Delbrück and nuclear Raman scattering of 10.83-MeV photons by deformed and spherical heavy nuclei

A beam of monoenergetic $\gamma$ rays, extracted from the CP-5 reactor, has been used in a high-resolution measurement of the differential cross sections for photon scattering by Tb, Ta, Pb, Bi, Th, and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$. The spectrum of scattered photons was observed with a resolution of ∼ 10 keV in a Ge(Li) detector whose sensitive volume was ∼ 60 ${\mathrm{cm}}^3$. For Pb and U extensive measurements of the angular distributions for elastic and inelastic scattering were measured. For scattering angles $\theta <{45}^{\circ }$ the elastic scattering is dominated by the Delbrück effect. With the possible exception of Ta, for $\theta >{90}^{\circ }$ (where only nuclear resonance and Thomson scattering are important) both the magnitude and angular dependence of the elastic scattering cross section are in good agreement with the values which follow from the most recent photoabsorption cross sections in the corresponding targets. To an accuracy of ∼ 10% the elastic and inelastic (nuclear Raman) scattering for U and Th can be deduced from the simple rotator model using the parameters implied by the photoabsorption data. A trend of Raman scattering to be 10% weaker than expected is suggested by the data. For Ta and Tb the Raman scattering is substantially weaker than expected.