Elastic photon scattering from carbon and calcium and its interpretation

Differential cross sections for the elastic scattering of tagged, monochromatic photons have been measured for targets of carbon and calcium between 19 and 52 MeV. The formalism for the interpretation of these cross sections in terms of the total photoabsorption cross section and its partitioning into E1 and E2 parts is presented. For carbon, we find excellent agreement between the photoabsorption deduced from the scattering and previous direct measurements. The fore-to-aft asymmetries in the cross sections indicate that the bulk of E2 strength is above 50 MeV and not between 30 and 45 MeV, as previously thought. For calcium, the scattering data indicate that the integrated photoabsorption cross sections per nucleon are closer to the nearly constant value reported for A>100 than previously thought. No compact E2 strength exhausting more than 0.5 classical sums is found below 50 MeV, although weaker or less compact strength cannot be ruled out