Natural Radioactivity

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Natural Radioactivity

• Due to spontaneous emission of particles or
  energy from an atomic nucleus.
• Radioactive decay is another term used.
• Something must make the nucleus unstable for
  radioactivity to occur.

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Review Atomic Structure

• Negatively charges electrons move around a
  positively charged nucleus.
• The nucleus contains protons with a positive
  charge and neutrons with no charge.
• Atomic number number of protons.
• Mass number protons neutrons

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Isotopic Symbols

• Use symbol of element
• In upper left corner, put mass number
• In lower left corner, put atomic number
• May put charge in upper right corner.
• Example 23892U Alternative U-238
• Isotopes have same atomic number but different
  mass numbers.

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Nuclear Equations

• Beginning isotopes ending isotopes
• Balance mass numbers and atomic numbers
• 23892U 23490Th 42He
• 21483Bi ______ 42He

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Source of Instabilities

• Too big
• Too many neutrons for the protons.
• Not enough neutrons for the protons.
• Too much excess energy

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Types of Radiation

• Four types of naturally occurring radiation have
  been experimentally identified.
• Named alpha (a), beta (ß), gamma (?) and positron
  (ß or e)
• First look at experiment to determine the charge
  on all but positron.

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Penetrating Abilities

• Use a Geiger Counter to detect radiation
• Background radiation occurs from small amount
  of radioactive isotopes in the environment plus
  cosmic rays from outer space that make it through
  the atmosphere.
• Look at ability of first 3 to penetrate Al and Pb.

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Alpha Radiation

• 2, largest mass, low ability to penetrate
• From isotopes that are too big.
• Helium nucleus 42He2
• Previously did an example decay

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Beta Emission

• -1, moderate ability to penetrate
• From isotopes with too many neutrons.
• An electron that originates when a neutron is
  converted to a proton.
• 146C 147N 0-1e-1

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Positron Emission

• 1
• From isotopes with too few neutrons.
• An anti-electron that looks like an electron,
  but has a positive charge. When an electron and
  a positron come together, they annihilate each
  other making 2 gamma rays.
• 189F 188O 01e

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Gamma Radiation

• Has no charge, most penetrating
• Isotope identification does not change
• High energy light.
• 222Rn 222Rn ?

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Practice Completing and Identifying

• 2411Na _____ 0-1e-1
• 22388Ra _____ 42He
• ____ 21990Th 42He
• 13153I 13154Xe _____

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Half-life

• Not all radioactive decay happens at one instant.
• Half-life is the time required for half the
  unstable isotopes to decay.
• There is a wide range of values, from fractions
  of a second to billions of years. (see table 11.3
  for some values.

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Decay

• Although the mass of the unstable isotope goes
  down, the mass of the daughter isotope
  corresponding increases so there is very little
  overall change in the mass of the sample.
• How long does it take to completely decay?
  Theoretically, forever. Once the radiation drops
  below background, it is of little concern.

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Calculations with Half-life

• The half-life of I-131 is 8 days. How much of a
  10 g sample remains after 32 days?
• How many days before it is 99 gone?
• Rather than brute force, the following equation
  can be used (1/2)X fraction left. Solving
  gives X ln(fraction)/ln(.5)

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Units of Radiation

• Curie 3.7 x 1010 disintegrations/second. Large
  so use micro-curies (µCi)
• Rem radiation equivalent man. Measures damage
  to a human body.
• Rad radiation absorbed dose. Measures amount
  of energy absorbed per unit mass.

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Biological Damage

• Absorbed energy ionizes molecules. This disrupts
  the chemistry of the cell and usually results in
  cell death.
• Some cells survive. The immune system removes
  most, but with repeated exposure, may result in
  cancerous cells.

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Low Dose

• Will have very little effects that are noticed.
• Long term exposure will increase likelihood of
  cancer.
• Often takes cancer years to fully develop.
• Individuals vary greatly in response. A lot
  depends of status of immune system.

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Medium Dose

• Suffer flu-like symptoms.
• May interrupt hair growth causing temporary loss
  of hair.
• Radiation treatments in this range. Cancerous
  cells are killed at a fast rate than healthy
  cells. May develop cancer from the treatment!

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High Dose

• Will cause physical burning of tissue like a burn
  caused by fire.
• Large cell death and very serous illness
  including high fever result.
• Death usually follows in a few days, but some may
  survive.

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Nuclear Energy

• When the mass of the daughter isotopes is
  carefully compared to the parent isotopes, there
  is a small loss of mass.
• The mass is converted to energy by E mc2
• Calculate how much energy there is in the loss of
  1g of matter.
• Nuclear fission and fusion result in large mass
  loss.

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Binding Energy

• If one calculates how much energy there is
  available to bind together nuclei and then
  divides by the atomic mass number, you find the
  binding energy per nucleon.
• The most stable isotope (has the highest binding
  energy per nucleon) is Fe. Smaller isotopes want
  to combine (fusion), larger ones want to
  dissociate (fission).

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Nuclear Fission

• Large nuclei will add a neutron and then fission.
• Three isotopes easily fission and release 2 to 3
  neutrons when they do fission generating a chain
  reaction
• U-232, U-235, Pu-239
• Only U-235 occurs naturally as 0.7 of uranium
  isotopes (most is U-238)

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Sustaining Chain Reaction

• If enough of U-235 is present, emitted neutrons
  will cause additional fissions to occur.
• Requires several hundred pounds of 3 U-235.
• Naturally occurring uranium must be enriched
  from 0.7 to 3, made into pellets and sealed
  into fuel rods for nuclear reactors.

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Branching Chain Reaction

• Need very high purity U-235 or Pu-239
• Need a minimum mass (critical mass) of about 35
  lbs for U-235 or 5 lbs for Pu-239
• Atomic bombs use Pu-239 as easier to purify.
• Bring two hemispheres of sub-critical mass
  together to initiate.

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Nuclear Reactors

• Use sustaining chain reaction
• Fuel rods provide U-235
• Control rods absorb neutrons to slow it down.
  (Boron)
• Coolant takes away heat (water)
• Danger meltdown if overheats and releases steam
  containing radioactive isotopes.
• Worry about disposing of waste and supply of fuel.

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Nuclear Fusion Reaction

• This is the energy of stars. Isotopes of
  hydrogen and other small isotopes fuse.
• Requires very high temperature and pressure.
• 1H 1H 2H e
• 2H 2H 3H n
• 3H 3H 4H 2 1H

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Hydrogen Bomb

• We have been able to do fusion in a hydrogen
  bomb.
• Atomic bomb is used to trigger tritium and
  deuterium isotopes.
• Small success at doing with very large magnets or
  very large lasers, but many technical
  difficulties.
• Very little radioactive waste, large fuel supply,
  no danger of meltdown.

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