The results of measurements of the one-dimensional angular correlation of the annihilation radiation (1D-ACAR) in Fz-Si and Cz-Si having different carbon content are presented. The analysis of data has been carried out on the basis of the independent particle model for outer shell core and valence electrons. The characteristic electron-positron ionic length of 1D-ACAR (to be characterised by both the ionic radii of atoms and the electron density in the lattice site where the annihilation of positron occurs ) has been obtained by the experimental findings for the investigated silicon as well as for some related materials. The systematic 1D-ACAR data have been used as a basis for the qualitative description of the 1D-ACAR spectra recorded for various materials of Fz-Si, Cz-Si, SiC, C(diamond), SiO_2 (single crystal quartz). It has been shown that the partial positron localization associated with the oxygen-related defects in silicon is dependent on both the configuration and number of carbon impurity atoms incorporated into the microstructure of positron-sensitive centers. Relatively high partial positron annihilation rate associated with the broad component of 1D-ACAR is intimately related to the presence of the carbon atoms in the oxygen related defects. The role of the effective charge of Si-O, Si-C, and C-O orbitals in the positron annihilation is briefly discussed.
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