Alpha, beta and gamma radiation
Alpha is a helium nucleus, beta is an electron and gamma is an electromagnetic wave. Ionising power falls, but penetrating power rises, from alpha to gamma.
Three kinds of radiation
Let the radiation from a radioactive source pass between two charged plates. It splits into three beams. One beam bends a little towards the negative plate: these are alpha (α) particles, so they are positive. One bends more strongly towards the positive plate: these are beta (β) particles, so they are negative. They bend more because they are much lighter. One goes straight on: these are gamma (γ) rays, which have no charge. A magnetic field separates them in the same way.
Properties compared
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| What it is | Helium nucleus (2 protons + 2 neutrons) | Fast electron given out by the nucleus | Electromagnetic wave of very short wavelength |
| Symbol | ₂He⁴ | ₋₁e⁰ | γ |
| Charge | +2e (positive) | −e (negative) | No charge |
| Mass | About 4 times a hydrogen atom | Very small (about 1/1836 of a hydrogen atom) | No mass |
| Speed | About 1/10 to 1/20 of the speed of light | Up to about 9/10 of the speed of light | Speed of light (3 × 10⁸ m/s) |
| Ionising power | Very high | Medium | Very low |
| Penetrating power | Very low: stopped by a sheet of paper or a few cm of air | Medium: stopped by a few mm of aluminium | Very high: reduced only by thick lead or concrete |
| In an electric or magnetic field | Bends a little towards the negative plate | Bends more, towards the positive plate | Not deflected |
Ionising and penetrating power
Ionising power is the ability to knock electrons out of atoms in air or in the body, making ions (charged atoms). Penetrating power is the ability to pass through matter. The two go in opposite directions. Alpha particles are heavy and highly charged. They ionise almost every atom they meet, lose their energy quickly and stop within a few centimetres of air. Gamma rays carry no charge, so they rarely disturb atoms. They ionise little but travel deep into matter. That is why gamma rays can reach deep tumours, and why their sources are stored in thick lead. All three affect a photographic plate and make substances like zinc sulphide glow (fluorescence).
How to draw the deflection of α, β and γ in an electric field
- 1Draw a lead block with a narrow hole. Put a dot for the radioactive source at the bottom of the hole.
- 2Above the block, draw two vertical plates on either side of the beam. Mark one + and the other −.
- 3Draw the α path bending slightly towards the − plate. Label it 'α (positive)'.
- 4Draw the β path bending sharply towards the + plate. Label it 'β (negative)'.
- 5Draw the γ path going straight up between the plates. Label it 'γ (no charge)'. Add a photographic plate at the top.
Some smoke detectors contain a tiny alpha source (americium-241): alpha particles ionise the air inside, yet the casing stops them. Cancer hospitals use gamma rays from cobalt-60 to reach tumours deep inside the body.
1 marks: “Which radiation has the highest penetrating power?” Gamma rays.
2 marks: “Why are alpha particles deflected towards the negative plate in an electric field?” They are helium nuclei with a positive charge (+2e), so they are attracted to the negative plate.
7 marks: “Compare the properties of alpha, beta and gamma radiation.” Table of nature, charge, mass, speed, ionising power, penetrating power and deflection in fields, with a labelled deflection diagram.
Alpha is A heavy atomic core (He), Beta is a Bit of charge (an electron), Gamma is a Ghost (no charge, no mass). Paper stops α, aluminium stops β, lead slows γ.