1. Institute of Cryogenics and Isotopes Technologies, Uzinei Str. No. 4, Rm. Valcea, Romania, Email: bidica@icsi.ro
2. TLK, Forschungszentrum Karlsruhe, Postfach 3640, D76021 Karlsruhe, Germany
3. University Politehnica of Bucharest, Splaiul Independentei 313, Bucharest, Romania