Fission fragment K x-ray emission and nuclear charge distribution for thermal neutron fission of 233U, 235U, 239Pu and spontaneous fission of 252Cf

Abstract

The emission of K X-rays by fission fragments within ≈ 1 nsec after fission has been studied as a function of fragment mass and nuclear charge for thermal neutron-induced fission of 233U, 235U, 239Pu and spontaneous fission of 252Cf. This work is based on a simultaneous measurement of the fragment kinetic energies to obtain the fragment masses and high-resolution measurement of the characteristic energy spectra of the K X-rays emitted by the fragments to determine their nuclear charges. For all four fissioning systems the K X-ray yields per fragment were found to be strongly dependent on nuclear structure. Near closed nuclear shells the K X-ray yields for odd- Z nuclei are enhanced relative to even- Z nuclei. Away from closed shells the K X-ray yields increase rapidly with Z and A, and the odd-even Z-fluctuations diminish. With an approximate parameterization for the dependence of K X-ray yields on Z and A, the average nuclear charges of the primary fragments represented as a linear function of their pre-neutron-emission mass (charge division) and the width of the charge distribution about the average values (charge dispersion) were obtained. The linear charge division functions obtained in this work are in good agreement with existing radiochemical data and with recent β-decay chain-length measurements. The average charge dispersion (before neutron emission) found in this work for 235 U( n, f) ( σ z = 0.40 ±0.05 charge units) is narrower than the charge dispersion (0.56±0.06) determined radiochemically (after neutron emission).