Applications of Radioactivity and Radioisotopes

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Radioisotopes find numerous uses in different areas such as medicine, chemistry, biology, archaeology, agriculture, industry and engineering.

Tracer Techniques

Radioisotopes are frequently used as tracers or tagged atoms in various fields. In tracer technique, a radioactive isotope is added to the reactants and its movement is studied by measuring radioactivity in different parts.

In medicinal diagnosis

In order to find if blood is circulating to a wound or not, a radioactive isotope is injected into the blood stream. After a time period, blood from the wound is examines for its radioactivity. If no radioactive isotope is detected, it means that passage of blood is hindered. The rate of circulation can also be detected by this method.

Tracer technique is also used for the detection of thyroid disorder and brain tumours.

In Agriculture

The uptake of phosphorous by plants is studied by mixing radioactive phosphorous with phosphatic fertilisers.

In Chemistry

Tracer technique is used

a) To find the solubility of sparingly soluble salt like lead sulphate. A lead salt containing known amount of radioactive lead is dissolved in water. Sulphuric acid is added to the aqueous solution to precipitate lead as lead sulphate.

The precipitate is filtered. The radioactivity of filtrate is measured which gives the amount of lead still remaining in solution. This gives a measure of the solubility of PbSO₄ i.e., the amount going into solution.

b) Tracer technique is also used to study the path or mechanism of the reaction.

Consider the reaction

The question is how does the elimination of water take place - does the oxygen atom in water come from the alcohol or acid. This is studied by labelling or tagging the oxygen in the alcohol molecule. In other words, the alcohol is prepared with O¹⁸. Results show that the ester formed has the radioactive oxygen. This shows that the starred oxygen comes from the alcohol. Thus the -OH group of the acid and the H atom of the alcohol are eliminated in the form of water.

Many other mechanistic applications have been reported. ¹⁴C has also been used as a radioactive tracer.

Cancer therapy

g - rays emitted by the radioisotopes can be used in the treatment of cancer. These radiations tend to destroy cancerous cells and the way can arrest the spreading of the cancerous cells. ⁶⁰CO is used in the treatment of tumours and cancers.

Neutron Activation Analysis

This technique is of gaining importance in analytical chemistry. In this technique, the sample containing very small amount of the stable isotope of the element to be investigated is bombarded with neutrons. This element is activated into its radioactive isotope. The radioactivity of this radioisotope is measured. Quantitative measurement of radioactivity and the knowledge of other factors such as half-life of radioisotope, efficiency of radiation detector, rate of neutron bombardment etc, help in the calculation of the amount of the element in the sample. This technique is advantageous because:

(i) Trace amounts of the elements can be determined, even one part of the element per billion parts of the sample.

(ii) Sample can be tested without destroying it.

(iii) Sample can be in any state of matter including biological material.

In Industry

a) Metal casting can be tested for cracks by putting them in baths of radioactive salts. The castings are then inspected for radioactivity to find out any penetration of salts into cracks. Absence of salt penetration indicates absence of cracks.

b) Radioactive isotopes can be used to detect any leakage in underground pipes carrying oils, gas or water. To check the point of leakage a small quantity of compound of radioactive isotope is introduced at the starting place and the detector is moved along the pipe. At the point of crack or leakage, the detector will show high level of radiations.

Age of Minerals and Rocks

The determination of age of minerals and rocks involves determination of either a species formed during a radioactive decay or of the residual activity of an isotope which is undergoing decay.

The former is illustrated by helium dating. Helium present in uranium mineral is almost certainly formed from a - particles. 1 gm of uranium on decay produces approximately 10^-7 g of helium per year. So if the helium and uranium contents of a mineral are known, the age of the mineral can be estimated.

In order to calculate the age of a rock containing

The equation used for the calculation is N= N_o e^-kt.

N_o = Amount of U-238 isotope originally present in a small quantity of the rock or mineral at the time the rock was formed.

N = Amount of U-238 still left undecayed after the lapse of time t which represents the age of the rock.

k = the decay constant of U-238.

We can determine the age of the rock by measuring the ratio of

and using the above equation.

Suppose the ratio of uranium-238 and lead-206 is unity. It implies that half of the uranium originally present has been converted into lead isotope. The age of the rock must therefore be equal to the half-life of uranium-238 i.e., 4.5 x 10⁹ years. Pb/U ratio of most rocks is

1.33 x 10^-2 indicating that their age is of the order of 10⁸ years.

The age of the earth - The abundance ratio of two isotopes of uranium i.e., ²³⁸U: ²³⁸U at present is 1:140. The half-life period of ²³⁸U is 7 x 10⁸ years. Assuming that in the beginning the proportion of two isotopes were equal, the above data determines the age of the earth as 5 x 10⁹ years.