Measurement of Low-Energy Gamma-Ray Pair Cross Sections in Scintillators Using a Scintillation Pair Spectrometer

Using a three-crystal pair spectrometer with pulse-height selection on the side crystals, absolute and relative gamma-ray pair production cross sections were measured in anthracene and sodium iodide, respectively, by making the center crystal of the spectrometer out of these substances. The experimental data were reduced to yield the following pair cross sections (gamma-ray energies in Mev given in brackets): carbon, $\mathrm{carbon}, \sigma \mathrm{}\left(1.17\right)=(1.16\mathrm{}\pm 0.05)\times {10}^{-28\mathrm{}} {\mathrm{cm}}^2, \sigma \mathrm{}\left(1.33\right)=(6.03\mathrm{}\pm 0.18)\times {10}^{-28\mathrm{}} {\mathrm{cm}}^2;$ iodine, $\mathrm{iodine}, \sigma \mathrm{}\left(1.17\right)=(1.36\mathrm{}\pm 0.05)\times {10}^{-26\mathrm{}} {\mathrm{cm}}^2, \sigma \mathrm{}\left(1.33\right)=(6.81\mathrm{}\pm 0.17)\times {10}^{-26\mathrm{}} {\mathrm{cm}}^2, \frac{\sigma \mathrm{}\left(2.76\right)\mathrm{}}{\sigma \mathrm{}\left(1.38\right)\mathrm{}}=15.0\mathrm{}\pm \mathrm{0.8.}$ These results are compared with those of other experimenters and with theoretical calculations.