The decay rate of a radioactive substance is characterized by its half-life (), a quantity that represents the time taken for a given amount of material to decay into half of its initial value. The amount of radioactive substance left in a sample at time is given by
,
which means that radioactive substances with big half-life are spent slower than those with small half-life.
The following graph shows a comparison of decay rates of some of the frequently encountered radioactive substances:
Fluorine-18
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synthesized, used as a body tracer in positron emission tomography
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Cesium-137
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product of nuclear weapons and reactors, great health hazard
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Technetium-99m
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synthesized, used as a body tracer for gamma cameras
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Americium-241
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present in nuclear waste, used in smoke detectors
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Radon-222
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biggest source of radiation exposure, second most frequent cause of lung cancer
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Radium-226
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highly radioactive, formerly used in luminescent paint, spas, and health products
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Iodine-125
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synthesized, used in radiation cancer treatment
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Carbon-14
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present in organic materials, used in archeology to date organic remains
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Polonium-210
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used in first nuclear weapons, highly toxic to humans
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Plutonium-239
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synthesized, fissile, used in nuclear reactors and weapons
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Cobalt-57
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synthesized, used in radiodiagnosis to detect vitamin B12 deficiency
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Uranium-234
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by-product of uranium enrichment, not fissile
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Cobalt-60
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synthesized, used for radiodiagnosis and sterilization
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Uranium-235
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fissile, used to sustain chain reactions in nuclear power plants and weapons, enrichment methods increase its concentration
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Strontium-90
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present in nuclear fallout and waste, used in bone cancer therapy
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Uranium-238
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most common uranium in nature, not fissile but fissionable by neutron absorption
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