820 



EOCZY 



[CHAP. 30 



In this equation, A^o is eliminated and we are able to use this method for 

 absolute age determination without making any theoretical assumption. The 

 uranium isotopes, 235 and 23S, produce protactinium-23] and thorium-230. 

 The ratio of the latter permits the determination of age when the two elements 

 have been separated from their mother elements. 



ivir= 



0.5 1.0 



1.5 



2.0 2.5 



3.0 3.5 4.0 



Fig. 4. Age determination by the ratio of two radio -nuclides that are produced by two 

 radio -isotopes of the same element. The ordinate gives the ratio of activities of the 

 two radio -nuclides and the abscissa gives the time scale in non-dimensional units as 

 multiples of mean lifetime. 



C. Theoretical Method of Age Determination, Type 3 



In this group, we combine all age determinations that employ an assumption 

 in order to estimate the original Nq. The most common assumption is that of a 

 constant ratio of the radioactive isotope to a stable isotope of the same element. 

 This includes the assumption of a constant rate of production of the radio- 

 isotope. The equation is then given by 



N 

 J 



No 



>-u 



(11) 



The radio-isotope is assumed to be replenished at a constant rate, and a constant 

 ratio of the radio -nuclide to the stable isotope is obtained when the nuclide 

 is added to a large reservoir in which the stable isotope is present. The degree 

 of geochemical separation of the isotopes must be known or we must be able 

 to determine it. When a part of this element is separated from the reservoir, it 

 decays and the change of the ratio gives the age : 



1 



A 



; No , N 

 lYi -f — 1^ T 



1 , No 



A N 



(12) 



This method is used with all isotopes produced by cosmic radiation, such as 

 14C, lOBe, 'Be and ^H. It has been shown (Lai and Peters, 1962) that the pro- 

 duction of these isotopes is constant over some million years. 



