Relative Cross Sections for Pair Production at 17.6 Mev

The relative cross sections for production of electron pairs by 17.6 Mev gamma-rays have been measured for Li, C, Al, Cu, Sn, and Pb. A gamma-ray pair spectrometer was used to count the number of pairs emitted from thin radiators of these materials when irradiated by 17.6 Mev gamma-rays produced in the ${\mathrm{Li}}^7(p,\gamma ){\mathrm{Be}}^8$ reaction. For each material, the number of pairs counted per atom of radiator has been extrapolated to a radiator of zero thickness, in order to obtain the relative pair cross section. From these measurements some information can be obtained about the probability of pair production in the field of the atomic electrons, and about the errors in the theoretical pair cross sections of heavy elements, which presumably arise from the use of the Born approximation. It is found that the ratio of the pair cross section of an electron to that of a nucleus of unit charge is 0.8±0.3. The data indicate that the pair cross sections of heavier elements are lower than predicted by the Born approximation, in qualitative agreement with measurements of the total absorption cross sections.