RADIOACTIVE ELEMENTS AXD DEEP-SEA SEDIMENTS 



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integration of the atoms of R. The number of atoms, N, remaining 

 after the passage of time / from to is given by the well-known 

 formula: 



N = No • e-^' 



Figure 1 shows this function. N/Nq is generally expressed, not 

 as a function of /, but as a function of T = 0.693/X, where T is the 

 half life and represents the time in which the number of atoms R 

 remaining has diminished by 50 per cent from the original number 



No. 



N/No 



Fig. 1. Law of radioactive decay. The unit of time is taken as the half life. 



Consider now a natural system (for example a mineral, a rock, 

 or a living being), which is completely formed at time to and which 

 stops all chemical exchange with the surrounding medium. Let us 

 suppose that at the instant to the imagined system includes a 

 number No of radioactive atoms of the type R. At any moment 

 later than to, the age of this system is the amount of time that has 

 passed from the moment that all chemical exchange or, more 

 precisely, all exchange of atoms of type R, or its ascendants or its 

 descendants, with the external medium had ceased. 



