A curie (Ci) is a unit of radioactivity that measures how quickly atoms in a radioactive material are breaking apart. One curie equals 37 billion atomic decays per second. The unit is named after Marie and Pierre Curie, who discovered radium and polonium in 1898, and it remains widely used in the United States for everything from medical imaging to industrial safety.
What a Curie Actually Measures
Radioactive materials are unstable. Their atoms spontaneously break apart over time, releasing energy in the process. The curie quantifies the rate of that breakdown: how many atoms are decaying each second in a given sample. One curie means exactly 37 billion atoms are decaying every second (3.7 × 1010 disintegrations per second), which also works out to about 2.22 trillion disintegrations per minute.
This number wasn’t chosen arbitrarily. It comes from the measured activity of one gram of radium-226, the element Marie and Pierre Curie isolated from uranium ore. Radium-226 has a half-life of 1,600 years, meaning it takes that long for half of its atoms to decay. Even at that slow pace, a single gram produces billions of decays every second, which gives you a sense of how many atoms are packed into even a tiny amount of material.
Why the Same Mass Can Mean Very Different Activity
One of the most counterintuitive things about radioactivity is that a heavier sample isn’t necessarily more active. What matters is how fast the atoms decay, which depends on the element’s half-life and atomic weight. A substance with a short half-life burns through its atoms quickly, so a small amount can register a high curie value. A substance with a long half-life decays slowly, so you need much more of it to reach the same activity level.
The contrast is striking. It takes only 0.001 grams of cobalt-60 to produce one curie of activity. For radium-226, you need about 1.4 million grams to reach the same level. Natural uranium requires roughly one gram per curie. The formula behind this relationship is called specific activity: it divides a material’s decay rate by its mass, producing a value in curies per gram. Short half-life, high specific activity. Long half-life, low specific activity.
Curies vs. Becquerels
The curie is not the only unit for measuring radioactivity. The international scientific community uses the becquerel (Bq), which is the official SI unit. One becquerel equals exactly one atomic decay per second, making it a much smaller unit than the curie. The conversion is straightforward: one curie equals 37 billion becquerels.
The International Commission on Radiation Units and Measurements recommended adopting the becquerel (along with the gray and sievert for other radiation measurements) as part of a broader push toward standardized SI units. Most countries now use becquerels as their primary unit. The United States, however, still uses curies extensively. The U.S. Nuclear Regulatory Commission accepts both units in its regulations, listing activity in curies or becquerels interchangeably.
Common Subunits
A full curie represents intense radioactivity, far more than most everyday applications require. In practice, you’ll encounter smaller subdivisions:
- Millicurie (mCi): one-thousandth of a curie, commonly used in medical imaging
- Microcurie (µCi): one-millionth of a curie, used for very small medical doses and laboratory work
- Nanocurie (nCi): one-billionth of a curie, used for trace-level environmental measurements
- Picocurie (pCi): one-trillionth of a curie, the unit you’ll see on home radon test kits
How Curies Show Up in Medicine
Medical imaging relies on small, carefully measured amounts of radioactive material injected into the body. These doses are typically expressed in millicuries. For a thyroid scan, you might receive 1 to 10 mCi of a technetium-based tracer. A bone scan uses 20 to 30 mCi. Heart perfusion imaging falls in the 15 to 30 mCi range. Pediatric doses are scaled down further, often calculated in microcuries per kilogram of body weight.
These numbers sound large when you remember that a millicurie still represents 37 million decays per second. But the tracers used in imaging have very short half-lives, often just a few hours. The material decays quickly, the imaging equipment captures the radiation it emits, and the activity in your body drops to negligible levels within a day or so.
Industrial and Scientific Applications
Outside of medicine, curie-level sources appear in industries that rely on penetrating radiation. Industrial radiography, which uses radiation to inspect welds and metal structures for hidden flaws, typically uses sources in the range of 20 to 150 curies of iridium-192. Sources used in the United States tend toward the higher end (100 to 150 Ci), while other countries generally use 20 to 50 Ci sources to limit worker exposure.
Oil and gas exploration uses radioactive sources for well logging, a technique that maps underground rock formations. These sources typically range from 1 to 3 curies of cesium-137, with some neutron sources containing up to 16 curies of americium-241. These are significant activity levels and are subject to strict regulatory controls, security tracking, and handling protocols.
The Curies Behind the Name
Marie Sklodowska-Curie (1867–1934) and Pierre Curie (1859–1906) announced their discovery of two new radioactive elements in 1898: polonium, named after Marie’s native Poland, and radium, named for its intense radioactivity. Marie Curie became the first woman to win a Nobel Prize and remains the only person to win Nobel Prizes in two different sciences (physics and chemistry).
Naming the unit after the Curies made practical sense. Their work with radium defined the field of radioactivity, and the decay rate of one gram of radium-226 became the physical basis for the unit itself. Though the becquerel has officially replaced it in international scientific literature, the curie persists in American regulatory documents, medical practice, and industrial standards, making it a unit you’re still likely to encounter regularly.