# Radioactive dating of rocks gcse biology

### BBC - GCSE Bitesize: More radioactive dating - higher tier only

Learn about and revise nuclear radiation, radioactive decay and half-life with tell when a particular nucleus will decay, but they can use statistical methods to. How do geologists use very long half-live values to date rocks? All of these Doc Brown's Chemistry KS4 science GCSE Physics Revision Notes. Practice. Discover how scientists determine the age of fossils, rocks, and other geologic phenomena by using the known half-lives of isotopes within.

The half-life of radioactive carbon is 5, years. If a sample of a tree for example contains 64 grams g of radioactive carbon after 5, years it will contain 32 g, after another 5, 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: So if the half-life is two days, four half-lives is 8 days. Example The half-life of cobalt is 5 years.

If there are g of cobalt in a sample, how much will be left after 15 years? Reveal answer up down Half of 1, ishalf of is So it takes two half-lives to drop from 1, Bq to Bq, which is 10 days. So one half-life is five days.

The decay follows are particular pattern, illustrated by the graph below, known as a decay curve. The graph will drop steeply for very unstable nuclei but show a very small gradient if more stable.

## Dating rocks

Every graph shows the same mathematical feature which is that for a particular time interval the amount of The half-life of a radioisotope is the average time it takes for half of the remaining undecayed radioactive nuclei atoms to decay to a different nucleus atom.

It means in one half-life of time, on average, half of the undecayed unstable nuclei of a particular isotope disintegrate. A short half-life means the activity radioactivity will fall quickly e. See the decay curve graph below representing the behaviour of relatively unstable radioactive-isotope with a half-life of 5 days.

- Uses and hazards of radiation
- More radioactive dating - higher tier only
- Radioactive dating

This means from radioactivity measurements we can analyse the data and calculate from the graph the half-life of a radioactive-isotope or some calculation based on an initial level of activity and a later measurement of the decreased activity.

Whatever method, you need accurate activity data linked to time. This also means that we can make predictions of activity The radioactivity of any sample will decrease with time as the unstable atoms decay to more stable atoms, though sometimes by complex decay series routes e. The older a sample of a radioactive material, the less radioactive it is.

The decrease in radioactivity follows a characteristic pattern shown in the graph or decay curve. Four Uses of decay data and half-life values 1 Determination of the half-life of a Radioisotope The radioactivity from a radioisotope is measured over a period of time. Graphical or mathematical analysis is performed to calculate the time it takes for the radioactivity of the isotope to halve. For short-lived radioactive isotopes, the radioactivity is likely to be measured in terms of the count rate.

**How Carbon Dating Works**

Therefore the half-life will be the time it takes for the count rate to halve. An example of what this means is shown in the diagram below.

### BBC - GCSE Bitesize: Dating rocks

You would use a Geiger-Marsden counter, or similar scintillation counter to make measurements of the radioactivity of a radioisotope. The graph shows the rapid decay of a very unstable radioactive isotope in terms of count rate per minute cpm versus minutes. Although not shown, before plotting the graph, you should do a blank test for the background radiation and subtract this from ALL the readings. You would do a blank test by taking several readings without the presence of the radioisotope and use the average to correct the readings.

An alternative to this is to use heavy lead shielding to protect the Geiger counter from background radiation, but should still do a blank test with the identical experiment setup.

## Radioactive decay

From the graph you can work out the time half-life it takes for half of the radioactive atoms to decay from the decrease in count rate. Suppose a sample of a radioisotope gives an initial activity of counts per minute cpm. If the activity has fallen to cpm after days, calculate the half-life of the radio-isotope.

The simple method just involves involving halving from the initial value of activity until you reach the final value. In terms of activity: From the definition of half-life, this means 3 half-lives elapsed for the activity to drop from cpm to cpm.

### BBC Bitesize - GCSE Physics (Single Science) - Uses and hazards of radiation - Revision 1

Use say 50 normal dice numbered shaken in a container. Just repeat a few times, removing all the sixes each time and counting the dice left. After, say six goes, plot the number of dice left versus the time interval 0, 1, 2, 3 etc.

You can then estimate the 'half-life' in time intervals. The time interval could represents, seconds, minutes, hours The half-life of a radioisotope is 10 hours. Therefore you just have to halve the amount three times!