Direct Evidence for Interstitial-Atom Trapping byCo57Impurities in Aluminum from Mössbauer-Effect Measurements after Low-Temperature Neutron Irradiation

The Mössbauer effect was used to study the effects of neutron irradiation on aluminum doped with ${\mathrm{Co}}^{57}$. Annealing between 32 and 70 K after irradiation at 4.6 K induces in the previously single-line Mössbauer source spectrum a new component with small quadrupole splitting, which has an isomer shift of + 0.40 mm/sec against the original single line. The new component disappears irreversibly on further annealing between 180 and 250 K. This component is interpreted as being due to interstitials trapped in the immediate proximity of the ${\mathrm{Co}}^{57}$ impurity atoms.