The independent fractional chain yields of Y^, Nb^®, and La^^ have been measured from fission of Th^ss with alpha particles. The excitation energy was varied from about 20 to 40 MeV. Yields are also calculated for Cs^*^ from data previously published. In addition, the independent fractional chain yields of Y^, Nb®®, and La}^ were measured from fission of U^^^ with 14.9-MeV neutrons. From these data, smoothly varying plots of independent yield versus excitation energy were constructed for Y*, Nb^®, Cs^'^ and La^^. The data from 14.9-MeV neutron fission of U^^ are in good agreement with those from alpha-particle fission of Th232 at the same excitation energy. It is shown that, within the Kmits of certain assumptions regarding the charge dispersion curves, the shift in Zp (the most probable charge for a given mass number) with increasing excitation energy varies among the nuclides studied. In particular, it appears that dZp/dE may be significantly greater for heavy fragments than for light fragments. The data for Y^, Nb^®, and La^' ' ® are consistent with linear plots of Zp versus £*, except that the value for Nb^^ for thermal-neutron fission of U^^s is off the curve by 0.1 charge unit. The value of dZp/dE for Cs^'^ decreases with energy in this energy range, possibly because of shell effects at low excitation energies.

Fission-product yields relative to the fission yield of Mo" were measured for the fission of Th 232 with He 4 ions of energies ranging from threshold to 45 MeV and for the fission of U 235 with 14-MeV neutrons. The linear relationships between fission yields predicted by the two-mode fission hypothesis and previously observed for fission of U 236 * at low excitation energies break down at the higher energies in many cases. The fission yield of Mo" and the total fission cross section were evaluated as a function of He 4 ion energy.

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It has been suggested that the effect of changing excitation energy on the shape of the fission product mass-yield curve from fission of a single nuclear species is due to the change in the relative amounts of two energy-independent modes of fission, each giving rise to its own characteristic mass-yield curve. It is shown here that this hypothesis predicts a linear relationship between the fission yields of any pair of fission products measured at a set of excitation energies. Linear relationships are also predicted between pairs of fission yields measured relative to the yield of some reference fission product. Fission product yields relative to the fission yield of Mo" were measured for fission of U 235 and of U 238 with neutron beams of mean energies ranging from 2 to 10 Mev. The predicted linear relationships w r ere observed in all cases. However results for fission yields from U 235 with thermal neutrons do not fall on the corresponding observed lines. The two-mode fission hypothesis is a possible explanation for the linear relationships observed but does not explain all of the data.

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