Master of Research
School of Physics
The aim of this project is to characterise an X-RAD 320 x-ray irradiator in the low energy spectrum range of 50-320kVp and develop methods for real-time 2d imaging of low energy x-rays using a MagicPlate-512 detector. The dose characteristics measured were the Output factor, depth dose, dose rate, and dose profile. The irradiator was characterised using the ‘American Association of Physicists in Medicine protocol for 40-300kV x-ray beam dosimetry’ (TG-61 protocol) to describe dose. First, the response was measured with a farmer chamber, using the TG-61 protocol the response was converted into dose. Using the farmer chamber the effect of different quantities on dose absorbed was tested including X-ray tube voltage, field size, SSD, and depth in water phantom. The ionisation chamber data allowed for the development of formula for dose absorbed by the MagicPlate-512 detector. EBT3 radiochromic film was used to compare the 2d dose imaging quality of the MagicPlate- 512. An attempt was made to develop a Geant4 Monte Carlo based simulation for comparison against detector materials, unfortunately the simulations did not show a similar relationship in dose deposited when compared to any of the other detector results. Other than the adjustable square collimator a 1cm (at 50cm SSD) pencil beam collimator was tested and found to be significantly rotationally unsymmetrical. To reduce electric noise the MagicPlate-512 detector was covered in aluminium tape, the tape and the detector housing was then connected to ground. To reduce the dose rate dependence, the detector was irradiated with 3Mrad, which improved the equalization of pixel response, comprehensive testing was not possible due to restricted lab access. When compared to a previous study by Rezvan which also used the X-RAD 320, once the difference in SSD and accounting for the backscatter factor the dose rate found experimentally using the farmer chamber was larger than Rezvan’s dose rate (21.2273mGy/s estimated at 50cm SSD compared to the 14.8333mGy/s found by Rezvan). Though this difference is significant the larger field size (24cm compared to 20cm) support that Rezvan’s dose should be lower. The MagicPlate-512 response illogically increased with depth in water phantom, at this low energy range the penetrating power of the x-rays should be relatively low meaning that the dose should be deposited at low depths.
Horsford, Reece, Small Animal X-ray Irradiator Characterisation, Master of Research thesis, School of Physics, University of Wollongong, 2022. https://ro.uow.edu.au/theses1/1545
FoR codes (2020)
5105 Medical and biological physics
Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.