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Question

Starting with a sample of pure 66Cu,78 of it decays into Zn in 15 minutes. The corresponding half-life is:

A
15 minutes
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B
10 minutes
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C
712 minutes
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D
5 minutes
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Solution

The correct option is D 5 minutes
The law of radioactive decay predicts how the number of the not decayed nuclei of a given radioactive substance decreases in the course of time.
Relation is N=N0(1/2)tτ
N is undecayed part
N0 is initial part
τ is half-life and t is time taken.

7/8 is decayed.so Remaining part is 1/8
N=1/8
N0=1
N=N0(12)tτ18=1×(12)tτ(12)3=(12)tτ3=tτ3=15ττ=5
Therefore, Half-life is 5 min.

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