Samarium doped alkaline earth halides as red-emitting scintillators and phosphors

<p>This thesis is concerned with the manufacture, spectroscopic characterisation, and radiation detection performance of three rare earth doped alkaline earth halides; these were designed for scintillation or phosphor detection of x-rays and γ-rays. The materials are transparent polycrystals of lanthanum or praseodymium stabilised cubic barium chloride ((La,Pr)₀.₁₂₅Ba₀.₈₇₅Cl₂.₁₂₅), BaCl₂ - SrCl₂ solid solutions, or single crystals of CaF₂. The primary dopant investigated was Sm²⁺ since this has a red emission in all the materials which is well matched to the spectral sensitivity of silicon photodiodes. The cubic structure of the polycrystalline materials is essential for optical transparency, and so the structural stability of the materials has been investigated using x ray diffraction and thermal analysis. For CaF₂ large single crystals were unintentionally produced without following the usual Bridgman-Stockbarger or Czochralski methods. All of the materials showed predominantly Sm²⁺ ions, and only in CaF₂ could evidence of Sm³⁺ ions also be seen. The spectroscopy of the 4f⁵5d¹ → 4f⁶ red emission, including lifetimes, and absorption of Sm²⁺ ions in all these materials is reported; a strong thermal cross over to 4f⁶ → 4f⁶ emission is observed and successfully modelled. A time correlated single photon counted system has been built to measure the scintillation decay time of these materials. The system yields decay times in excellent agreement with the literature values. The performance of the materials as scintillators is limited to varying degrees by the formation of colour centres which slow the electron-hole recombination process after x-irradiation. Ba₀.₃Sr₀.₇Cl₂:Sm was found to be a bright and fast x-ray phosphor. The integrated intensity (per x-ray half thickness of material) of the radioluminescence is ~ 30 % that of the commercial material, the scintillation lifetime is ~ 30 μs (c.f. milliseconds for Gd₂O₂S:Tb³⁺) and the imaging resolution is 6 LP/mm (c.f. 4.2 LP/mm for Gd₂O₂S:Tb³⁺). CaF₂:Sm²⁺ was shown to be a red-emitting scintillator with a decay time of ≤ 1 μs and a light output of 15,000 photons/MeV when cooled by dry ice. The x-ray imaging resolution was high at 8.5 LP/mm. Several of the materials have been tested for performance as neutron detecting phosphors by adding neutron capture elements such as gadolinium or lithium, the strongest emission observed was 6 % the integrated intensity of the standard material ⁶LiI(Eu²⁺).</p>

Samarium doped alkaline earth halides as red-emitting scintillators and phosphors

<p>This thesis is concerned with the manufacture, spectroscopic characterisation, and radiation detection performance of three rare earth doped alkaline earth halides; these were designed for scintillation or phosphor detection of x-rays and γ-rays. The materials are transparent polycrystals of lanthanum or praseodymium stabilised cubic barium chloride ((La,Pr)₀.₁₂₅Ba₀.₈₇₅Cl₂.₁₂₅), BaCl₂ - SrCl₂ solid solutions, or single crystals of CaF₂. The primary dopant investigated was Sm²⁺ since this has a red emission in all the materials which is well matched to the spectral sensitivity of silicon photodiodes. The cubic structure of the polycrystalline materials is essential for optical transparency, and so the structural stability of the materials has been investigated using x ray diffraction and thermal analysis. For CaF₂ large single crystals were unintentionally produced without following the usual Bridgman-Stockbarger or Czochralski methods. All of the materials showed predominantly Sm²⁺ ions, and only in CaF₂ could evidence of Sm³⁺ ions also be seen. The spectroscopy of the 4f⁵5d¹ → 4f⁶ red emission, including lifetimes, and absorption of Sm²⁺ ions in all these materials is reported; a strong thermal cross over to 4f⁶ → 4f⁶ emission is observed and successfully modelled. A time correlated single photon counted system has been built to measure the scintillation decay time of these materials. The system yields decay times in excellent agreement with the literature values. The performance of the materials as scintillators is limited to varying degrees by the formation of colour centres which slow the electron-hole recombination process after x-irradiation. Ba₀.₃Sr₀.₇Cl₂:Sm was found to be a bright and fast x-ray phosphor. The integrated intensity (per x-ray half thickness of material) of the radioluminescence is ~ 30 % that of the commercial material, the scintillation lifetime is ~ 30 μs (c.f. milliseconds for Gd₂O₂S:Tb³⁺) and the imaging resolution is 6 LP/mm (c.f. 4.2 LP/mm for Gd₂O₂S:Tb³⁺). CaF₂:Sm²⁺ was shown to be a red-emitting scintillator with a decay time of ≤ 1 μs and a light output of 15,000 photons/MeV when cooled by dry ice. The x-ray imaging resolution was high at 8.5 LP/mm. Several of the materials have been tested for performance as neutron detecting phosphors by adding neutron capture elements such as gadolinium or lithium, the strongest emission observed was 6 % the integrated intensity of the standard material ⁶LiI(Eu²⁺).</p>

Study on Aberration Correction of Adaptive Optics Based on Convolutional Neural Network

The existence of aberrations has always been an important limiting factor in the imaging field. Especially in optical microscopy imaging, the accumulated aberration of the optical system and the biological samples distorts the wavefront on the focal plane, thereby reducing the imaging resolution. Here, we propose an adaptive optical aberration correction method based on convolutional neural network. By establishing the relationship between the Zernike polynomial and the distorted wavefront, with the help of the fast calculation advantage of an artificial intelligence neural network, the distorted wavefront information can be output in a short time for the reconstruction of the wavefront to achieve the purpose of improving imaging resolution. Experimental results show that this method can effectively compensate the aberrations introduced by the system, agarose and HeLa cells. After correcting, the point spread function restored the doughnut-shape, and the resolution of the HeLa cell image increased about 20%.

Computing elastic properties of organic-rich source rocks using digital images

Digital rocks are 3D image-based representations of pore-scale geometries that reside in virtual laboratories. High-resolution 3D images that capture microstructural details of the real rock are used to build a digital rock. The digital rock, which is a data-driven model, is used to simulate physical processes such as fluid flow, heat flow, electricity, and elastic deformation through basic laws of physics and numerical simulations. Unconventional reservoirs are chemically heterogeneous where the rock matrix is composed of inorganic minerals, and hydrocarbons are held in the pores of thermally matured organic matter, all of which vary spatially at the nanoscale. This nanoscale heterogeneity poses challenges in measuring the petrophysical properties of source rocks and interpreting the data with reference to the changing rock structure. Focused ion beam scanning electron microscopy is a powerful 3D imaging technique used to study source rock structure where significant micro- and nanoscale heterogeneity exists. Compared to conventional rocks, the imaging resolution required to image source rocks is much higher due to the nanoscale pores, while the field of view becomes smaller. Moreover, pore connectivity and resulting permeability are extremely low, making flow property computations much more challenging than in conventional rocks. Elastic properties of source rocks are significantly more anisotropic than those of conventional reservoirs. However, one advantage of unconventional rocks is that the soft organic matter can be captured at the same imaging resolution as the stiff inorganic matrix, making digital elasticity computations feasible. Physical measurement of kerogen elastic properties is difficult because of the tiny sample size. Digital rock physics provides a unique and powerful tool in the elastic characterization of kerogen.

Optimized integrated design of a high-frequency medical ultrasound transducer with genetic algorithm

Designing efficient acoustic stack and elements for high-frequency (HF) medical ultrasound (US) transducers involves various interrelated parameters. So far, optimizing spatial resolution and acoustic field intensity simultaneously has been a daunting task in the area of HF medical US imaging. Here, we introduce optimized design for a 50-MHz US probe for skin tissue imaging. We have developed an efficient design and simulation approach using Krimholtz, Leedom and Matthaei (KLM) equivalent circuit model and spatial impulse response method by means of Field II software. These KLM model and Field II software are integrated, and a GA algorithm is used to optimize the design of the US transducer to obtain the best imaging performance. As a result, a 50-MHz single element probe is effectively optimized with 5 mm acoustic focal length, 72 $$\upmu {\text{m}}$$ μ m lateral, and 42 $$\upmu {\text{m}}$$ μ m axial imaging resolution, with an enhancement in imaging resolution over the conventionally designed and simulated probe by 10%. This work has the potential to benefit many applications that require a fast, high-resolution and strong US focus in skin imaging.