Parity Measurements of Nuclear Levels Using a Free-Electron-Laser Generated $\mathit{\gamma }$-Ray Beam

The quality and intensity of $\gamma$ rays at the High Intensity $\gamma$-ray Source are shown to make nuclear resonance fluorescence studies possible at a new level of precision and efficiency. First experiments have been carried out using an intense $({10}^7\gamma /\mathrm{s})$ beam of $100\%$ linearly polarized, nearly monoenergetic, $\gamma$ rays on the semimagic nucleus ${}^{138}\mathrm{Ba}$. Negative parity quantum numbers have been assigned to 18 dipole excitations of ${}^{138}\mathrm{Ba}$ between 5.5 MeV and 6.5 MeV from azimuthal $\gamma$-intensity asymmetries.