31 research outputs found
3D shape based reconstruction of experimental data in Diffuse Optical Tomography
Get PDFDiffuse optical tomography (DOT) aims at recovering three-dimensional images of absorption and scattering parameters inside diffusive body based on small number of transmission measurements at the boundary of the body. This image reconstruction problem is known to be an ill-posed inverse problem, which requires use of prior information for successful reconstruction. We present a shape based method for DOT, where we assume a priori that the unknown body consist of disjoint subdomains with different optical properties. We utilize spherical harmonics expansion to parameterize the reconstruction problem with respect to the subdomain boundaries, and introduce a finite element (FEM) based algorithm that uses a novel 3D mesh subdivision technique to describe the mapping from spherical harmonics coefficients to the 3D absorption and scattering distributions inside a unstructured volumetric FEM mesh. We evaluate the shape based method by reconstructing experimental DOT data, from a cylindrical phantom with one inclusion with high absorption and one with high scattering. The reconstruction was monitored, and we found a 87% reduction in the Hausdorff measure between targets and reconstructed inclusions, 96% success in recovering the location of the centers of the inclusions and 87% success in average in the recovery for the volumes
Development of instrumentation and methods for positron spectroscopy of defects in semiconductors
Get PDFInstrumentation and methods for positron annihilation spectroscopy of point defects in semiconductors have been developed. In particular, techniques to enhance the stability of positron lifetime spectrometers have been investigated. The ageing of the photomultiplier tubes (PMT) of the scintillation detectors can be slowed down by lowering the operating voltages over the PMTs and by compensating the lower gain with fast preamplifiers. The timing characteristics of the apparatus are preserved if the voltages in the input electron optics of the PMTs are high enough and the pulse amplitudes above some tens of millivolts. A positron lifetime spectrometer stabilized against fast inherent drifts in time zero is constructed. An artificial time reference peak in the lifetime spectrum is produced by feeding light pulses from a light-emitting diode onto the photomultipliers via optical fibers of different lengths. The reference peak serves as a basis of stabilization in a digitally stabilized multichannel analyzer.
Positron thermalization in Si and GaAs at low temperatures (8-100 K) is investigated both by experiments and theoretical calculations. Thermalization in GaAs is observed to be noticeably slower than in Si. The mass density of a material is found to play an important role in thermalization since the positron scattering rate off longitudinal-acoustic phonons is inversely proportional to it.
Point defects have been investigated by positron annihilation spectroscopy in Si and CdF2. V-As and V-P pairs are observed in electron-irradiated silicon. Native V-As3 complexes are found to be formed when the As-concentration exceeds 1020cm–3. The ionization level V2–2/– of the silicon divacancy is detected at Ec–0.40 eV by measurements under illumination with monochromatic light. An open-volume defect is observed to be a constituent of the deep-state atomic configurations of the bistable donors In and Ga in CdF2. The size of the open volume is at least half of a Cd monovacancy.reviewe
