EXPERIMENTAL-DETERMINATION OF WALL CORRECTION FACTORS .1. CYLINDRICAL IONIZATION CHAMBERS

Wall correction factors were measured for different types of ionisation chamber. The procedure was to compare the reading of an unknown chamber with that of a reference chamber (graphite Farmer chamber with graphite build-up cap), both in air in a 60Co gamma-ray beam and in a water phantom irradiated by a 20 MeV electron beam. A comparison of the authors' values with other experimental values shows good agreement, within 0.3%, after normalisation on the reference chamber values, for most chambers. However, a 0.9% lower value was observed for a nylon-walled Farmer chamber suggesting that differences in thickness of the inner surface of the thimble might occur. The use of such an inhomogeneous ionisation chamber as a reference instrument for absorbed dose determinations at the reference point in a phantom should therefore not be recommended. Generally good agreement, within 0.5%, exists between experimental and calculated wall correction factors.