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Applications of Radioactivity and Radioisotopes

It has already been pointed out that each radioactive isotope has a specific half-life period. The radiations that it emits can be detected and measured. Also the chemical properties of isotopes of a given element are identical. This makes it possible to incorporate a small amount of radioisotopes in a system and trace the fate of particular element or a compound in a series of chemical or physical changes. Radioisotopes have large number of applications in  different fields like medicine, engineering, biology, chemistry, archeology, agriculture and industry. Some of the applications of radioactivity have been discussed as follows:

AGE OF MINERALS AND ROCKS

During the nuclear disintegration, the end product in natural radioactive disintegration series is an isotope of lead. Each disintegration step has a definite half-life and hence a definite decay constant. Thus, by determining the amounts of parent radioactive element and the isotope of lead in a sample of rock, the age of rock can be calculated. For example, if we wish to determine the age of rock containing U (half life = 4.5 x 109 years), we measure the ratio of the concentrations of  U and its end product Pb. We assume that the rock did not contain any lead isotope initially.

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 rock must therefore be equal to the half-life of uranium-238 i.e., 4.5 x 109 years. Pb/U ratio of most of the rocks is 1.33 x 10-2 indicating that their age is of the order of 10s years.

Age of earth. The abundance ratio of the two isotopes of uranium, i.e., 235U : 238U at present is 1 : 140. The half-life period of 235U is 7 x 108 years. Assuming that in the beginning the proportion of two isotopes was equal, the above data determines the age of earth as 5 x 109 years.

 

RADIO CARBON DATING

It is a method of determining the ages of the archeological objects (wood, dead plants and animals). This technique was developed by Willard Libby. It helps in determining the date at which a particular plant or animal died. Libby was awarded Nobel Prize for his technique.

The principle of this technique lies in the fact that due to bombardment of cosmic rays, nitrogen atoms present in the upper atmosphere are converted into radioactive carbon, 14C according to the reaction:

 

 

Carbon-14 is radioactive and has half-life period of 5770 years. It is oxidised in the air to give radioactive carbon dioxide. Thus, the atmospheric carbon dioxide contains a small proportion of 14C carbon dioxide (14C02) which is assimilated by plants and animals. Living plants and animals have a definite and constant proportion of 12C and 14C. When plant or animal dies, no fresh L: C nuclei as 14C02 are received by the plant or animal. The C present in these, decays according to the reaction

The amount of 14C in a sample can be accurately determined by counting the number of 13-particles emitted per minute by one gram of the sample. By knowing 14C content and half-life period, the age of the sample can be determined.

 

It may be noted that older the sample is, smaller is  count rate. Ages of samples up to 50,000 years old can be determined accurately by this method.

 

 

 

TRACER TECHNIQUES

 

Radio isotopes are frequently used as tracers or tagged atoms in may processes in different fields like surgery, medicine, agriculture, industry and chemistry. In tracer techniques, a radioactive isotope is added to the reactants and its movement is studied by measuring radioactivity in different parts. The common examples of tracer techniques are:·

 

(i) In medical diagnosis. For example, in order to find out if blood is circulating to a wound or not, a radioactive isotope is injected into the blood stream. After suitable time gap, the blood from the wound is examined for its radioactivity. If no radioactive isotope is detected, it means that passage of blood is hindered. The rate of circulation of blood can also be detected by this method.

 

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

 

(ii) In Agriculture, the uptake of phosphorus by plants is studied by mixing radioactive phosphorus with phosphatic fertilizers.

 

(iii) In Chemistry, the use of tracer technique in chemistry is based on the fact that in chemical reactions the radioactive isotopes have same behaviour as ordinary isotopes. Some examples are:

 

(a) Solubility of sparingly soluble salt. The solubility of lead sulphate can be determined as follows. A lead salt centaining known amount of radioactive lead is taken and is dissolved in water. Sulphuric acid is then added to the aqueous solution to precipitate lead as lead sulphate.

 

 

 

Pb * ( NO3)2 + H2SO4 à Pb* SO4 + 2 HNO3

 

 

 

The precipitate is filtered. The radioactivity of filtrate is measured. From this the amount of lead still remaining solution is calculated.

 

 

(b) Tracer technique can be used to study the mechanisms or rates of chemical reactions. For example, consider the esterification of benzoic acid

In order to decide whether oxygen of ester comes from alcohol or from acid we use radioactive alcohol containing 180 isotope. The ester formed is found to contain O18 isotope but no radioactivity is detected in H2O. This indicates that reaction follows the II path and not the I. This implies that during esterification oxygen comes from the alcohol and reaction involves replacement of OH part of acid with OCH3 part of alcohol.

CANCER THERAPY

 

 

 

γ-rays emitted by the radio isotopes (Co) can be used in the treatment of cancer. The basis of such a treatment is, that the radiation tend to destroy cancerous cells more easily than the normal cells. Thus, a carefully controlled beam of y-rays of appropriate doses may be used to arrest the growth of cancerous cells. Radiophosphorus (P-32) is also used in the treatment of Leukaemia.

 

 

 

NEUTRON ACTIVATION ANALYSIS

 

This technique is gaining importance in analytical chemistry these days. It can help in the detection of even one part of the element per billion parts of the sample. In this technique, the sample containing very small amount of the stable isotope of the element to be investigated is bombarded with neutrons and the element of interest is converted (i.e., 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 quite advantageous because:

 

 

 

(i) trace amounts of the elements can be determined;

 

(ii) a sample can be tested without destroying it; and

 

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

 

 

 

IN INDUSTETRY

 

 

 

(a) Metal castings 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.

 

 
 

 

HAZARDS OF RADIATIONS

 

Although the nuclear radiations are quite useful to mankind, yet they are on~ of the major causes of atmospheric pollution. When living organisms are exposed to these radiations, the complex organic molecules in the body get ionised, break up and ultimately disrupt the normal functioning of the living organism. The damage caused by the radiations, however, depends upon the doses of radiations received. The radiations damage the living organisms in the following ways:

 

 (a) Pathological damage. It is a permanent damage produced in the body which ultimately leads to death. Larger doses of radiations cause immediate death whereas smaller doses can cause the development of diseases like cancers or leukemia which are fatal.

 

 (b) Genetic damage.This type of damage is produced when radiations affect the chromosomes of the cellular nuclei and injure the genes in reproductive cells.

 

 One of the major problem before the scientists is to check and deal with the radioactive waste formed in nuclear reactors. Any accidental leakage of the radioactive radiations can prove

 

to be disastrous. One such accident occurred in 1986 in U.S.S.R. which led to many deaths and caused injuries to several others.

 

 

 

Problems Associated with Nuclear Waste Disposal

 

From radioactive materials, very harmful radiation rays are released. When the human body is exposed to radiation, it can cause tumours and can do extreme damage to the Reproductive organs. For this reason, problems associated with radioactivity can be passed on to the victim’s children as well. That is why radioactive waste produced by nuclear power plants is so dangerous.

 

 Many scientists have argued about a long term storage for global nuclear waste. So far, continuing debates have prevented much of anything from being done about nuclear waste. Unfortunately, after buried underground, the nuclear waste can take millions of years to decay.

 

 
 

 

EVALUATE YOURSELF

 

 

 

I. Objective Type Questions

 

Select the most appropriate choice from the options given as

 

(a), (b), (c) and (d) after each question:

 

 1. Radioactive decay follows

 

(a) first order kinetics

 

(b) zero order kinetic

 

(c) second order kinetics

 

(d) third order kinetics.

 

 2. The bombarding projectile in the following transformation

(a) proton

(c) neutron

(b) deutron

(d) 0-1e

 

3. Which is the missing particle in the following nuclear reaction,

(a) proton                                (b) deutron

(c) positron                               (d) a.-particle.

 4. The following nuclear transmutation:

(a) (n, a.) type                          (b) (p, n) type

(c) (a., n) type                          (d) (d, p) type.

 

5. If f… is decay constant, then half-life period (t ½ ) for the radioactive decay is equal to

(a) λ/0.693                               (b) 0.693/λ

(c) 2.303 log λ             . (d) 0.693λ.

 6. The nuclear fission reaction involves the

(a) combination of two nuclei to form a single nuclide

(b) splitting of single nuclide into two nuclei

(c) combination of a. and 13-particles

(d) combination oft and deutron.

 

7. t ½  of a radioactive element is 5 years. The amount of it remaining out of 10 gin 20 years will be

(a) 0.625 g                   (b) 1.25 g

(c) 0.3125 g                 (d) It will totally disappear.

 

8. The reaction  1D +1T           He + 0n is an example of

(a) nuclear fission                    (b) nuclear fusion

(c) artificial radioactivity        (d) radio disintegration.

 

9. The radioactive decay of X by a β-emission produces an unstable nuclide which spontaneously emits a neutron. The final product is

(a) 37 X88                                  (b) 35Y89

 (c) 34Z 88                                  (d) 36 W87.

 10. In the reaction, 93 Np239                 94 Pu239 + ? The missing particle is

(a) neutron                               (b) proton

(c) positron                              (d) electron.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

It has already been pointed out that each radioactive isotope has a specific half-life period. The radiations that it emits can be detected and measured. Also the chemical properties of isotopes of a given element are identical. This makes it possible to incorporate a small amount of radioisotopes in a system and trace the fate of particular element or a compound in a series of chemical or physical changes. Radioisotopes have large number of applications in  different fields like medicine, engineering, biology, chemistry, archeology, agriculture and industry. Some of the applications of radioactivity have been discussed as follows:

 

AGE OF MINERALS AND ROCKS

 

During the nuclear disintegration, the end product in natural radioactive disintegration series is an isotope of lead. Each disintegration step has a definite half-life and hence a definite decay constant. Thus, by determining the amounts of parent radioactive element and the isotope of lead in a sample of rock, the age of rock can be calculated. For example, if we wish to determine the age of rock containing U (half life = 4.5 x 109 years), we measure the ratio of the

concentrations of  U and its end product Pb. We assume that the rock did not contain any lead isotope initially.

 

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 rock must therefore

be equal to the half-life of uranium-238 i.e., 4.5 x 109 years. Pb/U ratio of most of the rocks is 1.33 x 10-2 indicating that their age is of the order of 10s years.

 

 

Age of earth. The abundance ratio of the two isotopes of uranium, i.e., 235U : 238U at present is 1 : 140. The half-life period of 235U is 7 x 108 years. Assuming that in the beginning

the proportion of two isotopes was equal, the above data determines the age of earth as 5 x 109 years.

 

RADIO CARBON DATING

 

It is a method of determining the ages of the archeological objects (wood, dead plants and animals). This technique was developed by Willard Libby. It helps in determining the date at which a particular plant or animal died. Libby was awarded Nobel Prize for his technique.

 

The principle of this technique lies in the fact that due to bombardment of cosmic rays, nitrogen atoms present in the upper atmosphere are converted into radioactive carbon, 14C

according to the reaction: