Unit 9, Chapter 30

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Unit 9, Chapter 30
CPO Science Foundations of Physics
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Unit 9 The Atom
Chapter 30 Nuclear Reactions and Radiation

• 30.1 Radioactivity
• 30.2 Radiation
• 30.3 Nuclear Reactions and Energy

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Chapter 30 Objectives

1. Describe the cause and types of radioactivity.
2. Explain why radioactivity occurs in terms of
   energy.
3. Use the concept of half-life to predict the decay
   of a radioactive isotope.
4. Write the equation for a simple nuclear reaction.
5. Describe the processes of fission and fusion.
6. Describe the difference between ionizing and
   nonionizing radiation.
7. Use the graph of energy versus atomic number to
   determine whether a nuclear reaction uses or
   releases energy.

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Chapter 30 Vocabulary Terms

• radioactive
• alpha decay
• beta decay
• gamma decay
• radiation
• isotope
• radioactive decay
• energy barrier intensity
• inverse square law

• shielding
• fission reaction
• CAT scan
• ionizing
• nonionizing
• ultraviolet
• fusion reaction
• Geiger counter
• rem
• nuclear waste

• neutron
• antimatter
• x-ray
• neutrino
• background radiation
• dose
• fallout
• detector
• half-life

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30.1 Radioactivity

• Key Question
• How do we model radioactivity?

Students read Section 30.1 AFTER Investigation
30.1
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30.1 Radioactivity

• The word radioactivity was first used by Marie
  Curie in 1898.
• She used the word radioactivity to describe the
  property of certain substances to give off
  invisible radiations that could be detected by
  films.

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30.1 Radioactivity

• Scientists quickly learned that there were three
  different kinds of radiation given off by
  radioactive materials.
• Alpha rays
• Beta rays
• Gamma rays
• The scientists called them rays because the
  radiation carried energy and moved in straight
  lines, like light rays.

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30.1 Radioactivity

• We now know that radioactivity comes from the
  nucleus of the atom.
• If the nucleus has too many neutrons, or is
  unstable for any other reason, the atom undergoes
  radioactive decay.
• The word decay means to "break down."

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30.1 Radioactivity

• In alpha decay, the nucleus ejects two protons
  and two neutrons.
• Beta decay occurs when a neutron in the nucleus
  splits into a proton and an electron.
• Gamma decay is not truly a decay reaction in the
  sense that the nucleus becomes something
  different.

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30.1 Radioactivity

• Radioactive decay gives off energy.
• The energy comes from the conversion of mass into
  energy.
• Because the speed of light (c) is such a large
  number, a tiny bit of mass generates a huge
  amount of energy.
• Radioactivity occurs because everything in nature
  tends to move toward lower energy.

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30.1 Radioactivity

• If you started with one kilogram of C-14 it would
  decay into 0.999988 kg of N-14.
• The difference of 0.012 grams is converted
  directly into energy via Einsteins formula E
  mc2.

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30.1 Radioactivity

• Systems move from higher energy to lower energy
  over time.
• A ball rolls downhill to the lowest point or a
  hot cup of coffee cools down.
• A radioactive nucleus decays because the neutrons
  and protons have lower overall energy in the
  final nucleus than they had in the original
  nucleus.

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30.1 Radioactivity

• The radioactive decay of C-14 does not happen
  immediately because it takes a small input of
  energy to start the transformation from C-14 to
  N-14.
• The energy needed to start the reaction is called
  an energy barrier.
• The lower the energy barrier, the more likely the
  atom is to decay quickly.

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30.1 Radioactivity

• Radioactive decay depends on chance.
• It is possible to predict the average behavior of
  lots of atoms, but impossible to predict when any
  one atom will decay.
• One very useful prediction we can make is the
  half-life.
• The half-life is the time it takes for one half
  of the atoms in any sample to decay.

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30.1 Half-life

• The half-life of carbon-14 is about 5,700 years.
• If you start out with 200 grams of C-14, 5,700
  years later only 100 grams will still be C-14.
• The rest will have decayed to nitrogen-14.

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30.1 Half-life

• Most radioactive materials decay in a series of
  reactions.
• Radon gas comes from the decay of uranium in the
  soil.
• Uranium (U-238) decays to radon-222 (Ra-222).

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30.1 Applications of radioactivity

• Many satellites use radioactive decay from
  isotopes with long half-lives for power because
  energy can be produced for a long time without
  refueling.
• Isotopes with a short half-life give off lots of
  energy in a short time and are useful in medical
  imaging, but can be extremely dangerous.
• The isotope carbon-14 is used by archeologists to
  determine age.

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30.1 Carbon dating

• Living things contain a large amount of carbon.
• When a living organism dies it stops exchanging
  carbon with the environment.
• As the fixed amount of carbon-14 decays, the
  ratio of C-14 to C-12 slowly gets smaller with
  age.

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30.1 Calculating with isotopes

• A sample of 1,000 grams of the isotope C-14 is
  created.
• The half-life of C-14 is 5,700 years.
• How much C-14 remains after 28,500 years?

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30.2 Radiation

• Key Question
• What are some types and sources of radiation?

Students read Section 30.2 AFTER Investigation
30.2
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30.2 Radiation

• The word radiation means the flow of energy
  through space.
• There are many forms of radiation.
• Light, radio waves, microwaves, and x-rays are
  forms of electromagnetic radiation.
• Many people mistakenly think of radiation as only
  associated with nuclear reactions.

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30.2 Radiation

• The intensity of radiation measures how much
  power flows per unit of area.
• When radiation comes from a single point, the
  intensity decreases inversely as the square of
  the distance.
• This is called the inverse square law and it
  applies to all forms of radiation.

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30.1 Intensity
Power (watt)
I P A
Intensity (W/m2)
Area (m2)
Intensity 7.96 W/m2
Intensity 1.99 W/m2
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30.2 Harmful radiation

• Radiation becomes harmful when it has enough
  energy to remove electrons from atoms.
• The process of removing an electron from an atom
  is called ionization.
• Visible light is an example of nonionizing
  radiation.
• UV light is an example of ionizing radiation.

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30.2 Harmful radiation

• Ionizing radiation absorbed by people is measured
  in a unit called the rem.
• The total amount of radiation received by a
  person is called a dose, just like a dose of
  medicine.
• It is wise to limit your exposure to ionizing
  radiation whenever possible.
• Use shielding materials, such as lead, and do
  your work efficiently and quickly.
• Distance also reduces exposure.

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30.2 Sources of radiation

• Ionizing radiation is a natural part of our
  environment.
• There are two chief sources of radiation you will
  probably be exposed to
• background radiation.
• radiation from medical procedures such as x-rays.
• Background radiation results in an average dose
  of 0.3 rem per year for someone living in the
  United States.

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30.2 Background radiation

• Background radiation levels can vary widely from
  place to place.
• Cosmic rays are high energy particles that come
  from outside our solar system.
• Radioactive material from nuclear weapons is
  called fallout.
• Radioactive radon gas is present in basements and
  the atmosphere.

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30.2 X-ray machines

• X-rays are photons, like visible light photons
  only with much more energy.
• Diagnostic x-rays are used to produce images of
  bones and teeth on x-ray film.
• Xray film turns black when exposed to x-rays.

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30.2 X-ray machines

• Therapeutic x-rays are used to destroy diseased
  tissue, such as cancer cells.
• Low levels of x-rays do not destroy cells, but
  high levels do.
• The beams are made to overlap at the place where
  the doctor wants to destroy diseased cells.

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30.2 CAT scan

• The advent of powerful computers has made it
  possible to produce three-dimensional images of
  bones and other structures within the body.
• To produce a CAT scan, computerized axial
  tomography, a computer controls an x-ray machine
  as it takes pictures of the body from different
  angles.

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30.2 CAT scan

• People who work with radiation use radiation
  detectors to tell when radiation is present and
  to measure its intensity.
• The Geiger counter is a type of radiation
  detector invented to measure x-rays and other
  ionizing radiation, since they are invisible to
  the naked eye.

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30.3 Nuclear Reactions and Energy

• Key Question
• How do we describe nuclear reactions?

Students read Section 30.3 AFTER Investigation
30.3
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30.3 Nuclear Reactions and Energy

• A nuclear reaction is any process that changes
  the nucleus of an atom.
• Radioactive decay is one form of nuclear reaction.

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30.3 Nuclear Reactions and Energy

• If you could take apart a nucleus and separate
  all of its protons and neutrons, the separated
  protons and neutrons would have more mass than
  the nucleus did.
• The mass of a nucleus is reduced by the energy
  that is released when the nucleus comes together.
• Nuclear reactions can convert mass into energy.

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30.3 Nuclear Reactions and Energy

• When separate protons and neutrons come together
  in a nucleus, energy is released.
• The more energy that is released, the lower the
  energy of the final nucleus.
• The energy of the nucleus depends on the mass and
  atomic number.

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30.3 Fusion reactions

• A fusion reaction is a nuclear reaction that
  combines, or fuses, two smaller nuclei into a
  larger nucleus.
• It is difficult to make fusion reactions occur
  because positively charged nuclei repel each
  other.

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30.3 Fusion reactions

• A fusion reaction is a nuclear reaction that
  combines, or fuses, two smaller nuclei into a
  larger nucleus.

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30.3 Fission reactions

• A fission reaction splits up a large nucleus into
  smaller pieces.
• A fission reaction typically happens when a
  neutron hits a nucleus with enough energy to make
  the nucleus unstable.

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30.3 Fission reactions

• The average energy of the nucleus for a
  combination of molybdenum-99 (Mo-99) and tin-135
  (Sn-135) is 25 TJ/kg.
• The fission of a kilogram of uranium into Mo-99
  and Sn-135 releases the difference in energies,
  or 98 trillion joules.

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30.3 Rules for nuclear reactions

• Nuclear reactions obey conservation laws.
• Energy stored as mass must be included in order
  to apply the law of conservation of energy to a
  nuclear reaction.
• Nuclear reactions must conserve electric charge.
• The total baryon number before and after the
  reaction must be the same.
• The total lepton number must stay the same before
  and after the reaction.

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30.3 Conservation Laws

• There are conservation laws that apply to the
  type of particles before and after a nuclear
  reaction.
• Protons and neutrons belong to a family of
  particles called baryons.
• Electrons come from a family of particles called
  leptons.

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30.3 Calculating nuclear reactions

• The nuclear reaction above is proposed for
  combining two atoms of silver to make an atom of
  gold.
• This reaction cannot actually happen because it
  breaks the rules for nuclear reactions.
• List two rules that are broken by the reaction.

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30.3 Antimatter, neutrinos and others particles

• The matter you meet in the world ordinarily
  contains protons, neutrons, and electrons.
• Cosmic rays contain particles called muons and
  pions.
• Thousands of particles called neutrinos from the
  sun pass through you every second and you cannot
  feel them.

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30.3 Antimatter, neutrinos and others particles

• Every particle of matter has an antimatter twin.
• Antimatter is the same as regular matter except
  properties like electric charge are reversed.
• An antiproton is just like a normal proton except
  it has a negative charge.
• An antielectron (also called a positron) is like
  an ordinary electron except that it has positive
  charge.

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30.3 Neutrinos

• When beta decay was first discovered, physicists
  were greatly disturbed to find that the energy of
  the resulting proton and electron was less than
  the energy of the disintegrating neutron.
• The famous Austrian physicist Wolfgang Pauli
  proposed that there must be a very light,
  previously undetected neutral particle that was
  carrying away the missing energy.
• We now know the missing particle is a type of
  neutrino.

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30.3 Neutrinos

• Despite the difficulty of detection, several
  carefully constructed neutrino experiments have
  detected neutrinos coming from nuclear reactions
  in the sun.

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Application Nuclear Power