1. Centre of Low Temperature Physics of the Faculty of Science UPJŠ, and Institute of Experimental Physics, SAS, 041 54 Košice, Slovakia
2. Institute of Physics AS CR, 18221 Prague, Czech Republic
3. The "Vinča" Institute of Nuclear Sciences, 11001 Belgrade, Serbia
4. Faculty of Chemical and Food Technology, STU, 812 43, Bratislava, Slovakia