AbstractExperimental measurements of the gamma-ray spectra emitted by the products of thermal-neutron fission of235U have been made at nine selected times (, 1, 2, 5, 10, 24, 48, and 72 h) after fission. A calibrated and highly collimated 5-×5-in. NaI(T1) detector was used.The 100-energy-binγ-ray spectra were unfolded from the pulse-height distributions by means of an iterative method. Extensive use was made of machine computation. The number of fissions in each sample was determined radiochemically.Significant differences between this work and calculated spectra were found. At the earlier times the experimental photon emission rate is higher than the calculated rate by as much as 40%. At later times the experimental rate is 20% lower than the calculated rate.Surprisingly large differences (as much as 33%) were found between the photon emission rates of products of fission by slow neutrons and by fast neutrons.