Interstitial atom clustering at57Co impurity atoms in aluminium after electron and neutron irradiation

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Abstract
From Mössbauer measurements on Al 57Co samples irradiated above stage I with 2.8 MeV electrons it is found that the temperature dependence of the Debye-Waller factor of 57Co impurities having trapped interstitials becomes weaker with increasing number of interstitials trapped at a 57Co atom. This behaviour is explained by a linear superposition of two phenomena: The dynamics of 57Co impurities with only one trapped interstitial (mixed dumbbell) is determined by jumps in an octahedral cage. When two or more interstitials are trapped, the cage jumps are suppressed and the mixed dumbbells can only perform low frequency vibrations. The temperature dependence of the Debye-Waller factor was used to pursue the clustering of interstitials at 57Co impurities in aluminium after fast neutron irradiation at 4.6 K (dose 0.7 × 1018 cm−2) as well as after isochronal annealing. We found that after irradiation cobalt impurities have trapped interstitial clusters. Annealing in stage 1 leads to substantial fraction of trapped single interstitials, thus indicating free migration of single interstitials. In stage II the fraction of trapped single interstitials is reduced while the fraction of trapped interstitial clusters grows. This can be explained by the mobility of interstitial agglomerates.