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?
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.
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 /