Surface deposition of radioisotopes on sands and silts

There are two major methods of using radioisotopes to study sedimentary movements: mass-labelling - the sediment is replaced by a glass containing an element that can be activated; surface labelling - a physico-chemical treatment makes it possible to deposit a radioisotope on the surface of the sediment. The second method is used mainly for sands, when a large number of labelled particles is necessary (for statistical purposes); and for silts, when it is necessary to retain the features associated with their dynamic properties. The aim of the present study was to develop simple and rapid methods of labelling which could be applied at the site of the experiment by persons possessing only limited equipment for handling radioisotopes. The authors describe their technique. The process recommended for labelling sands by using 198Au is a combination of the Petersen method (treatment with tin chloride) and that of Campbell (treatment with silver) after pre-treatment with nitric acid and soda. By this method, it is possible to deposit 125 mg of gold per kilogram of sediment with an efficiency of 95% (i.e. 10 Ci/kg) using a solution of chlorauric acid. For marking sands with 51Cr a film of silica gel is first deposited and radioactive lead chromate is then adsorbed onto it. In this way, 500 mg of chromium per kilogram of sand can be fixed with more than 90% efficiency (i.e. 40 Ci/kg). The authors discuss the mechanical stability of the deposits, their value and their reproducibility for a given mineral composition of the sand: The labelling of silts by using 198Au is achieved with a neutralized chlorauric acid solution. The gold is reduced on contact with the silt on which it is fixed. Ten Ci/kg of sediment can be fixed with 100% efficiency. To mark silts with 51Cr chromium hydroxide is precipitated on contact with the silt. This results in a deposit of 450 mg of chromium per kilogram of sediment, i.e. 25 Ci/kg of silt, with 100% efficiency. The authors discuss the validity of the labelling, its effect on the dynamic behaviour of the silt and the number of particles labelled. As quantitative movement studies necessitate mass-labelling (constant specific activity), the authors indicate the transformation required to make surface labelling tantamount to mass-labelling. Finally, they indicate the future prospects and the probable lines which they will follow in further work. (author)