Spectrum of Photons Emitted in Coincidence with Fission ofU235by Thermal Neutrons

The absolute energy spectrum of prompt photons emitted from the fission of ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ by thermal neutrons was measured in the range from 0.01 to 10 MeV by using single-crystal, Compton, and pair NaI(Tl) scintillation spectrometers. Each was operated in ≤69-nsec coincidence with a fission chamber exposed to thermal neutrons from a reactor. The pulse-height response functions of the spectrometers were constructed in detail by exposing the spectrometers to radioactive sources of known disintegration rates. These data were used to "unfold" the measured pulse-height spectra to give the absolute differential energy spectrum and its random uncertainties. A careful analysis of systematic uncertainties was also performed. The average number of photons per fission is 8.13 ± 0.35 and the average photon energy release per fission is 7.25 ± 0.26 MeV, both over the energy region from 10 keV to 10.5 MeV. The results obtained here are in approximate agreement with the recent measurement by Verbinski and Sund in the energy region above 140 keV. From 1.5 to 4 MeV the calculation of Zommer, Savel'er, and Prokofiev gives results which are close to the measurements. The observed total energy release in photon emission per fission has been predicted by two published calculations which treated statistical evaporation theory in different ways to enhance the emission of photons. The $K$-shell x-ray intensities for the light- and heavy-fragment groups were found to be 0.08 ± 0.02 and 0.23 ± 0.02 photons/fission, respectively. The x-ray intensities are consistent with internal conversion of the observed $\gamma$-ray spectrum for various assumed mixtures of $E1\mathrm{}$, $E2\mathrm{}$, and $M2\mathrm{}$ transitions, including roughly equal $E1\mathrm{}-E2\mathrm{}$ mixtures; the intensities are not consistent with all the transitions having any single multipolarity.