to the next higher organism in the chain; i.e., materials which have been absorbed or metabolized once are more likely to be absorbed in the next step than are surface contaminants. In an evaluation of the sources of radioactive contamination, the tissues of an organism may be grouped into the following categories: (1) tissues, such as liver, bone, and muscle, which have only those isotopes absorbed from the blood, and (2) tissues, such as skin, gill, shell, and digestive tract, which may have surface contamination from externally adsorbed or adhering materials in addition to absorbed isotopes. Radioactive materials in the digestive tract are considered surface contaminants aa long as they have not been absorbed. The immediate sources of surface contamination are direct and indirect. The direct sources are the fall-out particles and the induced radioactive materials in the sea water, air, or substrate, and isotopes of those materials that are soluble in water. Indirect sources are other radioactive organisms which are ingested by the specimen or commensal with it. 4.9.1 “— Speck Contamination Autoradiographs have shown that the distribution of radioactivity in the samples is often Hmited te isolated areas or specks, most of which are assumed to be fall-out particles. The term “speck” contamination is used to denote spotty activity on organisms, presumably caused ’ from insoluble radioisotopes. The identification and distribution of specks in sand, plankton, algae, invertebrates, fish, and land plants are discussed in the following paragraphs. (a) Sand, An autographic technique found useful for locating these radioactive particles involved spreading sand on scotch tape, inverting to remove loose particles, and exposing with firm contact against fast film. After the film was developed, a positive transparency was printed on the film to be placed beneath the sand sample sothat, when in perfect registry, the radioactive particles would be Uluminated if viewed by transmitted light. Engebi beach sand showed spots that were associated apparently with only the finer sand particles. Some of the active particles were isolated by successive dichotomous division of a sample of sand and retention of the more active half, as determined by the end-window survey meter, until the individual particles which contribute most of the radioactivity could be picked out under the microscope. In Figs. 4.13a to d, sand samples and active and nonactive particles which have been separated from the samples are shown. Counting rates for the parti- “7 cles are given in the legends. In Biijiri dredged sand, radioactive particles were different in appearance from inactive particles. Active particles were chalky looking and lacked even the slight hyaline luster characteristic of most inactive sand particles. One of the larger of these, as well as the sand sample and autoradiograph by means of which it was located, is shown in Fig. 4.14. Autoradiographs of plates of ashed biological samples were made to compare the nature of the distribution of the activity found in these specimens with that of the Engebi and Biijiri sand samples (see Fig. 4.15). Activity of the tissues with absorbed radiation was diffuse. For those tissues with possible surface contamination, the distribution of activity was spotty and similar to the sand samples. Photographsof the plates (Fig. 4.16) show that the ash is evenly distrib- uted and that the unexposed portions of the autoradiographs are not due to the absence of ash. (0) Plankton, The spots on plankton autoradiographs from samples dried on filter paper were associated primarily with a white amorphous material of cheesy consistency, which may be the counterpart in the water of the chalky material in the sand. The autoradiographs also showed someactivity associated with organisms. However, almost every kind of organism that showed activity in one individual would, in another case, fail to show it. Thus among foraminifers, gastropods, mysids, and other crustaceans, there could be found some radioactive t and some nonradioactive individuals. Activity tended to be proportional to mass of organisms. This suggests that minute particles suspended in the water or possibly even a certain amount of dissolved radioactive material may accumulate on the surface of plankton organisms and that, in addition, there are larger particles (the cheesy material) suspended in the water. 67 _ UNCLASSIFIED 7