title: High-energy-photon dose measurements using exposure-calibrated ionization chambers.
abstract: The current method for determining absorbed dose from high-energy photons is to use in water a Cobalt-60 exposure-calibrated ionization chamber and Clambda. Recently questions have arisen about the correctness of currently recommended Clambda values and whether or not the Cobalt-60 buildup cap should be used for in-water dose measurements. The effect of the buidup cap on Clambda was obtained by measuring Farmer chamber response in water with and without the buildup cap. Results show no measurable change at Cobalt-60 or 4 MV but an increase of 1%-2% above 20 MV. The effects of the wall materials and the displacement factors on Clambda are also considered. It is shown that at high energies errors of 3%-5% can be made in determining dose in water using commercially available Farmer or Farmer-like chambers and currently recommended Clambda. The exact size of the error depends upon the materials and thicknesses of the wall and cap, the chamber cavity volume, and whether or not the buildup cap is used.
mesh_terms: D004601:Elementary Particles; D011874:Radiometry / Q000295:instrumentation*; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy