The average number and average energy of $\gamma$ rays emitted within ∼5 nsec after fission have been determined as functions of fragment mass and as functions of fragment mass and total kinetic energy in two-dimensional representations. In a four-parameter experiment, energies of coincident pairs of fission fragments were measured with surface-barrier detectors and $\gamma$-ray energies were measured with a large NaI(Tl) detector, which was located 89 cm from a thin ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ target and positioned coaxially with the fragment detectors. The time difference between detection of a fission fragment and a $\gamma$ ray was measured to allow time-of-flight discrimination against fission neutrons. The $\gamma$-ray data were analyzed with a "weighting method" proposed by Maier-Leibnitz to deduce average numbers and energies of $\gamma$ rays from measured pulse heights. The Doppler shift in the laboratory angular distribution of $\gamma$ emission was utilized to obtain the number and energy of $\gamma$ rays as functions of single fragment mass. The results, for both average number and average energy as functions of single fragment mass, are characterized by a sawtooth behavior similar to that which is well known for neutron emission. The over-all average number and energy of $\gamma$ rays emitted per fission were found to be 6.51 ± 0.3 and 6.43 ± 0.3 MeV, respectively, giving an average photon energy of 0.99 ± 0.07 MeV.

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