Radioactivity

1
Radioactivity

• Physical Science

2
Radioactivity

• Radioactivity is defined as the spontaneous
  release of both particles and radiation (energy)
  due to an unstable nucleus.
• Atoms with too many or too few neutrons are
  unstable and may undergo decay.
• Nuclei with more than 83 protons are always
  unstable.

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Health Issues

• Ionizing radiation may produce changes in living
  tissue, hair loss, skin lesions, burns
• Radiation sickness nausea, vomiting, malaise,
  hemorrhage, lowering of bodys resistance to
  disease and infection, death
• Extreme case fatal, increased chance of cancer,
  developmental effects on unborn children

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Units of Measure

• REM measures radiation absorbed by man or
  animals which produces a physiological effect
  equivalent to absorption of one roentgen of
  radiation
• mrem millirem
• lt 10 000 mrem no statistical effect
• gt 10 000 lt 100 000 mrem grey area
• gt 100 000 mrem statistical effects

5
Natural Radioactivity

• Alpha particles charged particle similar to
  stripped helium nuclei
• Can be stopped by skin or sheet of paper
• Problem is with ingestion
• Ionize matter (remove electrons) as they pass
  through matter
• Positively charged (2), weight (4)

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Natural Radioactivity

• Beta particles
• Negatively charged, high speed electrons
• Can be stopped by aluminum foil
• Can ionize atoms causing them to lose energy
• Comes from neutron decay

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Natural Radioactivity

• Gamma Radiation from nuclear excitation as
  electromagnetic energy
• Gamma rays can only be stopped by several inches
  of lead or a foot of concrete
• As radiation, has no charge or mass but is energy
  and travels at speed of light
• Does not ionize matter but can cause damage

8
Neutron Emission

• Neutrons do not ionize matter. This allows them
  to travel farther through matter than alphas or
  beta particles.
• Takes 15 cm of lead to stop most neutrons.

9
Chain Reactions

• This occurs when one atom fissions and the
  products cause the fission of additional atoms.
• Uncontrolled chain reactions are called bombs.
• Neutrons bounce off lead elastically (must use
  cadmium/boron as absorbers)

10
Nuclear Reactors

• Slow neutrons are directed at enriched uranium to
  create controlled chain reaction. The neutrons
  release continue the chain reaction. Control
  rods absorb the neutrons to slow/stop process.
• There is never enough uranium-235 (called
  critical mass) to allow uncontrolled reaction.
• Critical mass minimum amount of fissionable
  isotope that can chain react

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Decay Schemes

• Alpha decay lose 2 protons and 2 neutrons
  (24He)
• Beta decay neutron decays into proton and
  release of Beta particle neutrino
• (-10e)
• Electron capture electron combines with proton
  to form neutron (10n)

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Fission/Fusion

• Fission bombardment of atomic nucleus with
  neutrons can result in splitting of nucleus
  releasing neutrons and energy
• Fusion two smaller nuclei fuse at high
  temperatures to form a larger nucleus and release
  large amounts of energy
• See examples of decay schemes and fission/fusion
  problems in class

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Fusion Reactors

• Abundant fuel in hydrogen
• Bad produce fast, dangerous neutrons
• Shielding would have to be replaced often
• Lithium, boron, cadmium can slow down neutrons
  but are rare and can be chemically reactive.

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Fission Reactors

• Produce electricity at a cheap rate
• Safe, at least in United States
• Products must be handled carefully and waste can
  have long half-lives
• Plants very expensive to build and have short
  life-span (as little as 40 years)

15
Added Notes

• Unstable nuclei are called radioisotopes.
• These may occur naturally or be produced
  artificially.
• Geiger counters are used to measure levels of
  radioactivity.

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Background Radiation

• Living tissues of most organisms are adapted to
  survive low levels of natural nuclear radiation.
• Nuclear radiation can cause burns to skin,
  destroy bone marrow cells and can severely damage
  delicate lining of bodys organs.
• Serious exposure loss of white blood cells,
  hair loss, sterility, bone necrosis, and cancer
• DNA damage repairs may be made improperly

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Beneficial Uses of Nuclear Radiation

• Smoke alarms
• Controlled doses help treat cancer by killing
  tumor cells
• Tracers test how fast water moves through soil or
  crops can look at underground water flow
• Power production

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Cabon-14 Radioactive Dating

• Carbon-14 is useful in dating organic materials
  up to about 50 000 years old.
• Nitrogen in atmosphere undergoes electron capture
  to form radioactive carbon-14. Living organisms
  maintain about 13 of this in their tissues.
  Upon death it undergoes beta decay at constant
  rate (t1/2 5730 years).

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Half-Life Calculations

• Amount of radioactive material remaining in a
  sample starting amt / 2n where n number
  of half-lives
• 40 g / 25 40g/32 1.25 g remaining of original
  40 grams after 5 half-lives