Radioactivity and half-life

Unstable nuclei give out radiation to become more stable. This is random, but large numbers of nuclei decay at a predictable rate, described by the half-life.

Key facts

  • Alpha (α): helium nucleus, stopped by paper, very ionising.
  • Beta (β): fast electron, stopped by a few mm of aluminium.
  • Gamma (γ): electromagnetic wave, reduced by thick lead.
  • Half-life: the time for the activity (or number of nuclei) to halve.
  • Alpha decay: mass number −4, atomic number −2. Beta decay: atomic number +1.

Worked examples

Example 1

A sample has activity 800 Bq and a half-life of 3 days. What is its activity after 9 days?

  1. 9 days = 3 half-lives.
  2. 800 → 400 → 200 → 100 Bq

Answer: 100 Bq

Example 2

Uranium-238 (atomic number 92) emits an alpha particle. What does it become?

  1. Mass number: 238 − 4 = 234. Atomic number: 92 − 2 = 90.
  2. Element 90 is thorium.

Answer: Thorium-234, atomic number 90