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Determination of poton fluence spectra from NPL Mobaltron 60Co unit.

Shipley, D R; Duane, S (1994) Determination of poton fluence spectra from NPL Mobaltron 60Co unit. NPL Report. RSA(EXT) 46

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Absorbed dose to water calibrations of secondary standard hospital dosemeters are carried out at NPL in both X-radiation from 4 MV to 19 MV and in 6OCO y-radiation. Measurements are performed using the primary standard graphite calorimeter and by inserting the secondary standard and working standard chambers at a reference depth in either a water or graphite phantom. Details of the dose distributions and the responses of these chambers in each phantom for the NPL 6OCO radiation quality are, however, unknown. A knowledge of the photon fluence spectra produced by the NPL 6OCO unit is therefore required to enable this analysis to be carried out. This report describes the work that was undertaken to determine the photon fluence spectra incident on the front face of the water and graphite phantoms under standard measurement conditions. The spectra were validated by comparing calculated tissue-phantom ratios with recent experimental values.
The calculated TPR values were found to agree with measurements to within 1-20" (total uncertainty), and also with independent estimates obtained using the program A VERMV. The observed discrepancies were primarily systematic in nature with the largest source of uncertainty being the change in beam quality, and hence chamber calibration factor, with depth in the phantom.

Item Type: Report/Guide (NPL Report)
NPL Report No.: RSA(EXT) 46
Subjects: Ionising Radiation
Ionising Radiation > Dosimetry
Last Modified: 02 Feb 2018 13:19
URI: http://eprintspublications.npl.co.uk/id/eprint/1110

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