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A curie is a unit of radioactivity which has been replaced with the becquerel in most applications, because the becquerel fits in better with the system of standardized units which many nations have adopted. Curie are used to provide information about how radioactive a given isotope is, with this information being used to determine what kind of safety precautions should be followed around the isotope to avoid injury. Tools for converting between curie and becquerel measurements are readily available online.
When measuring radioactivity, people look at how many times per second an unstable nucleus gives off ionizing particles. The more particles emitted per second, the more radioactive the isotope is. Radioactivity, in other words is measured by disintegrations per second. The number of disintegrations per second varies in part on a time scale, and in part on the amount of the isotope present. Because the value can change over time, sometimes values are given within a particular time scale.
In the case of the curie, one curie equals the amount of an isotope needed to produce 37 billion disintegrations per second. Abbreviated as Ci, the higher the number, the more disintegrations happening every second, and the higher the radioactivity. Something with 1,000 Ci of a radioactive element, for example, is more dangerous than something with 100 Ci. Radioactive materials are commonly packaged with warnings indicating their radioactivity level so that people are aware of the risk involved.
The curie was based originally on the radioactivity of a single gram of radium, an element studied by Peter and Marie Curie. This was used as the baseline of the measurement for some time before researchers switched to declaring that the curie equaled 3.7 times 1010 disintegrations per second, a measurement roughly equivalent to that of radium. Under the international units system preferred by many researchers, the becquerel (Bq) is used. One becquerel equals one disintegration per second, or 2.7 times 10-11 curie. A curie, conversely, is 37 billion becquerel.
Prefixes may be added to this unit of measurement to convey orders of magnitude, as seen with kilocurie, millicurie, and microcurie. The same types of prefixes can be seen in use with the becquerel, for convenience, so that people do not have to write out a number like 100,000,000 becquerel (which equals 100 megabecquerel). Because the becquerel is such a small unit of radioactivity, it is common to see it listed with a prefix indicating a higher order of magnitude.