plastic ring and disc holders with a 2-in. flat Anton Geiger tube in a lead shield. The background of these counters was higher than expected and precision of measurement was therefore reduced. In spite of low precision, the data show considerable internal consistency and the average of group samples should be reliable. Sea Water Analyses Sea water samples were transferred from the Nansen bottles directly to polyethylene bottles. A 200-ml aliquot was measured in a graduate and poured into a 400-ml polyethylene beaker. A sufficient quantity of saturated sodium carbonate was added to precipitate all the calcium present and the precipitate collected on a 47-mm glass fibre filter. The precipitate and paper were covered with Mylar film of about 0.0005 in, thickness and mounted on a plastic ring and disc holder. Tests were made to determine if potassium, normally present in sea water to the extent of 600-700 d/min/liter would co-precipitate. Normal potassium was added at a level of 6000 d/min/200 mland the standard procedure carried out. No measurable increase inactivity was found, Itis believed that the interference of potassiumis negligible. One of the major constituents of mixed fission products more than a year oldis cesium which should not be co-precipitated with calcium carbonate. Tests were run in the laboratory to check possible co-precipitation but as in the case of potassium, no detectable cesium was found. The large amount of precipitate obtained in this procedure causes a considerable loss in measured activity by self-absorption. The data shown in Figure 6 were obtained by precipitating the calcium completely in various volumes of sea water after the addition of mixed fission products. Figure 6 indicates that self-absorption reduced the activity by a factor of 2. Comparison of the value extrapolated to zero absorption (3450 d/min) withthat obtained by direct measurementof the spike (4500 d/min) indicates that 23 percent of the activity escapes precipitation. ACTIVITY d/min/ SAMPLE The mixed fission products used in the spike were obtained from the Oak Ridge pile and were approximately 18 months old. The percentage of Csl37-Bal37 activity in material of this age should be 5.4 percent. Other isotopes in aged fission 6000 products that partially escape precipita| tion are Ru!06-Rh106 and Nb95. Their 5000 + = percentage in the Oak Ridge material ACTIVITY OF SPIKE should be 6.4 and 5.3 percent, respec1000] 4 tively. Pm147, at 9.8 percent, would not be counted efficiently. The total which might escape precipitation or 3000 counting could be as high as 27 percent, compared to 23 percent found. Piankton Analyses 2000 i000 | | ia ee ep SEA WATER PRECIPITATED, ml 208 Figure 6, EETaeons eae ee The plankton sample obtained from each net tow was quartered and onequarter used for activity measurement, This was ordinarily contained in about 200 ml of water. The plankton was filtered litered off off on a 47 *fermm Millipore M1tilpor filter € and dried at 110°C. The dried sample was covered with Mylar and mounted on