15 

 STUDIES IN DETECTING RADIO-ACTIVE FALL-OUT 



E. C. Winkler 

 Physics Department, Peter MacCallum Clinic, Melbourne 



World-wide measurement of radio-active fall-out has been in progress for 

 some time now, and a steady increase in the number of stations can be 

 observed. The commonest method in use is the sticky-paper method and 

 our interest, since the Melbourne area is included in the network operated 

 by the Australian Atomic Weapons Test Safety Committee, has been in 

 other methods of detecting fall-out. 



Large cjuantities of radio-active fission-products are released on the 

 explosion of a nuclear weapon. These products can return to earth in three 

 ways. Firstly, they can become attached to particles of matter thrown up 

 in the explosion and fall out in the immediate vicinity. Secondly, they can 

 be thrown up into the stratosphere and fall out within a few hundred miles 

 of the test site. Thirdly, mainly with a thermo-nuclear weapon, they can 

 be forced into the stratosphere where they circle the earth and fall out only 

 gradually over a period of years. Since Melbourne is 1000 miles from the 

 nearest testing site, and several thousand from most, we are primarily 

 interested in this third section of the fission-products. 



It was our aim to study methods of detection of the presence of fission- 

 products in the atmosphere in Melbourne. This involved the investigation 

 of any samples of unusual activity, to ascertain that they were actually 

 bomb debris and also, if possible, to trace their origin. 



materials 



The apparatus used is shown in Figure 1. 

 Collection was made in four ways. 



(7) For the collection of airborne material 



Two screens of cheesecloth, gauge 30 x 50 threads per in. size 6 in. x 

 12 in., were mounted on wire supports attached to a horizontal rod which 

 was turned by the wind against a Perspex vane so that the cheesecloth was 

 always facing into the wind. 



The cheesecloth was directly protected from rain by a 4 ft. square roof 

 having very slender supports to give minimum obstruction. For 64 collec- 

 tions one piece of cheesecloth was earthed and the other was insulated from 

 the ground. 



{2) For the collection of 'fall-out ' material 



The term 'fall-out' is generally used to include all the fission-products 

 produced in explosions which reach the earth, but in this paper, for want 

 of a better term, it is used in a restricted sense, to refer to radio-active 



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