THE ELASTIC SCATTERING OF Co60 γ-RAYS

Cross sections have been measured for 90° elastic scattering of Co60 γ-rays (1.33 and 1.17 Mev) from aluminum, copper, and tin. They agree with theoretical predictions for Rayleigh and nuclear Thomson scattering. A continuum between the Compton and the elastically scattered γ-rays interfered with the measurements for heavier elements, since it increased rapidly with Z.The continuum was investigated by observing the γ-rays scattered through 60° by thin gold foils. It was found to be produced directly by the γ-rays themselves, not by the electrons they knock out of the target atoms. The experimental cross section was compared with that expected for incoherent scattering from the bound atomic electrons. It agrees within a factor of 2 with the cross section given by the form factor approximation, although it is more than an order of magnitude larger than a modification of the form factor, which is based on comparison with elastic scattering calculations.The spectrum of elastically scattered γ-rays from lead was examined by a special method in order to reduce the contribution of the inelastic continuum; two photomultipliers faced a single scintillator and pulses from one photomultiplier were counted only when they corresponded to the largest pulses from the other. The cross section between 12° and 150° was found to be much lower than that of most previous measurements. It agrees with the calculated cross section for Rayleigh and nuclear Thomson scattering within the possible errors in the calculations. Therefore the measurements yield no evidence for the presence of Delbrück scattering, the upper limit on its intensity now being set primarily by the uncertainties in the theory.