Curie (unit)

The curie (symbol Ci) is a non-SI unit used to measure radioactivity. Radioactivity is the process of radioactive decay, where atoms release energy. This unit is vital for understanding the strength of radioactive materials. It helps scientists and doctors manage substances used in medicine and industry. While newer units exist, the curie remains widely used in the United States. It is still common in many government and medical settings today.

To understand how a curie works, you must look at the rate of decay. A curie measures how many atoms decay in a specific amount of time. Currently, one curie is defined as 37 billion decays per second. This is also expressed as 37 GBq, using the SI unit called the becquerel. One becquerel (Bq) represents just one single nuclear decay per second. Therefore, 1 Bq is approximately equal to 27 picocuries (pCi).

Measuring radioactivity also involves understanding the specific activity of a substance. Specific activity is the number of decays that occur in one gram of a radionuclide. This number changes depending on the type of atom you are measuring. Because the probability of decay is a fixed physical quantity, it is predictable. Scientists can use the decay constant, or lambda (λ), to convert activity into an actual number of atoms. This math allows researchers to know exactly how many atoms are present in a sample.

The history of the curie is tied to the famous scientists Pierre and Marie Curie. The unit was originally defined in 1910 to honor their work. At the first meeting to define the unit, some people proposed a smaller amount. They suggested that one curie should equal 10 nanograms of radium. Marie Curie initially accepted this idea, but she later changed her mind. She insisted that the unit should be based on one gram of radium. Some believed she felt the name 'curie' was too grand for such a tiny amount.

Different radioactive materials have vastly different levels of strength. For example, a radiotherapy machine might use 1,000 Ci of caesium-137 or cobalt-60. Such high levels of radioactivity can cause serious health effects. This can happen with only a few minutes of unshielded, close-range exposure. Some materials are even more dangerous if they are swallowed. The median lethal dose (LD-50) for ingested polonium-210 is only 240 μCi, which is about 53.5 nanograms.

Interestingly, the human body contains small amounts of natural radioactivity. We have about 0.1 μCi of naturally occurring potassium-40 in our bodies. We also contain about 0.1 μCi of carbon-14. Together, these result in a total of roughly 0.2 μCi inside a person. This means about 7,400 decays happen every second within the body. Most of these are beta decays, though some are gamma decays.

The way a curie relates to mass depends on the half-life of the element. The half-life is the time it takes for half of the atoms to decay. For bismuth-209, one curie would weigh 11.7 billion tonnes because it decays very slowly. In contrast, one curie of polonium-210 weighs only 223 micrograms. This is because polonium-210 decays much faster than bismuth. These differences show how the curie connects the concept of time, mass, and energy in nuclear physics.