Corrections for scattered photons in free-air ionisation chambers

Abstract

Although most recent interest and development in radiation dosimetry has been centered on higher energy radiations, calibrations of conventional x-ray machines for radiotherapy are still based on free-air ionisation chamber measurements. Such measurements require a correction for ionisation produced by secondary photons scattered out of the primary beam, and although the correction is relatively small (generally less than 1%) the data for deriving it are neither as complete or satisfactory as might be wished. This paper describes the derivation of theoretical expressions which allow the calculation of corrections, and compares calculated values with experimental results. It is apparent from the study that in determining the photon gain correction, account should be taken not only of the chamber geometry but also of photon scattering from the wall materials and, for higher energy beams in particular, of contributions from diaphragm scatter and possibly stray radiation penetration of the chamber.

New National Air-Kerma Standard for Low-Energy Electronic Brachytherapy Sources

“…Thus, an uncertainty associated with k dia = 1.0 is estimated to be 0.001 for the Ritz FAC. As McEwan [ 42 ] has pointed out, the correction factor should vary as the inverse of the diaphragm diameter, d . More completely, the correction factor should vary as the ratio of the area of the cylindrical sides of the diaphragm to the defining area of the diaphragm, which is 4 h / d , with h the thickness of the cylindrical diaphragm aperture.…”

Efficient Monte Carlo simulations in kilovoltage x-ray beams

“…The observation that aperture leakage must be taken into account, noted in previous work, 124,129 ' 130 is confirmed by this investigation. The computed aperture correction for the MEES varies between 1.00055 and 1.0013 for the beam qualities investigated.…”

“…However, A.b, the correction for photon backscattering into the scoring field, becomes non-negligible for low-energy x-ray beams and should be taken into account. The correction factor A ap is ignored in the NRC standard, although McEwan,129 Grimbergen et al,124 and Kurosawa and Takata130 have pointed out the need to take into account photons leaking through, or bouncing off, the aperture diaphragm to enter the FAC. The definitions of ^4 sca t and A e i oss given here are consistent with previous MC calculations of these factors 124 " 126 ' 128 (except that Burns 128 splits 74 scat into separate scatter and fluorescence corrections).…”

Free-air ionization chambers

“…McEwan [34] made measurements using a similar method as well as an alternative method, proposed by Kemp [2], in which the ionization produced between concentric cylinders was measured. McEwan also carried out a theoretical analysis of photon scatter.…”