The small-angle scattering of photons of about 100 MeV energy

Abstract Differential cross-sections have been measured for the scattering of photons of mean energy 87 MeV by uranium at eight angles in the range from 1.18 to 4.48 mrad and by aluminium, silver, tungsten, lead and uranium at angles of 1.89 and 4.24 mrad, using a narrowly collimated bremsstrahlung beam from the Oxford 110 MeV synchrotron as the primary source of photons. A biased total-absorption Čerenkov counter was used to detect photons with energies near the peak energy of the bremsstrahlung spectrum, and absolute differential cross-sections were measured by comparing counting rates for photons in the primary and scattered beams. The experimental results, with the differential Compton scattering cross-sections of Klein and Nishina subtracted, were analyzed in terms of their variation with angle and atomic number, and can be described as consisting of a sharply peaked angular distribution with absolute cross-sections varying from element to element as Z4, together with a uniform distribution varying nearly as Z2; these distributions were identified with Delbruck scattering and with bremsstrahlung from secondary electrons in the scattering target, respectively. The Delbruck scattering cross-section thus determined is slightly more sharply peaked than the cross-section predicted by an approximate theory of Bethe & Rohrlich.