Physics 6.10 - 6.11, 6.15 - 6.16 - Ionising radiation
Unstable nuclei emit particles or electromagnetic radiation at unpredictable moments. Connect the physical nature of each emission to ionisation and its ability to penetrate matter.
Unstable nuclei and their emissions
Radioactivity begins in the nucleus. An unstable nucleus can change spontaneously and emit radiation; it does not need to be heated, struck or observed first. The process is random: it is impossible to predict exactly when one particular unstable nucleus will emit radiation, even though an emission can be detected after it happens.
The word radiation covers both particles and electromagnetic waves. Keep the radioactive source separate from what leaves it: the unstable nucleus is radioactive, while the alpha or beta particle, gamma ray, positron or neutron is the emitted radiation.
| Emission from an unstable nucleus | What is emitted | Ionisation |
|---|---|---|
| alpha (α) | a helium nucleus: two protons and two neutrons, with no electrons | ionising |
| beta-minus (β−) | a high-speed electron emitted from the nucleus, not an electron from an outer orbit | ionising |
| beta-plus (β+) | a positron emitted from the nucleus | ionising |
| gamma (γ) | electromagnetic radiation emitted from the nucleus | ionising |
| neutron radiation | a neutron emitted from the nucleus | can cause ionisation indirectly through interactions with matter |
In the comparison that follows, beta means beta-minus: an electron emitted from the nucleus. Beta-plus radiation is also ionising. Nuclear changes that produce these particles are developed in the nuclear-equations lesson.
Ionisation and penetration
Ionisation
Ionisation happens when radiation transfers enough energy to remove one or more electrons from an atom. The atom is left with a net positive charge and is therefore an ion.
Ionisation happens in the material through which radiation travels; it is not the same as making that material radioactive. Each interaction transfers energy from the radiation to matter. Radiation that causes many ionisations over a short distance loses its energy quickly and therefore penetrates less far.
This gives a useful causal chain:
More frequent ionising interactions transfer energy to matter over a shorter distance, giving a shorter range and lower penetration.
| Radiation | Physical nature | Relative ionising ability | Relative penetration |
|---|---|---|---|
| alpha | helium nucleus; relatively large and charge | highest; causes many ionisations over a short path | lowest; travels only a short distance in air and is stopped by paper or skin |
| beta-minus | electron; much smaller and charge | intermediate | intermediate; passes through paper but is stopped by thin aluminium |
| gamma | electromagnetic radiation; no charge | lowest of these three, but still ionising | highest; passes through paper and thin aluminium and is reduced by thick lead or concrete |
Continue this lesson.
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