Gamma-ray spectrometry is a very powerful tool for radioactivity measurements. For low activity samples, using extended sources in gamma-ray spectrometry, will improve the sensitivity of detection. In order to obtain correct results, self attenuation of source matrix which describing the fraction of gamma-rays not registered in the full-energy peak, due to scattering or absorption within the sample, must be taken into consideration. This work includes the calculation of the self attenuation of source matrix alone and the full-energy peak efficiency including this factor.