PromptKX Rays as a Function of Fragment Mass and Total Kinetic Energy in the Thermal Fission ofU235

In order to obtain information about the deexcitation of fission fragments and the division of nuclear charge in fission for the case of thermal-neutron fission of ${\mathrm{U}}^{235}$, a measurement of the prompt $K$ x rays (0 to∼1 nsec after fission) was performed. A thin foil of uranium was caused to fission by a beam of neutrons from a beam port of a nuclear reactor. The fission fragments were detected by silicon surface-barrier detectors, and the x rays, by a NaI(Tl) scintillator. A three-parameter analyzer recorded the energies of the two fragments and the energy of the x ray emitted in coincidence with the fragments. The x-ray energies were sorted "off line" according to fragment mass or total kinetic energy in order to obtain x-ray spectra for different mass groups and different energy groups. From these spectra, x-ray yields were found as a function of fragment mass and as a function of total kinetic energy of the fragments. The results are compared, where possible, with those obtained in other laboratories for the thermal-neutron fission of ${\mathrm{U}}^{235}$ and with results for the spontaneous fission of ${\mathrm{Cf}}^{252}$. Differences and similarities are noted. The most probable charge versus mass was also estimated from the spectra, and these results are in agreement with radiochemical analysis.