Half life
The process in which unstable atomic nuclei break apart or change, releasing radiation as they do so. is a random process. A block of When unstable atoms give off particles that can be harmful to humans. material will contain many trillions of The nucleus controls what happens inside the cell. Chromosomes are structures found in the nucleus of most cells. The plural of nucleus is nuclei. and not all nuclei are likely to decay at the same time so it is impossible to tell when a particular nucleus will decay.
It is not possible to say which particular nucleus will decay next, but given that there are so many of them, it is possible to say that a certain number will decay in a certain time. Scientists cannot tell when a particular nucleus will decay, but they can use statistical methods to tell when half the unstable nuclei in a sample will have decayed. This is called the The time it takes for the number of nuclei of a radioactive isotope in a sample to halve. Also defined as the time it takes for the count rate from a sample containing a radioactive isotope to fall to half its starting level..
The illustration below shows how a radioactive sample is decaying over time.
From the start of timing it takes two days for the count to halve from 80 down to 40. It takes another two days for the count rate to halve again, this time from 40 to 20.
Note that this second two days does not see the count drop to zero, only that it halves again. A third, two-day period from four days to six days sees the count rate halving again from 20 down to 10.
This process continues and although the count rate might get very small, it does not drop to zero completely.
The half-life of radioactive carbon-14 is 5,730 years. If a sample of a tree (for example) contains 64 grams (g) of radioactive carbon after 5,730 years it will contain 32 g, after another 5,730 years that will have halved again to 16 g.
Calculating the isotope remaining - Higher
It should also be possible to state how much of a sample remains or what the activity or count should become after a given length of time. This could be stated as a fraction, decimal or ratio.
For example the amount of a sample remaining after four half-lives could be expressed as:
- a fraction - a ½ of a ½ of a ½ of a ½ remains, which is ½ × ½ × ½ × ½ = 1/16 of the original sample
- a decimal - 1/16 = 0.0625 of the original sample
- a ratio - given in the form 'activity after n half-lives : initial activity' , in this case 1:16
This could then be incorporated into other data. So if the half-life is two days, four half-lives is 8 days. So suppose a sample has a count rate of 3,200 Becquerel (Bq) at the start, what its count rate would be after 8 days would be 1/16th of 3,200 Bq = 200 Bq.
Example
The half-life of cobalt-60 is 5 years. If there are 100 g of cobalt-60 in a sample, how much will be left after 15 years?
15 years is three half-lives so the fraction remaining will be \((\frac{1}{2})^3 = \frac{1}{8} = 12.5g\)
As a ratio of what was present originally compared to what was left, this would be 100:12.5 or 1:0.125
Question
What is the half-life of a sample where the activity drops from 1,200 Bq down to 300 Bq in 10 days?
Half of 1,200 is 600, half of 600 is 300. So it takes two half-lives to drop from 1,200 Bq to 300 Bq, which is 10 days. So one half-life is five days.