STUDIES IN DETECTING RADIO-ACTIVE FALL-OUT 



From our figures, the sticky paper appears to be more efficient than the 

 method of collection of dust deposited on the roof, but it is less sensitive 

 because of the smaller exposed area. 



Efficiency 

 The counting system has been calibrated by using a ^*C1 source supplied by 

 the Commonwealth X-Ray and Radium Laboratory and calibrated by the 

 Laboratory against a 204^1 standard calibrated by the A.W.R.E. 



The efiicicncy of collection using cheesecloth probably varies with the 

 wind velocity. Blillord, Lockhart and Baus-'', using stainless-steel screens, 

 found that the activity collected did not vary regularly with the mesh of the 

 screen. This could be due to the fact that increase in collecting area for 

 smaller mesh size was compensated by the reduced air flow. 



These workers determined the efiiciency of 40 mesh stainless-steel screens, 

 comparable w'ith cheesecloth, by backing a screen with efficient filter paper 

 and drawing air through this filter. Assuming the efficiency of the filter 

 paper to be 100 per cent, the efficiency of the screen was found to be 11 per 

 cent. 



From this value, and an average wind velocity, the actual activity corres- 

 ponding to the collections has been calculated: for example, for the week 

 preceding the 24th October, 1957 there was a sharp rise in activity in all 

 cases. 



These correspond to: 



an airborne activity of 1 [jL[i,C/m^ 



a ground activity of 16 mC/km^ 



a rainwater activity of 18 [jt,[xC/ml. 



In general, the results obtained by us are in good agreement with the 

 published results of the Safety Committee: hence a further assessment of 

 hazards is unnecessary. As the Safety Committee have stated, the dose levels 

 involved at the present time, are very small compared with naturally 

 occurring radio-activity. This is partly due to the fact that there is very 

 little mixing between the winds of the Northern and Southern Hemispheres, 

 which, incidentally, makes it relatively easy in Australia to relate unusual 

 activity to specific explosions, as most of the test sites are in the Northern 

 Hemisphere. This made the Christmas Island tests particularly interesting 

 as it has a latitude of 2°N. 



Evaluation of Methods 

 The standard methods of detection of fall-out used by the Safety Committee 

 are gummed paper and, to a lesser extent, rainwater measurements, air-filter 

 measurements, and measurements of the activity of sheep's thyroids. 



This committee has found that gummed paper and air-filter activities do 

 not show close correlation with each other. We have found that when an 

 increase in activity has been obtained by one method each of the other 

 methods has shown a corresponding increase. 



The cheesecloth screen for the detection of airborne particles has the 

 advantage that it is much less inclined to break down than an air pump, it 

 requires no power, which is of some importance at outback stations, and it 



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