What is Radioactive Decay?
General
Public
Radioactive Decay
The
atomic structure for certain atoms is unstable; these unstable atoms are
termed radioactive atoms or radionuclides.
Radioactivity
is the natural and spontaneous process by which radionuclides transform or decay
to a different state, and emit or radiate excess energy in the form
of particles or waves called radiation. Depending on how the radionuclide releases this excess energy,
either a lower energy atom of the same form results, or a completely
different atom will be formed.
Radionuclides
decay in a random fashion, but at a characteristic rate. The
length of time this takes, the number of steps required and the kinds of
radiation released at each step are well known and are unique to that
radionuclide. For instance
phosphorus-32, a radionuclide, decays to a stable, or non-radioactive atom
of sulfur-32, accompanied by the emission of a beta particle (electron) with
energy up to 1.71 million electron-volts (MeV).
Half-Life
The
half-life is the time required for the number of
radioactive atoms to decrease by one half. After one half-life the number of
radioactive atoms is halved, after two half-lives it is reduced to one
quarter, after three half-lives to one-eighth and so on.
The
activity of any radionuclide is reduced to less than 1% after 7 half-lives.
This
half-life is a time that is unique time to each radionuclide. Half-lives
can range from less than a millionth of a second to millions of years.
Activity
The
quantity of radioactive atoms present is generally measured in
terms of activity rather than number of atoms or mass. Activity
is a measurement of the number of radioactive decays, called disintegrations,
a given number of radionuclides undergoes in a given period of time. Activity
is related to mass, because the greater the mass of radioactive material, the
more radioactive atoms are present to undergo radioactive decay.
The
two most common units of activity are the Curie or the Becquerel (international
units).
-
1 Curie (Ci)
= 3.7 x1010 disintegrations per second (dps)
-
1
Becquerel (Bq) = 1 disintegration per second (dps)
-
1
Millicurie (mCi) = 1/1,000 (one thousandth) of a curie
-
1
Microcurie (mCi)
= 1/1,000,000 (one millionth) of a curie
-
1
Nanocurie (nCi) = 1/1,000,000,000 (one billionth) of a curie
-
1
Picocurie (pCi) = 1/1,000,000,000,000 (one trillionth) of a curie
A picocurie is one
trillionth of a curie. 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.
Sources
Princeton University;
http://web.princeton.edu/sites/ehs/osradtraining/radiationproperties/radiationproperties.htm#Decay
Michigan State University;
http://www.pa.msu.edu/courses/1997spring/PHY232/lectures/radioactive/halflife.html
Links to external resources are provided as a public
service and do not imply endorsement by the
Washington State Department of Health.
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