Radioactive Decay
Radioactive Decay: Alpha, Beta, and Gamma
Some atomic nuclei are unstable. They become more stable by emitting radiation — this process is called radioactive decay. It is random and spontaneous (it cannot be predicted or controlled by external conditions like temperature or pressure).
Why Nuclei Are Unstable
A nucleus can be unstable if it has:
- Too many neutrons relative to protons
- Too many protons relative to neutrons
- Too much overall energy
An unstable nucleus will emit radiation to move towards a more stable configuration.
Types of Radiation
There are three main types of nuclear radiation:
Alpha Radiation (symbol: alpha)
- An alpha particle is a helium nucleus: 2 protons and 2 neutrons
- It is written as helium-4 (mass number 4, atomic number 2)
- Charge: +2 (two protons, no electrons)
- Relative mass: 4 (heavy)
When a nucleus emits an alpha particle:
- Mass number decreases by 4
- Atomic number decreases by 2
- The element changes (transmutation)
Example: Uranium-238 decays by alpha emission to become Thorium-234. The mass number drops from 238 to 234, and the atomic number drops from 92 to 90.
Beta Radiation (symbol: beta)
- A beta particle is a high-speed electron emitted from the nucleus
- It is written with mass number 0 and charge -1
- Charge: -1
- Relative mass: negligible (very light)
In beta decay, a neutron in the nucleus turns into a proton and an electron. The electron is ejected as the beta particle.
When a nucleus emits a beta particle:
- Mass number stays the same (a neutron becomes a proton — total nucleons unchanged)
- Atomic number increases by 1 (one more proton)
- The element changes
Example: Carbon-14 decays by beta emission to become Nitrogen-14. The mass number stays at 14, but the atomic number rises from 6 to 7.
Gamma Radiation (symbol: gamma)
- Gamma rays are electromagnetic waves — part of the EM spectrum
- They have no mass and no charge
- They are emitted when a nucleus has excess energy after an alpha or beta decay
When gamma rays are emitted:
- Mass number does not change
- Atomic number does not change
- The element stays the same — the nucleus just loses energy
Comparing the Three Types
| Property | Alpha | Beta | Gamma |
|---|---|---|---|
| What is it? | 2p + 2n (He nucleus) | High-speed electron | Electromagnetic wave |
| Charge | +2 | -1 | 0 |
| Mass | 4 | ~0 | 0 |
| Speed | Slow (up to 10% of c) | Fast (up to 99% of c) | Speed of light |
| Ionising power | Strongest | Moderate | Weakest |
| Penetrating power | Weakest — stopped by paper or skin | Moderate — stopped by aluminium (few mm) | Strongest — reduced by thick lead or concrete |
| Deflection by electric/magnetic fields | Deflected (positive charge) | Deflected (negative, opposite to alpha) | Not deflected |
Key relationship: The more ionising the radiation, the less penetrating it is. Alpha particles interact strongly with matter (causing lots of ionisation), so they lose their energy quickly and cannot travel far.
Identifying Radiation Types
An experiment to determine which types of radiation a source emits uses absorbers:
1. Measure the count rate with no absorber — record the reading
2. Place a sheet of paper between the source and detector — if the count rate drops significantly, alpha is present
3. Replace with a few mm of aluminium — if it drops further, beta is present
4. Replace with thick lead — if some radiation still gets through, gamma is present
Always subtract the background radiation count from all readings.
Nuclear Equations
In any nuclear equation, both the mass numbers (top) and atomic numbers (bottom) must balance on both sides.
Exam Tips
- In nuclear equations, always check that mass numbers and atomic numbers balance
- Do not say beta particles come from the electron shell — they are created IN the nucleus when a neutron converts to a proton
- Ionising power and penetrating power are inversely related — state both when comparing