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 3.0 per minute. The half life of the source is

• A. 120 min
• B. 80 min
• C. 30 min
• D. 20 min

Answer 1

The correct option is D

20 min

$\frac{A}{A_o}=\frac{1}{2^n}\Rightarrow \frac{30}{240}=\frac{1}{2^n}\Rightarrow 2^n=8\Rightarrow n=3Asn=3i.e,\frac{t}{T}=3\Rightarrow \frac{1}{T}=3\Rightarrow T=\frac{1}{3}hr=20minutes$