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AQA physics past paper may 2013 question 5(c)(ii)- can someone explain please?

the question states:
A sample of rainwater contaminated with iodine-131 gives a count rate of 1200 counts per second

Calculate how many days it will take for the count rate from the sample of rainwater to fall to 75 counts per second

Half-life of iodine-131 = 8 days

Show clearly how you work out your answer

Reply 1

varun1212

0 days -1200 counts
8 days -600 counts
16 days -300 counts
24 days -150 counts
32 days -75 counts

each time you basically add on a half life, and therefore when you add it on the counts halves so you end up with 32, but it starts from 0, because that's when it's 1200. Hope it helps

Reply 2

starry_n3bula

Original post by Cara200

Hey,

In physics we measure the rate at which a radioactive substance (called an "isotope") decays by working out how long it takes for half of it to decay -this is known as a 'half-life'.

To give you an (crude) example, say I give you 10 grams of a radioactive isotope called 'X' .

X has a half-life of 1hour.

At 0 hours you have 10 grams of X
After 1 hour you have 5 grams (10/2) of X (as half of it has decayed)
After 2 hours you only have 2.5 grams (5/2) of X (as half of it has decayed..again)
After 3 hours you only have 1.25 grams (2.5/2) of X (as half of it has decayed..yet again)
and so on.

The question is telling you that iodine-131 has a half life of 8 days. When iodine-131 decays it gives off radiation. The amount of radiation it gives off it dependant on how much of it there is i.e. the more iodine-131 there is the more radiation it gives off. So, as iodine-131 decays there’s less of it in the rainwater and less radiation.

The rate at which iodine-131 gives off radiation is measured in counts per second.

Initially there’s enough iodine-131 in the rainwater to give off radiation at a rate of 1200 counts per second
After 8 days half of the iodine-131 will have decayed, so there will only be enough of it left in the rainwater to give off radiation at a rate of 600 (1200/2) counts per second
After 16 days there will only be enough iodine-131 left in the rainwater to give off 300 (600/2) counts per second
After 24 days there will only be enough iodine-131 left in the rainwater to give off 150 (300/2) counts per second
After 32 days there will only be enough iodine-131 left in the rainwater to give off 75 (150/2) counts per second

So it will take 32 days for the count rate from the sample of rainwater to fall to 75 counts per second.

Hope this helped :-)

(edited 8 years ago)

Reply 3

Cara200

Thankyou that has really helped!