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We describe a GPU-accelerated framework that efficiently models spatially (shift) variant system response kernels and performs forward- and back-projection operations with these kernels for the DIRECT (Direct Image Reconstruction for TOF) iterative reconstruction approach. Inherent challenges arise from the poor memory cache performance at non-axis aligned TOF directions. Focusing on the GPU memory access patterns, we utilize different kinds of GPU memory according to these patterns in order to maximize the memory cache performance. We also exploit the GPU instruction-level parallelism to efficiently hide long latencies from the memory operations. Our experiments indicate that our GPU implementation of the projection operators has slightly faster or approximately comparable time performance than FFT-based approaches using state-of-the-art FFTW routines. However, most importantly, our GPU framework can also efficiently handle any generic system response kernels, such as spatially symmetric and shift-variant as well as spatially asymmetric and shift-variant, both of which an FFT-based approach cannot cope with.

The production of gamma-ray Cherenkov-transition radiation (GCTR) by charged particles in the photon energy region 0.8÷2 MeV is studied theoretically using the results of the recent discovery that in the above mentioned region the dielectric constant or the refraction index of some materials is greater than 1 due to Delbruck scattering on Coulomb field of nuclei. Using the results of the carried out numerical calculations, the possibility of observing GCTR and some of its applications are discussed.

Analytical and numerical results on the coherent X-ray Cherenkov radiation (CXCR) produced by microbunched beams in the region near the K-, L-edges of materials are obtained. The results show that CXCR can serve as a suitable mechanism for production intense beams of photons in the "water window" region as well as for studying the important microbunching process at FLASH TESLA, LCLS and other FELs.

It is well known that when some conditions are fulfilled the intensity of the radiation produced by high energy particles in stratified media or the so-called "parametric Cherenkov radiation is enhanced. On the other hand, it has been predicted and observed that in some materials the dielectric constant is greater than 1 in X-ray region near the absorption edges, and quasirnonochromatic X-ray Cherenkov radiation (XCR) photons are produced with certain angular and narrow spectral distributions. Recently new formulae have been derived by the coherent summation method for the radiation produced by fast charged particles in multilayers consisting of thin alternating layers of two materials with different dielectric constants. In difference from the coherent transition radiation formulae obtained in WKB approximation or coherent summation method the new formulae take into account the reflection from the interfaces and is valid when the dielectric constants are greater and/or less than 1 . In this work we study the angular and spectral distributions of XCR using these formulae, compare the obtained results with the results obtained theoretically and experimentally in other works and discuss some possible experiments.