High-Resolution Studies of K X-Ray Emission and the Distribution of Mass and Nuclear Charge in the Thermal-Neutron-Induced Fission of 233U, 235U and 239Pu

The emission of K X-rays by fission fragments as a function of fragment mass A and nuclear charge (element) Z has been investigated in the thermal-neutron-induced fission of 233U, 235U and 239Pu. Fission-fragment K X-ray spectra were obtained with a Si(Li) detector in coincidence with a pair of surface-barrier detectors which measured the kinetic energies of the complementary fragments for the determination of their masses. The data were recorded event-by-event in a multiparameter system. The yields of K X-rays per fission emitted by fragments in t < 1 ns are as follows: 233U - 0.037 (light fragment group), 0.125 (heavy group); 235U - 0.043, 0.120; and 239Pu - 0.070, 0.134. The high resolving power of the X-ray detector (FWHM » 800 eV) makes possible a computer analysis of the characteristic K X-ray spectra giving the relative contribution from each element. Near closed nucleon shells it is found that the average K X-ray yields for odd-Z nuclei are enhanced by a factor of two to three relative to the yields from even-Z nuclei. Away from the closed shells the yields increase rapidly with Z and A while the odd-even Z fluctuations tend to be substantially smaller. Differences in average K X-ray yields for a given Z among the three fissionable nuclei are consistent with an observed A (isotopic) dependence and a fixed yield of K X-ray emission for a given fission fragment nucleus (Z, A) in thermal-neutron-induced fission. In spite of the strong dependence of the K X-ray yields on Z and A, information on the primary charge distribution in fission in terms of average nuclear charges as a function of the fragment masses and on charge dispersion width can be obtained. The deviations of the mean primary charges of the fragments from that given by the charge densities of the fissioning nuclei lie in the range of 0.4 to 0.7 charge units for the complementary mass regions 85-110 and 130-155. The primary charge dispersion is narrower (FWHM = 1,10 ± 0.15 charge units) than the post-neutron-emission charge dispersion (FWHM = 1.56 ± 0.12) determined radiochemically. These results appear to have little dependence on total fragment kinetic energy at given mass splits. (author)