Extra Lecture - Nuclear Physics

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Transcript Extra Lecture - Nuclear Physics

Goal: To understand the
basics of nuclear physics
1) Atomic number and weight
2) Size of nucleus
3) Fusion
4) Fission
5) Binding Energy
6) How all the elements are made
7) Hydrogen Bomb and other uses
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
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.
Size of a nucleus
• Each proton and neutron gets squeezed
• 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
• 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
Fusion + Fission
• Fusion is taking two atoms and combining them
• This is the power source that powers stars.
• Fission is breaking an atom into more than 1
• This is what powers nuclear plants.
• Often time an “alpha” particle is released –
which is just a helium nucleus.
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
• Each element has some amount of binding
• If you divide this energy by the # of particles in
its nucleus you get a binding energy per
Which to do?
• If you go to higher binding energy with
larger atoms you gain energy through
• If you go to higher binding energy with
smaller atom you gain energy through
• At what point are you no
longer able to get energy?
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!
Heavier than Iron
• Once you get to Iron other processes take
• 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.
• 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.
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
• 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
• We have shown that stars use fusion for
• However it is very tough.
• In the core of our sun it is 100 million
degrees and 100 times the density of
• What usually happens when 2 protons find
themselves on a collision course?
• 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.
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.
• 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
• This gives a runaway affect.
• But only Uranium 235 reacts this way (neutron in means
3 neutrons out).
• 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!
Nuclear plants
• Now you have what you need to generate
• However, the fissioning U235 atoms fire
neutrons which move too fast.
• The U235 atoms can absorb them and not
• So, you have to have some substance to
slow down the neutrons (such as water).
Reactor at Critical!
• Now, how many neutrons do you want?
• If you get less than 1 on average, your reactions won’t 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 – don’t 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.
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
• 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”
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
• This generates a lot of energy – enough to fuse a lighter
element such as Hydrogen (which you can provide using
• 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.
• We learned everything there is to know
about the basics of nuclear power.
• We can now all apply for Homer
Simpson’s job 
• (picture from wikipedia)