Ionization Chamber Dosimetry for High Energy Photon and Electron Beams: An Experimental Study

Abstract

<p> A number of recently published papers have been critical of the values of CE and Cλ by the ICRU as absorbed dose conversion factors for ionization chambers used in electron or photon beams.</p> <p> This report examines the subject in light of these recent works. Presented is a review of both theoretical and experimental results published to date along with the results of experimental work carried out at this facility. The present study concentrated on the measurement of the absorbed dose conversion factors for electrons and 25 NW photons. It also attempted to determine how the conversion factors might be influenced by such factors as chamber design buildup cap thickness and composition, and phantom composition.</p> <p> Measurements were made using electrons with 10 to 32 MeV initial energy and a 25 MeV photon beam, all produced by a Sagittaire Model Therac 40 linear accelerator. Checks on the stability of the measurement system were made using a 60Co source. Fricke ferrous sulfate dosimetry was used for absorbed dose measurement in the determination of CE.</p> <p> The results of the study would indicate that there is little or no effect on the measured values of CE or Cλ introduced by either the sleeves or phantoms for the radiation qualities studied. The results of the absorbed dose conversion factor study would indicate that the values recommended by the ICRU are sufficiently accurate for use in most instances. The data would tend to support the arguement that if there is an error in one of the ICRU derivations, the error in theory lies in the CE determination rather than that for Cλ.</p>