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Positron annihilation lifetime spectroscopy (PALS) is a direct probe of size and concentration of nano-scale defects in materials, because it is very sensitive to electron density. The lifetime of positron ( ) and its intensity (I) can be used to characterize the defects concentration. In the present work, PALS has been applied to measure the variation of positron lifetime parameters for polycrystalline samples with chemical formula Zn0.5Cu0.5Fe1.98R0.02O4 (R = Gd, Sm, Nd and La). The variation of positron annihilation lifetime parameters (I1, I2,  av and k) with the ionic radius of rare earth ions, homogeneity, grain size and electrical resistivity have been studied. The inter-granular pores and grain boundaries defects are increased with the increasing of the ionic radius of the rare earth ions and decreased for Gd-sample. The defects inside and outside the grains are distributed homogeneously for Sm-sample only. A positive correlation has been found between positron lifetime parameters and ionic radius of rare earth ions as well as the electrical resistivity. Ferrites have very important applications according to their electrical and magnetical properties. The substitution effect and the change of the preparation condition are allowed to improve some ferrites with different composition to be used in wide frequency range, from microwaves to radio wave frequencies. A considerable amount of work has been carried out on Cu -Zn ferrite substituted with rare earth ions of different kind and concentration (1-3). It was found that, the initial permeability and the homogeneity of the composition Zn0.5Cu0.5Fe1.98R0.02O4 were increased for the samples with R = Nd, Sm and Gd while the resistivity was increased for the samples with R = La and Nd relative to the unsubstituted one, R = none. The improvement of the resistivity was attributed to the decrease of grain size and the increase of the porosity of the samples. From this point of view, we aimed in this paper to investigate the microstructure and the defects on the atomic scale of the composition Zn0.5Cu0.5Fe1.98R0.02O4 (R = Gd, Sm, Nd and La) by using the positron annihilation lifetime spectroscopy (PALS) analysis. (PALS) is a valuable nuclear method to investigate the materials without damage them (4, 5). PALS has a high sensitivity for probing vacancy defects through measurements of positron lifetime. It is based on the high sensitivity of positron to localize at low electrons density regions of a material and the emission of annihilation gamma rays that escape from the system without any interaction. These gamma rays hold information about the defects around the annihilation site. A correlation is established between the positron lifetime parameters and the ionic radius of the rare earth ions, grain size as well as the electric resistivity. The PALS is a good

Study the Properties of Cu-Zn Ferrite Substituted with Rare Earth Ions by Using Positron Annihilation Analysis