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Calculation of the response of a NE2561 ion chamber in a water phantom for high energy photons.

Nutbrown, R F; Shipley, D R (2000) Calculation of the response of a NE2561 ion chamber in a water phantom for high energy photons. NPL Report. CIRM 40

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Abstract

Absorbed dose to water calibrations of NE Technology Type 2561 chambers are carried out at the National Physical Laboratory (NPL) in both X-radiation from 4 - 19 MV and in 60Co -radiation. Secondary standard ion chambers are calibrated against the primary standard graphite calorimeter by inserting these chambers at reference depths in a water phantom. The first part of this report describes the work which was undertaken in an attempt to reproduce the measured absorbed dose calibration factors for a secondary standard NE2561 ion chamber for a range of NPL photon qualities under standard calibration conditions. This calculated data did not reproduce the observed variation in calibration factor with beam filtration in the measured results. However, had this data been reproducible, in particular at 4 and 6 MV, then the possible causes of the observed variation in the measured results such as effects on the calibration due to chamber components would have been investigated. By comparing calibration factors and depth dose curves for NPL X-ray beams with a simulated clinical quality the practical impact on clinical dosimetry of these observed differences will be looked at.
The majority of this work was carried out using Monte Carlo techniques based on the EGS4 code system. The simulations were validated in several ways. Firstly, comparisons of calculated and measured depth dose curves in water were made at energies from 4 - 19 MV and in 60Co -radiation. Then, data calculated with codes used in this work was compared with data produced by more well tested and well-established codes. The final validation method involved calculating the air kerma calibration factor of a NE2561 ion chamber in 2 MV X-rays and then comparing the calculated ratio of 2 MV air kerma calibration factor to 60Co absorbed dose calibration factor with the measured value.
The calculated normalised absorbed dose to water calibrations were found to agree with measured values of kQ, the ratio of absorbed dose calibration in quality Q to that in 60Co, to within 1.0% (which corresponds to 2 s in the total uncertainty). The observed discrepancies were primarily random in nature. However, it was not possible to distinguish between the results for light and heavy filtration. This could have been due to large uncertainties on the calculated absorbed dose to water calibrations or to an undetected problem in the calculations or measurements. It was therefore not possible at this stage to investigate the observed change in calibration factor with beam filtration. The calculated calibration factor for a 6 MV X-ray beam from a Varian 2100C does, however, lie close to the NPL curve for heavily filtered beams (with a TPR20/10 and depth dose curve similar to that of a NPL 6 MV (H) photon quality). The calculated and measured ratio of 2 MV air kerma calibration factor to 60Co absorbed dose calibration factor agreed within 0.66% (which corresponds to less than 2 s in the total random uncertainty).

Item Type: Report/Guide (NPL Report)
NPL Report No.: CIRM 40
Subjects: Ionising Radiation
Ionising Radiation > Dosimetry
Last Modified: 02 Feb 2018 13:17
URI: http://eprintspublications.npl.co.uk/id/eprint/1615

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