A 25-Mev betatron is used to measure the photoneutron thresholds of 77 isotopes by direct detection of neutron yields as a function of maximum bremsstrahlung energy. The neutrons, moderated in paraffin, are observed with a B${\mathrm{F}}_3$ proportional counter and time delay circuits. Threshold energies for 31 isotopes are repetitions of previously reported values from other laboratories and those for 46 isotopes are either new or more precise determinations.

• Received 12 July 1951

DOI:https://doi.org/10.1103/PhysRev.84.387