Raman scattering studies were carried out on the ferroelectric and paraelectric phases of HCl, DCl, HBr, and DBr at temperatures from ~72 to ~160 K. Changes in the lattice region spectra at temperatures near Tc confirm the order–disorder nature of the ferroelectric transitions and the importance of anharmonic effects in these materials. The structural transition occurring in HCl near 120 K was observed by internal mode depolarization measurements which change irreversibly; these changes are attributed to macroscopic disorder in the sample below this temperature. Observations of the internal modes of HBr show a thermal hysteresis around an intermediate cubic phase and analogous results obtained for DBr imply the existence of a new phase which is similar to the intermediate phase in HBr.
Pixellated CZT detectors provide a new opportunity to improve the image quality of SPECT detector systems. Their performance has to be evaluated in terms of resolution and efficiency, in a similar way as done earlier for NaI detectors.We have developed an analytical model for spatial resolution and geometric efficiency of collimators specifically for pixellated CZT based detectors. We derive an exact description for static and rotating detector concepts, use NEMA performance criteria for detection efficiency, and adapt measures for spatial resolution of pixellated detectors, based on the sampling of the single pixel response function.Tradeoffs among resolution, efficiency, and signal-to-noise ratio (SNR) have been investigated for different applications. Our analysis shows that the concept of rotating collimators suffers from noise accumulation, except for purely hot spot imaging. We propose multi-pinhole, slat-slit or fan beam-slit collimators in a demagnification mode for optimum efficiency and image quality using pixellated solid-state detectors for SPECT cameras.
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