Wall correction factors, P{sub wall}, for thimble ionization chambers
- Ottawa-Carleton Institute of Physics, Carleton University, Ottawa, K1S 5B6 (Canada)
The EGSnrc Monte Carlo user-code CSnrc is used to calculate wall correction factors, P{sub wall}, for thimble ionization chambers in photon and electron beams. CSnrc calculated values of P{sub wall} give closer agreement with previous experimental results than do the values from the standard formalism used in current dosimetry protocols. A set of P{sub wall} values, computed at the reference depth in water, is presented for several commonly used thimble chambers. These values differ from the commonly used values by up to 0.8% for megavoltage photon beams, particularly for nominal beam energies below 6 MV. The sleeve effect, which is not currently taken into account by the TG-51 dosimetry protocol, is computed to be up to 0.3% and is in some cases larger than the P{sub wall} correction itself. In electron beams, where dosimetry protocols assume a wall correction of unity, CSnrc calculations show P{sub wall} values of up to 0.6% at the reference depth, depending on the wall material. P{sub wall} is shown to be sensitive to the depth of measurement, varying by 2.5% for a graphite-walled cylindrical Farmer-like chamber between a depth of 0.5 cm and R{sub 50} in a 6 MeV electron beam.
- OSTI ID:
- 20775070
- Journal Information:
- Medical Physics, Vol. 33, Issue 2; Other Information: DOI: 10.1118/1.2161403; (c) 2006 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
- Country of Publication:
- United States
- Language:
- English
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