Absolute total ($p, n$) cross sections for the $\mathrm{T}(p, n){\mathrm{He}}^3$ and ${\mathrm{Li}}^7(p, n){\mathrm{Be}}^7$ reactions have been remeasured from threshold to 2.4 Mev, with special attention to the cross section near threshold. The $\mathrm{T}(p, n){\mathrm{He}}^3$ total cross section follows the behavior expected from either direct interaction or Breit-Wigner analysis over most of the range studied. Departure from predicted behavior occurs below $E_p\left(\mathrm{lab}\right)=1.10\mathrm{}$ Mev. The ${\mathrm{Li}}^7(p, n){\mathrm{Be}}^7$ cross section can be fitted over the entire energy range studied by assuming a broad $2^{-}$ resonance near threshold, a $3^{+}$ resonance at 2.25 Mev, and a small reaction background due to distant $l=1\mathrm{}$ resonances. The usual assumption of equal reduced widths for neutron and proton, however, cannot be used. Data for both reactions extrapolate to the values determined by reciprocity from the inverse reactions with thermal neutrons.

• Received 12 August 1957

DOI:https://doi.org/10.1103/PhysRev.109.105