Temperature measurement in an electromagnetically hostile environment, such as a magnetic resonance imaging (MRI) scanner, is challenging. The monitoring of a patient's temperature within an MRI is necessary to ensure the patient is in a comfortable condition due to the following problem. Unfortunately, the radio frequency (RF) radiation in the imaging cavity of an MRI will induce heat into both the body tissue of a patient as well as into metallic elements of any sensors. This thesis presents a feasibility study on the production of an optical phosphor-based sensor for temperature monitoring within such a hostile environment. The concept of a desired optical sensor is based on the thermal dependence of phosphor luminescence. A pulsed ultraviolet light source is used to excite phosphors coated on a thin membrane which is in contact with the patient's skin. The skin temperature (over 27-37 ·C) can then be determined by calculating the exponential decay of luminescence intensity with time after the excitation pulse has ceased. In this research, two thermographic phosphors have been investigated, namely europium doped lanthanum oxysulphide (La202S: Eu) and terbium doped lanthanum oxysulphide (La20 2S: Tb). A wide range of dopant concentrations, 0.005-15 mol% for La20 2S: Eu and 0.005-50 for mol% La202S: Tb, have been characterised in terms of photoluminescent emission, decay time and crystallinity in determining their temperature dependent characteristics. Both phosphors have shown a dependency to dopant concentration through variance of peak intensity and decay time measurements over a low temperature range of 5 to 60°C. Although maximum brightness of the temperature dependent lines is achieved at dopant concentrations of 1 and 10 mol% for La202S: Eu and La2~S: Tb respectively, results have shown that optimum temperature dependency is at a lower mol% of 0.1 for La202S: Eu with a high quenching rate of 24.03 moe-I. Therefore, 0.1 mol% L1l202S: Eu appears to be an ideal candidate for use within an optical sensor. A key aspect of this research is that in comparison to conventional phosphor temperature dependent characteristic, it has been shown for the first time that La20 2S: Tb has an increase in decay time with respect to temperature for concentrations above 2 mol%. A hypothesis of this discovery based on the differences in energy transfer pathways is described. X-ray diffraction (XRD) analysis has demonstrated a linear relationship between the c-axis lattice parameter of a unit cell of the phosphors with dopant concentration. As dopancy increases, the (100) and (002) reflections merge and there is a reduction in the c-axis parameter as well as the crystallite size. In addition, a positive thermal expansion behaviour is observed for a and c parameters as well as the volume of the unit cell over the temperature range of interest
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:510264 |
| Date | January 2009 |
| Creators | Yap, Sook Voon |
| Publisher | Nottingham Trent University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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