Fractional independent (FIY) and fractional cumulative (FCY) yields in the isobaric chain 99 from the thermal neutron induced fission of 235U have been investigated using rapid chemical separations of niobium and zirconium. The following results were obtained: FCY:(31.4±:3.9)%; FIY(99Zr):(56.9±4.3)%; The fraction (Pg) of 99Zr decaying to 99gNb amounts to 0.71 ± 0.03. The parameters of the nuclear charge distribution are found to be Zn = 39.78 ± 0.04 and σZ = 0.59 ± 0.02. The independent yield of the high spin isomer 90gNb relative to the l total independent yield of 99Nb has been determined to be FH = (FIY (high spin)/FIY (total)) = 0.06−0.06+0.14 in sharp contrast to a predicted value of 0.812. The results are discussed in the framework of fission yield systematics and of models for the prediction of isomeric ratios.

Neutron-rich rubidium isotopes were produced by thermal-neutron induced fission of235U and were isolated by surface ionization and mass separation. The new isotope (51 ±17) ms100Rb was identified. The half-lives of the rubidium isotopes with mass numberA=96 toA=100 were measured using neutron multiscaling or gamma multispectrum analysis. Measurements of the delayed-neutron emission probabilityP n of99Rb and of the fission yields of97Rb,98Rb, and99Rb were performed.

Radiochemically determined cross sections $\ensuremath{\sigma}(Z, A)$ were used to construct charge and mass distributions for the reaction of 1785-MeV $^{238}\mathrm{U}$ ions with thick $^{238}\mathrm{U}$ targets. Fission of the colliding nuclei is found to dominate. For the surviving uraniumlike fragments an enhancement of yields compared to the Kr + U and Xe + U reactions is observed. The formation of heavy actinides is shown to be associated with the low-energy tails of the broad excitation energy distributions in damped collisions.

Radiochemically determined cross sections sigma (Z, A) were used to construct charge and mass distributions for the reaction of 1785-MeV /sup 238/U ions with thick /sup 238/U targets. Fission of the colliding nuclei is found to dominate. For the surviving uraniumlike fragments an enhancement of yields compared to the Kr + U and Xe + U reactions is observed. The formation of heavy actinides is shown to be associated with the low-energy tails of the broad excitation energy distributions in damped collisions.

Neutron-proton population ratios for quasielastic and inelastic processes in the reactions of 761-MeV $^{129}\\mathrm{Xe}$, 769-MeV $^{132}\\mathrm{Xe}$, and 795-MeV $^{136}\\mathrm{Xe}$ ions with thick $^{197}\\mathrm{Au}$ targets were determined radiochemically. Completely relaxed $\\frac{N}{Z}$ ratios are observed for damped collisions involving the transfer of $\\ensuremath{\\Delta}Zg~1$ charge units. The limiting condition $\\ensuremath{\\Delta}Z\\ensuremath{\\approx}1$ corresponds to a characteristic time of the order of ${10}^{\\ensuremath{-}22}$ sec which indicates the absence of dissipative forces in the equilibration of the charge-asymmetry mode.

Neutron-proton population ratios for quasielastic and inelastic processes in the reactions of 761-MeV $^{129}\mathrm{Xe}$, 769-MeV $^{132}\mathrm{Xe}$, and 795-MeV $^{136}\mathrm{Xe}$ ions with thick $^{197}\mathrm{Au}$ targets were determined radiochemically. Completely relaxed $\frac{N}{Z}$ ratios are observed for damped collisions involving the transfer of $\ensuremath{\Delta}Z>~1$ charge units. The limiting condition $\ensuremath{\Delta}Z\ensuremath{\approx}1$ corresponds to a characteristic time of the order of ${10}^{\ensuremath{-}22}$ sec which indicates the absence of dissipative forces in the equilibration of the charge-asymmetry mode.