-206.

Services for Science and Education – United Kingdom

Correction Factors for the LSDG Free-Air Ionization Chamber for

Low and Medium-Energy X-Rays with Monte Carlo Calculations

Thierry Narcisse Kouagou Bangassi

Department of Physics, University of Bangui, Central African Republic

Odette Ngano Samba

Department of Physics, University of Dschang, Cameroon

Nick Raynaert

Laboratory Standard Dosimetry of Gand, University of Ghent, Belgium

ABSTRACT

Advances in radiation dosimetry continue to improve the accuracy of calibrating

photon and electron beams of radiotherapy. This accuracy in the dose delivery to

the patient in radiotherapy depends to a large extent on the absolute dosimetry of

the irradiation unit. This necessitates the use of an ionization chamber such as Free

air ionization chambers (FAC), calibrated in a recognised Standard Dosimetry

Laboratory [1]. FACs are widely used by Primary Standard Dosimetry Labs (PSDL)

as primary standard for absolute measurement of air kerma in beam of low and

medium energy x-rays. It can accurately measure air kerma of x-ray beams of

energy up to about 300kV. The purpose of this investigation, was to determine the

correction factors of the Free Air Chamber (FAC) using the Monte Carlo calculations.

Key words: FAC corrections, X-rays, EGSnrc, Penelope, Monte Carlo simulation, radiation

transport

INTRODUCTION

Free-air chambers are in worldwide use by Primary Standard Dosimetry Labs (PSDL) as

primary standard measuring devices for x-rays produced by conventional x-ray tubes and

generating voltages in the range from about 5kV to 300kV. The FAC signal reading must be

corrected for several effects before it can be related to the air-kerma [2]. Typically, these

corrections account for scatter contribution to the signal, for the contribution of fluorescent

photons to the signal, for the leak and transmission contribution of photons through the

collimator to the signal, signal reduction due to electron loss in the collecting volume and

attenuation of the photon beam along the distance between the point of measure to the centre

of collecting volume (CV). While such correction factors have been determined experimentally,

measuring some of them directly involves modification of the primary standard. This is the case

for the corrections accounting for photon scatter, photon fluorescence and electron loss.

Therefore several authors have proposed to estimate these three correction factors by means

of a Monte Carlo simulation. The correction for attenuation, which is particularly important in

the soft x-ray region, is still obtained experimentally using the displacement technique (e.g.,

employed at Laboratory Standard Dosimetry of Gand (LSDG)) an evacuated-tube technique

(e.g., employed at National Research Council (NRC) of Canada) or a variable pressure tube