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Amorphous tracks induced by monoatomic and cluster ion beams are compared in LiNbO 3 and Y 3 Fe 5 O 12 . In agreement with the prediction of our model, at high energy deposition R e 2 /S e ϭconst ͑R e is the effective track radius, S e is the electronic stopping power͒ for monoatomic and cluster ion irradiations. This indicates that the considerably higher energy density in the experiments with cluster ion beams does not modify the track formation. The analysis shows that the width of the thermal spike does not depend on the ion velocity in the range 0.02-20 MeV/nucleon. On the other hand, a strong variation of the efficiency of the energy deposition into the thermal spike g is observed around 2 and 4 MeV/nucleon for LiNbO 3 and Y 3 Fe 5 O 12 , respectively. The results cannot be explained by the variation of the ␦-electron distribution with the specific ion energy E. The importance of the electron-lattice ion and ion-ion interactions are emphasized. The relative shift of the g(E) curves is related to the different fraction of heavy elements in the two insulators. ͓S0163-1829͑99͒04129-6͔

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