R.\i)i()-.\c;i i\ i: 1 issioN-i'KoDUcrrs i\ iiii: hlmax loou chain 



In 1954, \vc ciUcicd a new phase, the age of megaton weapons with their 

 vastly greater j^ower. These throw their debris to much greatei" heights and 

 much of the material is injected into the stratosphere from which the rate of 

 deposition is \ery nnu h slower than hoin the troposjihere. The half-time 

 of deposition may he as long as ten years, though the prevailing opinion 

 seems to be that this estimate is unduly long. In some ways this injection 

 into the stratosphere can be considered as a safety-factor in that the shorter 

 lived fission-products decay in the stratosphere and do not reach us in their 

 radio-active state. On the other hand, because the yield of the weapon is 

 so very much larger, very much greater quantities of long-lived radio-active 

 materials are injected into the stratos]iheric 'bank', whence they are paid 

 out slowly into the iropospherc. 



Two of the longest-lived fission-products are ^"strontium and ^^''caesium. 

 Unhappily, of course, the oxides of both arc readily soluble in water and 

 biological fluids, so that when they contaminate food-stuffs they are readily 

 taken into the body. The oxides of rather less long-lived fission-products, 

 like cerium and some of the rare earths, are not soluble even in digestive 

 fluids; therefore only minute traces of these, relative to strontium and 

 caesium, arc absorbed. 



In the United Kingdom, Stewart finds a very definite correlation between 

 the deposition of fission-products and rainfall-. Data are available for a 

 number of years from a station at Milford Haven on the Welsh coast. More 

 recently another half a dozen stations have been set up. Table 1 shows 

 Stewart's results from rainfall normalized for each station against a value of 

 100 at Milford Haven, where the actual precipitation is about 37 inches per 

 year, considerably greater than the average for the whole country. There 

 is an eight-fold range between Felixstowe on the East Coast and the high 

 ground of Mount Snowdon in Wales. The total deposition of ■"'Sr is also 

 normalized to 100 at Milford Haven. 



Table 1. Normalized ""Sr deposition and rainfall— 1957 to 1958 



Stewart's other notable finding is that the concentration of ""Sr in rain- 

 water varies anything up to six-fold between autumn and spring. His 

 explanation is that there is a general atmospheric circulation. The airmass 

 in the stratosphere tends to move toward the Pole in the autumn; it concen- 

 trates as a heavy blanket over the Pole during the long winter night and 

 enters the troposphere with the beginning of spring; it moves towards the 

 Equator, and en route the contained radio-activity is washed out in rain, 

 particularly in latitudes 50 to 60°N. By the time this air reaches the Equator 

 it has lost its radio-activity; in the Tropics, even though the rainfall is high, 

 the contamination on the ground is low. The same phenomenon is seen, 

 but to a less marked extent, in the Southern Hemisphere. 



210 



