1. a Department of Physics , University College for Women, Osmania University , Sultan Bazar, Hyderabad , 500 001 , India
2. b Department of Physics , University College of Science, Osmania University , Hyderabad , 500 007 , India
3. c Radiochemistry Division , Bhabha Atomic Research Centre, Trombay , Bombay , 400 085 , India