B: Radioactive Decay

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B Radioactive Decay
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• There are about 350 isotopes of 90 elements found
  in our solar system.
• Of these, about 70 are radioactive.
• Naturally occurring radioisotopes (elements with
  atomic nos. of 83 or less) emit background
  radiation in relatively small amounts.

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B.2 Natural Radioactive Decay

• Alpha particles have two protons and two
  neutrons.
• They are massive8000 x heavier than beta
  particles, but can only cause damage over very
  short distances.
• They are easily blocked, even by skin.

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• Radium-226 loses two protons, so its atomic
  number drops from 88 to 86.
• Atoms of 2 elements, He and Rn can be formed.

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• Beta particles are fast-moving negatively charged
  particles released during radioactive decay.
• During beta decay, a neutron changes into a
  proton and an electron. The proton remains in
  the nucleus but the electron (beta particle) is
  ejected at high speed.
• A third particle called an antineutrino is also
  released.

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• A beta particle has a mass number of 0 and an
  atomic number (nuclear charge) of -1.
• The overall result of beta emission is that a
  neutron is converted into a proton.
• Notice that the atomic number increases and
  Radium turns into Actinium.

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v

• Gamma emissions do not change the mass or charge
  balance in a nuclear equation because the rays
  have no mass or charge.

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• Of the three emissions alpha, beta and gamma,
  alpha radiation is the most damaging at short
  range but easy to protect against. Beta and
  gamma do less damage over a long range but its
  harder to protect against them.

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• New isotopes created from radioactive decay can
  also be radioactive and undergo further decay.
• Eventually, the result will be a stable atom. In
  this case Uranium decays into stable leadit just
  takes 14 steps to do so.

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B.3 Ionizing RadiationHow Much is Safe?

• Radiation can be measured with a Geiger counter
  in counts per minute (cpm).
• It can also be measured in rems when evaluating
  its effect on humans. Rems stand for Roentgen
  equivalent man and measures the ability of
  radiation to cause ionization in human tissue.
• A third way to measure radiation levels is in
  Becquerels.

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1986 Nuclear Meltdown at Chernobyl

• The accident released 1.1 x 10 19 Becquerels of
  radioactive material (one Becquerel is one
  molecule undergoing radioactive decay per
  second).
• About 116,000 people were evacuated from the
  area around the plant, and 134 were hospitalized
  for acute radiation sickness. The 31 deaths were
  mainly among fire fighters who fought the first
  fire without radiation protection.
• The major health hazards came from iodine 131
  (I-131) , which accumulates in the thyroid gland,
  and cesium 137 (Cs-137), which gathers in bone.
  The accident released 1,760 10 15 Becquerels of
  I-131 and 85 10 15 Becquerels of Cs-137.

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Radiation Damage Now and Later

• Ionizing radiation tears molecules apart by
  breaking chemical bonds.
• Damage to proteins and nucleic acids are
  particularly detrimental because of their
  importance in body functions and structures.

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• Nucleic acids in DNA can be damaged in two ways
• Mutations result in the synthesis of altered
  proteins. Can result in killing the cell or if
  its a sperm or ovum, cause birth defects.
• If the exposure is high enough and proteins
  cannot be synthesized, death follows.

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Sources of Radiation

• Cosmic rays
• Radioisotopes from rocks and soils
• Radioisotopes from the atmosphere
• Fallout from nuclear weapons testing
• Increased exposure to cosmic rays through air
  travel.
• Radioisotopes released from nuclear power
  technologies.

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Radioisotopes in the Environment

• On average, people in the U.S. receive above 360
  mrem per year, of which 82 is naturally
  occurring.
• Radiation standard by fed. Gov. is 500 mrem per
  year.
• Maximum exposure recommended by federal
  government is 5000 mrem (.5 rem) per year.

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Radioisotopes in the Human Body
Thyroid I-131
Lungs Rn-222 U-238 Pt-239 Kr-85
Muscles K-40 Cs-137
Liver Co-60
Bones Ra-226 Sr-90 P-32 C-14
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Radon in Homes

• Radon is a decay product of uranium and is always
  present in the atmosphere.
• Some radon dissolves into groundwater from the
  soil.
• Inhaled radon gas goes through radioactive decay
  causing health problems such as lung cancer.

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The Discovery of Radioactivity
Use with worksheet (910) Becquerel Rays
(910)