E. C. WINKLER 



over the past few years, with a great increase over the past several months, 

 the activity measured can never be attributed solely to one explosion. 



(ii) The method of rapid dating of fission-products using the continuous 

 change in jS-ray energy distribution with ageing of the sample has been 

 investigated. Assuming the dates deduced from comparison with the /-^'^ 

 curve and subsequent correction from known dates of explosions are correct, 

 the ratio of the intensity through 0-025 mm aluminium absorber to the 

 intensity through 0-125 mm aluminium has been plotted against age. 



An endeavour was made to determine the age of the material using change 

 in ^-ray energy distribution. 



A 90Sr-^''Y source was measured for the same ratio to give an approximate 



< 



E 

 E 



o 



■ 02 



O 



< 



E 



E 

 in 



01 



o 



8 V 



Ok 



<\ 



>.. 



•^°Sr 



10 100 1000 



Age in days 



Figure 5. Determination of age using change in /3-ray energy 



distribution 



calibration figure, but as at no stage are these the only jS-emitting isotopes 

 present in the fission-products, the figure could only give the order of ratio 

 to be expected {Figure 5). 



The probable error due to randomness in counting is as shown for two 

 points on the graph. This becomes important because the index of age is the 

 ratio of two quantities of the same order. The only way for this error to be 

 reduced for samples of this activity would be to greatly increase the counting 

 time. The discrepancies between successive evaluations of the same sample 

 can be explained by this probable error but the result is a very wide scatter 

 of points. It could be postulated that there was a trend in ratio with age in 

 the expected direction but, from this experience, it would seem that this 

 method by itself would be of little use as a criterion of age but may possibly 

 be of some use as a guide to confirmation. 



{Hi) To obtain some idea of the energy of the radiation, the absorption in 

 aluminium was plotted for one sample. The thickness of aluminium needed 

 to decrease the intensity from 100 per cent to 50 per cent was 0- 1 mm and 

 from 50 per cent to 25 per cent was 0-7 mm. 



From the graphs of relative activities of the nucleide products of simul- 

 taneous slow neutron fission of ^ssy ^g given by Hunter and Ballou^, the 

 isotopes present at the time after fission corresponding to the age of the 



10 133 



