SECT. 1] 



ARTIFICIAL RADIOACTIVITY IN THE SEA 



79 



mixed layer and the rate of mixing of waters across the thermocline. Assuming 

 that the thickness of the mixed layer is 100 m and most of the radioactive fall- 

 out remains within the layer for a few years, the concentration of 90 Sr in sea- 

 water from fallout averages 0.17 p.fxc/1. in the Northern Hemisphere, while it 

 is only 0.05 [jljxc/I. in the Southern Hemisphere. 



Table I 



The Percentage of Activity in Curies of the Principal Radioisotopes from Slow 



Neutron Fission of 235 U (1 kg) 



Twenty 

 days 



9.8 x 10 6 curies 



One 

 year 



3.1 x 10 4 curies 



The ratio of immediate or direct fallout of artificial radioactivity to the far- 

 reaching tropospheric and stratospheric fallout is uncertain but it is estimated 

 to be roughly 1 : 2 to 2: 1. According to the estimation by the U.S. Atomic 

 Energy Commission the total amount of 90 Sr produced to date (1960) is equal 

 to 9.2 megacuries, of which about 3 megacuries was immediate fallout. On the 

 other hand, fifty explosions of multimegaton class were detected in Japan by 

 our barometric oscillations up to the end of 1958. Assuming that the average 

 energy of the explosions is five megatons and that half of the energy is produced 

 by the fission process, the total amount of 90 Sr produced so far is equal to 

 12.5 megacuries, which is a little larger than the estimated value of the 

 U.S.A.E.C. Based on the measurement of radioactivity in oceanic waters in 

 the Pacific (Miyake and Saruhashi. 1960), the total activity released directly 

 into sea-water from the test areas was estimated to be about 3 megacuries in 

 1954. Since no larger surface or sub-surface tests were made subsequently in 



