1. a Physics Department , University of Zululand , Kwa-Dlangezwa 3886, South Africa
2. b Department of Physics , University of the Witwatersrand , Johannesburg , 2001 , South Africa
3. c WITS-CSIR Schonland Research Centre for Nuclear Sciences, University of the Witwatersrand , Johannesburg , 2001 , South Africa
4. d Solid State Physics Research Unit, University of the Witwatersrand, Johannesburg, 2001 and Physics Department, University of Zululand , Kwa-Dlangezwa 3886, South Africa