The quasi‐steady variation of uranium and radium isotopes from the decay of 238U, 235U, and 232Th has been determined along an idealized, one‐dimensional aquifer which is a distributed source of activity and from which the transfer of radionuclides to solution occurs by alpha recoil and chemical exchange. The model includes the effects of dispersive flow and retardation by adsorption. The importance of these effects on the variation of U and Ra activity ratios along the model aquifer is shown. Predicted limiting activity ratios (for large flow times) of 226Ra/223Ra, 224Ra/228Ra, and 226Ra/228Ra are 21.4, 1.5, and (∼1.7–2.0) (U/Th)s, respectively (where (U/Th)s is the activity ratio 238U/232Th in the aquifer rocks), although in many cases, an aquifer may not be long enough for the ratios involving 226Ra to achieve their limiting values.
R6sum6.-Le spectre Mossbauer du 57Fe d'un certain nombre de cristaux de Beryl nature1 (Be3AlzSi6018) indique la presence de 2 sites independants Fez+ et d'un site Fe3+. Les mesures de dependance en temperature des raies d'absorption montrent que les 2 sites Fez+ observes sont substitutionnels, et des etudes sur monocristal, on conclut que l'un est le site octakdrique Al3+ et l'autre un tetraedrique (Be2+ ou Si4+). Le spectre d'absorption Fe3+ n'etait pas incompatible avec ces ions occupant le site Al3+ comme l'avaient suggkre les mesures de RPE. Preuve en est que l'on observe la reduction thermique Fe3+-+Fez+. Contrairement aux etudes antbieures, les intensites des pics optiques et d'absorption Mossbauer dans diffkrents Bchantillons indiquaient une association possible entre la bande optique 810 nm a polaris& du Fez+ en site tetraedrique et entre les autres bandes n polarisees a 810 nm et l 000 nm du Fez+ en site octakdrique.
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