We argue that due to various restrictions only necklaces and monopolonia survive as potential sources of ultra-high energy cosmic rays, among the topological defects studied so far. Other promising sources which are similar to topological defects are relic superheavy particles. All these sources have an excess of pions (and thus photons) over nucleons at production. We demonstrate that in the c...

A measurement of the longitudinal polarization of the electron beam at HERA utilizing coherent interactions of high energy photons in crystals is described. Modii-cation of existing facilities would allow an independent polarization measurement and a veriication of birefringence phenomena in crystals for 20{30 GeV photons. Relevant experimental issues and systematic uncertainties are also prese...

The total cross section for scattering virtual photons at high energy is sensitive to pomeron physics. If the photons are su ciently virtual, QCD perturbation theory applies, so that the reaction probes the short distance pomeron. We study this reaction for present and future e e colliders. Typeset using REVTEX

PET (positron emission tomography) is a powerful imaging technique that can provide quantitative information on the distribution of positron emitter labeled radiopharmaceuticals (PET radiopharmaceuticals) in the body. Positrons (ß+) are positively charged beta particles. They are emitted when the atom is proton rich. A positron has only a transient existence. After losing all of its kinetic ene...

PET (positron emission tomography) is a powerful imaging technique that can provide quantitative information on the distribution of positron emitter labeled radiopharmaceuticals (PET radiopharmaceuticals) in the body. Positrons (ß+) are positively charged beta particles. They are emitted when the atom is proton rich. A positron has only a transient existence. After losing all of its kinetic ene...

In the cosmological blast-wave model for gamma ray bursts (GRBs), high energy ( > ∼10 GeV) γ rays are produced either through Compton scattering of soft photons by ultrarelativistic electrons, or as a consequence of the acceleration of protons to ultrahigh energies. We describe the spectral and temporal characteristics of high energy γ rays produced by both mechanisms, and discuss how these pro...

One of the last frontiers of the gamma-ray sky is that characterized by the distribution of TeV photons. These photons can be detected relatively easily with ground-based detectors (constituting a TeV ”window” in the atmosphere) and thus the detection of sources did not have to await the availability of space platforms. In practice although the technology was available at an early date, it requ...

We explore several models which might be proposed to explain recent possible detections of high-energy (TeV) gamma rays in association with low-energy gamma-ray bursts (GRBs). Likely values (and/or upper limits) for the source energies in lowand high-energy gamma rays and hadrons are deduced for the burst sources associated with possible TeV gamma-ray detections by the Project GRAND array. Poss...