An electron spin resonance (ESR) spectrum induced in magnesium oxide crystals by pile irradiation has been interpreted as arising from $F$ centers. After an oxide ion is displaced from its normal position by neutrons, an electron donated by ever-present divalent impurities may become trapped at the vacancy. Those centers having only ${\mathrm{Mg}}^{24}$ and ${\mathrm{Mg}}^{26}$ neighbors give a single ESR line with $g=2.0023\mathrm{}$, while those with one or more ${\mathrm{Mg}}^{25}$ neighbors give hyperfine patterns corresponding to the nuclear spin value of $\frac{5}{2}$. The magnitudes of the isotropic and anisotropic parts of the hyperfine interaction show strong localization of the electron in the vacancy.

• Received 10 June 1957

DOI:https://doi.org/10.1103/PhysRev.107.1535