Radioluminescence of NaMgF₃ : characterisation for radiotherapy.
Thesis DisciplineMedical physics
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This study seeks to evaluate doped synthetic NaMgF₃ as a luminescence material for applications in radiotherapy dosimetry. Two largely independent investigations were undertaken: a basic radiotherapy characterisation of a pre-existing prototype system incorporating NaMgF₃ activated with europium and manganese, and the fundamental characterisation of infrared emitting species in the same crystal host.
During the radiotherapy characterisation it was found that the tested system was in many regards unsuitable for use in radiotherapy, suffering from radiation damage, stem effects, and statistical uncertainty imposed by optical inefficiency. Radiation damage effects were found to be predominantly caused by radiation induced F–centres in NaMgF₃, while stem effects were dominated by Cerenkov radiation as seen in other fibre coupled systems at megavoltage x-ray energies.
It was identified that both of these difficulties could be substantially mitigated by exploiting luminescence at near infrared rather than visible wavelengths, spurring the subsequent investigation into infrared emitters.
A range of rare earth and transition metal dopants were then investigated in the same crystal host, of which divalent nickel was identified as the most promising. With this dopant a broad emission band occurs in the region of 1600 nm, at which wavelength the Cerenkov stem effect is theoretically reduced by at least an order of magnitude with respect to that detected in the near ultraviolet with a europium dopant. No colour centre absorptions were observed in undoped NaMgF₃ beyond 1000 nm, and the radiation damage effects measured with the nickel dopant are correspondingly negligible, on the order of 10⁻⁶ sensitivity loss per Gy of absorbed dose.
Samarium was also identified as a dopant with potential for dosimetry applications. Line emissions were observed from both divalent and trivalent ions, each with markedly different evolution of intensity with absorbed dose. In principle, tracking of total absorbed dose is possible by monitoring the intensity of each, although this is likely to be complicated by the radiation induced F–centre absorptions. The manganese dopant remains of interest as it provides relatively high sensitivity for a low atomic number, while activation with europium yields an efficient phosphor which may have applications in reusable planar dosimeters. The near ultraviolet emission of the latter is detected against both a larger background of Cerenkov radiation and the F–band absorption in NaMgF₃ rendering it unsuitable for real-time fibre coupled dosimetry.
Engineering refinements to facilitate infrared luminescence and improve optical efficiency of the prototype dosimetry system are also reported.