The Luminescence of Lanthanide-doped NaMgF3 Nanoparticles: Radioluminescence, Radiophotoluminescence and Optically Stimulated Luminescence
Authors:   , Nalumaga Hellen 
Published: August 2022

<p><b>NaMgF3 is a host material that has demonstrated luminescent properties suitable for radiation dose measurements when doped with lanthanide (Ln) ions. NaMgF3 has a response to ionising radiation similar to that of tissue, and hence it has potential application in radiotherapy where a high level of accuracy is required. This thesis focused on investigations into the suitability of NaMgF3 nanoparticles for radiation dose measurements. Nanoparticles display advantages that can be exploited to increase spatial resolution in two-dimensional dosimetry. Additionally, nanoparticles exhibit new and different distributions of optically active traps, and their luminescence can be sensitised to increase the signal to noise. Hence, this thesis presents results obtained from the synthesis and characterisation of Ln-doped NaMgF3 nanoparticles. The nanoparticles were synthesised via the hydrothermal, reverse micelle and thermolysis techniques. Structural studies were carried out, followed by investigations on the effect of X-ray irradiation on the luminescent properties of the nanoparticles. It was observed that compared to the bulk materials, the nanoparticles exhibited increased oxygen (O2-)-related defect luminescence and favoured substitution of trivalent ions over the divalent ions. The behaviour was attributed to the stability of trivalent ions in aqueous solutions during nanoparticle synthesis, which encouraged increased incorporation of O2--related defects to create charge neutrality. O2--related defects allowed for O2- → Yb3+ charge transfer process, hence Yb3+ emission in the infrared region. X-ray stimulation resulted in Yb3+ → Yb2+ radiophotoluminescence (RPL), but the increase in Yb-dopant concentration revealed no significant effect on the structural and luminescent properties.</b></p>
<p> Most of the studied materials exhibited multiple luminescence properties for monitoring radiation doses, which would increase the dose accuracy. Ce-doped nanoparticles exhibited the first reported optically stimulated luminescence (OSL) in Ln-doped near tissue-equivalent NaMgF3 nanoparticles suitable for total radiation measurements. OSL occurred through a series of processes, including the creation of F-centres through X-ray irradiation and optical stimulation to release electrons that recombined with holes trapped at Ce4+ sites, followed by Ce3+ emission. In addition, the Ce3+ → Ce4+ RPL effect suitable for cumulative radiation monitoring was observed from isolated and aggregated Ce3+ sites in the Ce-doped nanoparticles, providing an additional method to increase dose accuracy. Furthermore, the Ce,Sm co-doped NaMgF3 nanoparticles demonstrated suitability for real-time and cumulative radiation dose monitoring via Sm radioluminescence after a priming dose and multiple Ce3+ → Ce4+, Sm3+ → Sm2+, and O2- → O- RPL signals through which cumulative radiation doses can be determined.</p>
<p> Finally, the applicability of singly doped oleic acid-capped NaMgF3:Eu and NaMgF3:Sm nanoparticles for radiation monitoring via the Eu3+ → Eu2+ and Sm3+ → Sm2+ RPL effects were demonstrated. The studies were followed by investigations of the sensitisation of Eu3+ and Sm3+ luminescence signals using 2-thenoyltrifluoroacetone (TTFA). The first successful sensitisation of NaMgF3 nanoparticles was demonstrated through remarkable TTFA → Eu3+ energy transfer. However, minimal TTFA → Sm3+ sensitisation was observed, attributable to a non-optimal energy difference between the TTFA triplet state and the Sm3+ resonance energy level, encouraging back energy transfer. TTFA → O2- sensitisation was additionally observed in the NaMgF3:Sm nanoparticles. Furthermore, enhancement of Sm3+ luminescence was additionally demonstrated in the Ce,Sm co-doped nanoparticles via energy transfer from the Ce3+ fully allowed 4f → 5d transitions. The studies resulted in a wealth of information, and where possible, detailed schematic models are presented to explain the observations, which will guide future research. Ultimately, nanoparticles display luminescent properties suitable for dosimetry applications, and the results encouraged further research into the application of nanoparticles as dosimeter materials.</p>