The K-series X-ray intensities of eight pure elements (from Al to Ge) and the L-series X-ray intensities of four pure elements (from Ni to Ge) excited by 15-30 keV electrons have been measured. Using the modified and simplified "full diffusion" electron diffusion model the ratios of the characteristic X-ray intensities) of these pure elements have been calculated. The back scattering effect is included in this model itself and the absorption correction factors are obtained from the stepped intensity-depth distribution derived from this model. Moreover the continuum fluorescence effect is also considered. The calculated values are in agreement with the experimental data, therefore the reliability of the model is verified.