Radionuclide Fragtionation in Fallout Particles

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Abstract
The radionuclide fractionation seen in the highly radioactive primary fallout particles which were produced in Russian air-bursts carried out in the autumns of 1961 and 1962 has been investigated for gamma-emitting nuclides by gamma-ray spectrometry and for 90Sr and 89Sr by conventional radiochemical analysis. It has been revealed that the radiochemical composition of these particles is much enriched in 95Zr + 95Nb and impoverished in 137Cs, 89Sr, 103Ru and 106Ru, while the other nuclides take a middle position, and that there is a rough trend that a particle of larger size, namely of higher activity, was more affected by the fractionation effect, in agreement with the finding that EDVARSON et al. made on masses of airborne fallout particles divided into two or three groups according to the size. The fractionation factor defined by EDVARSON et al. was calculated in order to express the degree of fractionation in a particle for simplicity. The fractionation between 141Ce and 144Ce was not detected, but the fractionation between 90Sr and 89Sr was revealed to be considerable. There is a general trend that a particle of larger fractionation factor for a given nuclide has larger fractionation factors also for other nuclides. 237U/95Zr + 95Nb and 239Np/95Zr + 95Nb composition ratios of a highly radioactive particle are smaller than those of a mean fallout sample, respectively, and seem to decrease as its fractionation factor for 141Ce + 144Ce decreases, although there are some exceptions. As the result of the present investigation, it is tentatively proposed that the order of increasingly larger fractionation factor for a nuclide in the fallout particles dealt with here is as follows: [137Cs, 89Sr, 103Ru, 106Ru] 140Ba + 140La, 141Ce + 144Ce, 90Sr, 99Mo, 95Zr + 95Nb where the order of the four nuclides in the brackets was not determined with confidence. A relation seems to exist between particle color and fractionation factor.

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