Radioactive Radiations

Natural and Artificial Radioactivity

The phenomenon of spontaneous emission of highly penetrating radiations from heavy elements occurring in nature is called natural radioactivity. The phenomenon in which radioactivity can be induced by artificial means through nuclear transmutation is known as artificial radioactive radiations. They are named as α, β, and γ radiations.

Radioactive radiations (Radiations Emitted by Radioactive Elements)

After the discovery of natural radioactivity, a large number of experiments were conducted to determine the nature of radiations emitted by the radioactive substances.

A small hole is drilled in a lead block in which a piece of the radioactive sample such as radium is placed and a narrow beam of radiation emerges out of the hole. The nature of the radiation is studied by applying an electric field or magnetic field as in the figure. In both cases, the narrow beam splits into following three components.

1. The component which bends towards the left plate i.e. negative charge is called α-particles.
2. The component which bends towards right consists of negatively charged particles called β-particles.
3. The component which goes straight consists of neutral photons called γ-rays.

Three types of emission from a radioactive nuclide are observed as discussed above. In addition to these, electron capture and positron emission is also considered as radioactive decay. Hence there are five kinds of radioactive decay. Examples of this decay are given below: ₃C¹²

1. Alpha decay; \(_{92} U^{238} \rightarrow _{90} U^{234} + _2 He^4 \)
2. Beta decay; \(_6 C^{14} \rightarrow _7 N^{14} + -e^0 \)
3. Gamma decay;\(_{87} Sr^{87*} \rightarrow _{38}Sr^{87} + \gamma \)
4. Positron emission;\(_{29} Cu^{64} \rightarrow _{28} Ni^{64} + 1^0 \)
5. Electron capture;\( _{29} Cu^{64} + -e^0 \rightarrow _{28} Ni^{64} \)

Properties of Radioactive Radiation

Following are the properties of radioactive radiations.

Properties of α-particles

1. α-particles are the nuclei of a helium atom.
2. They are positively charged particles having charge + 2e = 3.2 × 10^-19C
3. Their rest mass is equal to 6.4 × 10^-27 kg
4. They move with high velocities which depend on the source emitting the particles.
5. They are deflected in an electric and magnetic field.
6. They affect a photographic plate.
7. The energy of α particles emitted from radioactive substance is about 6 MeV.
8. They ionize the gas through which they pass. The ionizing power is higher than β-particle.
9. They get scattered while passing through the thin metal foil.
10. They produce fluorescence and phosphorescence on some materials such as zinc sulphate.
11. They penetrate through matter, but the penetrating power is lower than that of β-particle.
12. They can induce artificial radioactivity.

Properties of β-particles

1. β-particles are the electrons of nuclear origin.
2. They are negatively charged particles having charge = -1.6 × 10^-19C
3. Their rest mass is equal to 9.1 × 10^-31kg
4. They move with velocities which of the order of 10⁸ ms^-1.
5. They are deflected in an electric and magnetic field.
6. They affect the photographic plate.
7. The energy of β-particles ranges 2 to 3 MeV.
8. They ionize the gas through which they pass, but it is about 1/100^th of that of β-particle.
9. They get scattered while passing through the thin metal foil.
10. They produce fluorescence and phosphorescence on some materials such as zinc sulphate.
11. They penetrate through matter, but the penetrating power is 100 times more than that of α-particle.
12. They can induce artificial radioactivity.

Properties of γ-particles

1. γ-particles are electromagnetic waves of very short wavelength.
2. They are a chargeless particle.
3. They move with the velocity equal to that of light.
4. They are not deflected by electric and magnetic field.
5. They affect a photographic plate.
6. They produce fluorescence on some materials.
7. They ionize the gas through which they pass, but it is about the only 1/100^th of that of β-particle.
8. They knock out the electrons from the surface on which they fall.
9. They produce heat on the surface exposed to them.
10. They can produce a nuclear reaction.

Cause of Radioactivity

There are two types of forces are acting there. One of force is attractive and other is repulsive. If attractive force dominates the repulsive force, the nucleus becomes stable but if repulsive force is too much greater than the attractive force, the nucleus becomes unstable. Such unstable nucleus emits radioactive radiations. Hence, the unstable nucleus is the main cause of radioactivity.

Reference

Manu Kumar Khatry, Manoj Kumar Thapa, et al. Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.

S.K. Gautam, J.M. Pradhan. A text Book of Physics. Kathmandu: Surya Publication, 2003.