Positron annihilation technique is recently considered as a good tool for studying the changes in the microstructure of materials. Accordingly, a variety of application have been employed in a wide field of physics, particularly, in material science. In the present work, this technique has been examined as a tool for investigation of the material defects; their type concentration in atomic scale, in addition to use it as a probe of the recovery process of defects. A Fast- Slow positron lifetime spectrometer has been constructed using fast plastic scintillation detectors and a constant fraction time pick-off method factors affecting the performance of the present spectrometer were investigated in order to set the optimal condition for fast and efficient detection of events. An optimal time resolution of 180ps were achieved at 10.