Charge Build-Up Effects in Insulating Phantom Materials

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Abstract
When a medium is irradiated with electrons, electric charges are injected into the medium. If this is an electric insulator and its extension in the beam direction is larger than the electron range, the electrons are trapped inside the medium. In this work the influence on the ionization chamber dosimetry of the charge trapping in insulating phantom materials, such as polystyrene and polymethyl-methacrylate (PMMA), has been investigated. It has been shown that the electron fluence inside a cylindric ionization chamber used in these materials increases when the number of electrons trapped in the phantom increases. The influence of various parameters, such as electron energy, accumulated absorbed dose, type and purity of the phantom material, radiation induced conductivity and ionization chamber construction are discussed. The effect can cause errors of several per cent in the dosimetry. We conclude, in agreement with the conclusion of Galbraith et coll., that insulating phantom materials should therefore be avoided in electron beam dosimetry.