Physics 12

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Radioactivity

• Physics 12

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Clip of the day

• Minutephysics ..Common Physics Misconceptions
• http//www.youtube.com/watch?vIM630Z8lho8

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What is radiation?

• Is the emission of energy as electromagnetic
  waves or as moving subatomic particles.
• either directly from unstable atomic nuclei or as
  a consequence of a nuclear reaction.

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Radioactive Isotopes

• It is also possible to have a nucleus that is not
  stable (meaning that it will fall apart)
• An unstable nucleus will decay following a few
  very specific processes
• We call this decay radioactivity and classify it
  into one of three types

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Alpha Decay

• An alpha particle is a helium nucleus (two
  protons and two neutrons)
• Thus, a nucleus that emits an alpha particle will
  lose the two protons and two neutrons
• Large nuclei will emit alpha particles
• They do not penetrate matter well and a sheet of
  paper or 5cm of air will stop most
• They can free electrons from atoms, meaning they
  are a form of ionizing radiation
• Ex Radium-222

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Writing an alpha decay reaction
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Beta Decay

• When a nucleus emits a beta particle, it appears
  to lose an electron from within the nucleus
• There are two types of beta decay (ß- and ß)
• Beta particles can penetrate matter to a greater
  extent than alpha particles they can penetrate
  about 0.1mm of lead or 10m of air
• They are also a form of ionizing radiation but
  less damaging than alpha particles

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Beta Decay (ß-)

• In this type of beta decay, a neutron becomes a
  proton and a beta minus particle (high energy
  electron) is emitted
• In addition an antineutrino is emitted
  (antimatter) along with the beta minus particle
• The nucleuss atomic number increases by one
  while the atomic mass number remains the same

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Beta Decay (ß)

• In this type of beta decay, a proton becomes a
  neutron and a beta plus particle (high energy
  positron (antielectron)) is emitted
• In addition a neutrino is emitted along with the
  beta plus particle
• The nucleuss atomic number decreases by one
  while the atomic mass number remains the same

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Gamma Decay

• When a nucleus goes through alpha or beta decay,
  the daughter nucleus is often left in an excited
  state
• In order to reduce the energy of the nucleus, it
  will go through gamma decay (high energy photon)
  to return to the ground state
• Gamma radiation can pass through 10cm of lead or
  2km of air
• It is the most damaging of all due to the energy
  of the gamma particle

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Decay Series

• When a large nucleus decays by alpha and beta
  radiation, the daughter nucleus will be more
  stable than the original nucleus
• However, the daughter nucleus may still be
  unstable and will itself go through alpha or beta
  radiation
• This leads to a decay series

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Rate of Radioactive Decay

• It is impossible to predict when a specific
  nucleus will decay
• You can describe the probability of decay
• The concept of half life is used with radioactive
  decay
• Using the half life equation, it is possible to
  determine how much of a sample would remain after
  a given period of time

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Half Life

• N ? sample remaining
• N0 ? original sample
• ?t ? elapsed time
• T ? half life

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Example

• You have a 160.0 µg sample of polonium-218 that
  has a half-life of 3.0 min.
• (a) How much will remain after 7.0 min?
• (b) How long will it take to decrease the mass
  of the polonium-218 to 8.0 micrograms?

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Try it ?

• Page 915
• 4-6

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Sources of radiation

• Natural
• Artifical