Nucleation of an interstitial loop at an impurity site in electron irradiated pure aluminum

Pure aluminum of three different purities were irradiated with 200 keV electrons at 140 K and the density of interstitial loops was examined by electron microscopy. It was found that the density depended sensitively on specimen purity. For each kind of specimens, the density was constant for flux 1.1 x 1018 to 3.5 x 1019 e/cm2sec. Assuming that a di-interstitial was the nucleus and that a vacancy does not move, chemical rate equations were integrated to find the number density of interstitial loops. For the homogeneous nucleation, it is found for the modelate flux that in a thin specimen the density should be proportional to the flux. It is found also that in an impure specimen the density is proportional to the smaller power of the flux than that in pure specimens. It is concluded that even in zone refined aluminum all interstitial loops are nucleated at impurity sites during a 140 K electron irradiation. (auth.)