# Curies

Since small amounts of material contain very large numbers of atoms, small samples can have a very large number of atoms disintegrating at the same time. It didn't take radiation scientists very long to decide that working with activities in the billions of disintegrations per second was too awkward. To make measuring activity more convenient, they developed a new unit, the curie, named in honor of Marie Curie, a pioneer in the study of radioactive materials.

#### How big is a curie?

A curie is defined as 37 billion disintegrations per second. The curie was originally a comparison of the activity of a sample to the activity of one gram of radium, which at the time was measured as 37 billion disintegrations per second. A radioactive sample that has an activity of 74 billion disintegrations per second, has an activity of 2 curies. When more accurate techniques measured a slightly different activity for radium, the reference to radium was dropped.

#### Are there smaller and larger units of activity?

The curie, abbreviated as "Ci", is a very large unit for some purposes and a very small unit for others. Scientists use the following fractions or multiples of a curie as well.

• picocuries (pCi) are 1 million millionth of a curie (1 x 10-12 Ci). Picocuries are used in measuring the typically small amount of radioactivity in air and water.
• megacuries (MCi) are 1 million curies (1 x 106 Ci).
Megacuries are used in measuring the very large amount of radioactivity released from nuclear weapons.
• Other fractions such as millicuries (mCi), or 1/1000 Ci and nanocuries (nCi), or 1 billionth of a curie, are used as needed.