Scattering of Gamma Rays by a Static Electric Field

In an effort to observe Delbr\"uck scattering (the scattering of photons by a static electric field), the absolute differential cross sections for the elastic scattering of 1.33-Mev gamma rays by lead, tin, and uranium and of 2.62-Mev gamma rays by lead and tin have been measured for angles between 15 and 105 degrees. The observed scattering is the coherent sum of Delbr\"uck, Rayleigh (bound electron), and nuclear Thomson scattering. The amplitude for the latter process is well known; recent calculations of Rayleigh scattering, which are in good agreement with data obtained previously in this laboratory for gamma-ray energies below 1 Mev, provide exact values of the amplitudes at 1.33 Mev and approximate values at 2.62 Mev. At 1.33 Mev, the difference between the observed scattering and that due to the Rayleigh and Thomson processes is not sufficiently large compared with experimental error to permit a definite identification of Delbr\"uck scattering to be made. At 2.62 Mev, for lead, the experimental cross sections at intermediate angles (30 to 75 degrees) are substantially larger than those calculated by extrapolation of the exact calculations, even when reasonable allowance for error in the extrapolation is made. The most probable explanation for this difference is an appreciable contribution from Delbr\"uck scattering.