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Question

The half-life of cobalt-60 is 5.25 yrs. After how long does its activity reduce to about one eight of its original value?


A

5.25 years

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B

10.50 years

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C

15.75 years

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D

21 years

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Solution

The correct option is C

15.75 years


The activity is proportional to the number of undecayed atoms: In each half-life, the remaining sample decays to half of its initial value. Since (12)×(12)×(12)=18, therefore, three half-lives or 15.75 years are required for the sample to decay to 18th its original strength.


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