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Scatter and heat conduction correction factors for two glass vessel geometries in sealed water calorimeters.

Palmans, H; Defloor, M*; Benmokhtar, S; Rosser, K E (2003) Scatter and heat conduction correction factors for two glass vessel geometries in sealed water calorimeters. In: Workshop on Recent Advances in Absorbed Dose Standards, 19-21 August 2003, Melbourne, Australia.

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If properly designed, sealed water calorimeter (SWC) glass vessels containing high purity water induce only small perturbation effects and thus require small corrections which can be determined with acceptably small uncertainties. Nevertheless, it is worth performing a systematic investigation yielding generic formulae as a function of vessel dimensions and/or energy for those correction factors.
Two perturbations are due to the presence of a glass vessel: firstly, the perturbation of the radiation field at the point of measurement due to the change in scatter conditions and due to attenuation of the primary beam and secondly, the perturbation due to an excess temperature rise in the glass vessel which produces excess heat to flow towards the point of measurement.
In this work, two SWC vessel geometries are investigated for 60 Co photon beams: the cigar type and the pancake type. In the cigar type, which is used in many laboratories and is also called the Domen type, a cylindrical vessel is positioned perpendicular to the beam direction and the thermistors are normally positioned on the cylinder axis. In the pancake type, which is used at NPL and PTB, a cylindrical vessel with two parallel flat faces at the top and bottom is positioned with its cylinder axis coincident with the beam axis. For the cigar type, a set of glass vessels with various diameters and wall thicknesses is used to systematically measure the relative effects of both perturbations. For the pancake type, the work is restricted to numerical work at present.
The experimental determination of the scatter perturbation factors was performed using a diamond detector positioned at the point of measurement by comparing its response in the presence of the vessel with that in the absence of the vessel. The experimental results were compared with numerical results from Monte Carlo simulations using the egsnrc code system and from a simplified analytical model. The results for the cigar type vessels obtained by these three methods are found to be in good agreement and are represented by a simple formula as a function of the vessel wall thickness and the vessel diameter. The same set of vessels was used to measure their relative responses in the calorimeter operation. These were compared with results from heat conduction simulations with programs from Ghent and from a commercial package (FEMLab).

Item Type: Conference or Workshop Item (UNSPECIFIED)
Subjects: Ionising Radiation
Ionising Radiation > Dosimetry
Last Modified: 02 Feb 2018 13:16
URI: http://eprintspublications.npl.co.uk/id/eprint/3485

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