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Question

A count rate meter shows a count of 240 per minute from a given radioactive source. One hour later the meter shows a count rate of 30 per minute. The half-life of the source is?

A
80min
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B
120min
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C
20min
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D
30min
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Solution

The correct option is C 20min
The number of counts left after time t
N=N0(12)t/T1/2
30=240(12)60/T1/2
[t=1h=60min]
or 30240=(12)60/T1/2
or (12)3=(12)60/T1/2
Comparing the powers, we get
60T1/2=3
T1/2=603
or T1/2=20min.

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