Cylindrical and spherical electrostatic analyzers have been used to measure the ratio of the threshold proton energy for the ${\mathrm{Li}}^7(p, n){\mathrm{Be}}^7$ reaction to the $Q$ of the ${\mathrm{Mg}}^{24}(p, p^{\prime }){\mathrm{Mg}}^{24*}$ reaction. This ratio was found to be 1.3734±0.0007. The energy of the gamma ray from ${\mathrm{Mg}}^{24*}$ has been compared earlier by Hedgran and Lind to other gamma rays whose energies have been measured absolutely by either the curved crystal spectrometer method (${\mathrm{Au}}^{198}$-Caltech) or proton-moment calibrated magnetic field (${\mathrm{Co}}^{60}$-Lindstrom et al.). The present data fix the $\mathrm{Li}(p, n)$ threshold as 1881.4±1.1 kev in terms of the Caltech value for the ${\mathrm{Au}}^{198}$ gamma, or as 1879.7±1.1 kev in terms of the Lindstrom et al. value for a ${\mathrm{Co}}^{60}$ gamma ray. The former value is in better agreement with currently used nuclear energy scales.

• Received 2 February 1954

DOI:https://doi.org/10.1103/PhysRev.94.947