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 Minimum detectable Net signal , ^ ^ 



in sees. concentration counts/min Total net Photons Photons 



t 7/sec/ml = 7/min/l CaVe=30Ca counts 30Cat .6 mev 1.5 mev 



Ca 



3 19 11,000 5.7 17 6.5X10^ 16X10^ 



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/ Vt 



(b) Minimum concentration detectable if backround were 15 CPM, i.e., an actual background signal ex- 

 perienced in deep water. 



Cb 



3 19 11,000 5.7 17 6.5X10"^ 16X10-" 



5 11 6,600 3.3 17 3.8 9.5 



60 010 590 .29 17 .33 .84 



180 0058 350 .17 32 .20 .50 



300 0049 290 .15 45 .17 .42 



600 0032 190 .096 58 .11 .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. 



Cc 



3 205 12,000 6.1 18 7.1 X 10"^ 18 X 10"^ 



5 133 8,000 4.0 20 4.6 12 



60 0222 1,330 .67 40 1.9 7.5 



180 0116 700 .35 63 .4 1.0 



300 0087 520 .26 78 .3 .74 



600 0059 354 .17 102 .2 .51 



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 



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- 



