NRC-CNRC NRC NRC - - CNRC CNRC Direct calibration of ion chambers in linac electron beams Malcolm McEwen & Carl Ross Workshop on Absorbed Dose and Air Kerma Primary Standards LNE‐LNHB, Paris 9‐11 May 2007 Ionizing Radiation Standards National Research Council Ottawa, Canada
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Direct calibration of ion chambers in linac electron beams
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Direct calibration of ion chambers in linac electron beamsMalcolm McEwen & Carl Ross
Workshop on Absorbed Dose and Air Kerma Primary Standards
LNE‐LNHB, Paris9‐11 May 2007
Ionizing Radiation Standards National Research Council
Ottawa, Canada
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Outline of project
• Aim - to obtain absorbed dose calibration coefficients for ion chambers in megavoltage electron beams from a clinical linac
• Follow on from photon beam measurements completed in 2005
In general, electrons are more troublesome
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Water calorimetry - electrons
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Water calorimetry - electrons
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Water calorimetry - electrons
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Water calorimetry - electrons
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Water calorimetry - electrons
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Issues for calorimetry
LINAC - Performance of monitor chamber
CALORIMETER – Vessel geometry
CHAMBER – Type, Ion recombination
Most of the time in calorimetry is spent measuring ΔT
But there are a number of other factors:
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• All clinical linacs use a multi-element ion chamber to control output – doserate, stability, uniformity
• Any variability in the performance of this ion chamber will increase the uncertainty in dose measurements
• Short-term very good –standard deviation on a set of 100 MU runs is 0.06%
• Need ± 0.1% stability over the course of a day –generally meet this requirement, worst case drift ~ 0.3% over 8 hours