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:

20min

120min

80min

30min

A

20min

B

30min

C

120min

D

80min

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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=20min

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