Unit 3 · Lesson 3.2.5
Penetration Properties Of Radiations
Describe alpha, beta, gamma and neutron radiation and compare their charge, range in air, penetrating power and ionising ability.
The types of nuclear radiation
Alpha (α)
- Alpha (α) An alpha particle is identical to a helium nucleus: two protons and two neutrons.
- It carries a charge of +2.
Beta (β)
- Beta (β) A beta particle is a fast-moving electron created when a neutron in the nucleus changes into a proton.
- It carries a charge of -1.
Gamma (γ)
- Gamma (γ) Gamma rays are electromagnetic waves with the highest energy in the spectrum.
- They have no charge.
Neutron (n)
- Neutron (n) A neutron is an uncharged particle that can also be emitted by an unstable nucleus.
Comparing alpha, beta and gamma
| Property | Alpha | Beta | Gamma |
|---|---|---|---|
| Make-up | 2 protons + 2 neutrons | High-energy electron | Electromagnetic wave |
| Charge | +2 | -1 | None |
| Range in air | A few centimetres | Around a metre or more | Very large, effectively unlimited |
| Stopped by | A sheet of paper or skin | A few millimetres of aluminium | Thick lead or very thick concrete reduces it |
| Ionising power | High | Medium | Low |
Trend from alpha to gamma
1Less ionising
- Less ionising Alpha ionises strongly, gamma only weakly.
2Loses energy slowly
- Loses energy slowly Radiation that ionises less uses up its energy less quickly.
3Travels further
- Travels further Range in air increases.
4More penetrating
- More penetrating It passes through thicker materials before being absorbed.
Showing penetration in a test
1
Place a source in front of a detector and note the count rate.
2
Put a sheet of paper between them; a large drop in count shows alpha was present.
3
Replace the paper with a few millimetres of aluminium; a further fall shows beta.
4
Add thick lead; any radiation still getting through is gamma.
5
Remember that beta passes through aluminium thinner than a few millimetres.
Why ionisation matters
Ionisation
- Ionisation Radiation ionises atoms by knocking electrons out of them, leaving positively charged ions.
- This can damage living cells or cause mutations.
Inside the body
- Inside the body Alpha is the most dangerous source if it is inhaled or swallowed, because all of its energy is absorbed by a small amount of tissue.
Outside the body
- Outside the body Gamma and beta are more dangerous from outside the body because they can pass through skin and reach internal organs.
Everyday uses linked to penetration
| Use | Radiation chosen | Reason |
|---|---|---|
| Smoke detector | Alpha | Easily absorbed by smoke particles, which triggers the alarm, and stopped by the casing |
| Thickness control of paper or foil | Beta | Partly absorbed, so the count rate changes with thickness |
| Medical tracer in the body | Gamma | Passes out of the body so it can be detected, and has a short half-life |
| Sterilising surgical equipment | Gamma | Penetrates sealed packaging and kills microbes |
Key terms
| Term | Meaning |
|---|---|
| Nuclear radiation | Particles or waves released by an unstable nucleus. |
| Penetrating power | How far radiation passes through a material before it is absorbed. |
| Ionisation | Knocking electrons from atoms to make charged ions. |
| Alpha particle | Two protons and two neutrons emitted from a nucleus. |
| Gamma ray | A high-energy electromagnetic wave from a nucleus. |
| Count rate | The number of radiation detections recorded per second or per minute. |
Avoid these mistakes
1
Writing that beta is stopped by any thickness of aluminium. It only stops beta if the sheet is a few millimetres thick.
2
Mixing up range and ionisation. The more a radiation ionises, the shorter its range.
3
Describing beta as an electron from the electron shells. It comes from the nucleus.
4
Saying gamma has mass or charge. It is an uncharged wave.
5
Thinking radioactive sources are safe because alpha is stopped by paper, when alpha is hazardous inside the body.