product decay constant of ge, As previously stated, the average gamma energy of the mixed fission products varies with time, becoming less during the first four days then gradually increasing dwring the next several days. It might be expected that random gross samples of mixed fission products brought down as fall-out would have avout the same decay rate and energy. This is not found to be the case because of the phenomenon of fractionation. Fractionation depends to a large extent upon volatility, the solu- bility, and other physical and chemical properties of the fission products, both for the first elements of a decay chain and its subsequent radioactive decay products. When a nuclear weapon is detonated on the ground, the duration of the fireball and the chemical composition of the soil are the predominant factors in determing the composi- tion of the fall-out particle. The larger the weapon and the longer time that heat is evolved in the fireball while it is in close proximity to the ground, the greater will be the effect on early formed products, and particularly on gaseous elements. Fractionation may take place within the fireball of an air burst but the results are not as apparent as for surface detonation fractionation. When particles fall to the ground, whatever the effect within the fireball, further fractionation is possible, due to the solubility of the specific radioactive isotopes present and their compounds, as well as the characteristics of the soil upon which the fall-out is deposited. Some of the most fruitful studies on the movement of various ions of biological importance through soils have been made by the Waste Disposal Group at the Hanford Atomic Products Operation .2L/ It was found from a study of several elements that movement of ions by the mechanism of adsorption and elution on the soil decreases with increasing acidity. Further investigation revealed that the presence of phosphate ions in = n/ Parker, H.M., Radiological Sciences Department Quarterly Progress Report, Research and Development Activities, Hanford Atomic Prod- ucts Operation, HW-34408, SECRET Restricted Data. 30