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Corrections to air kerma rate measurements from a 192Ir high dose rate brachytherapy source to free space conditions.

Nutbrown, R F; Sander, T (2000) Corrections to air kerma rate measurements from a 192Ir high dose rate brachytherapy source to free space conditions. NPL Report. CIRM 39

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Abstract

The purpose of this experiment was to determine the combined air scatter and air absorption correction necessary to apply to measurements of the air kerma rate made at a distance of 1 metre from a 192 Ir source free in air. This correction needs to be determined as the National Physical Laboratory (NPL) intends to set up a calibration service whereby secondary standard ionisation chambers can be calibrated against a primary standard in terms of air kerma rate at 1 metre in free space from a 192 Ir source. Measurements were made with 2 types of chamber. Firstly measurements were made with a 100 cm 3 spherical graphite transfer chamber and then measurements were repeated using a 1.79 cm 3 cylindrical cavity chamber which is one of a set of 3 chambers which form the primary standard for 60 Co and 137 Cs maintained by the NPL. As both these chambers were originally intended for use with photons of higher energies the graphite caps of the chambers provide a sufficient build up region to develop charged particle equilibrium. The 192 Ir source used is a pellet encapsulated in a stainless steel casing, with diameter 1.1 mm and length 5 mm. The source was exposed using a High Dose Rate (HDR) microSelectron brachytherapy unit manufactured by Nucletron. Results from measurements made using the transfer chamber gave an air attenuation correction to be applied to the measured charge at a source to chamber distance of 1 metre of (1.2 ± 0.3)%. Measurements made using the cylindrical cavity chamber could not be used to determine an estimate of the air attenuation correction for this set up.

Item Type: Report/Guide (NPL Report)
NPL Report No.: CIRM 39
Subjects: Ionising Radiation
Ionising Radiation > Dosimetry
Last Modified: 02 Feb 2018 13:17
URI: http://eprintspublications.npl.co.uk/id/eprint/1597

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