Goal: To understand the basics of nuclear physics

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Goal To understand the basics of nuclear physics

• Objectives
• Atomic number and weight
• Size of nucleus
• Fusion
• Fission
• Binding Energy
• How all the elements are made
• Hydrogen Bomb and other uses

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Atomic number (Z)

• The atomic number is the number of protons an
  atom has in its nucleus (and electrons if it is
  not ionized).
• Each different element has its own atomic number.

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Atomic Weight

• Each atom has some number of neutrons.
• The of neutrons is N.
• Most atoms lighter than iron have 1 neutron per
  proton.
• Atoms with a lot more neutrons than protons tend
  to be unstable.

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Size of a nucleus

• Each proton and neutron gets squeezed together.
• The nucleus will almost always have the same
  density which is the density of matter.
• This is 1014 g/cm3 or 100 trillion times the
  density of water.
• As for the radius, since the volume of the
  nucleus depends on the cube of the radius and the
  number of particles therefore
• r r0 A1/3
• And r0 1.2 fm

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Atoms

• The density for the total atom includes the
  electrons, so it is mostly empty space (like
  finding the density of our solar system).
• Larger atoms have their electrons closer to the
  atoms, so their densities are larger.
• So, while atoms all have the same density of
  nucleus, they do not have the same density.

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Fusion Fission

• Fusion is taking two atoms and combining them
  together.
• This is the power source that powers stars.
• Fission is breaking an atom into more than 1
  atom.
• This is what powers nuclear plants.
• Often time an alpha particle is released
  which is just a helium nucleus.

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Binding energy

• It takes some amount of energy to glue the atoms
  together.
• If you slam them together or break them apart you
  can either loose energy or gain energy depending
  on what the new atom needs to be formed.
• Each element has some amount of binding energy.
• If you divide this energy by the of particles
  in its nucleus you get a binding energy per
  nucleon.

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Which to do?

• If you go to higher binding energy with larger
  atoms you gain energy through fusion.
• If you go to higher binding energy with smaller
  atom you gain energy through fission.
• At what point are you no
• longer able to get energy?

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How the elements are made

• There are a few different methods to make
  different elements.
• Hydrogen was formed in the big bang when energy
  formed into quarks and the quarks formed into
  Hydrogen.
• Nuclear fusion in the early universe created most
  of the Helium.
• For all of the other elements iron and lighter
  they were all formed via fusion in the cores of
  massive stars!

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Heavier than Iron

• Once you get to Iron other processes take over.
• The first involves a massive bombardment of
  neutrons onto a Iron atom.
• This is called the fast process.
• The neutrons then decay and emit an electron to
  become a proton.
• This is called beta decay.

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Reverse

• Sometimes a neutron can capture an electron and
  become a proton.
• This is called electron capture.
• So, a heavy Nitrogen atom can capture an electron
  in the nucleus and become a light oxygen atom.

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Alpha Decay

• Some atoms are radioactive.
• What this means is that in some given time
  (called a half life) half of the atoms will
  release a particle (we have seen the beta decay
  example already).
• Usually though a mean lifetime is used, after
  which only 37 of the original atoms stay
  original.
• Many radioactive materials release a helium
  nucleus (2 protons 2 neutrons) in an attempt to
  become more stable.
• A problem with this is that the nucleus is bigger
  than it should be in size.
• This is called an excited state.
• To unexcite itself it will usually emit one or
  more gamma rays.

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Uses

• We have shown that stars use fusion for power.
• However it is very tough.
• In the core of our sun it is 100 million degrees
  and 100 times the density of water.
• What usually happens when 2 protons find
  themselves on a collision course?

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NOTHING!

• Even at that temperature their energy is still
  not enough to collide.
• Eventually their repulsive force brings both to a
  screeching halt and then they go the other way.
• But, it turns out that there is some small chance
  that they are really located somewhere else and
  can fuse thank you quantum mechanics.

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Our uses for fusion

• Well, not much.
• We have tried, but we cannot generate a sustained
  burst which gives more energy that it takes.
• Heck, even for the sun it takes an average of 10
  billion years for each Hydrogen atom to fuse.

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Fission

• We use it for nuclear power.
• We use it for bombs.
• The bombs that were used in WWII would have been
  pure fission bombs.
• Basically you have some radioactive material that
  you hit with a neutron.
• That makes it split into 2 atoms 3 neutrons.
• The resulting neutrons then hit other atoms
  making them split.
• This gives a runaway affect.
• But only Uranium 235 reacts this way (neutron in
  means 3 neutrons out).

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Difficulties

• If you had pure U235 this would be pretty easy,
  but luckily for us U235 comes with a LOT of U238
  which is pretty harmless.
• Also the U238 absorbs neutrons, so if you have a
  normal breakdown (99.3 U238) then the neutrons
  quickly all get eaten up by U238 atoms and the
  chain reaction ends.
• To get it to work you have to enrich the U235
  to make it a few percent.
• But this is EXPENSIVE!

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Nuclear plants

• Now you have what you need to generate power.
• However, the fissioning U235 atoms fire neutrons
  which move too fast.
• The U235 atoms can absorb them and not fission!
• So, you have to have some substance to slow down
  the neutrons (such as water).

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Reactor at Critical!

• Now, how many neutrons do you want?
• If you get less than 1 on average, your reactions
  wont last long and will die out.
• This is called subcritical.
• If you get on average exactly 1 neutron then you
  can keep it going at a constant pace.
• This is called critical and a reactor at
  critical is actually a GOOD thing dont listen
  to Hollywood.
• Will it blow? Well if you produce more than 1
  the reaction rate will INCREASE with time. Other
  than when it is turned on this is very bad. If
  this happens you need to absorb some neutrons by
  inserting a control rod.
• This is called supercritical.

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Problem in N. Korea

• Here is the problem with nuclear power in
  countries that cannot be regulated by the UN
• Sure, you produce energy without greenhouse
  gasses.
• This is good, but
• When the U238 absorbs a neutron (and it will from
  time to time there is a LOT more of it) then it
  will become U239 which is not stable.
• The U239 beta decays into plutonium 239.
• Pu239 can be used to make nuclear weapons!
• This is NOT a good thing clearly.
• These can be called breeder reactors

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H bomb

• The modern version of the atomic bomb uses both
  fission and fusion.
• The first part of the bomb is a fission bomb
  (U235 or Pu239).
• This generates a lot of energy enough to fuse a
  lighter element such as Hydrogen (which you can
  provide using water).
• Yes this takes hundreds of millions of degrees!
• This fusion reaction is an uncontrolled reaction
  and generates even more energy than the original
  bomb.
• This type of bomb destroyed the Bikini atoll.
• Stronger bombs than this were banned by the
  Geneva convention because any stronger than this
  and the blast wave would reach outer space and
  throw some of our atmosphere into space.

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Conclusion

• We learned everything there is to know about the
  basics of nuclear power.
• We can now all apply for Homer Simpsons job ?
• (picture from wikipedia)