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Fact Sheets How Big is a Picocurie? August 1998

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The curie is a standard measure for the intensity of radioactivity contained in a sample of radioactive material. It was named after French scientists Marie and Pierre Curie for their landmark research into the nature of radioactivity.

The basis for the curie is the radioactivity of one gram of radium. Radium decays at a rate of about 2.2 trillion disintegrations (2.2 x 102) per minute. A picocurie is one trillionth of a curie. Thus, a picocurie (abbreviated as pCi) represents 2.2 disintegrations per minute.

To put the relative size of one trillionth into perspective, consider that if the Earth were reduced to one trillionth of its diameter, the "picoEarth" would be smaller in diameter than a speck of dust. In fact, it would be six times smaller than the thickness of a human hair.

The difference between the curie and the picocurie is so vast that other metric units are used between them. These are as follows:

Millicurie = l/l ,000 (one thousandth) of a curie
Microcurie = 1/1,000,000 (one millionth) of a curie
Nanocurie = 1/1,000,000,000 (one billionth) of a curie
Picocurie = l/l,000,000,000,000 (one trillionth) of a curie

The following chart shows the relative differences between the units and gives analogies in dollars. It also gives examples of where these various amounts of radioactivity could typically be found. The number of disintegrations per minute has been rounded off for the chart.

Unit of Radioactivity Symbol Disintegrations per Minute Dollar Analogy Examples of Rad. Materials
1 Curie Ci 2 trillion 2 times the annual federal budget Nuclear Medicine Generator
1 Millicurie mCi 2 Billion Cost of a new inter-state highway from Atlanta to San Francisco Amount used for a brain or liver scan
1 Microcurie uCi 2 Million All-Star baseball player’ s salary Amount used in thyroid tests
1 Nanocurie nCi 2 Thousand Annual home energy costs Consumer products
1 Picocurie pCi 2 Cost of a hamburger and Coke Background environment levels

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