The general problem of determining photonuclear reaction energies in the region below 25 Mev is discussed, and the need for absolute calibration of betatron energy scales is demonstrated. A system for accomplishing such a calibration is briefly described and the results of some measurements made with it are given. The ${\mathrm{Cu}}^{63}(\gamma , n){\mathrm{Cu}}^{62}$ threshold was observed to be at 10.78±0.03 Mev, and two well-defined "breaks" in the ${\mathrm{O}}^{16}(\gamma , n){\mathrm{O}}^{15}$ yield curve are at 16.19±0.04 and 17.25±0.04 Mev. These "breaks" lead to the assignment of energy levels in ${\mathrm{O}}^{16}$ which are in excellent agreement with those observed in the ${\mathrm{N}}^{15}(p, n){\mathrm{O}}^{15}$ reaction.

• Received 15 December 1958

DOI:https://doi.org/10.1103/PhysRev.115.420