Total cross section of the3H(p,n)3Hereaction from threshold to 4.5 MeV

We report new measurements of the total cross section for the ${}^3\mathrm{H}{\mathrm{}(p,n)\mathrm{}}^3\mathrm{He}$ reaction from threshold ${(E}_p=1.02\mathrm{}$ MeV) to $E_p=4.5\mathrm{}$ MeV. The experiment utilized specially prepared $\mathrm{Ti}{-}^3\mathrm{H}$ targets, and neutrons were detected using a $4\pi$ detector. A weak resonant structure due to an excited state in ${}^4\mathrm{He}$ is observed which was not seen in previous cross section measurements. A new expression for the ${}^3\mathrm{He}{\mathrm{}(n,p)\mathrm{}}^3\mathrm{H}$ thermonuclear reaction rate for temperatures below 10 GK is presented which will allow for more accurate calculations of the yields of light elements produced by big-bang nucleosynthesis.