Question

An artificial element shows activity of 12.5 cps(counts per second) at its formation as recorded in a G.M. counter. After 10 min activity is 7.5 cps. In the absence of element, G.M.counter records activity 0f 2.5 cps. What is its half life period ?

• A. half-life $=10$ min
• B. half-life $=13$ min
• C. half-life $=20$ min
• D. None of these

Answer 1

The correct option is A half-life $=10$ min

Initial activity is 12.5 cps. From this 2.5 cps (the activity in absence of element) is subtracted to obtain corrected initial activity.
Hence, the corrected initial activity is $12.5-2.5=10cps$
The final activity is 7.5 cps.
From this 2.5 cps (the activity in absence of element) is subtracted to obtain corrected final activity.
Hence, the corrected final activity is $7.5-2.5=5cps$
In 10 min, the activity decreases from 10 cps to 5 cps. Hence, in 10 min, the activity is reduced to one half.
Hence, the half life period is 10 minutes.