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 iveapons' 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 



