A method for measuring the ionization fraction due to the chamber wall (alpha) and assessing its characteristics

To calibrate a megavoltage therapy beam using an ionization chamber, it is necessary to know the fraction of the ionization arising in the chamber wall when this is made of a material different than the medium. A method for measuring the ionization fraction produced by electrons arising in the chamber wall (alpha) is presented here. The method uses three measurements at the same point in a medium in order to calculate alpha. These measurements are made using the examined chamber with and without a buildup cap and one reference chamber of wall material equivalent to the medium (i.e., in our case, A1 and A-150 were used as wall materials for the examined and the reference chamber, respectively). Using this method, it is possible to calculate alpha in the medium for a series of irradiation conditions and assess its characteristics. Two main conclusions came out of this assessment. The first one is the independence of alpha from the wall material, even if this is aluminum (alpha is only dependent on wall thickness expressed in g cm-2). The second one is that alpha depends on the irradiation conditions; it increases with field size and depth.