energy unit learning goal #4: understand the meaning of half-life and be able to determine graphical...
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ENERGY UNIT
Learning Goal #4: Understand the meaning of half-life and be able to determine graphical data relating to half-life and age of a radioactive substance.
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MARIE CURIE
http://www.aip.org/history/curie/brief/index.html
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A A DecaDeca
y y SerieSerie
ss
A radioactive nucleus reaches a A radioactive nucleus reaches a stable state by a series of stepsstable state by a series of steps
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Decay Series• Often radioactive nucleus cannot
achieve stability state through a single decay process.• A decay series will
occur until a stable nuclide is formed.
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Half-life• The time required for half of
the original sample of nuclei to decay.
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Half-life ConceptHalf-life Concept
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KINETICS OF RADIOACTIVE
DECAYFor each duration (half-life), one half of the substance
decomposes.
For example:
Ra-234 has a half life of 3.6 dayIf you start with 50 grams of Ra-234
After 3.6 days > 25 gramsAfter 7.2 days > 12.5 grams
After 10.8 days > 6.25 grams
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LEARNING CHECK
The half life of I-123 is 13 hours. How much will a 64 mg sample of I-123 is
left after 39 hours?
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RADIOCARBON DATINGRadioactive C-14 is formed in the upper atmosphere
by nuclear reactions initiated by neutrons in cosmic radiation
14N + 1on 14C + 1HThe C-14 is oxidized to CO2, which circulates
through the biosphere.When a plant dies, the C-14 is not replenished.
But the C-14 continues to decay with t1/2 = 5730 years.
Activity of a sample can be used to date the sample.
http://www.pbs.org/wgbh/nova/tech/radiocarbon-dating.html
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Sample Half-LivesSample Half-Lives