Radioactive half-life

Radioactive half-life

Question:

If you inject a mouse with radioactive material of current activity of 256 Bq, what will be the activity after completion of 6 half-lives?

  1. 4 Bq
  2. 8 Bq
  3. 16 Bq
  4. 24 Bq

Rephrasing the Question:

How do you calculate radioactive half-life?

Answer:

4 Bq (option 1)

Explanation:

Radioactive half-life

A half-life is the time it takes for a radioactive material to degrade to half its original amount. So, if you start off with 100 Bq worth of material, after one half-life, you will have 50 Bq left. After 2 half-lives, you will have 25 Bq left.

The formal equation for this is:

N(t) = N0 (1/2)n

where N(t) is the quantity of substance remaining, N0 is the initial quantity of the substance, and n is the number of half-lives elapsed.

But the easiest – and quickest! – way to answer these questions is to just calculate the quantity step-by-step.

So for this question, we would do this:

At the beginning (after 0 half-lives) = 256 Bq

After 1 half-life = previous amount / 2 = 256 Bq / 2 = 128 Bq

After 2 half-lives = previous / 2 = 128 Bq / 2 = 64 Bq

After 3 half-lives = previous / 2 = 64 Bq / 2 = 32 Bq

After 4 half-lives = previous / 2 = 32 Bq / 2 = 16 Bq

After 5 half-lives = previous / 2 = 16 Bq / 2 = 8 Bq

After 6 half-lives = previous / 2 = 8 Bq /2 = 4 Bq

Therefore, after 6 half-lives there will be 4 Bq remaining.

Source:

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