E. C. WINKLER 



provides a means of continuous monitoring of the atmosphere. For this 

 reason it would be particularly useful for inter-trial periods when it is not 

 vitally important to know exactly on what day any increase in activity 

 appears, but it is desirable to know whether any increase occurs. A weekly 

 collection would certainly be preferable to a daily one in these cases. Cheese- 

 cloth has the additional advantage over gummed paper that it is much less 

 likely to be affected by precipitation. 



Although the counting system has a fairly low efficiency, the sensitivity is 

 higher than that of some standard systems because of the low background 

 counting rate and the longer period of collection. It has been the custom to 

 carry out two assays of each sample but only the second of these is necessary. 



In conclusion it is felt that the methods of detection reviewed here, and in 

 particular that using the cheesecloth screen can be recommended as practical 

 and meaningful techniques. 



I would like to thank Dr. J . H. Martin of the Cancer Irutitute,for his guidance and 

 encouragement throughout this work. 



REFERENCES 



iTajima. E. and Doke, T. Science 123 (1956) 211 



^Hunter, H. F. and Ballou, N. E. Nucleonics 9 (1951) C-3 



3BLIFFORD, I. H., LocKHART, L. B. and Baus, R. a. Science 123 (1956) 1120 



DISCUSSION 



Professor Titterton: I should like to ask whether your peak in June 1957 can be 

 correlated with the Australian Atomic Weapons Test Safety Committee global 

 fall-out results which appeared in Melbourne on 17th June. Would your weekly 

 change give us enough overlap for that ? 



Mrs. Winkler: No, it would not, because our high activity screen was collected 

 on the 6th, so the activity would be from the week preceding the 6th. 



Mr. Jackson : How would you correct for the variation in wind velocity if you used 

 a cheesecloth collector? The collection would depend on wind velocity enormously. 



Mrs. Winkler: Yes, that is very true, but from figures obtained from the Bureau of 

 Meteorology, I found that the wind velocity over a period of a year — in units of 

 monthly miles that the wind travels — varies from about 5500 to about 6300. I was very 

 surprised to find that it actually was as constant as that over the whole year. .So it 

 looks as though averaged over the week, variation in wind velocity would make little 

 difTerence. 



137 



