Experimental determination of wall correction factors. Pt. 1

Wall correction factors were measured for different types of ionisation chamber, 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 agreement, within 0.3%, after normalisation on reference chamber values, for most chambers. 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. Generally good agreement, within 0.5%, exists between experimental and calculated wall correction factors. For two chambers constructed from A-150 plastic larger deviations (0.6% and 1.4%) were observed. Due to these unexplained discrepancies the use of an average of calculated and measured values of wall correction factors in dosimetry protocols is recommended. (author)