Electron-Paramagnetic-Resonance Linewidth of Fast-Neutron-Irradiated ${\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)}_{1-x}{\left({\mathrm{Cr}}_2{\mathrm{O}}_3\right)}_x$

Electron-paramagnetic-resonance (EPR) absorption studies of single crystals of ${\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)}_{1-x}{\left({\mathrm{Cr}}_2{\mathrm{O}}_3\right)}_x$, with less than 0.01%, have been conducted under controlled fast-neutron treatments (1×${10}^{18}$ to 1×${10}^{19}$ nvt) near 300°K. The observed linewidth of the ${\mathrm{Cr}}^{3+}$ resonance has indicated a dependence on both the polar angle and the azimuthal angle of the external magnetic field. Furthermore, the center of resonance of individual EPR lines has been found to shift (0.2% at most) relative to the position of the corresponding lines before irradiation. The observed anisotropy of the linewidth can be explained most satisfactorily in terms of a random distribution of strain-producing radiation defects, and less satisfactorily in terms of a random distribution of point electric charge sources. However, the observed shift of the position of the EPR lines after irradiation is not consistent with the charge-source model. It appears that the model with radiation-induced strain as the source is most satisfactory for the irradiated samples. The optimum rms values of the strains are as follows: $\Delta {\epsilon }_1=\Delta {\epsilon }_2=\Delta {\epsilon }_3=\Delta {\epsilon }_4=\Delta {\epsilon }_5=\frac{\Delta {\epsilon }_6}{\sqrt{2}}=1.8\mathrm{}\times {10}^{-4\mathrm{}}$