Nuclear resonance fluorescence in238U and a new approach to Doppler-shift-attenuation using high speed rotation

Using⁵⁶Co sources, a 3.254 MeV level in²³⁸U was photoexcited by aγ-line emitted in the decay of the 4.100 MeV level in⁵⁶Fe. 10γ-transitions deexciting this 3.254 MeV level were identified. Spin and parity were determined to be 1⁻. By moving the⁵⁶Co source in a high speed centrifuge the profile of the 3.254 MeV emission line was investigated. By applying a new variety of Doppler shift analysis a lifetime ofτ _Fe=55±25fs was obtained for the 4.100 MeV level in⁵⁶Fe. The cross section for nuclear resonance fluorescence yields a lifetime ofτ _U =0.33±0.12 ps for the 3.254 MeV level in²³⁸U.