Gamma de-excitation of fission fragments: (II). Delayed radiation

Abstract

The delayed part of the gamma radiation emitted from fission fragments of Cf 252 has been studied. The fission fragments were detected with semiconductor counters and the gamma radiation with a scintillation spectrometer. Mass-ratio selection was employed so that the properties of the gamma radiation could be studied as a function of fragment mass. The delayed radiation was found to have an intensity of 6% relative to the prompt one. The half-life depends on the mass and on the delay. It varies in the range 15–100 nsec. The mass spectrum of the fragments associated with delayed gamma radiation was found to exhibit a pronounced fine structure with most of the delayed gamma emission concentrated to deformed and magic fragments. The gamma-ray spectra have a shape consistent with a rotational cascade in deformed nuclei and a vibrational cascade in magic nuclei, respectively. Of special interest is the fact that the gamma emission from some of the fragments in the mass range 92–110 has the same characteristics as the emission from the deformed fragments in the rare-earth region. It is therefore concluded that these fragments have a stable deformation. Various theoretical and experimental evidence for the existence of this new region of deformation is discussed.