E P Popov (Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria) E P Popov (Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria) J Waliszewski (Faculty of Physics, University of Bialystok,14 Maria Sklodowska-Curie, 15-089 Bialystok, Poland) M.N. Mirzayev (Institute of Radiation Problems, ANAS, 9 B. Vahabzade Str., AZ 1143 Baku, Azerbaijan)