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Re-establishing the absorbed dose primary standard for photon beams on the NPL clinical linac.

Pearce, J A D; Shipley, D R (2010) Re-establishing the absorbed dose primary standard for photon beams on the NPL clinical linac. NPL Report. IR 24

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Abstract

The NPL Elekta clinical linac was officially opened in November 2008. Since then, graphite calorimetry with the existing primary standard calorimeter and ionisation chamber measurements in both graphite and water phantoms have been made at each of the seven available x-ray energies. Monte Carlo simulations of the NPL Elekta linac, the existing primary standard calorimeter and both graphite and water phantoms have also been completed in support of the measurements to enable the complete realisation of absorbed dose to water for photon beams. This report describes the work that was carried out to re-establish the existing UK primary standard of absorbed dose to water on this new facility for use in the photon therapy level calibration services offered by NPL. A set of reference 2611 type ionisation chambers were calibrated in terms of absorbed dose to water, Nw , with a total standard uncertainty of 0.49% (k=1). The values of kQ , the ratio of the calibration factor at a beam quality Q to that in a cobalt-60 beam, show good agreement (within 0.4%) with kQ values currently in use with a standard uncertainty 0.56% (k=1). For now, 2611 ionisation chamber calibrations will continue to be reported by normalising the existing kQ curve, however this will be reconsidered when the new NPL primary standard calorimeter is commissioned and sufficient data from this new calorimeter is available.

Item Type: Report/Guide (NPL Report)
NPL Report No.: IR 24
Subjects: Ionising Radiation
Ionising Radiation > Dosimetry
Last Modified: 02 Feb 2018 13:14
URI: http://eprintspublications.npl.co.uk/id/eprint/4840

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