E. C. WINKLER 



Admittedly this is not a long period of time for comparison to be made, 

 but there is no reason to believe that it is not a typical time interval. 



The fall-out and airborne activities collected are to a certain extent 

 complementary, but from this study it seems that, in general, if there is an 



100 





C 



c 

 ° 50 



u 



1000 



500 



June 



Counts/minute/litre 



Counts /minute/weekly 



rainfall 



Dec 



June 



1957 1958 



Figure 3. Activity of rainwater 



increase in one there is a corresponding increase in the other. These in- 

 creases may not be quantitatively the same but are of the same order of 

 magnitude. 



Properties of Active Samples 



(i) Further investigations were carried out on most of the samples 

 exhibiting unusually high activity. 



Probably the best way of ascertaining that these activities are due to 

 fission-products is to plot their decay curve. Several recounts were made on 

 samples over a period of about twelve months after collection and the curves 

 of their decay were compared with the curve of the —1-2 power of time 

 which is commonly held to be the rate of decay of fission-products. 



A comparison of measured and theoretical decay is shown in Figure 4, 

 plotted for two of our typical samples. 



The close agreement shows that there is very good reason to believe the 

 activities collected were due to fission-products. Since the slope of this 

 curve is continually changing, the approximate time elapsed between the 

 explosion and collection can be obtained from the comparison, 



131 



