Hyperfine Interactions at the Final Positions ofFe56(n, γ)Fe57Recoil Nuclei in bcc Iron-Aluminum Alloys

The results of M\"ossbauer-effect measurements with 14.4-keV $\ensuremath{\gamma}$ rays of $^{57}\mathrm{Fe}$ emitted after thermal-neutron capture by $^{56}\mathrm{Fe}$ in iron-aluminum alloys (\ensuremath{\sim}25 to \ensuremath{\sim}50 at.% Al) are reported. High-energy $\ensuremath{\gamma}$ rays emitted in transitions from the capture level at 7.6 MeV to the lowest excited state at 14.4 keV impart recoil energies up to 549 eV to the $^{57}\mathrm{Fe}$ nuclei. Thus the M\"ossbauer $\ensuremath{\gamma}$ rays are emitted by nuclei which have been displaced from their lattice sites. The hyperfine spectra deviate significantly from the spectra obtained with a Cu: $^{57}\mathrm{Co}$ source and the alloy specimens as absorbers. The fraction of $^{57}\mathrm{Fe}$ recoil atoms that come to rest at lattice sites, the number of point defects, and the distribution of iron and aluminum atoms in the immediate neighborhood of their final positions are estimated with a simple collision model. Preliminary estimates of the influence of point defects on hyperfine fields are deduced from the experimental data in conjunction with the model results.