Tosawat Seetawan Department of Physics, Faculty of Science And Technology, Sakon Nakhon Rajabhat University, 680 Nittayo Road, Muang District, Sakon Nakhon 47000, (THAILAND) Phone:+66-0-4320-2222 Fax:+66-0-4320-2374 E-mail: weerasak1963@yahoo.co.th Weerasak Somkhunthot, Thanusit Burinprakhon, Ian Thomas, Vittaya Amornkitbamrung Department of Physics, Khon Kaen University, 123 Mittapap Road, Muang District, Khon Kaen 40002, (THAILAND) Department of Physics, Faculty of Science And Technology, Sakon Nakhon Rajabhat University, 680 Nittayo Road, Muang District, Sakon Nakhon 47000, (THAILAND) E-mail : weerasak1963@hotmail.com Received: 14 January, 2007 Accepted: 19 January, 2007