60 WORLDWIDE EFFECTS OF ATOMIC WEAPONS SAMPLING AND RESEARCH PROGRAM 61 considerable amount of dirt with hydrofluoric acid to guarantee the dissolving of the colloidal fission particles is a most unpleasant task. There does not seem to be any obvious alternative at the moment, however, to the complete dissolution of a dirt sample followed by a treatment with hydrofluoric acid solution. The sampling principles for the biosphere are fortunately less than annoying. If the strontium has been tncorporated in the biosphere, it certainly is tn chemically active form, Therefore, the direct ignition of the biological material followed by the, dissolving of the ash in a slightly actd solution and the radiochemical processing of the solution ts a perfectly allowable procedure. carbonate precipitate with dilute nitric acid, add a small amountofferric nitrate, and neutralize with ammonium hydroxide. This would produce a precipitate of ferric hydroxide that should scavenge all yttrium and rareearth activities very effictently. To make certain of the purification of the calcium-strontium carbonate precipitate for yttrium, one might have to add little yttrium before, or together with, the ferric nitrate. Actually lanthanum nitrate would serve as well, and it is cheaper. After purification of the strontium-calcium solution for yttrium, one should reacidify with dilute nitric acid and allow the system to stand for regrowth of the 61-hour yttrium activity, At this point a few milligrams of yttrium could be added and the solution could be treated again with ammonium hydroxide until alkaline; the yttrium precipitate could then be removed by filtration and Chemical Processing of Samples After the samples from the atmosphere, the hydrosphere, the lithosphere, and the biosphere have been properly dissolved, the subsequent processing should be accomplished approximately as follows: 1. The addition of a comfortable amount of nonradioactive strontium salt to serve as a carrier, together with other salts to serve as holdback carriers for the other fission products. 2. The separation of a pure salt of strontium from the solution. This might, in the case of the majority of samples, consist in the precipitation of carbonates by the use of sodium carbonate solution, followed by appropriate purification, such as by ton exchange or by any of the well-known precipitation methods. We are fortunate in the nature of the decay scheme of radioactive strontium, which proceeds as follows: Sr? > Y" > Zr”, The strontium activity corresponds to a pure B-ray spectrum of 0.54 menttrelemnteeltetelhenhisietemthanesl imenstmenliemblneasiclamehnraeh transformation involves an emission of 2.18 Mev of £ rays with a halflife of 61 hours. No y radiation is involved in either. Following the precipitation of the carbonate mixture, one could either purtfy the strontium as indicated above or, without further purification, redissolve the counted. The only other fission-productactivity likely to be present in this yttrium precipitate is La'*®, which arises from the 12.8-day Ba'®. This, therefore, would be important only in relatively young samples. The possibility that Ra*** and Pb*"" (RaD) would also be present in the calcium-strontium precipitate should be considered. The RaE growing from RaD might appear in the yttrium sample and interfere with the counting of Y°°. In this eventuality it would be necessary to separate the strontium from lead before growth of the Y"’. Counting Procedures It is clear from the procedure outlined above that the yttrium milking technique allows one to concentrate the yttrium: daughteractivity in equilibrium with any given amount of strontium in very smal! amountsofsolid yttrium sample. Therefore, one can envisage the use of Geiger counters with the lowest backgrounds and the maximum sensitivity. The penetrating nature of the radiation of Y°° allows one to use samples as thick as several milligrams per square centimeter without serious self-absorption, j: . c . se ee aos Serre ty 1 wy backscattering from supports made of elements of the largest atomic numbers, such as gold or platinum. In other words, mounting thin layers of yttrium salt on gold or platinum should increase the chance of recording disintegrations occurring in the solid yttrium samples to something around