Nuclear excitation ofLu176by positron annihilation

An experimental study on the nuclear excitation process of ${}_{}{}^{176}{}_{}{}^{}\mathrm{Lu}$ by positron annihilation has been made. After irradiation of a natural lutecium foil by positrons from ${}_{}{}^{64}{}_{}{}^{}\mathrm{Cu}$, the 88.35-keV $\gamma$ rays from ${}_{}{}^{176}{}_{}{}^{}\mathrm{Lu}_{}^m{}_{}{}^{}$ were observed by the Ge(Li) detector. The cross section of the annihilation-excitation process has been evaluated to be (9.0 ± 3.2) × ${10}^{-22\mathrm{}}$ ${\mathrm{cm}}^2$. The result is compared with the theoretical value in the two-step model and possible reasons for the discrepancy between theory and experiment are discussed.