Model 3440 D-3
Radioactivity
Radioactivity is the spontaneous transformation (or
disintegration) of an unstable nucleus into a more stable
configuration accompanied by the emission of radiation.
The quantity of a radioactive material is measured in terms of
the average number of nuclear disintegrations per unit time.
The traditional unit of measure for radioactivity (or activity) is
the curie (Ci), which is defined as 3.7 u 10
10
disintegrations per
second. The activities of the radioactive sources in nuclear
gauges are so small that they are typically measured in
millicuries (mCi), which is one-thousandth of a curie, or
microcuries (PCi), which is one-millionth of a curie.
In the Standard International (SI) (or metric) system, the unit of
activity is the becquerel (Bq), which equals one disintegration
per second. Because the becquerel is such an extremely small
unit, the activity of sources in nuclear gauges is normally
expressed in megabecquerel (MBq), which is one million
becquerels, or gigabecquerel (GBq), which is one billion Bq.
The radioactivity of a source is not constant, but decreases with
time as the source decays. The time it takes for one-half of the
original atoms to disintegrate is called the half-life. In successive
half-lives, the activity decreases to 1/2, 1/4, 1/8 and so on of the
initial value. After seven half-lives, less than 1% of the original
radioactive atoms remain. Each radioisotope has a characteristic
half-life, which can range from seconds to billions of years. The
half-lives for the typical radioisotopes used in nuclear gauges
are:
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