1. Wigner Research Center for Physics , Konkoly-Th. M. u. 29-33, 1121 Budapest , Hungary
2. Department of Geophysics and Space Science, Eötvös Loránd University , Pázmány P. s. 1/C, 1117 Budapest , Hungary
3. Budapest University of Technology and Economics, Institute of Nuclear Techniques , Műegyetem rkp. 9, 1111 Budapest , Hungary