Radioactive decay

Worked example

100 mCi source, half-life 8 days, after 24 days.

100 × e^(−(0.693 ÷ 8) × 24) = 12.5 mCi

24 days is three half-lives: 100 → 50 → 25 → 12.5.

Where it appears on the exam

• ASP: ASP11 Domain 1, Mathematical Calculations

  Blueprint: ASP11 Domain 1, task 15: radiation exposure (half-life)

  Practice: Radiation Calculations: Decay, Distance, Shielding

• CSP: CSP11 Domain 6, Occupational Health and Applied Science

  Blueprint: CSP11 Domain 6, task 1: occupational exposures (radiation)

  Practice: Applied Science: Radiation, Ventilation, Heat and Force

• CHST: CHST5 Domain 1, Hazard and Risk Identification and Control

  Blueprint: CHST5 Domain 1, knowledge 19: radiation exposure

  Practice: Noise, Hearing Protection, Radiation and Lasers

Source

Questions

What is the formula for radioactive decay?

At = A0 × e^(−λt); λ = 0.693 ÷ T½. A0 = starting activity; t = elapsed time; T½ = half-life, in the same time units as t.

Will this formula be given on the BCSP exam?

BCSP says the formula is included in the question whenever one is needed, unless the question is testing whether you know the formula. You can't bring a formula sheet into the exam, so learn what each formula does and when to use it.