Thursday, October 15, 2009


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Naturally occurring radioactive substances have high nucleon number. It is possible to make artificial radioactive substances by bombarding lighter nuclides with - particles, protons or neutrons. The radioactive substances produced in this manner are known as radioisotopes.

A nuclide is any species of atom of which each atom has an identical proton number and also an identical nucleon number. Different nuclides, which have the same proton number (but different nucleon numbers) are called isotopes (isotopic nuclides). The first radioisotope was an unstable isotope of phosphorus. It was produced in 1934 by bombarding aluminium with - particles, i.e.,

Phosphorus - 30 was produced, together with a neutron. Notice that on each side of the equation the sum of the nucleon number is 31 and the sum of the proton number is 15. Phosphorus - 30 decays by ejecting a positron and has a half-life of about 3 minutes.

The positron has not been mentioned before because it does not occurring natural radioactivity. It has a mass equal to that of the electron, and a positive electron. It is denoted by

When magnesium is bombarded by neutrons a radioisotope of sodium is formed. The reaction is

The sodium decays with the emission of a - particle.


The important point is that it is now possible to produce any radioisotope. Most of those produced have short half-life periods. This is very important because the activity and hence the danger from radioactive emissions does not last very long.

Uses of Radioactive Isotopes

All isotopes of a substance have the same chemical properties and behave in an identical manner. The advantage of a radioisotope is that its position can be detected very easily by the radiation which it emits. It has wide applications in various fields like:

In Medicine

Radio isotopes are used in detection of diseases and also in radio therapy:

  • The rays from radium is used in the treatment of skin diseases.
  • Radiation from Co60 ( - rays) is used to diagnose and treat thyroid disorders.
  • Radio iodine (I131) is used to diagnose and treat thyroid disorders.
  • Radio phosphorus (P32) is used in the treatment of leukaemia and tumours.
  • Radio sodium (Na24) in the form NaCl is used to study circulation of blood.

In Agriculture

  • Radioactive phosphorus (P32) is used in the study of metabolism of plants.
  • Radioactive sulphur (S35) helps to study advantages and disadvantages of


  • Pests and insects on crops can be killed by - radiations.,
  • - rays are used for preservation of milk, potatoes etc.
  • Yield of crops like carrot, root, apples, grapes can be increased by irradiation with radioisotopes.

In Industry

  • In is used in the manufacture of paper, plastic and metal sheets. (Used to control the thickness of the sheets.)
  • Radioisotopes can be used to estimate the amount of wear in bearings.
  • Leaks in pipes may be traced by introducing a small quantity of radioisotopes into the fluid in the pipe.
  • It is also used to detect the cracks in the welding, casting etc.

Natural Radioactive Isotopes - Radiocarbon and Carbon 14 Dating

There are a small number of radioisotopes of low proton number, which occur naturally. They are produced by bombardment by radiation from outer space (cosmic rays). The most well known of these is radioactive carbon-14, which is produced when nitrogen is bombarded by neutrons.

Carbon-14 decays with the emission of a

b - particle, and reverts to nitrogen

In nature, radiocarbon is formed when high energy atomic particles called cosmic rays break down the atoms in the atmosphere into electrons, protons, neutrons and other particles. Some of the neutrons strike the nuclei of nitrogen atoms in the atmosphere get converted into radiocarbon atoms.

Carbon-14 has a long half-life of about 5600 years. It is reasonable to assume that equilibrium has been reached between the rate at which carbon - 14 forms in the atmosphere and the rate at which it decays, and that the amount of it in the atmosphere is constant. When plants photosynthesise they take in CO2 from the air. The carbon 14 atoms in these molecules slowly decay to Nitrogen. Human beings and other animals take in radiocarbon chiefly from the food provided by plants. Thus, all living things contain radiocarbon. Plants are utilised as cotton or linen; or might form coal. But whatever happens the C - 14 in it gradually decays. When the plant or animal dies fresh carbon is no longer taken in and the C - 14, which is present, decays. Thus, the length of time a specimen has been dead may be determined by the activity of the C - 14, which remains in it. Carbon - dating has therefore become an important tool for archaeologists and anthropologists.

Radiocarbon or C -14, is a radioactive isotope of carbon. It is used to determine the age of fossils and other ancient organic matters.

So by finding out how much carbon 14 is there in an object, we can approximate the age of the sample. The less the C - 14 compared with carbon 12, the older the sample is. C - 14 dating is widely used in Archaeology to determine the age of archaeological samples like tools, ornaments, paintings, furniture etc.

Carbon Dating

The age of fossils of plant or animal origin can be determined by carbon dating technique developed by Willard Libby in 1949. The radioactive isotope of carbon (6C14) is used to determine the date at which an animal or plant had died.

Carbon Dating

Carbon Dating

The method of measuring the age of archaeological materials that contain matter of living origin using the radioactive isotope 6C14 is called carbon dating. It is continuously formed in the upper strata of the atmosphere by the action of neutrons in the cosmic rays on 7N14.

By photosynthesis, plants take up CO2 from the atmosphere which contains small amount of radio isotope 6C14. It is used by plants to build carbohydrates which are then consumed by living animals.

Because of the natural plant - animal carbon cycle, an equilibrium will be set up and all living matter will contain a constant equilibrium concentration of C - 14, if the intensity of cosmic rays reaching the earth remains constant over a long period of time.

Once the plant or animal dies, the process of incorporation of 6C14 stops and 6C14 already present begins to decay.

(t1/2 = 5770 years)

Thus, by knowing the equilibrium concentration of 6C14 in a living plant and the concentration of 6C14 in a dead piece of organic matter at a particular time, the age of the material can be determined.

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