Types of Radioactive Decay: Revision Notes (Physics 2 15.8)

Short on time? This page is the recap. For explanations, the decay chart and the worked examples, use the full study guide.

Recap

The isotope decides the type of decay. Two isotopes of one element can decay differently.
Alpha particle: helium-4 nucleus, 2 protons + 2 neutrons, charge +2e.
Positron: same mass as an electron, charge +e.
Neutrino ν and antineutrino ν̄: no charge, negligible mass. They feel only the weak force and gravity, so they pass through matter almost untouched.
Gamma follows an alpha or beta decay that leaves the nucleus excited. The nucleus drops to a lower energy state and emits a photon.
In beta decay three particles share the energy, so the electrons or positrons have a range of energies. In alpha decay two particles share it, so the alpha energy is fixed for each final state of the daughter.

Decay	General equation	ΔA	ΔZ
α	ᴬ_Z X → ᴬ⁻⁴_(Z−2) Y + ⁴₂He	−4	−2
β⁻	ᴬ_Z X → ᴬ_(Z+1) Y + ⁰₋₁e + ν̄ (n → p)	0	+1
β⁺	ᴬ_Z X → ᴬ_(Z−1) Y + ⁰₊₁e + ν (p → n)	0	−1
γ	ᴬ_Z Y* → ᴬ_Z Y + γ	0	0

Particle	e⁻	ν	e⁺	ν̄	α, γ, nuclei
Lepton number	+1	+1	−1	−1	0

When sharing energy between two products from rest, take each mass as proportional to its nucleon number.
Only helium-4 counts as an alpha particle. Electron capture and neutron emission are not part of this course.

Electron from the shells. The β⁻ electron is created in the nucleus.
Wrong partner. e⁻ goes with ν̄; e⁺ goes with ν.
Changing A in beta decay. A stays the same.
Changing the element in gamma decay. It stays the same nucleus.
Counting a decay chain the wrong way round. Find the number of alphas from ΔA first, then the betas from Z.

Complete: ²²⁶₈₈Ra → ? + ⁴₂He. (²²²₈₆Rn)
A nucleus with A = 40 and Z = 19 undergoes β⁻ decay. Give the daughter’s A and Z. (A = 40, Z = 20)
Which particle is emitted with a positron? (a neutrino)
Why do neutrinos pass through the Earth? (No charge, negligible mass; they interact only through the weak force and gravity)