Measurements of the pair production cross section close to the threshold energy

We have measured the total cross section for production of e+e- pairs by photons in the Coulomb field of Ge nuclei. Up to 1016 γ quanta per second were produced by thermal-neutron capture reactions of a natural gadolinium target, placed inside the high-flux reactor of the ILL Grenoble. Out of this stream of γ rays various strong discrete γ lines were extracted using the GAMS5 Bragg spectrometer of the ILL. A well-collimated beam of reflected γ rays irradiated a germanium detector placed inside an anti-Compton shield, consisting of eight bismuth germanate crystals, assembled as a pair spectrometer. Using this new technique we have measured, for the first time, the pair production cross section at only 18.2 keV above the 1022-keV threshold. Our measurements indicate that the pair production cross section, expressed in units of Bethe-Heitler, grows monotonously toward low energies and does not support the claim of other works of a maximum at ∼65 keV.