For direct space X-ray imaging with an intrinsic spatial resolution of less than 10 µm scintillating detectors is used almost universally. The spatial resolution is limited by the thickness of the scintillator and by the numerical aperture of the optical connection between scintillator and photo detector. This establishes an inverse correlation between the spatial resolution and the detection efficiency which limits the performance of existing x-ray detectors.
The purpose of this Ph.D. project is to explore alternative paths of research, to develop x-ray detectors for the 30-100 keV energy range with single micrometre resolution without compromising efficiency. A number of detector types have been evaluated for this purpose. Structured scintillators are found to exhibit a high potential in terms of performance and also in terms of realizing an actual detector. The structured scintillator consists of a silicon matrix of holes filled with the phosphor material, CsI:Tl. The generated luminescence of the phosphor is guided by the silicon structure through total internal reflection. The spatial resolution of the scintillator is given by the size of the holes which is fabricated down to a distance of 1 µm between pores.
The potential of the structured scintillator is explored through Monte Carlo simulations. A spatial resolution of 1 µm is obtainable and for scintillators with a resolution between 1 µm and 8 µm the efficiency could be more than 15 times higher than a regular scintillator with corresponding spatial resolution.
The porous silicon substrate for the structured scintillator is realized both with advanced electrochemical etching and highly refined plasma etching. These techniques can produce structures with an aspect ratio of more than 100. The process of filling scintillator into pores was developed to produce single crystals with a limited number of air bubbles. To obtain a smooth homogenous surface from the two materials; silicon and CsI, an index matching coating has been applied.
Resolutions down to 1.3 µm at 50 keV have been measured and compared to conventional scintillators a 3-5 times gain in efficiency has been found. The homogeneity of the structured scintillators can be comparable to regular scintillators although so far the yield of useable samples from the coating process is low.
The radiation hardness of the scintillator has been measured. Doses of more than 107 Gy from synchrotron white beam gives no decrease in performance on uncoated samples. However for coated samples the decrease in efficiency is above 50 % at the same dose and the polymer surface coating shows severe structural damage.
- Materials characterization and modelling
- Materials research