Nuclear Changes

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Nuclear Changes

• (This is the stuff that can make you glow in the
  dark.)
• 100 trillion watts of fusion power...

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7.1 What is radioactivity?

• Some elements, particularly those with very large
  nuclei (note those at the bottom of the Periodic
  table), tend to have an unstable nucleus.
• Occasionally they lose parts or pieces in the
  form of particles and energy.
• The process of an unstable nucleus emitting
  particles or energy is called radioactivity.
• The charged particles or energy released is
  called nuclear radiation.
• There are four types of Nuclear Radiation Alpha
  particles, Beta particles, Gamma rays, and
  Neutrons. These particles fly out of the nucleus
  and interact with surrounding matter, depending
  on their charge, energy and mass. (see Table7.1)

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• Alpha particles- consist of two protons and two
  neutrons (2 charge). They are essentially
  helium nuclei.
• They are very large for a subatomic particle and
  dont penetrate very far into materials. They
  can travel thru a piece of paper.
• Alpha particles ionize matter (take away
  electrons) as they pass thru it.
• Example emitted by Uranium-238.

Small carbon-covered silicon- carbide fuel
pellets for use in the Modular Helium Reactor, a
power system meant to be safer than present
nuclear power plants. A penny provides size
comparison.
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• Beta particles- are -1 charged, and essentially a
  high speed free electron.
• Formed by neutrons decaying to form a proton and
  an electron. Electron is then ejected from the
  nucleus.
• More penetratinggo thru paper easily, but
  stopped by 3mm of Al or 10mm of wood.
• Able to move fast, but ionize other materials and
  slow down in them.

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• Gamma rays- no mass, no charge. This isnt made
  of matterits energy in an electromagnetic wave,
  like light, emitted by nucleus.
• Very powerful wave, very penetrating (more than
  alpha and beta). Can go thru 60cm of Al or 7cm
  of Pb.
• Pose risk to health, due to penetrating
  qualities.
• Ex found in radium, (discovered by Marie Curie).

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• Neutron radioactivity occurs in a neutron-rich
  nucleus.
• A high-energy neutron is emitted from the nucleus
  in its decay.
• Neutrons have no charge, so theres no charge to
  slow it down.
• Neutron radiation is the most penetrating
  radiation formcan be very dangerous. It takes
  about 15cm of solid lead shielding to stop most
  neutron radiation.

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• When an unstable nucleus emits alpha or beta
  particles, the number of protons or neutrons
  changes (see p.223.in beta a neutron turns into
  a proton).
• Not a chemical equationchanging elements.
  However, notice that the masses still add up
  equally on each sidemass is still accounted for.
• Ra -----gt Rn He
• Beta decay equation works same/no mass/-1.
• C -----gt N e
• Gamma rays no change in atomic , only energy
  level of the nucleus changes.
• Neutron emission discussed later..

Mass
Alpha
226 88
222 86
4 2
226 222 4 88 86 2
Atomic
14 6
14 7
0 -1
14 14 0 6 7 (-1)
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• It is possible to predict the age of rock by
  looking at its radioactive decay.
• Possible to accurately predict time for 1/2 the
  radioactive part of a rock to decay. This is
  called its half-life.
• After its first half-life is over, half of the
  remaining 50 will decay over the next half-life
  (down to a quarter of the original). The
  radioactive mat. decays into new element.
• Of that quarter, half will decay over the next
  half-life.
• In short, a predictable pattern of change
  results. If one knows how much radioactive
  material was present to start with, one can
  predict how old the object is.
• Radioactive materials can have 1/2lives from part
  of a second to billions of yearsdifferent ones
  are used for measuring different times.
  Carbon-14 is used for once-living organisms.
  (see p.226-227 and Table 7-2)

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Time out Why are nuclear issues such strong
issues with environmentalists and humanitarians?
8/12/45, Nagasaki after atomic explosion...
8/7/45, Nagasaki before atomic explosion.
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7.2 Nuclear Fission and Fusion

• Protons and neutrons are tightly packed into
  nucleus.
• Some nuclei are unstable and decay. There may be
  stable and unstable isotopes of an element.
• Stability of a nucleus depends on nuclear force
  that holds it togetheracts between protons and
  neutrons.
• Force of interaction between protons and neutrons
  that holds nucleus together is called the strong
  nuclear force. Much stronger than repulsion
  between protons, but acts over a very short
  distancewidth of 3 protons.
• Neutrons help stabilize a nucleus, but too many
  or too few will cause instabilitydecay.
  Anything over 83 protons is always
  unstabledecayrelease energized particlesrest
  of energy emitted as gamma rays.

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Nuclear Fission

• Fission is the process of a nucleus splitting
  into 2 or more smaller pieces.
• U-235 hit by a neutron breaks into Ba-137 and
  Kr-84 and releases 15n and energy. (can split to
  make several combos).
• Produces large amounts of energy (HIGH order of
  magnitude).
• When measured, some mass turns out missing. Its
  exception to the law of consv. of mass/energy,
  because some of the mass is turned into energy!

1st atomic bomb-Hiroshima
Fat man bomb -Nagasaki
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• Albert Einstein (1905) actually explained this in
  the Mass-energy equation.
• E mc2
• Energymass X (spd.of light)2
• Mass and energy can be converted into each other.
• c is constant and very large, so even a small
  amount of mass will equal a very large amount of
  energy.
• Mass is usually very stable (thank goodness)
  except concentrations of large, unstable element
  nuclei.

March 26, 1954, Bikini atoll, 11 megaton
explosion.
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• Neutrons released by fission can run into other
  nuclei and cause them to fission, releasing more
  neutrons.chain reaction.
• Nuclear bombs work on a nuclear chain reaction
  principle. Two or more masses of U-235 are
  inside. An explosion crushes them close together
  to make a critical mass that will start and
  sustain a chain reaction. Pure, weapons-grade
  U-235 is very dangerous, b/c it easily starts an
  uncontrolled chain reaction.
• Devastating energy in short time.

Nuclear detonation energy release is very
damaging to environment and life-forms over a
large area. Can leave harmful radioactives
behind.
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Nagasaki bomb damage, 1945.
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• The more neutrons and the more crowded the nuclei
  the more chance of a successful chain reaction.
• Some materials can absorb neutrons and slow a Rx
  down. This slower reaction can be used to
  generate electrical power.
• Graphite rods slow reactions in nuclear plants to
  control the reaction.

Three
Mile Island Cooling Towers During 1979 Accident

Steam blows from the cooling towers of the
Three Mile Island nuclear power plant on the
night of March 28, 1979, during the most serious
accident in U.S. nuclear power history. One of
the plant's reactor cores was exposed after a
series of equipment failures complicated by human
error, resulting in the production of radioactive
gases. People in nearby homes (bottom) were
evacuated for safety, but fortunately most of the
gas was contained.
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Nuclear Fusion

• Energy can also be obtained by combining smaller
  nuclei to make a larger nucleus. (Fusion)
• Stars (sun) use fusion to fuse 4hydrogens to make
  a helium atom and tremendous amounts of energy as
  gamma rays.
• Large amount of energy needed to overcome
  repulsion and push them together (stars,
  heat/pressure).

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7.3 Dangers/Benefits of Nuc. Radiation

• We are exposed to background radiation every day
  from sun, soil, water, plantssource cosmic rays
  and radioactive materials in ground. This is a
  normal occurrence.
• Skin protects us somewhat, but internal damage
  from radon gas or penetrating/excessive radiation
  is dangerous.
• Long periods or hi-intensity exposure is worst,
  can result in radiation sicknesshair loss,
  sterility, cancer, low WBCs, death of bone, etc.
• Penetrating radiation damages DNA. If DNA is
  badly damaged, its nitrogen bases are repaired
  wrongly. Cells cannot function, and reproductive
  cells damagemutations.
• Short-lived isotopes are often used for
  medical/geology/agriculture as radioactive
  tracers to locate infections, water with
  radiation-sensitive detectors.

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Diagnostic Cobalt 60, a radioactive tracer,
glowing in a liquid bath.
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• Small beams of gamma rays can kill tumors.
• Nuclear power does not pollute air, is a
  long-term resource, and is used in dozens of
  countries.
• Nuke power uses U-235, creating radioactive
  byproducts. These must be handled carefully
  closely regulated. Power plants in the U.S. may
  operate for 40 years before dismantling. Many
  operate less due to political opposition. This
  and expense prevent their widespread use.

Nuclear fusion experimental chamber.
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Nuclear fuel waste glowing deep under a liquid
protective bath...
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• Nuclear plants produce waste--dangerous long
  time, must be stored well.
• Bury in very deep shafts. Best if sparsely
  populated, free from earthquakes, far from
  groundwater. (Utah, Nevada) Radioactive for
  100s or 1000000s of years, very hard to ensure
  long-term safety.
• Nuclear fusion, based on hydrogen, seems best
  option. Difficult to do...

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The End
...Of a nuclear fusion fuel pellet imploding
under an ultraviolet laser light.