1. Department of Radiobiology, National Center of Radiobiology and Radiation Protection, 132 Kl. Ohridski Boulevard, Sofia 1756, Bulgaria; Institute of Polymers, Bulgarian Academy of Sciences, Sofia 1113; Department of Biophysics and Radiobiology, University of Sofia, 8 Dr. Tsankov Street, Sofia 1421; and Michael M. Szwarc Polymer Research Institute and Department of Chemistry, College of Environmental Science and Forestry, State University of New York, Syracuse, New York 13210