Question

What is radioactivity? What is the difference between natural and induced radioactivities? Give the properties of different types of rays emitted by radioactive substances.

Answer 1

Radioactivity: The phenomenon of spontaneous disintegration of the nucleus of an atom with the emission of some radiations is called radioactivity. There are a number of heavy elements occurring in nature whose nuclei are unstable and undergo spontaneous disintegration with emission of some radiations. The rate of emission or intensity of radiations is not influenced by any external agency such as the temperature or pressure. The substances having this property are called radioactive substances.

Natural and induced (or artificial) radioactivities. The radioactivity exhibited by some naturally occurring elements is called natural radioactivity. All elements with atomic number greater than 82 are radioactive. It is, however, possible to induce radioactivity in the elements with atomic number less that 83 by artificial means. This type of radioactivity is called induced or artificial radioactivity.

Properties of α-rays

1. The α-rays are shot out from the radioactive material with large velocities ranging from 1.4 x 10⁷ to 2.3 x 10⁷ ms^-1.
2. They produce ionisation in the gas through which they pass. The ionising power is 100 times greater than that of ß-rays and 10,000 times greater than that of γ-rays.
3. They affect a photographic plate. The effect is very-very feeble.
4. They produce fluorescence in substances like zinc sulphide, barium platinocyanide etc.
5. The α-rays are scattered when they pass through thin sheets of mica, gold foil etc. The angle of divergence of α-particles from its straight path is 2 to 3 degrees. Geiger and Marsaden found that a few particles, sometimes, were deflected by more than 20°. This was explained by Rutherford to be due to the repulsion between α-particle and the positively charged nucleus of the atom.
6. The α-rays are deflected by electric and magnetic fields showing that they are charged particles as shown in in Fig. 
7. They produce a heating effect. A quantity of radium always maintains itself at a temperature higher than that of the surroundings. The evolution of heat is due to the stoppage of α, ß and γ rays by the radioactive substance.
8. The body suffers incurable burns when exposed to α-rays.

Properties of ß-rays

1. ß-rays are shot out from radioactive elements with very high velocities ranging from 1% to 99% of the velocity of light. The velocity of all ß-particles given out by an element is not the same.
2. They produce ionisation in air but the number of ions produced is very less than those of α-rays. Although their velocity is very large they possess a comparatively small mass than that of α-particle and hence they have small kinetic energy. As ß-particles are slowed down by collision with the atoms of the gas and change their path, their tracks in a Wilson Cloud Chamber are scattered and not continuous as those of α-particles.
3. They affect a photographic plate and their effect is greater than those of α-rays.
4. ß-rays produce fluorescence in barium, platinocyanide, calcium, tungsten, willemite etc.
5. Because of their small mass, they can penetrate through large thickness of matter, e.g., they can easily pass through 1 cm of thick aluminium sheet.
6. They are more easily scattered when they pass through matter, because their mass is very small as compared to the mass of the atomic nuclei.

Properties of γ-rays

1. The velocity of γ-rays is the same as that of , light i.e. 3 x 10⁸ ms^-1.
2. They produce ionisation in gases through which they pass but their effect is very small as compared to that of α-rays and ß-rays. This is due to the fact that their mass is very small as compared to that of α and ß-ray particles.
3. They affect photographic plate and their effect is greater than those for ß-rays.
4. They produce fluorescence in barium, platinocyanide, etc.
5. They are more penetrating that even ß-rays and can pass very easily through 30 cm thickness of iron.
6. They are diffracted from crystals in a way similar to X-ray diffraction.
7. They are not affected by electric and magnetic fields. This shows that they are neutral particles; further experiments have shown that they are similar to e.m. rays.
8. γ-rays can be absorbed by the matter.