Boron isotopes, chemical fractionation

This review deals with studies performed at the Oak Ridge National Laboratory concerning the chemical fractionation of boron isotopes between BF3 and its molecular addition compounds. This research resulted in the development of a new separation process which was superior to methods previously employed. The work also led to a theoretical explanation of the exchange reaction which accounted for the anomalous concentration of boron-10 in the molecular addition compound, and the observed variations of the isotopic equilibrium constants as a function of different donors. The model predicted maximum isotopic equilibrium constants for the exchange reaction which were consistent with the experimental data. It also predicted the behavior of the other boron halides. 

Thus, the fractionation of boron isotopes between boron trifluoride and its molecular addition compounds may be explained in terms of unique characteristics of the boron and fluorine atoms. The model presented here adequately describes the direction of enrichment as well as the magnitude of the equilibrium constant. It accounts for observed variations in the size of fractionation factor for different donors as well as for different substituents on the same donor. The model correctly predicts the isotopic behavior of other boron halides when these are substituted for BF3 in the exchange reaction. Finally, the proposed model provides insight into the design of a practical chemical exchange system for the separation of boron isotopes. 

The fundamental chemical principle behind the isotopic offset between the two aqueous boron species relates primarily to vibrational and rotational energy differences between the two isotopes, such that the heavier isotope is preferentially incorporated into the trigonal species. The motivation for studying this fractionation was for the nuclear industry, as has a very high cross section for neutron captme, and so is used as a neutron flux absorber. Mechanisms for enriching from natural boron led to studies that showed it could be separated from "B on ion exchange columns. Theoretical approaches were used to calculate the fractionation factor a, which describes the isotopic offset between two phases. 

Critical measurements are the exact spectra of the most common elements, hydrogen and helium, the fraction of antiparticles (antiprotons and positrons), isotopic ratios of elements such as neon and iron, the ratio of spallation products such as boron to primary nuclei such as carbon as a function of energy, the chemical composition near the knee, at about 5 x lO eV, and beyond, and the spectrum and nature of the particles beyond the ankle, at 3 x 10 eV, with special emphasis on the particles beyond the CZK cutoff, at 5 X 10 eV. 

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