Half-Life and Nuclear Reactions

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Half-Life and Nuclear Reactions
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Review

• We learned that all radioactive atoms eventually
  decay into stable isotopes.
• We did not talk about how long this takes.

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Half-Life

• Cannot predict when a radioisotope (RI) will
  decay.
• Can only give probability.
• Half-life Amount of time during which one-half
  of a radioactive substance will decay.
• During a RIs half-life, every atom has a 50
  chance of decaying.
• Half-lives can be as short as a fraction of a
  second or as long as billions of years.

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Half-Lifes

• N-13 is radioactive.
• It decays into C-13 via a process called electron
  capture.
• The half-life of 13N is 10 minutes.
• If you start with 1000 atoms of N-13, approx. how
  many will remain after 10 minutes?
• 10 minutes 1 half-life, so there would be about
  500 atoms left.
• Approx. how many will remain after 20 minutes?
• 20 minutes 2 half-lives, so there would be
  about 250 atoms left.
• Approx. how many will remain after 30 minutes?
• 30 minutes 3 half-lives, so there would be
  about 125 atoms left.

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Half-Lifes
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Half-Lifes

• The half-life of 26Al is 710,000 years.
• If you start with 28.0 grams of 26Al, how much
  will remain after 1,420,000 years?
• At start, 28.0 grams remain.
• At 710,000 years, 14.0 grams remain.
• At 1,420,000 years, 7.0 grams remain.

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Half-Lifes

• The half-life of 61Fe is 6.0 minutes. How much
  time must pass before a 600-mg sample decays to
  75 mg?
• At start, 600 mg remain.
• At 6.0 minutes, 300 mg remain.
• At 12.0 minutes, 150 mg remain.
• At 18.0 minutes, 75 mg remain.

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Half-Lifes

• In 48 minutes, 12 mg of 212Rn will decay to 3 mg.
  What is the half-life of 212Rn?
• At start, 12 mg remain.
• At (1st half-life), 6 mg remain.
• At (2nd half-life), 3 mg remain.
• It takes 2 half-lives for 12 mg to decay to 3 mg.
• 2 half-lives 48 minutes
• 1 half-life 24 minutes

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Nuclear Fission

• A heavy nucleus is struck by a neutron.
• It becomes unstable and splits into smaller
  fragments, releasing energy.
• In some cases extra neutrons are also released.
• The neutrons can strike other nuclei and cause
  them to split.
• Chain reaction.

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Nuclear Fission
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Uses for Nuclear Fission

• Nuclear reactors use fission to produce energy.

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Uses for Nuclear Fission

• Weapons the bombs that destroyed Hiroshima and
  Nagasaki, Japan, were fission bombs.

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Nuclear Fusion

• Two light nuclei are joined to produce a larger
  nucleus.
• Energy is released.

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Nuclear Fusion
Energy
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Uses for Nuclear Fusion

• Nuclear fusion can be performed at low
  temperatures, but it does not produce much
  useable energy.
• Fusion only produces significant energy at
  extremely high temperature and pressure.
• The Sun produces energy via nuclear fusion.

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Fusion in the Sun

• The Sun is mostly made of hydrogen.
• Intense heat and pressure at its center causes
  nuclear fusion to occur.
• This releases energy that supports the mass of
  the Sun and prevents it from collapsing.

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The Sun
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Uses for Nuclear Fusion

• Weapons Thermonuclear (hydrogen) bombs.
• Use a conventional fission bomb to trigger
  nuclear fusion in a lithium hydride shell.
• Much more destructive than fission bombs.
• Never used in war.