1. 1Ankara Nuclear Research and Training Center, Turkish Atomic Energy Authority, 06100 Beşevler-Ankara, Turkey
2. 2Ankara University, Faculty of Engineering, Department of Engineering Physics, 06100 Bes¸evler-Ankara, Turkey
3. 3Institute for Materials Science, Darmstadt University of Technology, Petersenstraße 23, D-64287 Darmstadt, Germany