Measurement of the Double Compton Cross Section

The differential cross section for the double Compton process has been measured as a function of the independent final photon energy with the two detectors at 90° to each other and to the incident beam. The energy of the incident photons was 661.6 keV as produced by a 6-Ci ${}_{}{}^{138}{}_{}{}^{}\mathrm{Cs}$ source. The measured differential cross section and the cross section integrated over energy agree with the theory, the latter to within 3.5% which is within an estimated experimental error of 5%.