The effect of thickness of the waterproofing sheath on the calibration of photon and electron beams

The TG 21 protocol recommends using a thin sheath for waterproofing an ion chamber used in the calibration of photon and electron beams. A thickness of 0.5 mm is suggested for a material having a composition and density close to that of water. This work investigates the effect on the calibration of photon beams ranging from Co^{6 0} to 25 MV, and electron beams ranging from nominal energies of 7–18 MeV, for changes in the thickness of the waterproofing sheath from 0.5 to 5.5 mm. For photon beams, a maximum change of 1.2% was found for the 25 MV x ray beam. For electron beams, a maximum change of 0.5% was found for 10 MeV electrons. It is concluded that the thickness of the waterproofing sheath is not a very sensitive variable, assuming the thickness is between 0.5 and 2.0 mm.