Chapter 2 



Natural Radiation of Selected Organisms 



31 



1949) that any potassium beta particle which 

 originates inside a small organism will deposit 

 most of its energy outside the organism; appar- 

 ently less than 10 per cent of the total ioniza- 

 tion can take place inside a sphere having a 

 mean radius of 0.1 mm, and perhaps from the 

 activity concentrated inside a phytoplankter hav- 

 ing a mean radius of 0.01 mm only 1 per cent 

 of the energy would be felt by the organism 

 itself. Thus we see that the constitution of the 

 surrounding medium dominates the life of the 

 marine microorganism in a radiological sense as 

 well as in those other manners more familiar to 

 biologists. 



Units used 



For quantitative statements concerning such 

 feeble radiations as these it is logical to use a 

 very small unit and preferably one which is 

 defined in terms of energy absorbed; the milli- 

 rad per year (mrad/yr) is such a unit and is 

 used here. The rad unit is only slightly larger 

 than the more familiar roentgen unit, since 1.0 

 rad by definition causes 100 ergs to be absorbed 

 per gram of matter, and this is approximately 

 the energy deposited by 1.1 roentgen of gamma 

 rays. For converting beta activity to equivalent 

 rad dosage the average beta energy of potassium 

 has been taken as being 0.5 mev. 



Comparison of natural doses in several dojnains 



Figure 2 attempts to bring into a single pic- 

 ture the magnitudes of the main components 

 making up the radiation in each of several do- 

 mains of interest. The approximate total dose 

 to the organism is listed below the figure so 

 that numerical comparisons can be made. In 

 the sea and in deep lakes the dose to small or- 

 ganisms must be evaluated separately from that 

 experienced by large organisms. Circumstances 

 in each domain are given in more detail in 

 Table 3. (See Figure 2 and Table 3.) 



Discussio7t 



Small organisms must be considered sep- 

 arately from large ones. Only a small fraction 

 of the energy coming from activity inside a very 

 small organism can be absorbed by the organ- 

 ism, whereas a large organism cannot escape so 

 well from its own radioactivity. 



Near the sea surface a large fish receives 



about half its total natural exposure from the 

 rays originating in the radio-potassium in its 

 own tissues. On the other hand near the sea 

 surface cosmic rays appear to outweigh all other 

 radiations received by a microorganism. 



At depths of the order of 100 meters the 

 attenuated cosmic rays no longer contribute sig- 

 nificantly to marine organisms either large or 

 small. However, the beta and gamma rays from 

 potassium in sea water can give small organisms 

 doses amounting to about ten per cent of the 

 total dose they receive at the sea surface; the 

 small marine organism cannot escape this expos- 

 ure to radioactivity in the surrounding water. 



It is the deep fresh water which makes pos- 

 sible the most extreme variation in natural ex- 

 posure. In the deeper waters living things can 

 hide from external bombardment; fresh water 

 generally contains such small amounts of radio- 

 activity that this source can be neglected even 

 in comparison with the feeble effect of cosmic 

 rays remaining at depths of several hundred 

 meters or more. 



In pure fresh water the total dose from 

 strongly ionizing rays depends largely upon the 

 size of the organism and upon its living habits. 

 If the organism is small in the sense already 

 discussed, if it lives in deeper waters, if it stays 

 away from the bottom sediments, if it avoids 

 the neighborhood of large masses of living tis- 

 sue or of detritus, and if it avoids as far as pos- 

 sible accumulating excessive amounts of those 

 elements which can be radio-active — then it 

 can remain remarkably free from the ionizing 

 bombardment received by all other living things. 



It would be interesting to find out how the 

 phytoplankton that seek the deeper portion of 

 the euphotic zone of clear lakes respond to their 

 extremely low external dose. If morphological 

 or other differences are discovered between sur- 

 face specimens and deep-water specimens, then 

 one of the origins of these differences might 

 possibly be the extremely different amounts of 

 strongly ionizing rays in the two biospheres. 



Geneticists should not overlook another as- 

 pect of the minute cell in feeble radiation; an 

 individual cell has an extremely small proba- 

 bility of being struck at all during one genera- 

 tion. In a deep lake the radiation intensity can 

 be so low that only one phytoplankter in about 

 five hundred would experience an ionizing ray 

 before it divided ; at least this is the probability 

 of a cosmic ray hitting an area 0.1 mm square 



