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Evaluation of factors to convert absorbed dose calibrations in graphite to water for mega-voltage photon beams.

Nutbrown, R F; Duane, S; Shipley, D R; Thomas, R A S (2000) Evaluation of factors to convert absorbed dose calibrations in graphite to water for mega-voltage photon beams. NPL Report. CIRM 37

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Abstract

The National Physical Laboratory (NPL) provides a high-energy (4-19 MV) X-ray calibration service for secondary standard dosemeters in terms of absorbed dose to water. As the primary standard that is used for this calibration service is a graphite calorimeter absorbed dose calibrations must be converted from graphite to water. The conversion factors currently in use were determined prior to the commencement of this service. Since this time it has been identified that clinical LINACs have more inherent filtration than the NPL LINAC and so calibrations are now carried out for heavily filtered qualities. The conversion factors for heavily filtered qualities were determined by interpolation and extrapolation of lightly filtered results as a function of TPR (tissue phantom ratio). This work aims to evaluate these factors for all mega-voltage photon energies provided by the NPL LINAC for both lightly and heavily filtered qualities in two ways. The first method involves the use of the photon fluence scaling theorem. This states that if two blocks of different material are irradiated by the same photon beam, and if all dimensions are scaled in the inverse ratio of the electron densities of the two media, then, assuming that all photon interactions occur by Compton scatter the photon attenuation and scatter factors at corresponding scaled points of measurement in the phantom will be identical. The second method involves making in-phantom measurements of chamber response at a constant target-chamber distance. Monte Carlo techniques are then used to determine the corresponding dose to the medium in order to determine the chamber calibration factor directly. Values of the ratio of absorbed dose calibration factors in water to graphite determined in these two ways agreed with each other to within 0.2% (1 s uncertainty). The best fit to both sets of results agrees with values determined in previous work to within 0.4% (2 o uncertainty).

Item Type: Report/Guide (NPL Report)
NPL Report No.: CIRM 37
Subjects: Ionising Radiation
Ionising Radiation > Dosimetry
Last Modified: 02 Feb 2018 13:17
URI: http://eprintspublications.npl.co.uk/id/eprint/1593

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