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A count rate meter shows a count of 240 ...

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

120 min

B

80 min

C

30 min

D

20 min

Text Solution

Verified by Experts

The correct Answer is:
D
`R= 30, R_(0)= 240`
`((R)/(R_(0)))= ((1)/(2))^(t//T_(1//2)) rArr (30)/(240) = ((1)/(2))^((1)/(T_(1//2)))`
`rArr 3= (1)/(T_(1//2)) rArr T_(1//2)= (1)/(3) hr rArr T_(1//2)= 20` min
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