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CHAPTER 12 ON THE TAGGING OF WATER MASSES FOR THE STUDY OF PHYSICAL PROCESSES IN THE OCEANS1 THEODORE R. FOLSOM, Scripps Institution of Oceanography, La Jolla, California and ALLYN C. VINE, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts FINDING, identifying, and plotting the courses of characteristic masses of water in the oceans are major activities of the physical ocea- nographer. Assistance from the new techniques that have come with the use of radioactive materials has been welcomed by him; some of these techniques have already been put into service for tracing the water. It is not gen- erally realized how much experience has been gained, beginning with the 1946 tests in Bi- kini Lagoon, in tracing water masses contam- inated with radioactive materials from weapons' tests. And many thoughts are now turning toward the radioactive tagging of ocean water by other means in regions where knowledge of underlying physical processes are meager, espe- cially in the very deep waters. Proposals for the disposal in the sea of atomic energy wastes cannot be properly evaluated until estimates can be improved concerning the motion of this deep water. Many of the advantages (familiar in the laboratory) of using radioactive identifying tags can be realized at sea, even though rendered difficult by the very large physical dimensions of the oceans. What appeals most to the ocea- nographer is his new ability, under certain cir- cumstances, to make very rapid identifications of water lying on the surface or deep below his ship; thus allowing large volumes to be sur- veyed in three dimensions and in more detail than ever before possible. Three layers in the ocean are distinguished 1 Contribution from the Scripps Institution of Oceanography, New Series, No. 905. Contribution No. 929 from Woods Hole Oceano- graphic Institution. Part of Table 2 was computed with the collaboration of John Harley of AEC, New York City Operations Office, who gave much other counsel for which the authors are grateful. clearly by structure and behavior: the mixed layer near the surface; the intermediate layer lying just below wherein the temperature changes rapidly with depth (this thermal stratification bringing about great stability); and finally, the large, nearly uniform bottom water mass ex- tending to the sea floor with so little variation in density that very stable stratification is not possible. Each water mass reacts differently when disturbed, and therefore, mixing occurs differently in response to currents and mass intrusions. Experiments conducted in any one of these domains must take into consideration the special features that exist, and must call upon equipment most suited to these sections. Equipment specialized for radiological survey work in any of these oceanographic domains is still primitive. However, it can be said that equipment for detecting and measuring radia- tion is not a bit less highly developed than are the equipment and techniques needed for navi- gating a ship, and for maneuvering detectors at sea, especially at great depth. So much in- formation now can be reported by radiological means in a short time that a ship now has more reason than heretofore for precise navigation. In some cases, the depth and position of the detector relative to the ship must be known instantly, and almost always must be controlled far better than has been accepted by traditional hydrography. It is difficult to record data in full detail in many cases, and in others it is difficult to evaluate features rapidly enough to alter maneuvers to best advantage; an ocea- nographer can now expect to be aware of a strongly active layer in less than one second after his electronic probe makes contact, and he even may make use of a fast moving airplane to outline radioactive areas on the surface. 121

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122 Atomic Radiation and Oceanography and Fisheries Instrument Sensitivity and Natural Backgrounds Many promising measuring schemes have been proposed. However, it is profitable to com- pare the equipment and techniques which have been used already at sea. At the top of Table 2 is presented the background radiations coming from cosmic rays and from the natural potas- sium in the sea, and the activity level now believed tolerable for drinking water also is given for comparison. At the bottom of Table 2 are listed, in the brief numerical form in which they are com- monly stated, the sensitivities of three measur- ing techniques which actually have been used for radiological exploration at sea. Many as- pects of the measurement problem are over- simplified by a comparison of this sort, but the table does indicate that present shipboard beta analysis is capable of measuring beta tracer ac- tivity below the background beta activity due to the potassium in the sea water, whereas gamma detectors so far have been limited at levels above the gamma backgrounds of the sea. On surveys covering large distances, such as on Operation TROLL (U. S. Atomic Energy Commission 1956), and on the SHUNKOTU MARU Expedition (Mujoke, Sugiura, and Ka- meda 1954), there is ample time for water analyses, and advantage can be made of beta techniques. Nevertheless, there are many circum- stances where direct measurements by gamma devices are necessary for rapidly locating small contaminated water masses, and it is likely that gamma techniques will be perfected so as to allow use at levels far below their present capability. There are occasions at sea in which a gamma detector must indicate the presence of tracer activity within a few seconds after making con- tact. The limitations imposed by this sort of time restriction in the presence of statistical fluctuations in the signals are discussed in Ap- pendix A, and are summarized in Table 3. Other important details concerning the radio- active background in the sea have not been thoroughly explored. It may be too late to estimate the background level that existed a decade ago for some isotopes, and this should not be forgotten in planning future surveys. Of particular interest are background condi- tions near the sea floor where radium and thorium activity are known to accumulate in sediments; but little is known in detail about the lateral distribution of bottom activity. More complete utilization of weapons' tests for the marine sciences It appears likely that large weapons will continue to be tested in oceanic areas and that radioactive materials will be strewn from time to time over the surface of the sea. Valuable oceanographic data already has come from such sources; for example, direct measurements have been made of the rate of mixing downward from the surface to the thermocline, and also, direct information has been obtained regarding mass motion and lateral mixing. One special feature of benefit in studying weapons tests is the unique initial boundary condition provided by the arrival of fallout activity almost simul- taneously over an area having dimensions very large compared with the depth of water in- volved; downward mixing appears as a rela- tively simple phenomena following this initial condition, and can be studied under almost ideal circumstances. Two expeditions mentioned above have proven that further information concerning lateral mixing and flow can be gained for many months after a weapons' test, and obviously this fact should be exploited fully by marine scientists of all nations. Ancillary benefits might come from more or less fixed monitoring stations; if, for example, following the 1954 test, repeated sampling had been done off Guam it would have furnished data of value for in- terpolating observations made in the two fol- low-up cruises mentioned. Bottom exploration following weapons' tests has not been given deserved attention, and in- sufficient attention has been paid to getting even purely oceanographic information from these sources into the form needed by those people who are charged with making decisions regarding the ominous waste disposal problem. Hazards involved in the deliberate tagging of ocean waters Safety of the research staff is always a con- sideration; at sea because of special circum- stances the handling of extremely large amounts of activity is not too difficult or hazardous. Pro- tection can be secured very cheaply by towing the larger sources of radioactivity aft of the

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Chapter 12 123 Tagged Water Masses for Studying the Oceans ship, preferably slightly submerged on a suit- able barge or special vessel. Bringing large quantities of activity to the waterfront prom- ises to be more expensive, but practical experi- ence in this should be valuable for later plan- ning of large-scale disposals. The more controversial question of how much radioactivity can safely be introduced into the sea is not without reasonable solutions; but the recommendations depend upon the cir- cumstances, especially, upon the particular part of the oceans to be studied. At the outset, barren areas of ocean rather than those produc- tive of things leading to human food must be selected since the former can yield equally good information regarding purely physical phenomena. Deliberate tagging of surface waters (Opera- tion PORK CHOP) Surface waters mix in a turbulent manner due to forces not yet fully understood. Better knowledge of this layer is badly needed justi- fying the consideration of water tracing experi- ments involving introduction of fairly large amounts of activity. Greatest care must be exercised here because these waters are those most close to humans, in several senses. Rate of mixing to the bottom of the mixed layer, and rate and character of lateral motion as functions of the usual parameters of the sea are of most immediate interest, and observa- tions lasting even a few days or few weeks would be of great value at the outset, especially if repeated frequently. A simple surface water experiment now will be proposed in briefest possible outline. Figure 1 presents schematically some of the procedure which might be used and some of the phenomena to be expected. Guided by suitable navigational aids, here represented by deep-anchored buoys No. 1 and No. 2, the ship A proceeds on a straight course while dropping two quantities of radioactive materials (a and a') mixed with enough surface water to leave near the surface a small contaminated patch having nearly neutral buoyance. These are essentially point-source initial conditions in this scale of dimensions; although, they are not as convenient as the plane-source initial conditions BUOY * 2 OPERATION "PORK CHOPS' FIGURE 1

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124 Atomic Radiation and Oceanography and Fisheries provided by fallout, they have some mathemati- cal simplicity. It would appear economical and informative to drop two sources almost simul- taneously, some distance apart — say one to ten kilometers; this would permit large-scale ad- vection also to be studied at little extra ship cost. From the sources s and s' will grow a larger more dilute patch of water finally ceasing to penetrate rapidly downward at depth d. The rate and lateral spread prior to this time as functions of wind velocity are of special in- terest. After further downward penetration is retarded, the areas a and a' move and expand to the larger areas A and A' conserving most of the original radioactive material, and the product of activity and area should be almost constant after correction is made for the known rates of decay of radioactive constituents. Dual ship operations Experience has shown that operations on the scale of this sort can scarcely hope to be suc- cessful unless more than one ship is used; even with the best facilities one ship may lose con- tact with the invisible patch and waste valuable time locating it. One ship, X, must stay in or near the tagged mass while the other one, Y, may survey the area in detail, inspecting sections across the mass, studying the bottom for ref- erence features, and chasing missing buoys if necessary. Ultimate disposal of hazard in surface waters Reduction of activity to a level below that of the natural activity of sea water is one cri- terion which has been used for planning dis- posals (Glueckoff 1955), and this is fairly reassuring provided the specifically dangerous and the long-lived activities are eliminated, for example, after radiostrontium and radiocesium are removed from raw fission wastes. Present evidence permits the conclusion that in the open ocean, when winds are above the critical white-cap level and under circumstances where mixing ceases at a depth of about 30 to 50 meters, as much as 1,000 curies would mix to a safe dilution in less than 40 days. An ex- ample of the dispersal rate in the open sea will now be given. Brief outcome of an experimental tagging of surface waters in the open sea Surface water made active by introducing fis- sion products concentrated within a few square kilometers was intercepted by a ship 36 days after inoculation and traversed for 10 days. After corrections were made for the drift of the water during the survey, and for radioactive decay, a synoptic picture could be drawn roughly locating the contours of activity. This estimate of radioactive distribution was referred to the time of 40 days after the start of dispersal. The contamination had mixed significantly only to about 30 to 60 meters, although the thermodine lay nearer to 100 meters depth. The following tabular description of this synop- tic sketch can be made. TABLE 1 APPROXIMATE DISTRIBUTION OF RADIO- ACTIVITY FOUND IN THE SURFACE WATERS OF THE OPEN SEAS 40 DAYS AFTER BEING INTRODUCED SUD- DENLY AS A "POINT SOURCE." (A SYNOPTIC PICTURE COMPUTED FROM MEASUREMENTS MADE ON SEVERAL DIFFERENT DAYS.) Concentration of radio- activity (as per cent of the maximum concentration measured). Areas inside the contours as percentages of the area of the maximum contour. Areas inside con- tours of equal concentration in square kilometers. 40,000 (km*) 100% 24,000 65 14,000 38 8,000 22 800 2 490 1 35 0.1 20 30 40 60 80 100 At the end of 40 days, the center of gravity of this distribution was about 120 miles from the point of inoculation and the pattern was about four times longer than broad. The wind was 3 and 4 of Beaufort's scale for the first 20 days, but was much calmer for the last 20 days. If the average mixing depths are taken as 50 meters, then, 1,000 curies distributed over 40,000 square kilometers would result in an average concentration of 1.5 x KHy/ic/ml. This would certainly be safe sea water in most senses; and even in the smaller areas where much less than the average dispersal took place the water should also be safe. In fact, the experiment indicated that it is likely that after 40 days, following the introduction of 1,000

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Chapter 12 Tagged Water Masses for Studying the Oceans 125 curies of activity into the surface waters of the by considerations of hazard to humans. Two open sea, only about 0.1 per cent of the total more difficult experiments will now be de- area should retain contamination above the scribed, tolerance concentration permitted for potable water, and even in this small region the residual Investigations in the thermoclme layer by use artmcial activity would amount to less than the Q± radjoactivih normal natural activity of sea water. It is evident from Table 2 and Table 3 that The thermocline lying between perhaps 100 shipboard beta measurements would suffice to meters depth on an average, and 800 meters detect the more radioactive spots if there were or more, can be thought of as being a lid which initially 1,000 curies of slowly decaying beta restrains deeper water from reaching the sur- activity; it is apparent, however, that direct face. Experiments in this stable region must measurements by gamma detectors might be take into consideration the fact that any liquid sufficient for several days or even weeks. Sur- introduced here will seek the level of its own face experiments are by far the easiest to con- density and will then spread out in a very thin duct and implement — they are limited largely layer. An experiment in this layer has been TABLE 2 APPROXIMATE SENSITIVITIES OF THREE DETECTING AND MEASURING TECHNIQUES PRESENTLY AVAILABLE FOR USE AT SEA COMPARED WITH THE ACTIVITY OF SEA WATER AND WITH THAT OF FRESH WATER. A Common background radiation levels: d/m/1 curies/1 microcuries/ml rad/hr2 mrad/yrS Activity in normal sea water due to potassium:1 Gamma rays 70 3 X 10'u 3 X 10-8 1 X 10'1 0.9 Beta rays 660 3.0 X 10'10 3.0X10^ — — Maximum permissible 4 concentration of unknown mixed beta activities in drinking water: Beta rays 220 11 X 10'" 1 X 10"* — — Cosmic ray background at sea surface: s At equator 61 — — — 33 At 55°N (mag) — — —37 B Sea water activities at which present measurements are significant. Shipboard water analysis 0 for mixed beta emitters, 60 minutes count after removal of potassium: 50 ±15 2 X 10'u 2 X 1 OCR for page 121
126 Atomic Radiation and Oceanography and Fisheries TABLE 3 COMPARISON OF MINIMUM DETECTABLE CONCENTRATIONS USING SEVERAL MEASURING TIMES AND ASSUMING SEVERAL BACKGROUNDS (a) Minimum detectable anomolous activity if potassium of the sea produced the only background, i.e, B= 1.2 X 10-* gammas/sec/ml. Rads/hour Counting time .,,,-. L,_ ,.. in sees. t Minimum detectable concentration = ,y/min/1 Net signal r counts/min Total net Photons Photons C.Ve=30C. counts 30C.t .6mev 1.5 mev 3 19 11,000 5.7 17 6.5 X 10-* 16 X 10" 5 11 6,600 3.3 17 3.8 9.5 60 010 600 0.3 18 .3 .8 180 0039 230 .12 22 .13 .33 300 0026 160 .078 23 .09 .22 600 0016 99 .048 30 .06 .14 Very large 0.025/rVt (b) Minimum concentration detectable if backround were 15 CPM, i.e., an actual background signal ex- perienced in deep water. 3 5 60 180 300 600 .19 .11 .010 .0038 .0049 .0032 11,000 6,600 590 350 290 190 3.7 3.3 .29 .17 .15 .096 17 17 17 32 43 58 6.5 X 10" 3.8 .33 .20 .17 .11 16 XU 9-5 .84 .50 .42 .28 Very large 0.067'/ vT' (c) Minimum detectable concentration if total background were 60 CPM, i.e., an actual background signal experienced at the sea surface. C. 3 205 12,000 5 133 8,000 60 0222 1,330 180 0116 700 300 0087 520 600 0059 354 Very large 0.13/"Vt~ described in some detail by Revelle, Folsom, Goldberg, and Isaacs (1955), and discussed in several of the accompanying papers. It will be discussed here only in the matter of difficulty of survey. Although mixing is known to be very slow in the thermocline, it is not certain how direct is the path from this fringe biosphere to human food supply, so that the hazard of a long remaining concentration of activity is not easily evaluated. Revelle et al., prefer to sug- gest the experimental use of the conservative amounts of 10 to 100 curies, and they then show that such small sources of radioactivity might be practical none the less. Actual field experience has shown that layers as thin as one or two meters thick are extremely difficult to sample for water analyses even after being located by gamma ray detectors. Folsom (1956) has emphasized that future deep sur- veys with radioactive tags must rely heavily upon discovery of radioactive water by means 6.1 4.0 .67 .35 .26 .17 18 20 40 63 78 102 7.1 X 1 4.6 1.9 .4 .3 3. 18 X 10-* 12 7.5 1.0 .74 .51 of gamma detectors, and has urged that special- ized forms of these be brought to perfection. In this particular layer, geometric factors are not adverse for maneuvering a detector into the water mass to be studied; a probe is dropped rapidly and more or less vertically so as to intersect and pierce a rather broad horizontal lamina, sharply confirming the activity. Some difficulty would be encountered in holding the probe in the thin layer long enough to permit accurate measurements after the activity falls to such a low level that statistical fluctuation becomes the predominant source of error; how- ever, the major difficulty even at these depths is holding the ship in the general area of active pools of small size. Any area of less than a square mile below the surface is a tiny detail in the open sea, and oceanographers never be- fore have realized how hard it is to navigate and maneuver to study areas so small. Multi- ship operation, the use of the best position-

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Chapter 12 127 Tagged Water Masses for Studying the Oceans locating gear, and careful crew training and teamwork are necessary for subsurface radio- logical surveys even at these moderate depths. Outline of tagging experiment in the thermo- cline layer Figure 2 illustrates certain features which must be considered in this region. The ship, A, may lower a gamma sonde through an activated pool and detect its presence by the receiving of a signal like that shown on the right side of the figure; the hydrographer may obtain a water sample by triggering electrically a water sampler at the moment the detector indicates that the sampler is within the active layer. The data in Table 3 make it clear that rapid response is important during this sort of measurement; a statistically significant signal must be accumu- lated in the short period during which the probe is passing through the active layer. Attention is called to the need for naviga- tional and maneuvering aids here by including schematically the parachute-drogue C. It is difficult to maneuver a weighted detector hori- zontally in order to study the lateral distribution in detail. The use is suggested of towed gamma detectors depressed to the desired level by hydrofoils controllable from the surface, more or less as illustrated schematically at the left of Figure 2. By means of a swivel-clamp, SC, a pennant several meters long containing a row of Geiger tubes or other gamma detectors, might be suspended above the depressor so as to pre- sent a vertical, linear array, thus giving a high probability of intersecting wide lateral distri- butions of activity. This sort of gear should not be too awkward nor fragile for deck handling at sea. Signals might be recorded partially, or entirely inside the depressor, or reported to the ship electronically or sonically. Ship A or a sister ship with similar gear might stay in the pool during the whole experi- ment, however, if the pool were lost after its depth was established, then Ship B would likely be the first to find it again with its towed detector. Difficulties in sounding and exploring very deep waters Bottom exploration so far has been confined largely to sonic plotting and sounding by solid cable; very deep wire casts are very time con- suming and difficult; the ship generally is moved laterally by surface currents before the OPERATION "POKER CHIP" FIGURE 2

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128 Atomic Radiation and Oceanography and Fisheries wire touches bottom. Oceanographers seldom hope to place their sondes and coring tools upon any pre-selected topographic detail of small area. However, it is quite likely that a technique can be perfected for dragging a de- tecting instrument along the bottom in many areas of the oceans' floor, and with a dragged detector a large region might be traversed rap- idly, and tagged water masses near the sea floor might be located and surveyed. A proposal for tagging bottom waters now will be outlined. Difficulties in tagging bottom waters Fortunately, little hazard to human popula- tions would result from putting into the deep bottom waters in certain latitudes almost any amount of activity which might be readily available in the near future, or which would be easy to handle safely ashore and on ordinary surface vessels. After all, these amounts would be only the feeble forerunners of what may have to follow. The problem is that of displaying even a rela- tively large radioactive source economically in face of the immensity of the abyssal reaches. One can think of many things which must not be done; heavy, radioactive liquid cannot be merely poured overboard, for example. Match- ing density at intermediate layers or attempting to insert a strata at a selected depth also would appear experimentally difficult in view of the limited knowledge presently available; an un- equilibrated liquid mass might wander about like a sinking dinnerplate — and soon become lost. In the absence of the restraining forces found in more stable waters, the pouring of streams of dense solution downward from a height above the bottom, or alternately the re- leasing of lighter material upward from the bottom would surely cause mass motion which might not cease until the streams had moved long distances and perhaps had curled into con- figurations quite unsuited as initial boundary conditions for water tracing experiments. Fur- thermore, activity spread initially in more or less vertical lines would make very poor targets for detectors trailing on the end of wires three miles long, and would be wasteful in terms of radioactive material and of expedition time. One might, of course, carefully select a per- fect basin, and might gently introduce into it a dense radioactive solution. This certainly should be considered since only a small amount of activity might suffice for tagging the waters in a small basin and valuable information re- garding motion and dispersion in basins might result, but results would not lead to a realistic picture of the large scale flow over bottom which may have to disperse the wastes dumped in the future. The results of an experiment set up in this way would be inadequate, and, in fact, might be misleading in a dangerous direc- tion. Production and use of horizontal line-sources near the bottom "Operation HARE and HOUND" It is evident that distribution of activity in a horizontal line near the bottom would be most easy to intercept by a detector dragged along the bottom, and it appears also to be something which would be relatively easy to produce, and economical. It should be possible to hold tagged water near the bottom by mixing it with a very dense solution; and there are two ways im- mediately evident for effectively spreading streaks of dense solution for long distances over the bottom terrain. Figure 3 illustrates the two methods proposed for tagging bottom water, and the method pro- posed for locating the tagged masses later. The Ship B' is shown dragging a "Hare" D, across the bottom leaving behind a streak of contami- nated water. Alternately, Ship B is shown just after it has dropped to the sea floor a specialized water blending device which might well be called a "quern" 1, C, which generates for a few minutes or hours, a stream of dense, radio- active solution on the slope of a carefully se- lected large topographical ridge b — d; this stream flows away very much like one of the submarine currents which are now called "tur- bidity currents" by geologists. Violence of this sort of free current might theoretically be con- trolled through wide limits by adjusting the densities of the solution. The essential features of a water-tagging quern are shown in the upper right of Figure 3. Radioactive material, AS, is combined in predetermined proportions with a heavy salt solution by metering pump, P, and the two are then fed to a fan-type mixer, and are there blended with a large volume of 1 Old English name for a mill for grinding all sorts of things. (Ruggoff, 1949.)

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Chapter 12 129 Tagged Water Masses for Studying the Oceans OPERATION "HARE AND HOUND" *, DISTANCE FIGURE 3 local water. There are several reasons for pre- ferring a design leading to inexpensive construc- tion and single use; the cost of decontamination of apparatus of this type would outweigh any benefit from repeated use. Suggestion is made of the use of a salt such as sodium nitrate which has both high solubility, and an endothermic heat of solution which would serve to overcome the adiabatic heat set free during lowering. It would appear that one or more tons of a nitrate salt, mixed into bottom water by use of a few kilowatt hours of energy, stored in oil-sealed accumulators, could produce a compact body of very heavy water which would rush like a freight train across the terrain dropping a streak of traceable radioactive eddies as it trav- eled. A fixed, water-mixing quern, of the sort described, might produce a tagged water mass behaving in a manner appearing realistic to both the disposal planner and the submarine geologist; however, its use is not likely to lead directly to the extremely simple results needed for the very first experiments. The employment of a dragged hare might be preferable at the outset — and its metering machinery might be somewhat less elaborate than that of the quern just described. One might contemplate using 1,000 or more curies for making streaks several kilometers long so that location would not be difficult with a simple gamma device dragged by a ship. In Figure 3, Ship A is shown dragging such a de- tector which might be called a "hound" for obvious reasons. For very great depths, no elec- trical wire is presently available with the dura- bility equal to that of an ordinary dredging cable. It would, therefore, be wise to first con- sider the use of a compact multichannel chart recorder inside the dragged pressure shell E so as to make permanent records of signals picked up by a set of gamma detectors suspended by an

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130 Atomic Radiation and Oceanography and Fisheries oil-filled float F. Numerous accessories might profitably ornament this sort of gear, but the one which might prove most rewarding would be a sound producer capable of reporting the moment of contact with the tagged water mass ; even a crude sonic signal sent from a transducer on the float, F to the ship, A, via the towed hydrophone, H, would suffice. Details of the gamma signals need only be recorded so that they might be inspected later on the recorder chart, however, it would be important for the navigator to recognize instantly when contact was made so that he could maneuver the ship economically. The operations proposed above are not un- like those used successfully by cable ships when retrieving submarine wires. Careful preliminary surveys of the whole area, the selection of iden- tifying landmarks, and the laying of the mark- ing buoys also appear essential for success in work of this type. The final results might have the general char- acter of the hypothetical signals shown graphi- cally at lower right in Figure 3. Change in amplitude and displacement, and skewness of the signal records should lead to estimates of both velocity and rate of mixing. If each survey included ten or more intersectings, and if each contact brought separate gamma signals from several detectors distributed along the hound's vertical "tail," then the data of the sort needed would accumulate quickly. Rough estimate of effectiveness of 1,000 curies for tagging bottom waters It appears possible to distribute radioactivity uniformly along the course of a device dragged over the sea bottom, and it would appear pos- sible also to deposit the material so gently that it would come to rest within a few meters of the precise course. If, for a rough evaluation, we assume that local diffusion sooner or later produced a uniform distribution within a radius of 10 meters, and that the total activity, M, was 1,000 curies, then the length of the water mass which might be tagged can be stated ship's speed is about 4 knots), then the equa- tion (9) of Appendix A indicates that a single detector like the 1955 SIO Geiger instrument could detect, in the presence of a realistic deep- water background of 15 cps, a limiting gamma source concentration of 0.061 disintegrations/ sec/ml, or C=0.061/3.7x 10-10 curies/ml, and the length of traverse which could be tagged with 1,000 curies would be, under these as- sumptions, /= 1000 where C is the average concentration of activity within the tagged mass. If now we assume that only 10 seconds can be allotted for traversing 20 meters (that is the It would appear feasible to locate and allocate by ordinary navigational means a geographical line in the deep sea floor of less than two kil- ometer's length, so that the hypothetical ex- ample just given suggests that 1,000 curies could equally well be used to produce a very concentrated streak of activity having a length of two or three kilometers which might still be detected with ease after it had diffused, mixed, or decayed to less than one percent of its initial concentration. Thus it can be concluded that 1,000 curies, or even less activity, put into bot- tom water would be quite adequate for tracing movements on a scale large enough to contrib- ute information useful in disposal planning. SUMMARY AND CONCLUSIONS 1. Consideration has been given some of the problems involved in tagging water masses in the open ocean. 2. The problems are different in the three major strata; the surface layers, the thermocline, and the deep water layer. 3. It appears that under certain circumstances water tagged with even moderate quantities of activity can be followed for at least several weeks; surface waters contaminated by large activities such as result from fallout can cer- tainly be followed for a year or more. 4. Much field experience in radiological ocea- nography has been gained already. A fairly clear direction for development of instruments has been indicated. 5. The need is seen for attention to the perfec- tion of navigational aids, for use of specialized vessels and gear, and for the use of several ves- sels simultaneously in oceanic surveys of this sort.

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Chapter 12 131 Tagged Water Masses for Studying the Oceans APPENDIX A In practice, many factors tend to limit the effectiveness of an under sea gamma detector, but the random fluctuation of a feeble radiation may alone prevent its recognition in the pres- ence of a background of similar magnitude. The lowest detectable concentration, limited only by statistical considerations, may be expressed in terms of the strength of the background, the time permitted for measurement, and the meas- uring efficiency of the instrument. Let the sea water be contaminated with a con- centration of radioactivity N curies/ml, and let this activity cause m counts/sec to be indicated by the instrument, and let the average back- ground be b counts/sec. The relative accuracy, n, of a single measurement made during t sec- onds will depend upon signal strength and background strength; if the fluctuations are purely random, the error, 95 per cent of the time will be equal to, or less than, _2a _ mt~ mt mt and solving for the net signal gives, mt = A.I A.2 Now, the counting efficiency of the instru- ment logically should be derived from the ratio of counts recorded to the photons striking the instrument. This ratio would be impossible to evaluate, but it is approximated when the instru- ment is small, and easily penetrated by, e = 3.7x1010Nf/ A.3 that is by the ratio of the net counts recorded to the photons emitted in a volume of liquid, v, equal to that displaced by the detector. Solv- ing this equation for concentration, XT mt N=^r= rrr^ : A.4 curies/ml, and substituting here the value for net count, mt, obtained in equation (2) when the background rate is b, and accuracy is, n, the limiting concentration can be expressed, N=- A.5 curies/ml, wherein b expresses the background rate actually indicated when the instrument is surrounded by clean sea water. If no other back- ground exists except that coming from a sur- rounding solution having specific activity B, and if the instrument counts this activity with the same efficiency, e, than the limiting detectable concentration becomes, in curies/ml, N= 2 + 2\/l + Bri-vet 3.7 x 1 A.6 Numerical examples applying to an actual un- dersea instrument The sensitive portion of the 1955 model of the Scripps Institution of Oceanography's Geiger instrument has a volume of about 1,000 ml. The ratio e, applying to hard gamma rays, was measured directly by submerging the in- strument in a tank containing potassium solu- tion of known concentration, and was found to be approximately 0.03. If by "detection" is meant the measurement of the concentration with an error of not more than 50 per cent, then, n = 0.5. Formulas (5) and (6) may now be applied to three characteristic background circum- stances: Case 1: Here no other background is evi- dent except that caused by a solution having specific activity B = 1.2x10-5 gammas/sec/ml such as comes from the natural potassium in normal sea water. From (6), the limiting de- tectable concentration, 2 + 2V1+0.009/ A.7 gammas/sec/ml, and when t becomes very large this approaches, Case 2: In deep water cosmic rays may be neglected, and the S. I. O. probe is likely to indicate a total background of about 15 CPM, or b=0.25 counts/sec, therefore, the concentra- tion just delectable is, 2 + 2V1 + 0.063/ A.9 gammas/sec/ml, which approaches as t in- creases to a large value, 0.067 A-10 Case 3: In shallow water where cosmic rays are unattenuated, the background on the S. I. O. probe amounts to about 60 CPM, or b=1.0

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132 Atomic Radiation and Oceanography and Fisheries counts/sec, therefore the minimum detectable concentration becomes, 2 + 2\/1+0.25/ 7.5/ A.11 gammas/sec/ml which approaches for very large values of t, C,=V/ A"2 Tabulations Table 3 compares the effect of increasing the period of measurement with the effect of di- minishing the background. It is evident that a substantial change in background has relatively small practical effect on any measurement made so rapidly that only a very poor sample is taken out of the fluctuating signal; however, when sufficient time can be alloted for good sampling, the background level becomes the limiting fac- tor. It should not be overlooked that in practi- cal field work, instrument imperfections may contribute to the overall error more or less pro- portionally with time of measurement, and that measurement time must be spent economically on almost all oceanographic expeditions. It is apparent therefore that efforts should be made towards increasing the counting rate, ve, while reducing the relative value of the background count by all possible means. Technique for cleanliness and for discrimination of back- ground by electronic means have not yet been fully developed for this purpose. REFERENCES FOLSOM, THEODORE R. 1956. Problems pecul- iar to direct radiological measurements at sea. Paper presented at Nat. Acad. of Sci- ence Meeting, 29 Feb.-1 Mar. 1956. Wash- ington, D. C. Proceedings (in press). GLUECKOFF. 1955. Long term aspects of fis- sion product disposal. United Nations Con- ference on the Peaceful Uses of Atomic Energy, Geneva. Paper No. 398: 11 pp. MIYAKE, Y., Y. SUGIURA, and K. KAMEDA. 1954. On the distribution of radioactivity in the sea around Bikini Atoll in June 1954. Paper in meteor and geophys., Me- teorol. Research Institute, Tokyo, 5:253— 262. REVELLE, R. R., T. R. FOLSOM, E. D. GOLD- BERG, and J. D. ISAACS. 1955. Nuclear science and oceanography. United Nations International Conference on the Peaceful Uses of Atomic Energy, Geneva. Paper No. 277:22 pp. RUGGOFF, MILTON D. (Editor) Why the sea is salt (an abstract from a translation from the Norse by Sir George Weble) pp 672- 676 in Harvest of World Folk Tales. XViii + 734 pages, Viking Press. U. S. ATOMIC ENERGY COMMISSION and OF- FICE OF NAVAL RESEARCH. 1956. Opera- tion TROLL. Health and Safety Labora- tory, U.S.A.E.C., New York Operations Office, NYO-4656, Ed. by J. H. Harley: 37pp. U. S. DEPARTMENT OF COMMERCE. 1953. Maximum permissible amounts of radio- isotopes in the human body, and maximum permissible concentrations in air and water. National Bureau of Standard Handbook 52:445 pp.