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Development of a portable graphite calorimeter for photons and electrons.

McEwen, M R; Duane, S (1998) Development of a portable graphite calorimeter for photons and electrons. NPL Report. CIRM 18

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The aim of this project is to develop a calorimeter for use in both electron and photon beams. The calorimeter should be more robust than the present NPL primary standard X-ray calorimeter and is designed to be sufficiently portable to enable measurements at clinical accelerators away from NPL. Although intended for therapy-level dosimetry, the new calorimeter can also be used for high-dose measurements at industrial facilities. The system consists of a front end (the calorimeter itself), means for thermal isolation and temperature control, and a measurement system based on thermistors in a DC Wheatstone bridge. The early part of the project focussed on the development of a temperature control system sensitive enough to allow measurements of temperature rises of the order of 1 mK. The control system responds to the calorimeter, phantom and air temperature and maintains the temperature of the calorimeter to within ±0.2mK over several hours. Initial operations at NPL in X-ray and electron beams show that the system is capable of measurements of 1 Gy at 2 Gy/mm with a random uncertainty of ±0.5% (1 standard deviation). The result of comparing the calorimeter with the present primary standard calorimeter in 6, 10 and 16 MV photon beams is very encouraging, giving agreement within 0.5% at all energies measured, which is within the measurement uncertainties. The results in a 16 MeV electron beam are somewhat confusing, showing a discrepancy of approximately 5%. There is no obvious reason for this discrepancy, although a possible cause is charged storage in the calorimeter core due to poor earthing. Further work is required to investigate this problem, as well as carry out a wider range of measurements in both photon and electron beams.

Item Type: Report/Guide (NPL Report)
NPL Report No.: CIRM 18
Subjects: Ionising Radiation
Ionising Radiation > Dosimetry
Last Modified: 10 Sep 2018 13:20
URI: http://eprintspublications.npl.co.uk/id/eprint/1002

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