Section 1 of 7The three types of emission
Alpha particle (α)
- Alpha particle (α) An alpha particle is two protons and two neutrons joined together, the same as a helium nucleus,
- so it has a relative charge of +2 and a relatively large mass.
- It is emitted from large, unstable nuclei.
Beta particle (β)
- Beta particle (β) A beta particle is a fast-moving electron that is created in the nucleus when a neutron changes into a proton.
- It has a charge of −1 and a very small mass compared with an alpha particle.
Gamma ray (γ)
- Gamma ray (γ) Gamma radiation is a wave of high-frequency electromagnetic radiation with no mass and no charge.
- It is often released after alpha or beta emission, when the nucleus still has surplus energy.
Section 2 of 7Comparing the properties
| Property | Alpha | Beta | Gamma |
|---|---|---|---|
| Relative charge | +2 | −1 | 0 |
| Stopped by | A sheet of paper or a few centimetres of air | A few millimetres of aluminium | Never fully stopped; thick lead or concrete reduces it a lot |
| Range in air | A few centimetres | Tens of centimetres to about a metre | Very long; it just gets weaker with distance |
| Ionising power | Very strong | Moderate | Weak |
| Penetrating power | Lowest | Medium | Highest |
Section 3 of 7Why the properties follow a pattern
1Radiation passes close to atoms
- Radiation passes close to atoms As it travels through a material it interacts with the atoms it meets.
2Electrons are knocked out
- Electrons are knocked out Removing an electron ionises the atom, leaving it with a net positive charge.
3Energy is lost at each ionisation
- Energy is lost at each ionisation Every ionisation takes energy from the radiation, so it slows down and eventually stops.
4Strong ionisers are poor penetrators
- Strong ionisers are poor penetrators Alpha ionises most, loses its energy fastest and so has the shortest range.
- Gamma ionises least and travels furthest.
Section 4 of 7Identifying a source from absorption data
1
Place a Geiger-Müller tube in front of the source and record the count rate, then subtract the background count measured with the source removed.
2
Put a sheet of paper between the source and the tube. If the count rate drops sharply, alpha particles are being emitted.
3
Replace the paper with a few millimetres of aluminium. If the count rate falls to near background, beta was present; if it hardly changes, gamma is being emitted.
4
Add thick lead. A source that still gives a small count rate through the lead is emitting gamma rays.
5
Combine the results: a source that is unaffected by paper but stopped by aluminium emits beta only.
Section 5 of 7Worked reasoning
Example
- Example A source gives a high count rate with nothing in front, almost the same count rate through paper,
- and a count rate close to background through 3 mm of aluminium.
- Paper did not reduce it, so it is not alpha. Aluminium stopped it, so it is not gamma. The source emits beta particles.
Watch the thickness
- Watch the thickness Beta is stopped only by aluminium that is a few millimetres thick.
- Very thin foil lets many beta particles through, so always check the thickness given in a question.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Ionisation | The removal of electrons from an atom, leaving it as a charged ion. |
| Penetrating power | How far radiation can pass through a material before it is absorbed. |
| Range | The distance radiation travels through a material, such as air, before it stops. |
| Background radiation | The low level of radiation that is always present from rocks, air, space and other natural and man-made sources. |
| Count rate | The number of radiation detections per second or per minute recorded by a Geiger-Müller tube. |
| Geiger-Müller tube | A detector that produces a click or count each time ionising radiation enters it. |
Section 7 of 7Exam technique
1
Saying alpha is the most penetrating because it is the biggest. It is the most ionising but the least penetrating, because it loses energy so quickly.
2
Claiming that gamma is stopped by lead. Thick lead only reduces the intensity; no thickness stops it completely.
3
Forgetting that beta passes through paper and thin aluminium. It is stopped only by a few millimetres of aluminium.
4
Not subtracting background count when working out what a material absorbs.
5
Describing a beta particle as coming from an electron shell. It is formed in the nucleus when a neutron turns into a proton.