E.V. Mitrofanov - Post-graduate Student, Trainee-researcher, Kotel\'nikov IRE of RAS (Moscow). E-mail: mitrofanov_ireran@mail.ru V.A. Cherepenin - Dr. Sc. (Phys.-Math.), Professor, Corresponding Member of RAS, Kotel\'nikov IRE of RAS (Moscow), Faculty of Physics, Lomonosov Moscow State University. E-mail: cher@cplire.ru