Q.

An isotope of potassium K40 undergoes two parallel types of decay, one by electron capture and other by β decay with half lives as 1.3 x 10 years and 1.39×109. If in a sample of a mineral, mass ratio of K : Ar : Ca is 5,1.5 and 16 then calculate age of the mineral if it is known that all Ca40 in the mineral is not from the radioactive decay of potassium.

[Express your answer in terms of 107 years for e.g. if you answer is 2 x 109 years fill 0200 in OMR sheet]

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answer is 26.

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Detailed Solution

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k1=ln21.39×109;k2=ln21.3×109

mCBBmAr=k1k2=9

mK5;mCa13.5;mAr1.5k=k1+k2=ln21.3×109[9+1]=ln21.3×108

ln21.3×108×t=ln5+13.5+1.55t=2.6×108 years =0026

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