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, i t is necessary to know the fraction of the ionization arising in the c hamber 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 calculat e alpha. These measurements are made using the examined chamber with a nd without a buildup cap and one reference chamber of wall material eq uivalent to the medium (i.e., in our case, Al and A-150 were used as w all 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 th e independence of a 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 ct depends on the irradiation conditions; it increa ses with field size and depth. (C) 1997 American Association of Physic ists in Medicine.