Study of GaAs detectors characteristics for medical imaging

In this work we present the results of a systematic study about SI GaAs detectors as a function of substrate and contact type, geometry and thickness. This study has been stimulated from the interest in using GaAs as a detector for medical imaging applications. GaAs detectors have been produced using crystals grown with different techniques and changing both the thickness (in the range 200 μm-1 mm) and the contacts type and geometry. We have measured the current-voltage characteristics and, using radioactive sources (109Cd, 20 keV photons, 241Am, 60 keV photons, 99mTc, 140 keV photons), we have studied the performance of our detectors in terms of charge collection efficiency and energy resolution as a function of the bias voltage. Besides we have also studied the electrical and spectroscopic properties of GaAs detectors with different types and concentrations of the dopants in the substrate. So we have found the optimal doping type and concentration to have the best spectroscopic performances and the higher breakdown voltage. Simulation programs made with Monte Carlo methods have been developed to describe the electric field distribution and the transport of charge carriers toward the electrodes in GaAs detectors. In these simulations we have considered the presence of deep energy levels in the bandgap, the thickness, the bias voltage and the charge deposition in the crystal after photon interaction

Albert Einstein ed il 1905 "annus mirabilis"

Coordinamento SIBA - Università del Salent

Applicazioni di Mathlab all'analisi di immagini telerilevate

ItI nuovi satelliti per l'osservazione della Terra offrono alla comunita' scientifica un potente mezzo per l'acquisizione di dati ambientali. La disponibilita' di informazioni telerilevate e' fondamentale per la modellazione di numerosi processi naturali (es. deformazioni del territorio, inquinamento atmosferico, fenomeni di stress della vegetazione) e culturali (es. modifiche dello stato di uso dei suoli dovute a fenomeni di antropizzazione). Infatti molti modelli sono basati su informazioni geospazializzate senza le quali non potrebbero essere operativi. L'estrazione di queste informazioni dalle immagini telerilevate viene spesso effettuata mediante l'applicazione di algoritmi matematici. Particolarmente utile ed efficace nell'analisi delle immagini satellitari e' il toolbox di image processing disponibile con il software MATLAB, attualmente tra i piu' diffusi per il calcolo scientifico. Esso integra funzionalita' di calcolo, visualizzazione e programmazione in un ambiente semplice da usare in cui e' possibile esprimere problemi e soluzioni mediante familiari notazioni matematiche. Scopo di questo lavoro e' illustrare il processo di elaborazione (rettificazione, correzione radiometrica e miglioramento visuale) delle immagini satellitari mediante il sistema MATLAB

Preparation of uniformly dispersed copper nanocluster doped silica glasses by the sol gel process

The synthetic mechanism of uniformly dispersed Cu-nanocluster doped (0.05-1.0 wt%) silica glasses (copper ruby glasses) was investigated by UV-VIS and FTIR spectroscopy, DSC, XRD, surface area and density measurements at different stages of drying/densification up to glass formation. The monolithic gels were prepared from Cu(NO3)2, H2N(CH2)2NH(CH2)3Si (OCH3)3 (DAMO) and acid hydrolysed Si(OH2H5)4 (TEOS). DAMO was used to immobilize the Cu2+ ions in the silica matrix. The formation and decomposition of Cu-DAMO complexes in the silica gel monoliths were studied. The doped gels were densified under H2 and He gas atmospheres. A maximum matrix (SiO2) density of 1.70-1.73 g cm-3 (77-79% of the theoretical density) could be achieved in an H2 atmosphere at 900°C. However a density close to silica glass 2.17 g cm-3 (>98.5% of theoretical) was achieved when the gels were densified in H2 up to 800°C followed by He gas at 980°C. The surface area data also confirmed this densification behaviour. Uniformly dispersed Cu metal nanoclusters were formed during the heat-treatment of the gels and as a result a surface plasmon (SP) band of Cu-nanoclusters (563-580 nm) was observed. The increase of heat-treatment temperature caused the growth of nanoclusters and as a consequence the SP band was blue-shifted. XRD data also confirmed this

Surface structural and morphological characterization of ZnTe epilayers grown on {100}GaAs by MOVPE

The surface structural and. morphological characterization of ZnTe epilayers grown on (100)GaAs by atmospheric pressure MOVPE is reported. Scanning electron microscopy (SEM), reflection high energy electron diffraction (RHEED) and reflection electron microscopy (REM) have been performed on the as-grown ZnTe epilayer surfaces. RHEED pattern allowed to estimate the ZnTe crystalline quality, whereas the layer-to-substrate epitaxial relationships have been demonstrated from both RHEED and X-ray diffraction. SEM and REM observations in combination with alpha-step surface profiling measurements allowed to obtain a quantitative description of ZnTe surface irregularities, whose dimensions range from a few nm to just above 30 nm in height, being several hundred nm wide across the surface. Also, the ZnTe surface roughness depends linearly on the epilayer thickness and approaches the initial surface roughness of the GaAs substrates when the thickness tends to zero. Finally, no evident surface anisotropy has been revealed in the present samples, apart from a characteristic [011] elongation effect of some growth defect features at the epilayer surfaces

Thin films of TiO2 nanocrystals with controlled shape and surface coating for surface plasmon resonance alcohol vapour sensing

This work explores the potential of thin films composed of shape-controlled colloidal TiO 2 nanocrystals in optical gas sensing applications. To this purpose, an optical characterization of nanocrystal thin films has been performed by means of UV-vis spectroscopy and surface plasmon resonance (SPR) technique. In particular, this technique has been used as optical transduction method to test the sensing properties of the oxide films towards alcohol vapours. Numerical fitting routines have been employed to evaluate quantitatively the changes in the characteristic optical parameters for the investigated films during the sensing process. It has been found that rod-shaped TiO 2 nanocrystals provided with close-packed monolayers of long-chain surfactants on their surfaces, maximize the sensor response, as they may help to achieve a higher degree of alcohol accumulation on the active material