There are many books available on Radio Active Physics which present the matter in a concise manner so that it is interesting and with in grasp of B.Sc. (Hons) and post-graduate students keeping the above approach in mind this book has been planned. It is hoped that it would be useful to all B.SC., M.Sc. and engineering students. This book consist of five chapters. At the end each chapter a set of problems has been given to help the student become familiar with the concept and principles presented in the text.