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.This could be stated as a fraction, decimal or ratio.įor example the amount of a sample remaining after four half-lives could be expressed as: 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. Calculating the isotope remaining - Higher 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. The half-life of radioactive carbon-14 is 5,730 years. This process continues and although the count rate might get very small, it does not drop to zero completely. A third, two-day period from four days to six days sees the count rate halving again from 20 down to 10. Note that this second two days does not see the count drop to zero, only that it halves again. It takes another two days for the count rate to halve again, this time from 40 to 20. The illustration below shows how a radioactive sample is decaying over time.įrom the start of timing it takes two days for the count to halve from 80 down to 40. Count-rate is the number of decays recorded each second by a detector, such as the Geiger-Muller tube. Half-life is the time it takes for half of the unstable nuclei in a sample to decay or for the activity of the sample to halve or for the count rate to halve.

Half life in 60 seconds