Yield ofKX Rays Emitted fromU236Fragments

Abstract

The yield of $K$ x rays from different fragment masses have been determined in the thermal-neutron-induced fission of ${\mathrm{U}}^{235}$. The energies of the pairs of fragments were measured with two semiconductor detectors placed on either side of a thin ${\mathrm{U}}^{235}$ foil. The $K$ x rays from the light and the heavy groups of fragments were separated by measuring the x-ray energies with a 1-mm NaI(Tl) crystal. The fragment mass distributions in coincidence with the light group of $K$ x rays and with the heavy group of $K$ x rays, and the unbiased mass distributions, were simultaneously recorded in different quarters of a 1024-channel analyzer memory. From these distributions, after suitable corrections for the background, x-rays detection efficiencies, and finite-energy-resolution effects, the number of x rays as a function of fragment mass has been determined. The x-ray yield per fission is found to be 0.08\ifmmode\pm\else\textpm\fi{}0.01 for the light fragment group, and 0.30\ifmmode\pm\else\textpm\fi{}0.02 for the heavy fragment group. The gross features of the yield as a function of mass are similar to those observed earlier for emission from ${\mathrm{Cf}}^{252}$ fragments. However, unlike the case of ${\mathrm{Cf}}^{252}$, for masses greater than 144 the striking increase in the yield is not observed. The present results are consistent with the interpretation that the x-ray yield depends both on the characteristics of the low-lying states and the initial spin of the fragments.