Development and analysis of a radiofluorogenic polymer material for recording depth-dose profiles related to radiation processing of surfaces

Low-energy (80-300 keV) electron beams find applications in many types of irradiation procedures focused on surface treatment, such as surface microbiological decontamination, sterilization of food packaging, pharmaceutical products and disposable medical devices. The very low penetration depth of these electrons, usually in the range of 50-400 μm in water, hinders an easy assessment of the absorbed dose in the material when measuring with commercial dosimetry systems like radiochromic or alanine films. Part of the dosimetry process involves the establishment of the depth-dose relationship while measuring optical density values of stacked radiochromic B3 films, each ~18 μm thick. The measured depth-dose curve has then to be extrapolated to the surface and combined with knowledge of the temperature and dose sensitivity of the alanine thin films to give the information about the surface dose with traceability to national standards. Consequently, direct information about the surface dose cannot be extracted, and the concept of correcting all measured doses to the average dose to water in the first micrometer, Dμ, needs to be introduced.
This PhD project aimed to develop a new 3D polymer material for use as a dosimeter in low-energy electron beam irradiations and to investigate fluorescence microscopy as a readout technique for dosimetry purposes, with a focus on depth-dose profiles and a more precise surface dose estimation. The chosen radiation-sensitive substance was pararosaniline leuco dye, which has a known radiochromic response used in the Risø B3 dosimeter film. The dye, once placed in a rigid polymer matrix and irradiated with low-energy electrons, becomes fluorescent and shows a depth-dependent fluorescence intensity response. The developed polymer material was prepared by means of a photopolymerization reaction, and the radiofluorogenic polymer material then obtained was irradiated with low-energy electron beams, with absorbed doses at the surface ranging up to 50 kilograys, and measured using two types of fluorescence microscopy: wide-field fluorescence microscopy and confocal laser scanning microscopy.
The optimal composition of the polymer matrix and preparation conditions were analyzed. The characterization of the material in terms of its thermal stability, glass transition temperature, rheological behavior, and water affinity was conducted, revealing a closely cross-linked polymer of high thermal stability and hydrophilic properties, which remains in a rubbery state while working at room temperature. The optical properties, such as refractive index, were assessed using optical coherence tomography. The proof-of-concept measurements of the depth-dependent fluorescence intensity response were evaluated using wide-field fluorescence microscopy. It was proposed that the influence of the out-of-focus signal would be solved by using confocal laser scanning microscopy, which possesses optical sectioning ability that enables 3D measurements of fluorescence intensity signals. The axial profiles of surface reflection and fluorescence signal were measured and theoretically analyzed, concluding that the specificity of the readout method and its limited resolution do not allow for a direct, precise assessment of the surface signal. The fluorescence signal increased linearly with the absorbed dose, but the very intense localized illumination from the laser caused significant photobleaching of the dye. The difficulty of quantifying the fluorescence intensity measurements is discussed, showing that several factors related to the microscope setup, fluorophore and sample contributed to the overall uncertainty of the measurement.
The findings indicate that the applicability of fluorescence microscopy as a technique for dosimetry of low-energy electron beams might be limited due to its complexity and poor reproducibility. However, investigating more sophisticated fluorescence methods, such as 2-photon light- sheet microscopy, could alleviate some of the challenges presented in this thesis.