104 



Atomic Radiation and Oceanography and Fisheries 



spread over a horizontal area of radius r. They 

 find that with the best of present instruments, 

 the horizontal spread in which the concentration 

 of the introduced radioactivity can be deter- 

 mined corresponds to r= 1 km. With the possi- 

 bility of improved instrumentation, and the use 

 of specially selected nuclides, it may be possible 

 to raise the area of detection and determination 

 of activity concentration to about 100 km^, an 

 area which is still negligible with respect to 

 oceanic expanses. 



The radioisotopes suitable for such measure- 

 ments must of course have a half-life compati- 

 ble with the mixing rates to be studied and yet 

 short enough so as not to constitute a perma- 

 nent hazard, namely of the order of a week to a 

 month. Moreover, they must be available in 

 multi-curie amounts at reasonable cost, should 

 form soluble ionic species in sea-water, have a 

 high specific activity, and, for instrumental rea- 

 sons, should be gamma emitters with energy be- 

 tween .2 and 1.3 Mev. Revelle, et al., were 

 able to list three such isotopes, which, together 

 with half-life, cost, and other data, are listed in 

 the following table: 



Cost 

 Half- per 



Isotope life curie 



Rb"* . . . 19.5 day $1000 

 P^ ... 8.0 day 750 

 Ba'*" ... 12.8 day 500 



Specific Gamma 



activity energy 



available Mev 



9 mc/gram 1.1 



Carrier-free 0.36, 0.72 



Carrier-free 0.16, 0.54 



Comparison of the cost of these isotopes with 

 the maximum area of detection cited above 

 shows that the study of large-scale transfer phe- 

 nomena in the oceans, using deliberately intro- 

 duced artificial radioactivity in the form of spe- 

 cific isotopes, is so costly as to be infeasible with 

 the estimated best instrumentation which will 

 be available in the near future. It is evident 

 that such isotopes are at best adapted only to 

 short-term, small scale studies of local phe- 

 nomena. The use of mixed fission products on 

 a large scale, discussed elsewhere in this report, 

 is somewhat more feasible but is beset with 

 many difficult problems of transportation and 

 handling. 



From these considerations it seems evident 

 that the critical data in studies of atmospheric 

 and oceanic mixing and interaction will come 

 from the use of the naturally occurring isotopic 

 tracers, which reflect in their material balance 

 adjustments the differential rates of transfer 

 from source, through reservoir, to sink, and 



loss by decay. It is from these transfer rates, 

 adjusted to the steady state geochemical and 

 geophysical cycles of the various elements, that 

 we can hope to gain an understanding of the 

 long period variations in natural transfer phe- 

 nomena. The importance of gaining a clear un- 

 derstanding of the long period transfer rates, 

 when problems such as storage of potentially 

 hazardous radioactive wastes and cumulative ef- 

 fects of nuclear detonations are considered, can- 

 not be overemphasized. 



In the following sections we discuss the pres- 

 ent status of our knowledge of the distribution 

 and properties of the various naturally occurring 

 isotopes which are useful for studies of atmos- 

 pheric and oceanic transfer phenomena. In ad- 

 dition, mention is made of the nuclides pro- 

 duced in nuclear detonations and supplied by 

 reactors which have properties such that they 

 are also useful in such studies and which have 

 been studied to some extent. 



II. Distribution of naturally occurring isotopes 

 of elements adapted for transfer studies 



In this section we discuss the production and 

 occurrence of radioactive and stable isotopes 

 showing measurable isotopic variations, and the 

 distribution factors which determine their rela- 

 tive concentrations in natural materials. 



Carbon 14 



Carbon 14 is formed in the atmosphere by 

 the reaction of neutrons with nitrogen, i. e. 



Ni4-}-n = C" + p + 620kev; 



the neutrons being the result of the interaction 

 of primary cosmic rays with the atmosphere 

 (Libby, 1955). The carbon 14 is naturally ra- 

 dioactive, decaying by ^S-emission back to nitro- 

 gen 14 with a half -life of 5570 years. Thus the 

 half-life is so short that radiocarbon depends, 

 for its existence, on the continual production in 

 the stratosphere, with which it is presumably in 

 steady state. The assumption of a steady state 

 condition for at least the last 15,000 years is 

 justified by the observation that radiocarbon 

 dates on historic samples agree with the calen- 

 dar dates. The steady state production rate, 

 which is equal to the steady state disintegration 

 rate, can be calculated from measurements on 

 the neutron flux in the lower stratosphere and 

 compared with the observed specific activity of 



