Emission ofKX Rays and Division of Nuclear Charge in the Spontaneous Fission ofCf252

Abstract

Measurements of the energies of the prompt $K$ x rays emitted by the fragments in the spontaneous fission of $^{252}\mathrm{Cf}$ have been made which give information concerning the division of nuclear charge in fission. The kinetic energies of the pairs of fragments and the coincident $K$ x rays emitted along the direction of fragment motion were recorded event by event, using a multiparameter analyzer. The energies of the fragments were measured by two semiconductor detectors, and the x-ray energies were measured with a lithium-drifted silicon detector operated at dry-ice temperature. The energy resolution of the x-ray detector as measured in terms of the full width at half-maximum of the 59.57-keV line of $^{241}\mathrm{Am}$ was 3.5 keV. The data were analyzed to obtain (a) the most probable charge versus the fragment mass, (b) the x ray yield per fragment versus the fragment mass and charge, and (c) the average half-life for x-ray emission versus the fragment mass and charge. The measured x-ray emission times and the variation of the x-ray yield per fragment with the fragment mass are found to be consistent with the view that these x rays are emitted as a result of the internal-conversion process during the de-excitation of the fragment nuclei. In the heavy group, the abrupt rise in the x-ray yield at fragment mass 144 appears to be connected with the onset of large stable nuclear deformations of fragment nuclei at neutron number about 88. The measured x-ray yield from the light-fragment group suggests that most of these nuclei make up a new region of deformation. The observed variation of the most probable charge with the fragment-mass ratio over a limited mass region is found to be very similar to that obtained in radiochemical studies of the thermal fission of $^{235}\mathrm{U}$.