QUIZ

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QUIZ7, HW7 Next week Tuesday
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Fission
Those who cannot learn from history are
condemned to repeat it.
The first use of fission was for war. A
visionary named Leo Szilard saw this possibility
more than 20 years before it occurred. He tried
to PATENT the fission chain reaction.
The discovery of fission
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In order to study nuclei, we throw things at them
and see what we get
Alpha particle
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In 1933, Joliot-Curie bombarded a thin Aluminum
foil with alpha-particles and created an isotope
of phosphorous that was radioactive (never
observed before).
Aluminum Foil
Alpha particle
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At about the same time, scientists bombarded
Beryllium (Be) with alpha particles and
discovered a new particle (chargeless, and about
the same mass as a proton).
Beryllium
NEUTRONS!
Alpha particle
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Alpha particle
Some nucleus
What is the force between the alpha-particle and
the nucleus?
They repel since they have the same sign of
charge!
Does this make it harder or easier for them to
collide?
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Can you think of any particle that would not feel
a repulsive force when it got close to the
nucleus?
Some nucleus
Neutron
In 1934, Enrico Fermi bombarded all elements up
through Uranium with Neutrons to see what would
happen.
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In 1934, Enrico Fermi bombarded all elements up
through Uranium with Neutrons to see what would
happen. No conclusions were made.
In 1938, Joliot-Curie reported that the neutron
bombardment of 92U produced an element like
Lanthanum 57La. This could not be explained!
World War II was starting up!
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Bohrs Liquid Drop model of the nucleus
The nucleons in a nucleus were held together like
the molecules in a drop of water
This led to the following description of fission
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Neutron

Kinetic Energy

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If one of these fission fragments was 56Ba
(barium) and the original nucleus was 92U
(uranium), what must the other fragment be?
36Kr
92U
56Ba
?
36Kr
92 56 36
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56Ba
92U
36Kr
Should the components Ba Kr have the same mass
as the original Uranium nucleus?
They DO NOT!! The fragments weigh LESS than the
original nucleus! What happened to the missing
mass??
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Einstein Again
E mc2
The missing mass was converted to energy!!
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Leo Szilard thought, well before WWII, that if
such a process produced a free neutron, that
neutron could cause another nucleus to fission,
and so on and so on
56Ba
neutron
92U
36Kr
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U
U
U
U
You cant get very much energy from this process..
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What if each fission process emitted its nuclear
fragments 2 neutrons??
BOOM
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In turns out that between 2 and 4 neutrons are
emitted in every fission process!! What a
different world it would be if only one or less
were emitted!!
It turns out that you have to have a minimum
amount of material to make this process possible
this is called the critical mass.
Uranium Mc 15 kg (about the size of a
grapefuit)
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It turns out that you have to have a minimum
amount of material to make this process possible
this is called the critical mass.
Uranium Mc 15 kg (about the size of a
grapefuit) Plutonium 5 kg (the size of an
orange)
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It turns out that
238
U
Does not fission very well.
92
However, its isotope does
235
U
92
Less than 1 of all uranium!
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235
U
92
Less than 1 of all uranium!
Almost all of the technology that is required to
make a nuclear fission weapon is focused of
separating the uranium isotopes.
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The code name for the project to build the atomic
bomb in WWII was called the Manhattan project
The technological requirements for the bomb
design was relatively simple.
Every country that has tested a nuclear weapon
has succeeded the FIRST TIME!
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Nuclear Proliferation The spreading of nuclear
weapons technology to more and more countries.
This really boils down to the transport of
weapons grade material and isotope separation
technology to countries that do not presently
have it.
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Another nuclear related threat
A conventional (TNT) bomb that is packed with
dangerous RADIOACTIVE materials.
Dirty Nuke
A fission bomb is used to start a fusion reaction
(much, much more powerful than a fission bomb).
Thermonuclear weapon
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Thermonuclear Weapon (FUSION)
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Calculating your grades
For each category, take the total number of
points that you got and divide that by the total
points possible. Multiply this number by the
percentage (25 for labs, etc.) Do this for each
category and add them up. Then compare to the
grade scale
Labs 25 Homework 20 Quizzes 30 In-Class
5 Final 20
85-100 A 75-84.9 B 65-74.9 C 55-64.9 D