Nuclear Elastic Scattering of Photons

A NaI(Tl) scintillation spectrometer biased to detect only photons in the upper energy tip of a betatron-produced bremsstrahlung spectrum is used to measure the differential nuclear elastic scattering cross section at 120 degrees as a function of photon energy from 4 to 40 Mev. The targets ranged in $Z$ from Na to U. Total cross sections are calculated by assuming a dipole angular distribution. The scattering cross sections tend to exhibit two maxima, one below the particle threshold that corresponds to the scattering by separate levels, and one that follows the giant resonance for photon absorption. Both the maximum cross section and the energy of the giant resonance vary smoothly with $A$ from Na to U and are roughly proportional to ${\left(\frac{\mathrm{NZ}}{A}\right)}^2$ and $A^{-\frac{1}{3}}$, respectively. The dipole dispersion relation is used to compare the scattering data with the neutron yield data in the giant resonance region.