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 thermocline layer by use 

 artificial activity would amount to less than the ^j radioactivity 



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/hr 2 mrad/yr ^ 



Activity in normal sea water due to potassium: ^ 



Gamma rays 70 3 X 10"" 3 X 10"* 1 X 10"^ 0.9 



Beta rays 660 3.0 X 10"'" 3.0 X 10"^ — — 



Maximum permissible * concentration of unknown mixed beta activities in drinking water: 



Beta rays 220 l\ X 10"'° 1 X 10"^ — — 



Cosmic ray background at sea surface: ^ 



At equator 61 — — — 33 



At 55°N (mag) — — — 37 



B Sea water activities at which present measurements are significant. 



Shipboard water analysis ^ for mixed beta emitters, 60 minutes count after removal of potassium: 



50 ± 15 2 X 10"" 2 X 10"* — — 



Uuderwater gamma detector,'' 1956 scintillation rate-meter of AEC-NYOO: 



220 (approx) — — 1.4 X 10"^ 1.2 



(0.6 MEV gammas assumed) 

 Underwater gamma detector,^ 1935 geiger instruments of SIO. {counting pulses): 



(See also table 3 for other cases) 

 Case A: Used in deep water where net background is 15 CPM, assume photons of 0.6 Mev; assume short 

 measurements required, t = 5 sec. 



6600 3 X 10"' 3 X 10^ 3.8 X 10^ 30 



Case B: Towed on surface, assume constant background 60 CPM, assume photons of 0.6 Mev; assume 

 long measurements permitted, t ^ 5 min. 

 520 2 X 10"'° 2 X 10"' 0.3 X 10^ 3 



1 Assuming normal sea water has 3.8 X 10"* gk/g sea water, that beta activity is 29 d/s/gk and that 

 gamma activity is 3 d/s/gk. 



'The rad unit is somewhat larger than the more familiar roentgen unit; 1 rad z= 1.1 roentgen approximately 

 for gamma rays. Values in this column were computed upon the assumption that the activity was uniformly 

 distributed in the water and that the detector was a meter or more from any boundary. 



3 Referring to beta ray activity in rad units in roentgen units is a dangerous practice — much further spe- 

 cification depending upon the individual experiment is required. 



* Handbook 52 of the National Bureau of Standards. The values given refer to the case where the nature of 

 the activity is unknown; certain radioisotopes can be tolerated at much higher levels. 



5 See Table 1 in the accompanying paper "Comparisons of Some Natural Radiations Received by Selected 

 Organisms" by T R. Folsom, and John H. Harley for variation of cosmic rays with depth and altitude. 



s Cosmic rays are counted by most geiger counters at the average rate of approximately one count/min/sq 

 cm of counter area. 



^ This information was supplied by J. H. Harley from personal communication with H. D. LeVine of 

 the New York City Operations Office of the Atomic Energy Commission who designed this equipment. 



s This detector was not intended previously for use at low intensities, but rather for measuring a wide 

 range of intensities of gamma rays. Additional geiger tubes might easily be added to increase the sensitivity 

 by at least five fold. Still more sensitive gamma devices are now used in oil well logging. 



