Interference between Rayleigh, Delbruck, and nuclear resonance scattering processes

The forward scattering cross section at θ=1°and 1.7°of E=4-10 MeV monoenergetic photons from Pb and Bi targets has been measured. The photon beam was obtained from the Fe(n, γ) reaction. The elastic cross section at such angles, being dominated by Rayleigh and Delbruck scattering processes, was measured relative to the Compton cross section. The fact that one of the γ lines of the Fe(n, γ) reaction happens to overlap the 7.28 MeV level in Pb208, yielding a strong nuclear resonance fluorescence signal, enabled us to observe, for the first time, an interference effect with the Rayleigh and Delbruck scattering processes. A theoretical expression of the nuclear resonance scattering amplitude for the specific case of the random photoexcitation process is derived. The interference effect of this latter process with Rayleigh and Delbruck scattering is calculated and an excellent agreement with measured values is obtained. NUCLEAR REACTIONS Pb, Pb208, Bi209(γ, γ), E=4.2-9.3 MeV, measured σ(θ), Delbruck and Rayleigh scattering, nuclear resonance fluorescence, interference effects.