A source contains two phosphorous radio nuclides 3215P (T1/2 = 14.3d) and 3315P (T1/2 = 25.3d). Initially, 10% of the decays come from 3315P. How long one must wait until 90% do so?


Half-life of


Half-life of



Thus the source have initially 10% of .


Suppose after some time t, this situation may be reversed.


Let,


Initial number of nucleus of


Initial number of nucleus of


For



……………(1)


For



……………(2)


On dividing equation (1) by equation (2) we get,







Thus,


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