Branching ratios for radiative alpha capture by tritium and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ have been studied in a kinematical direct capture model for the purpose of obtaining an independent derivation of the ${}_{}{}^3{}_{}{}^{}\mathrm{H}$(α,γ${)}^7$Li branching ratio and possibly resolving an apparent discrepancy in the experimentally measured value. From the mirror symmetry between ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Be}$, and the observed branching ratio R(0)=0.38±0.04(1σ) for the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$(α,γ${)}^7$Be reaction, the branching ratio for the ${}_{}{}^3{}_{}{}^{}\mathrm{H}$(α,γ${)}^7$Li reaction is constrained to be R(0)=0.47±0.07, which agrees best with the higher measured value. The implication of these results for big-bang nucleosynthesis in standard and nonstandard models is discussed.

• Received 10 January 1989

DOI:https://doi.org/10.1103/PhysRevC.40.525