the Nal(Tl) crystal is very suitable for the detection of gamma-radiation because very large crystal can be obtained and the high atomic number of iodine gives high absorption coefficient. Pure Nal emits light in the UV region; addition of 1% TI shifts the wavelength of the emitted light to values above 400 nm and also increases the conversion efficiency. Some proporties of the Nal(Tl) are the following : conversion efficiency = 10% , Wavelength = 410 nm, time constant = 0.3 and density