Fission Yield Studies and Closed Shell Effects in Atomic Nuclei

Abstract

The relative fission yields of the isotopes of krypton and xenon have been determined mass spectrometrically. Abnormal fission yields, resulting in fine structure in the mass fission yield curve, have been found in both mass ranges. A shift of the fine structure to lower masses has been observed in going from U^235 + n fission to U^238 + n fission. From this shift in fine structure, it has been possible to determine the proportion of U^235 and U^236 neutron fission that have occurred in the sample. Evidence is presented to show that the observed fine structure and the shift in this fine structure is the result of a combination of two effects involved the extra stability of closed neutron shells of 50 and 82 neutrons which fall in the Kr and Xe ranges respectively. The capture of thermal neutrons by Xe^135, which modifies the fission yields in the 135 and 136 mass chains, has been studied so that the observed fission yields at these masses may be suitably corrected. Finally, the branching ratio between the isomeric states of Kr^85 and the half-life of the long-lived isomer have been re-determined and found to be 0.29 and 10.27 ± 0.18 yrs. respectively.