What is the difference between activity and dose?

Activity (Bq, Ci) measures decay rate of a source—disintegrations per second. Dose (Sv, rem) measures energy absorbed by tissue and biological effect. A high-activity source may give low dose if far away or shielded. Dose depends on activity, distance, shielding, exposure time, and radiation type.

How do I calculate activity from mass?

A = λN = (ln2/t½) × (m/M) × Nₐ, where m is mass, M is molar mass, Nₐ is Avogadro's number, and t½ is half-life. Shorter half-life means higher specific activity (activity per gram). 1 gram of Ra-226 (t½ = 1600 y) ≈ 1 Ci; 1 gram of I-131 (t½ = 8 days) ≈ 124,000 Ci.

What is half-life and how does it relate to activity?

Half-life (t½) is the time for half the radioactive atoms to decay. Activity decreases by half every half-life: after 1 t½, 50% remains; after 2 t½, 25%; after 10 t½, ~0.1%. Half-lives range from fractions of a second (Po-214: 0.16 ms) to billions of years (U-238: 4.5 billion years). Short half-life = high activity but quick decay.

Why do nuclear medicine and industry still use curies?

Historical inertia and convenient scale. The curie was established before SI units and is deeply embedded in US regulations, equipment calibrations, and medical practice. Converting documentation and retraining would be costly. The curie also has convenient magnitude for many applications—mCi doses are common in nuclear medicine imaging.

Activité radioactive Converter

About Radiation Activity Conversion

Radioactivity (or activity) measures the rate of radioactive decay—the number of nuclear disintegrations per unit time. It quantifies how "active" a radioactive sample is, regardless of the type of radiation emitted or its energy. Activity tells you about the source itself, not about the hazard it poses: a highly active source of weak beta emitters may be less dangerous than a lower-activity gamma source. Understanding activity is essential for handling radioactive materials, planning nuclear medicine procedures, and managing radioactive waste.

The SI unit is the becquerel (Bq), equal to one disintegration per second. The older unit curie (Ci) is still widely used, especially in medicine and industry—named after Marie and Pierre Curie and originally defined by the activity of 1 gram of radium-226. Activity decreases exponentially with time according to the decay law: A(t) = A₀ × e^(-λt), where λ = ln(2)/t½. Half-life is the key property relating activity to time.

Our converter handles radioactivity units used in nuclear physics, nuclear medicine, and radiation safety.

Common Radiation Activity Conversions

From	To	Multiply By
Ci	Bq	3.7 × 10¹⁰
Bq	Ci	2.703 × 10⁻¹¹
mCi	MBq	37
MBq	mCi	0.027
μCi	kBq	37
kBq	μCi	0.027
Bq	dps	1
Ci	dpm	2.22 × 10¹²

Radiation Activity Unit Reference

Becquerel (Bq) – The SI unit of radioactivity, named after Henri Becquerel who discovered radioactivity in 1896. 1 Bq = 1 nuclear disintegration per second. The becquerel is a tiny unit—even a banana contains about 15 Bq of potassium-40. Common multiples: kBq (10³), MBq (10⁶), GBq (10⁹), TBq (10¹²). Nuclear medicine doses typically range from MBq to GBq.

Curie (Ci) – Historical unit still widely used in US medicine and industry, named after Marie and Pierre Curie. 1 Ci = 3.7 × 10¹⁰ Bq = 37 GBq exactly. Originally defined as the activity of 1 gram of radium-226. A large unit: 1 Ci is a significant radioactive source requiring careful handling. Common submultiples: mCi (10⁻³ Ci = 37 MBq), μCi (10⁻⁶ Ci = 37 kBq), nCi (10⁻⁹ Ci = 37 Bq), pCi (10⁻¹² Ci = 0.037 Bq).

Disintegrations per minute (dpm) – Direct measurement unit for laboratory counting. 1 Bq = 60 dpm exactly (60 seconds per minute). Liquid scintillation counters often report results in dpm. Also written as d/min. Related: disintegrations per second (dps) = Bq.

Specific activity – Activity per unit mass (Bq/g or Ci/g). Shorter half-life means higher specific activity: 1 gram of I-131 (t½ = 8 days) has about 4.6 PBq activity, while 1 gram of U-238 (t½ = 4.5 billion years) has only 12.4 kBq.

Rutherford (Rd) – Obsolete unit occasionally found in older literature. 1 Rd = 10⁶ Bq = 1 MBq exactly. Named after Ernest Rutherford. Rarely used in modern practice.