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Type: Thesis
Title: The characterisation of NaCl crystal for a reusable radiation dosimeter
Author: Chapsky, Alexandra
Issue Date: 2020
School/Discipline: School of Physical Sciences : Medical Physics
Abstract: Dosimetric properties of radiation sensitive materials have long been of interest for applications such as retrospective and accidental dose indicators. Their dose storage capabilities can also be exploited in radiotherapy. Dose verification is required to ensure that the planned treatment dose is accurately delivered. The dose deposition during quality assurance or treatment plans verification in radiotherapy is measured using point detectors, located in phantoms, and film dosimeters. A 3D dosimeter could enable the measurement of a dose distribution with a high resolution. A concept for a reusable 3D dosimeter, based on detection and localization of optical signal emitted from an irradiated 3D crystal upon laser stimulation has been developed in this work. Several transparent optical window materials (NaCl, BK7, ZnS cleartran, borosilicate glass, SiO₂, IR SiO₂, fused silica, ZnSe, MgO, CaF₂, ZnS FLIR, AlON, BaF₂, MgF₂, MgAl₂O₄, Al₂O₃) were tested for their thermoluminescence response to photon irradiation. NaCl was shown to have the desirable properties of a dosimeter and further properties essential for its application in clinical radiotherapy were characterised. Dose response of the optically stimulated luminescence (OSL) from grained NaCl showed high sensitivity, repeatability and near linear dose response, with variations in dose sensitivity between samples. An optical setup was designed in order to perform 2D imaging of radiation dose deposition. 10 Gy from a superficial x-ray generator was applied to a NaCl crystal in two patterns – uniform and a strip. Optical stimulation was provided using a 100 mW, 488 nm laser, and the emitted signal, characteristic to OSL, was detected in compliance with the deposited dose distribution, with optical signal detected only from the irradiated volume. The behavior of large NaCl crystal under exposure to ionizing radiation was examined in more detail using optical stimulation from a 475 nm, nominal 25 mW laser for doses in the range of 1 – 15 Gy, showing repeatable, near linear dose response of all crystals. Calibration factors were derived to correct for discrepancies between crystals. Efficiency of various stimulation wavelengths was tested for their capability of removal of radiative traps in a crystal. The high efficiency of wavelengths <550 nm, and almost negligible trap removal by wavelength >700 nm propose the potential application of two methods for 3D dose measurements: by stimulation of OSL using short wavelength or by shining the crystal with long wavelength and collection of the photons, scattered from the dose dependent colour centres. Imaging of localised dose deposition based on detection of photons scattered from the colour centres during illumination of the crystal with 852 nm laser (rather than OSL stimulation with 475 nm laser) showed linear dose dependence but requires some fine tuning of the optical setup to improve SNR and accuracy. Imaging of localised dose deposition combined with scanning of the detector in the vertical direction and at different depths of the sensor could potentially provide the third dimension with continuous dosimetric data.
Advisor: Spooner, NA
Santos, Alexandre
Rutten, Thomas
Dissertation Note: Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 2020
Keywords: Dosimetry
3D luminescence imaging
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