Alpha (α), beta (β) and gamma (γ) radiations are emitted during the radioactive decay of unstable atomic nuclei. They differ in their nature, charge, mass, penetrating power, ionizing power and behaviour in electric and magnetic fields.
1. Alpha Rays (α)
Alpha rays consist of positively charged particles called alpha particles. Each alpha particle contains two protons and two neutrons and is identical to the nucleus of a helium-4 atom.
Properties of Alpha Rays
- They consist of two protons and two neutrons.
- They carry a charge of +2e.
- Their mass is approximately 4 atomic mass units (u).
- They travel at high speeds but are generally slower than beta particles and gamma rays.
- They have high ionizing power because of their relatively large mass and double positive charge.
- They have low penetrating power and can generally be stopped by a sheet of paper or the outer dead layer of skin.
- They are deflected by electric and magnetic fields toward the negative plate in an electric field.
- They produce fluorescence in suitable materials and can affect photographic film.
2. Beta Rays (β)
Beta rays consist of high-speed charged particles emitted during certain types of radioactive decay. Beta-minus (β-) radiation consists of electrons, whereas beta-plus (β+) radiation consists of positrons.
Properties of Beta Rays
- Beta-minus rays consist of electrons, while beta-plus rays consist of positrons.
- The electron and positron have equal rest masses of approximately 1/1836 of a proton mass (~0.00055 u).
- The charge is -e for beta-minus particles and +e for beta-plus particles.
- They travel at speeds that may approach the speed of light.
- Their ionizing power is generally lower than that of alpha particles but higher than that of gamma rays under typical comparisons.
- Their penetrating power is greater than that of alpha particles but generally lower than that of gamma rays.
- They are deflected by electric and magnetic fields. Beta-minus particles bend toward the positive plate, while beta-plus particles bend toward the negative plate.
- They can be attenuated by suitable thicknesses of plastic or aluminium, depending on their energy.
3. Gamma Rays (γ)
Gamma rays are high-energy electromagnetic radiation emitted by excited atomic nuclei. They consist of photons and do not possess electric charge or rest mass.
Properties of Gamma Rays
- They are electromagnetic radiation, not material particles like alpha and beta particles.
- They have no electric charge and no rest mass.
- They travel at the speed of light in a vacuum, approximately 3 × 108 m/s.
- They have very high penetrating power compared with alpha and beta radiation.
- Their ionizing power is relatively low per unit path length compared with alpha particles.
- They are not deflected by electric or magnetic fields.
- They can be attenuated by thick lead, concrete or other suitable shielding materials.
- They can produce fluorescence, ionization and effects on photographic materials.
4. Comparison of Alpha, Beta and Gamma Rays
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Nature | Helium nucleus | Electron or positron | Electromagnetic radiation |
| Symbol | 42He2+ | e- or e+ | γ |
| Charge | +2e | -e or +e | Zero |
| Rest mass | Approximately 4 u | Approximately 0.00055 u | Zero |
| Speed | High, but generally below beta and gamma | May approach the speed of light | Speed of light in vacuum |
| Ionizing power | Very high | Intermediate | Relatively low |
| Penetrating power | Low | Moderate | Very high |
| Electric field | Deflected toward negative plate | Deflected according to charge | Not deflected |
| Magnetic field | Deflected slightly under comparable conditions | Deflected; curvature depends on energy and charge | Not deflected |
| Typical shielding | Paper or suitable thin barrier | Plastic or aluminium of suitable thickness | Thick lead or concrete, as appropriate |
5. Ionizing and Penetrating Power
Order of Ionizing Power
Alpha particles usually produce the greatest ionization over a short distance because of their relatively large mass and double positive charge. Gamma rays are indirectly ionizing and generally produce less ionization per unit path length.
Order of Penetrating Power
Alpha particles are readily stopped by thin barriers. Beta particles penetrate farther, while gamma rays may pass through considerable thicknesses of matter and require suitable shielding.
These are the usual comparative orders for introductory study. Actual ionization and attenuation depend on radiation energy and the material through which the radiation passes.
6. Nuclear Equations
A. Alpha Decay
During alpha decay, the mass number decreases by four and the atomic number decreases by two.
B. Beta-Minus Decay
During beta-minus decay, a neutron in the nucleus is converted into a proton, emitting an electron and an electron antineutrino. The mass number remains unchanged, while the atomic number increases by one.
C. Gamma Emission
An excited nucleus releases excess energy by emitting a gamma photon. Neither the mass number nor the atomic number changes.
7. Important Points for Examinations
- Alpha particles have a charge of +2e and a mass of approximately 4 u.
- Beta-minus particles are electrons; beta-plus particles are positrons.
- Gamma rays are uncharged electromagnetic radiation.
- Alpha particles have the greatest ionizing power among the three in the usual introductory comparison.
- Gamma rays have the greatest penetrating power among the three in the usual introductory comparison.
- Gamma rays are not deflected by electric or magnetic fields.
- In alpha decay, the mass number decreases by 4 and the atomic number decreases by 2.
- In beta-minus decay, the mass number remains unchanged and the atomic number increases by 1.
- Gamma emission does not change the atomic number or mass number of the nucleus.
Conclusion
Alpha, beta and gamma radiations differ in their physical nature, charge, mass, ionizing power and penetrating power. Alpha particles are helium nuclei, beta particles are electrons or positrons, and gamma rays are high-energy photons. Understanding these differences is essential for studying radioactivity, nuclear reactions and radiation protection.