You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
Radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting radiation. The three main types of nuclear radiation are alpha (α), beta (β), and gamma (γ). This lesson covers the nature, properties, and penetrating power of each type, along with the role of neutrinos and antineutrinos in beta decay. You must be able to write balanced nuclear equations for all decay types. This material is assessed in AQA sections 3.2.1 and 3.8.1.
A nucleus is unstable when the balance of protons and neutrons does not allow the nuclear forces to hold the nucleus together in its lowest energy state. The nucleus can become more stable by emitting particles or electromagnetic radiation. Radioactive decay is:
An alpha particle is a helium-4 nucleus: two protons and two neutrons bound together. It is written as ⁴₂He or ⁴₂α.
| Property | Value |
|---|---|
| Charge | +2e = +3.20 × 10⁻¹⁹ C |
| Mass | 4 u ≈ 6.64 × 10⁻²⁷ kg |
| Speed | Typically 5–10% of the speed of light (~1.5 × 10⁷ m s⁻¹) |
| Ionising ability | Strong — ionises about 10⁴ ion pairs per cm in air |
| Penetrating power | Very low — stopped by a few cm of air or a sheet of paper |
| Deflection in fields | Deflected by electric and magnetic fields (low specific charge, so deflected less than beta) |
In alpha decay, the parent nucleus loses 2 protons and 2 neutrons:
ᴬ_Z X → ᴬ⁻⁴_(Z−2) Y + ⁴₂α
Example — Uranium-238 alpha decay:
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂α
Check: Nucleon number: 238 = 234 + 4 ✓. Proton number: 92 = 90 + 2 ✓.
Example — Radium-226 alpha decay:
²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂α
Check: 226 = 222 + 4 ✓. 88 = 86 + 2 ✓.
Alpha decay is most common in heavy nuclei (A > 200) where the nucleus is too large for the strong nuclear force (which is short-range) to overcome the long-range Coulomb repulsion between all the protons.
In beta-minus decay, a neutron in the nucleus transforms into a proton, emitting an electron (the beta-minus particle) and an electron antineutrino (ν̄ₑ).
At the quark level: a down quark changes into an up quark via the weak interaction.
n → p + e⁻ + ν̄ₑ
or in quark terms: d → u + e⁻ + ν̄ₑ
| Property | Value |
|---|---|
| Charge | −1e = −1.60 × 10⁻¹⁹ C |
| Mass | 0.000549 u ≈ 9.11 × 10⁻³¹ kg |
| Speed | Up to ~99% of the speed of light |
| Ionising ability | Moderate — about 100 ion pairs per cm in air |
| Penetrating power | Moderate — stopped by a few mm of aluminium |
| Deflection in fields | Strongly deflected (high specific charge, opposite direction to alpha) |
ᴬ_Z X → ᴬ_(Z+1) Y + ⁰₋₁e + ⁰₀ν̄ₑ
The nucleon number A stays the same (a neutron is replaced by a proton). The proton number Z increases by 1.
Example — Carbon-14 beta-minus decay:
¹⁴₆C → ¹⁴₇N + ⁰₋₁e + ⁰₀ν̄ₑ
Check: A: 14 = 14 + 0 + 0 ✓. Z: 6 = 7 + (−1) + 0 ✓.
Example — Strontium-90 beta-minus decay:
⁹⁰₃₈Sr → ⁹⁰₃₉Y + ⁰₋₁e + ⁰₀ν̄ₑ
When beta decay was first studied, it appeared that energy, momentum, and angular momentum were not conserved. The emitted electrons had a continuous spectrum of energies (from zero up to a maximum), rather than a single fixed energy as expected. In 1930, Wolfgang Pauli proposed that a third, undetected particle was carrying away the missing energy and momentum. Enrico Fermi named this particle the neutrino ("little neutral one"). The neutrino (and its antiparticle, the antineutrino) has:
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.