Z-Dependence of the Pair Production Cross Section at 1.33 and 2.62 Mev

The Z-dependence of the pair production cross section has been measured at 1.33 Mev for Al, Cu, Sn, and Pb, and at 2.62 Mev for Be, C, Al, Cu, Sn, and Pb. A target is used which is thick for the positrons produced in it but thin for the incident gamma ray beam. The positrons stop in the target and are detected by observing their annihilation radiation with two NaI(Tl) scintillation counters in coincidence. At both energies the Z-dependence of the cross section is best represented by an equation of the form aZ²+bZ⁴. If it is assumed that the Born approximation calculations of the cross section give the correct value in the limit of low Z, then in lead at 2.62 Mev the measured cross section is 23 percent higher than the value calculated from the Born approximation, and 104 percent higher at 1.33 Mev. All these results are in good agreement with the exact numerical calculations of Jaeger and Hulme.The present measurement shows that near threshold the pair production cross section in lead is considerably higher than the Born approximation calculation, whereas at energies between 17 Mev and 280 Mev the value has been measured to be 10 percent to 15 percent lower than that calculated from the Born approximation. The crossover appears to occur at around six Mev.