The World of Nuclear Science

Archaology and Geology

The principals of radioactive decay are employed widely in many fields of archaology and geology to determine the nature and age of materials, artifacts and rocks.

The principals of radioactive decay are applied in the technique of radioactive dating, a process widely used by geologists and archaeologists to determine the age of materials and artifacts.

Radioactive carbon-14 atoms exist naturally. They are everywhere around us: in our clothes, in the food we eat, even in the air we breathe. However, there are not many of these - only 1.3 × 10^-12 percent of all carbon atoms are the carbon-14 isotope. This is why they do not pose danger to us - there are so few of them.

The ratio of radioactive carbon-14 atoms to stable carbon-12 atoms in the atmosphere has remained constant over thousands of years. Although carbon-14 naturally decays, it is also continually being formed. Carbon-14 atoms are formed when neutrons from the sun's cosmic radiation collide with nitrogen-14 atoms in the atmosphere:

Thus the decay of carbon-14 is reasonably balanced with its production, resulting in a constant ratio of carbon-14 to carbon-12.

Carbon dioxide (CO₂) molecules in the air can contain either isotope of carbon. This CO₂ is continually used by plants to grow. Because the ratio of carbon-14 to carbon-12 in atmospheric CO₂ is constant, the intake of CO₂ by a plant results in a constant ratio of the two isotopes in the plant's body while it is alive. However, when the plant dies it will no longer take in CO₂. As a result, the carbon-14 decaying in the dead plant will not be replenished by a "fresh supply" of more CO₂, resulting in the ratio of carbon-14 to carbon-12 decreasing over time.

Because animals eat plants, the ratio of carbon-14 to carbon-12 in them also decreases once they die, since the carbon-14 cannot be replenished.

This process of dating using carbon-14 is used by paleontologists. Paleontologists burn a small sample of a fossil to react the carbon in it with oxygen, to form CO₂. The CO₂ that contains carbon-14 will be radioactive, and the amount can be easily measured using a radiation counter. Burning is done to facilitate measuring the level of carbon-14.

Carbon-14 has a half life of about 5730 years. This means that in a given sample of a carbon-containing substance, (without the carbon-14 being replenished) the ratio of carbon-14 to carbon-12 will decrease by half every 5730 years. Suppose for example, some archaeologists uncovered ancient manuscripts and found that the ratio of carbon-14 to carbon-12 in the paper was half of that found in living trees. This would mean that the manuscripts would be about 5730 years old.

The use of radioactive carbon-14 for dating was first done by William Libby, an academic at the University of Chigaco, USA, in 1947.

The relatively short half-life of carbon-14 (5730 years) means that the amount of carbon-14 remaining in materials and objects older than about 80,000 years is too small to be measured with today's equipment. Thus carbon dating is limited to objects which are not older than this. However, the abundance of other atoms with longer half-lives, such as uranium-238 (half-life 4.5 × 10⁹ years) can be measured in place of carbon-14. Geologists measure the amounts of other radioactive metal isotopes such as uranium-238, rubidium-84 and potassium-40 (see below) found in rocks to determine their age. Measurements show that the oldest rocks on Earth are about 4.6 billion years old - which is a reasonably accurate estimate of the Earth's age. Similarly, analysis of fossilised plants shows that they first occurred on Earth about 3 billion years ago.

The major problem using radiocarbon dating is the chance of getting carbon from the samples mixed up with "fresh" carbon.

As well as using carbon-14 to carbon-12 decay, geologists also measure the decay of potassium-40 to argon in dating rocks. However, this method is not accurate for rocks that have been heated above 120°c (250°f) because the argon diffuses out from the rock at these temperatures. The decay of rubidium-87 to strontium-87 is used to check potassium-argon dates, and is much more accurate because neither isotope is diffused by heat. This rubidium technique was used by scientists to determine the age of the moon. Measurements using uranium-238 were used to determine the age of the Earth.

Radioactivity is used to identify the location of deposits of uranium and other radioactive minerals. This is useful in mining exploration. The intensity of detected radiation also is an indication of the amount of uranium that may be located there.

The mining industry employs radioactivity in its routines. One example is identification of rocks and minerals. X-rays from a radioactive material can induce other materials to emit fluorescent X-rays. These subsequent X-rays can have their energies measured, and then this gives an indication of the elements present in the original material. The intensity of these X-rays also is an indicator of the amount of the element present.

This technique is done by placing probes into the slurry - water that contains sediments of minerals, etc. The probes contain a radioisotope and a detector. The radiation from the isotope causes metals in the slurry to emit fluorescent X-rays - these are identified by the detector also located on the probe. The probe's input is then analysed to give an indication of the types and amount of metals present in the slurry. Metals that are detected this way include lead, copper, tin, zinc, nickel and iron.

Elements that can absorb neutrons will release gamma rays. These gamma rays can be analysed for their energies. Specific energies correspond to specific elements - thus this is another way of identifying the metals and minerals that may be present.