Double Positron-Electron Pair Creation by Photons: An Experimental Limit at Eγ=6.13 MeV

A search has been made for double positron-electron pair creation by photons of 6.13 MeV. γ rays from the reaction ¹⁹F(p,α)¹⁶O were incident on Ge(Li) detectors of either 15- or 4-cm³ volume and the pulse-height spectra were examined for the presence of a four-escape peak. Such a peak was, in fact, observed using the 15-cm³ detector, but is accounted for by secondary effects, namely, the production of a second pair by bremsstrahlung from the first pair and the direct production of a second pair by the members of the first pair. These secondary effects are evaluated in the appendixes. An upper limit on the cross section for the genuine production of two pairs (σ₂) in one photon-nucleus encounter at 6.13 MeV relative to the formation of a single pair (σ₁), is represented by the experimentally determined ratio σ₂σ₁=-(2+/-5)×10^{- 5}; this is consistent with the theoretical expectation of order (απ)².