Pronounced structure in x-ray excited luminescence (XEL) has been observed in dilute Tb-doped Y2O3 (Y2O3:Tb) nanocrystals. This effect affords a means to assess different energy transfer mechanisms in the nanocrystals and also an opportunity for novel device applications. Sharp jumps and oscillations are found in the XEL output with the incident x-ray energy around the absorption edges of Y and Tb. When compared with a bulk Y2O3:Tb sample, these effects are attributed to some unique electronic and optical properties of doped nanocrystals related to quantum confinement of charge carriers, and the main features can be explained by a proposed model of multichannel energy transfer. Extended x-ray absorption fine structure techniques have also been employed to study the effect of size variation and chemical doping on the local structures in Y2O3 and Y2O3:Tb nanocrystals. The local environment surrounding Y and Tb in the nanocrystals is compared with that in the respective bulk material. The results indicate that Tb impurity atoms substitute for Y sites in bulk Y2O3, while doping in the nanocrystals is complicated by the large fraction of surface atoms and local disorder.
Seismic footstep detection based systems for homeland security applications are an important additional layer to perimeter protection and other security systems. This article reports seismic footstep signal characterization for different signal to noise ratios. Various footstep signal spectra are analyzed for different distances between a walking person and a seismic sensor. We also investigated kurtosis of the real footstep signals under various environmental and modeled noises. We also report on the results of seismic signal summation from separate geophones. A seismic signal sum spectrum obtained was broader than that obtained from a single sensor. The peak of the seismic signal sum was broader than that from the footstep signal of the single sensor. The signal and noise spectra have a greater overlap for a seismic signal sum than that from a single sensor. Generally, it is more difficult to filter out the noise from the sum of the seismic signals. We show that the use of the traditional approach of spectrum technology and/or the statistical characteristics of signal to noise of reliable footstep detection systems is not practical.
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