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The Radioactivity Unit Converter translates between SI Becquerel (Bq, 1 disintegration per second — the modern standard), the older Curie (Ci, defined as the activity of 1 gram of radium-226 = 3.7 × 10¹⁰ Bq exactly), the obsolete Rutherford (Rd, 10⁶ Bq), and Gigabecquerel (GBq, 10⁹ Bq, commonly used in nuclear medicine). These are units of activity — the rate at which a radioactive sample decays — not dose (which uses Sievert/Sv) or absorbed energy (Gray/Gy). The Curie was the original radioactivity unit, established by Marie Curie and named in honor of her and Pierre. The value 3.7 × 10¹⁰ Bq was chosen because that's the activity of 1 gram of radium-226 — Marie Curie's signature element. The SI Becquerel (named after Henri Becquerel, discoverer of radioactivity) replaced the Curie as the official scientific unit in 1975 but Curie persists in medical and industrial use. 1 Ci = 37 GBq. Typical activity ranges: natural human body contains ~6,000 Bq from internal potassium-40 (about 10 disintegrations per second per gram of tissue from natural sources). Smoke detector americium-241 source: ~30,000 Bq. Medical iodine-131 thyroid treatment: ~5 GBq (5 × 10⁹ Bq). Cobalt-60 cancer therapy source: ~50 TBq (50 × 10¹² Bq). Chernobyl reactor at meltdown: ~10²⁰ Bq. The activity scale spans roughly 20 orders of magnitude — Becquerel works well for small biological samples while Curie/GBq are more practical for medical and industrial sources. What activity tells you: half-life × natural log of 2 × original sample atoms = total decay events that will eventually occur. Activity at any moment tells you the current decay rate. Short half-life isotopes (technetium-99m, used in medical imaging, 6 hour half-life) have very high activity briefly. Long half-life isotopes (uranium-238, 4.5 billion years) have very low activity per gram. Both can be hazardous — high-activity sources need careful shielding; long-half-life isotopes persist in environment for geological timescales.
- 1Step 1 — Identify the activity in your source document (Becquerel SI, Curie legacy, GBq medical)
- 2Step 2 — Select source unit in the calculator
- 3Step 3 — Enter the numeric value
- 4Step 4 — Calculator converts to Becquerel base: 1 Ci = 3.7 × 10¹⁰ Bq, 1 GBq = 10⁹ Bq, 1 Rd = 10⁶ Bq
- 5Step 5 — Computes equivalents in all four units
- 6Step 6 — Outputs in scientific notation for very large or small values
- 7Step 7 — Verify against expected magnitudes (medical doses GBq, environmental TBq, natural Bq)
The traditional reference unit. 1 gram of radium-226 has exactly this activity by definition.
Therapeutic iodine-131 dose for hyperthyroid treatment. Activity is high but isotope half-life is short (8 days) so radiation declines rapidly.
Natural human body radioactivity from potassium-40, carbon-14, etc.
Our bodies are always slightly radioactive — natural isotopes in food and tissue. This is harmless background.
Americium-241 in smoke detector. Very small but enough to ionize air for detection. Safely sealed; broken detectors should be disposed properly.
Chemistry / physics homework and lab work
Nuclear medicine dose calculation
Reading radioactive source labels
Translating between SI and US units in radiology
Environmental sampling (Bq/L drinking water, Bq/kg soil)
Reading regulatory documents (FDA uses Ci; international uses Bq)
When is each unit used?
Becquerel (SI): scientific and regulatory standard. Most physics, biology, environmental science papers. Curie: legacy in medicine and US industrial. Many older textbooks, FDA documents. Gigabecquerel (GBq): nuclear medicine standard worldwide — easier to write '5 GBq' than '5 × 10⁹ Bq' or '0.135 Ci'. Rutherford (Rd): essentially obsolete; appears only in very old French/European literature.
Is high activity always dangerous?
Not necessarily — depends on isotope, half-life, exposure type, distance, and shielding. 5 GBq of technetium-99m (6-hour half-life, gamma emitter, used for imaging) decays harmlessly in days, mostly outside the body. 5 GBq of plutonium-239 (24,000 year half-life, alpha emitter, lung-damaging if inhaled) is dangerous for centuries. Activity is one factor; dose (Sievert) and exposure pathway determine actual biological hazard.
What does specific activity mean?
Activity per gram of isotope (Bq/g or Ci/g). Helps distinguish 'concentrated' vs 'dilute' radioactivity. Plutonium-239 specific activity: 2.3 GBq/g. Uranium-238: 12 kBq/g (much lower — long half-life). Same TOTAL activity in different specific activities means very different physical mass and handling concerns.
How is activity different from dose?
Activity (Bq, Ci) = decay rate of source. Dose (Sievert, mSv) = biological effect on tissue. Two related but different: same activity in alpha emitter vs gamma emitter produces vastly different doses (alpha is biologically more damaging per energy when ingested). For health/safety, dose matters most. Activity helps quantify source strength for shielding, transport, and disposal decisions.
Why is 1 Curie such an awkward number (3.7 × 10¹⁰)?
Historical accident. Marie Curie measured radium-226's activity per gram and assigned the unit name. The value 3.7 × 10¹⁰ disintegrations/second wasn't chosen for mathematical convenience — it's just the natural physical property of radium. SI committee chose round-number Becquerel (1 dps) when standardizing, but Curie was already entrenched. Both persist.
Pro Tips
When working with both activity and dose, keep units explicit at every step — confusion between Bq (decay rate) and Sv (biological effect) is the most common error in radiation work. Activity tells you 'how much source'; dose tells you 'how much biological harm.' They're related but not interchangeable.