Application Of Radioactivity
Category : JEE Main & Advanced
Radioisotopes find numerous applications in a variety of areas such as medicine, agriculture, biology, chemistry, archeology, engineering and industry.
(1) Age determination : The age of earth has been determined by uranium dating technique as follows. Samples of uranium ores are found to contain \[P{{b}^{206}}\] as a result of long series of a- and b-decays. Now if it is assumed that the ore sample contained no lead at the moment of its formation, and if none of the lead formed from \[{{U}^{238}}\] decay has been lost then the measurement of the \[P{{b}^{206}}/{{U}^{238}}\] ratio will give the value of time t of the mineral.
\[\frac{\text{No}\text{. of atoms of }P{{b}^{206}}}{\text{No}\text{. of atoms of }{{U}^{238}}\text{ left}}={{e}^{-\lambda t-1}}\]
where l is the decay constant of uranium-238
Alternatively,
\[t=\frac{2.303}{\lambda }\log \frac{\text{Initial amount of }{{U}^{238}}}{\text{Amount of }{{U}^{238}}\text{ in the mineral present till date}}\]
Similarly, the less abundant isotope of uranium, \[{{U}^{235}}\] eventually decays to \[P{{b}^{207}};\,T{{h}^{232}}\] decays to \[P{{b}^{208}}\] and thus the ratios of \[P{{b}^{207}}/{{U}^{235}}\] and \[P{{b}^{208}}/T{{h}^{232}}\] can be used to determine the age of rocks and minerals.
\[_{6}{{C}^{14}}\] (half-life 5760 years) was used by Willard Libby (Nobel lauret) in determining the age of carbon-bearing materials (e.g. wood, animal fossils, etc.) Carbon-14 is produced by the bombardment of nitrogen atoms present in the upper atmosphere with neutrons (from cosmic rays).
\[_{7}{{N}^{14}}+{{\,}_{0}}{{n}^{1}}\to {{\,}_{6}}{{C}^{14}}+{{\,}_{1}}{{H}^{1}}\]
Thus carbon-14 is oxidised to \[C{{O}_{2}}\] and eventually ingested by plants and animals. The death of plants or animals puts an end to the intake of \[{{C}^{14}}\] from the atmosphere. After this the amount of \[{{C}^{14}}\] in the dead tissues starts decreasing due to its disintegration.
\[_{6}{{C}^{14}}\to {{\,}_{7}}{{N}^{14}}+{{\,}_{-1}}{{e}^{0}}\]
It has been observed that on an average, one gram of radioactive carbon emits about 12 b-particles per minute. Thus by knowing either the amount of C-14 or the number of b-particles emitted per minute per gram of carbon at the initial and final (present) stages, the age of carbon material can be determined by using the following formulae.
\[\lambda =\frac{2.303}{t}\log \frac{{{N}_{0}}}{{{N}_{t}}}\] or \[t=\frac{2.303}{\lambda }\log \frac{{{N}_{0}}}{{{N}_{t}}}\]
where t = Age of the fossil, l = Decay constant, \[{{N}_{0}}\]= Initial radioactivity (in the fresh wood), \[{{N}_{t}}\]= Radioactivity in the fossil
The above formula can be modified as,
\[t=\frac{2.303}{\lambda }\log \frac{\text{Initial ratio of }{{C}^{14}}/{{C}^{12}}\text{ (in fresh wood)}}{{{C}^{14}}/{{C}^{12}}\text{ ratio in the old wood}}\]
Similarly, tritium \[_{1}{{H}^{3}}\] has been used for dating purposes.
(2) Radioactive tracers (use of radio–isotopes) : A radioactive isotope can be easily identified by its radioactivity. The radioactivity can, therefore act as a tag or label that allows studying the behaviour of the element or compounding which contains this isotope. An isotope added for this purpose is known as isotopic tracer. The radioactive tracer is also known as an isotopic tracer. The radioactive tracer is also known as an indicator because it indicates the reaction. Radioisotopes of moderate half-life periods are used for tracer work. The activity of radioisotopes can be detected by means of electroscope, the electrometer or the Geiger-Muller counter. Tracers have been used in the following fields,
(i) To diagnose many diseases : For example, Arsenic – 74 tracer is used to detect the presence of tumours, Sodium – 24 tracer is used to detect the presence of blood clots and Iodine –131 tracer is used to study the activity of the thyroid gland. It should be noted that the radioactive isotopes used in medicine have very short half-life periods.
(ii) In agriculture : The use of radioactive phosphorus \[^{32}P\] in fertilizers has revealed how phosphorus is absorbed by plants. This study has led to an improvement in the preparation of fertilizers. \[^{14}C\] is used to study the kinetics of photo synthesis.
(iii) In industry : Radioisotopes are used in industry to detect the leakage in underground oil pipelines, gas pipelines and water pipes. Radioactive carbon has been used as a tracer in studying mechanisms involved in many reactions of industrial importance such as alkylation, polymerization, catalytic synthesis etc.
(iv) In analysis : Several analytical procedures can be used employing radioisotopes as tracers.
(a) A small amount of radioactive isotope is mixed with the inactive substance and the activity is studied before and after adsorption. Fall in activity gives the amount of substance adsorbed.
(b) The solubility of lead sulphate in water may be estimated by mixing a known amount of radioactive lead with ordinary lead.
(c) Ion exchange process of separation is readily followed by measuring activity of successive fractions eluted from the column.
(d) By labelling oxygen of the water, mechanism of ester hydrolysis has been studied.
(e) The efficiency of analytical procedures may be measured by adding a known amount of radio-isotopes to the sample before analysis begins. After the completion, the activity is again determined. The comparison of activity tells about the efficiency of separation.
(3) Use of \[\gamma \]rays : \[\gamma \]rays are used for disinfecting food grains and for preserving food stuffs. Onions, potatoes, fruits and fish etc., when irradiated with \[\gamma \]rays, can be preserved for long periods. High yielding disease resistant varieties of wheat, rice, groundnut, jute etc., can be developed by the application of nuclear radiations. The \[\gamma \]rays radiations are used in the treatment of cancer. The \[\gamma \] radiations emitted by cobalt –60 can burn cancerous cells. The \[\gamma \] radiations are used to sterilize medical instruments like syringes, blood transfusion sets. etc. These radiations make the rubber and plastics objects heat resistant.
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