Search for Dipole States in $^{235,238}$U

There is considerable interest in isotope-specific material identification. The presence of a particular isotope can be inferred by observing deexcitations of nuclear levels of $\gamma$-ray transitions characteristic for the isotope of interest using nuclear resonance fluorescence techniques. These high energy $\gamma$-transitions would penetrate protective shielding, thus acting as an identifier of hidden nuclear materials. Nearly monoenergetic, high-intensity and 100\% polarized $\gamma$-ray beam from the HI$\gamma$S facility was used to search for low-spin states in $^{235,238}$U at excitation energies between 3 and 5 MeV. The resulting data on the distribution of dipole strength below particle emission threshold will be presented.