Prompt Gamma Rays Emitted in the Thermal-Neutron-Induced Fission ofU235

Abstract

The average number and average energy of $\ensuremath{\gamma}$ rays emitted within \ensuremath{\sim}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 $\ensuremath{\gamma}$-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 $\ensuremath{\gamma}$ ray was measured to allow time-of-flight discrimination against fission neutrons. The $\ensuremath{\gamma}$-ray data were analyzed with a "weighting method" proposed by Maier-Leibnitz to deduce average numbers and energies of $\ensuremath{\gamma}$ rays from measured pulse heights. The Doppler shift in the laboratory angular distribution of $\ensuremath{\gamma}$ emission was utilized to obtain the number and energy of $\ensuremath{\gamma}$ 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 $\ensuremath{\gamma}$ rays emitted per fission were found to be 6.51 \ifmmode\pm\else\textpm\fi{} 0.3 and 6.43 \ifmmode\pm\else\textpm\fi{} 0.3 MeV, respectively, giving an average photon energy of 0.99 \ifmmode\pm\else\textpm\fi{} 0.07 MeV.