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# The half life of Iodine... watch

1. Struggling on an AS question, any help would be much appreciated aswell as an explanation on how to do it! thank you!

The half life of Iodine 131 is 8 days. A sample manufactured for use in hospitals has an original count rate of 16 000 counts per minute. It can be used as a tracer as long as its count rate is at or above 500 counts per minute.

For how long after manufacture Iodine 131 be used as a tracer.
2. Do you understand half-life?
They've made the numbers easy - work out what will be the count rate after 1 half life then repeat for 2,3...
3. dont understand it no ! could you explain?
4. Half life is the rate in which the concentration of the substance reduces by half. i.e. if the half life is 1 hour for substance A and you have 8 molecules of it, in 1 hour you will have 4 molecules and in two 2 molecules and so on.

Does the question make sence now?
5. Hmm If you'd said 8 billion molecules (or in this example nulei) it'd have been even better imo. Each decay is an independent random event so it's quite likely that for instance 4 could decay to 0 (or stay at 4) in one half life, in the same way as it's much more likely that you could coin toss 4 heads in a row than say 4billion.

It's something you could get marked down for even though you haven't done the maths yet.

But anyway in gross quantities of a radioactive isotope (like the question) it's the time taken for half the nuclei to decay.
6. I haven't done this yet but it seems right. Did you not do this in GCSE Physics?
0 days: 16,000
8 days: 8,000
and so on until:
40 days: 500
7. I might be barking up the wrong tree here as it's years since I did this...

Every eight days the count should halve, therefore:

After 0 days, the count is 16000.
After 8 days, the count is 8000
After 16 days, the count is 4000
After 24 days, the count is 2000
After 32 days, the count is 1000
After 40 days the count is 500.

We know this is the lowest possible count at which iodine can be used as a tracer, so the answer is 40 days.

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Updated: October 1, 2010
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