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# 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:20 min120 min80 min30 min

A
20 min
B
30 min
C
120 min
D
80 min
Solution
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#### Since in radioactive decay, the decay rate is proportional to the amount of substance. Hence, 2402N=30, where N is number of half lives passed in one hour. This gives, N=3. Hence half life: t1/2=603min=20 min

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