In radiation protection, three units appear everywhere: becquerel (Bq), gray (Gy), and sievert (Sv). They are often confused because they describe the same broader domain, but each unit measures a different physical quantity.

1) Bq: how many decays happen

Becquerel measures activity: 1 Bq means one nuclear decay per second. It tells you how active a source is, not how much energy your body absorbs.

2) Gy: how much energy is absorbed

Gray measures absorbed dose: 1 Gy equals 1 joule per kilogram of material. This is a physical energy-deposition quantity and applies to tissue, water, air, or electronics.

3) Sv: what the biological impact is

Sievert measures equivalent/effective dose, which weights absorbed dose by radiation type and tissue sensitivity. For the same absorbed dose, alpha radiation is generally biologically more damaging than X-rays.

  • Bq: source activity (decays per second)
  • Gy: absorbed energy per kilogram
  • Sv: estimated biological effect and risk

In practice, a high Bq value does not automatically mean a high Sv value. Real risk depends on distance, shielding, exposure time, radiation energy, and radiation type.