Method of deriving the response curve of an ionisation chamber from experimental values of its air kerma calibration coefficients measured in standardised X-ray beams

Abstract

Air-equivalent ionisation chambers are the reference instruments for measuring air kerma in dosimetry calibration laboratories. The energy dependence of the response of a given ionisation chamber is usually characterised by a discrete set of values of its air kerma calibration coefficients, N_K,a, measured in standardised photon beams. For photon energies below 300 keV, the most commonly used standardised calibration beams are the X-ray beams, i.e. beams with continuous spectra.

The energy dependence of a given ionisation chamber could alternatively be characterised by the response function f_IK(E) which describes the dependence of the magnitude of the electrical response of the chamber on the photon energy, E, at a constant air kerma rate. In practice, this method of characterising the energy dependence of the ionisation chamber response is rarely used because monoenergetic calibration photon beams covering the above mentioned energy range are not routinely available and it is therefore difficult to measure the response function f_IK(E) experimentally.

In this work, we show that:

a) the response function f_IK(E) of a given ionisation chamber can be derived from the values of its air kerma calibration coefficients, N_K,a, measured in standardised X-ray calibration beams;

b) the proposed method can reveal possible internal inconsistencies in the set of air kerma calibration coefficients, N_K,a, of an ionisation chamber;

c) the response function f_IK(E) of a given ionisation chamber derived from a set of its N_K,a values can be used to predict the N_K,a values of the chamber for other X-ray beams.