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A substance has a half-life of 3min. After 6min, the count rate was observed to be 400. What was its count rate at zero time?
A. 1600
B. 1200
C. 200
D. 2400
QUICK ANSWER…
A
DETAILS…
3min = half-life,
6min = half of half-life = 400
If the half of half-life has a count rate of 400, it then follows that 3min earlier, the count rate was twice.
So, 3min = 400 × 2 = 800
0 min = 800 × 2 = 1600
Now for the right answer to the above question:
- Option A is correct. 1600
- Option B is incorrect.
- C is incorrect.
- D is not the correct answer.
KEY-POINTS…
You may please note these/this:
- Simple understanding of what half-life is all about will help you trace the initial count rate.
- Half-life simply means the time taken for half of a radioactive substance to decay.
- Half-life for the substance above is 3min, after 6min, 400 counts remained, this implies that at 3min, 800 would have remained, and at the beginning, 1600 would have remained.
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/ culled from 2020 JAMB-UTME physics past question 40 /
