Measurement of internal conversion cascading in 235U fission fragments

Abstract

Internal conversion cascading in fragments from thermal-neutron-induced fission of 235U was studied through the measurement of the x rays emitted by the fragments. Two high-resolution Si(Li) semiconductor detectors were used to measure the x rays which are emitted when internal conversion occurs during the deexcitation of a fragment nucleus. The energies of the two coincident x rays and the kinetic energy of a fission fragment, if present, were recorded event by event as three-parameter data. The yield of x rays was obtained for the events when two or more x rays were emitted from the same element for the cases of fission and nonfission coincidences and for the events when both fission fragments emitted one or more x rays in coincidence with fission. The yield of x rays from the different elements produced in fission was also measured for single x-ray fission detections. From the results of the experiment, it was found that the odd-Z elements of the heavy-fragment region had a higher proportion of their x rays emitted in multiple-emission events than did the even-Z elements. An increase in the single-x-ray yield per fragment above Z=57 was observed for the even-Z elements 58 and 60. This rise was not present in the multiple-x-ray yields and hence was attributed to the possible enhancement of single low-energy transitions in the even-even nuclei of these two elements. It was also observed that, on the average, for a given element split x-ray emission is anticorrelated; there is a preference for x-ray emission from one or the other of the two complementary fission fragments.