Nuclear Physics
atom – the smallest particle of an element that
retains the chemical properties of that element
Atom Basics
The number of protons determines the element
An atom is composed of…
•
Nucleons
– Protons
Subatomic
– Neutrons
Particles
•
Electrons
Neutrons and protons are about the same weight
A electron
An
l t
is
i about
b t 1/2000 th
the mass off a proton
t
A teaspoon of nuclei would weigh more than a
battleship!
The nucleus is held together by nuclear forces
Nucleons – protons and electrons in the nucleus
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Subatomic Particles
Atom Basics
Symbol Charge Mass(u)
The electrons determine the size of the atom
e-
-1
≈0
p+
+1
1
n0
0
1
Electron
Electrons move so fast in such a tiny area they
make the atom seem solid (Like a moving fan
blade)
Size: If the nucleus was a marble… then the entire
atom would be the size of a football field.
Sizes of atoms are measured in picometers
1,000,000,000,000 picometers = 1 meter
Proton
1
1
Neutron
1
0
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Basics of Atoms
Reading a Periodic Table
Atomic Number
“Z Number”
((number of p
protons))
Atomic Mass
(weighted average)
1) A neutral atom has the same number of electrons as
it has protons.
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Li
Lithium
6.941
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Symbol
2) Atoms of the same element can have different
masses due
d tto different
diff
t numbers
b
off neutrons.
t
Name
3) Elements with different masses are called isotopes.
Nuclide is a term for an isotope.
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Nuclear Physics
Designating Isotopes
Designating Isotopes
Mass Number = Protons + Neutrons
There are two common ways to present isotopes
Example: an isotope of uranium has a mass of 235 and
an atomic number of 92.
→ Mass Number – Protons = Neutrons
Example: an isotope of uranium has a mass of 235 and
an atomic number of 92. How many neutrons?
mass
Z
235 – 92 = 143
mass
Nuclear Symbol
protons neutrons
235
92
U
Hyphen Notation
Uranium-235
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How to remember Nuclear Symbol
Correct
235
92
Isotopes of hydrogen
Electron
Incorrect
92
235
U
U
Proton
The nuclear symbol should look like an easy
subtraction problem to calculate neutrons.
235
92
143
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Neutron
Protium
Deuterium
Tritium
mass = 1
mass = 2
mass = 3
H
2
1
0.015%
1
1
99.985%
U
H
9
H
3
1
Rare &
radioactive
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Relative Atomic Mass – a separate unit of
measurement is used for mass of
individual atoms.
Heavy Water
Water composed of deuterium hydrogen is
often labeled D2O rather than H2O. Because
each molecule is heavier but still the same
size, D2O is more dense.
amu – Atomic Mass Unit (u)
I cubes
Ice
b off D2O in
i liliquid
id H2O will
ill sink!!
i k!!
1 amu = 1.660540 x 10-27 kg
The amu is based off 1/12th the mass of Carbon-12
Regular ice
Proton – 1.007276 u
Neutron – 1.008665 u
D2O ice
$65 / 100g
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2
Nuclear Physics
Nuclear Force – force that acts between
protons and neutrons.
Binding Energy – The difference in energy
between individual nucleons and an
assembled nucleus (always negative)
This is about 100 times stronger than the
electromagnetic
l t
ti fforce that
th t causes protons
t
to repel one another.
Nuclear forces only act at a distance of about
the radius of a proton.
Binding
Bi
di energy iis ttypically
i ll given
i
iin electron
l t
volts (eV) or million electron volts (MeV)
1 eV = 1.60217 x 10-19 J
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Mass Defect - The difference in mass between the total
of nucleons and the assembled nucleus.
E  mc
Find the binding energy of 42 He in MeV
The mass of a helium-4 nucleus is 4.002603 u. There
are 2 neutrons and 2 protons.
Einstein related the binding energy to the mass
defect.
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E = energy
m = mass defect
c = 3 x 108 m/s
Find the nucleon masses
Protons 2 x 1.007276
1 007276 u
Neutron 2 x 1.008665 u
4.031882 u
Find the Mass Defect
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4.002603
002603 u
- 4.031882 u
- 0.029279 u
Convert using the binding energy of 1 u
E = (-0.029279 u)(931.49 MeV / u) = -27.273 MeV
Binding Energy for 1 u = 931.49 MeV
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Binding Energy and Stability
The most tightly bound nucleus is iron-56.
Nuclear reactions for elements lighter than 56 will
tend to increase the mass. Stars will transform
hydrogen into helium,
helium carbon
carbon, and other heavier
elements.
Nuclear reactions with for heavier elements will tend
to decrease the mass. An atom of Uranium-238
will decay into thorium-232.
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3
Nuclear Physics
Three Types of Decay
Penetration Power of Radiation
Alpha Emission – A helium nucleus (positively charged) is
emitted from a large atom. This will drop the mass by four
and the atomic number by two. The extra electrons from
the large atom are quickly absorbed into the environment.
Beta Emission – There are three types of beta emission.
This will not effect the mass number but will affect the
number of protons or neutrons.
Gamma emission – This will involve the release of energy
to stabilize the atom. This often accompanies alpha or beta
decay.
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Band of Stability
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Alpha Emission (  )
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Beta Emission (  )
Electron Emission
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Beta Emission (  )
Positron Emission
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Nuclear Physics
Beta Emission (  )
Electron Capture
Gamma Emission (  )
This often accompanies alpha and beta decay.
This releases excess energy without affecting
the mass or atomic number.
Gamma
Emission
Beta Decay
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Thorium – 232 Decay Series
Half Life
Half Life is the time it takes for half of the atoms of
an isotope to decay.
To find the amount of material remaining:
t
 1  t1 / 2
A  Ao  
2
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A = amount
Ao = initial amount
t = time
t1/2 = half life
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Nuclear Physics
Smoke Detectors
Smoke detectors use Americium-241 as an alpha source
to continually ionize the air between two charged plates.
This allows current to flow between them.
Smoke particles neutralize the air particles and stop the
electric current
current. This will set off the alarm
alarm.
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Nuclear Fission
Nuclear Fission
Large amounts of energy are produced by fission
Fission is the process of splitting an atomic nucleus
into fission fragments of smaller atomic nuclei and
neutrons.
1
0
92
141
1
n  235
92 U  36 Kr  56 Ba  3 0 n  200 MeV
An atom that can be split called fissionable.
Fissile nuclei a special type of fission that can have a
chain reaction. Not all fissionable nuclei are fissile.
Fissile nuclei are generally heavy atoms with large
numbers of nucleons. The nuclei are struck by
neutrons initiating the fission process.
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Nuclear Chain Reaction
A classic example of a fission reaction is that of UU235 where stray neutron strikes an atom of UU-235.
It absorbs the neutron and becomes an unstable
atom of UU-236.
236 This undergoes fission.
fission
More neutrons are released in the reaction. These
neutrons can strike other UU-235 atoms to initiate
their fission.
Uranium 238 on the other hand is not fissile 
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Nuclear Physics
Natural Fission
Nuclear Power Generation
The fission process is an a natural one as a French
researcher found a natural uranium reactor in Gabon, West
Africa; it has been estimated to be over 2 billion years old.
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Fuel Rods
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Control Rods
A fuel assembly consists of a square array of 179 to
264 fuel rods, and 121 to 193 fuel assemblies are
loaded into an individual reactor. A reactor could have
over 50,000 fuel rods storing 200 tons of uranium!!
Control rods slide in and out between fuel rods. They
are used to moderate the nuclear reaction by
absorbing neutrons. When completely inserted, they
can absorb enough neutrons to stop the chain reaction.
Cadmium, Indium, Silver and a variety of rare earth
metals are used for control rods.
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Nuclear Fusion on the Sun:
Proton-Proton Chain
Fusion
Fusion is a nuclear reaction where two light atomic
nuclei fuse or combine to form a single heavier nucleus.
Fusion reactions do not occur naturally on our planet
but are the principal type of reaction found in stars
The large masses, densities, and high temperatures of
stars provide the initial energies needed to fuel fusion
p
forces of p
protons,, fusion will only
y
Due to the repulsive
take place when nuclei have extreme high thermal
energy. The proton-proton chain of the Sun requires
temperatures at about 20,000,000 K !!!
Fusion on the Sun involves turning four protons into a
helium nucleus. This process gives off 25 MeV.
The fusion reaction generates a tremendous amount of
energy due to protons losing mass to form larger nuclei.
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By comparison, one dynamite molecule can give of
about 20 eV, or less than one millionth the energy.
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Nuclear Physics
Nuclear Fusion on the Sun
Proton-Proton Chain
Attempting Nuclear Fusion
Nuclear fusion has been made possible on Earth using
a fission reaction for ignition.
Because temperatures required for fusion are so great,
confining and harnessing power has not been
achieved.
Currently the most powerful nuclear devices are
hydrogen bombs, also called thermonuclear bombs.
These use a fission reaction to start the fusion process.
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Hiroshima
Little Boy – Messed up Hiroshima
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Little Boy
Play Video
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Little Boy
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Nuclear Physics
Fat Man
Fat Man – Messed up Nagasaki
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Fat Man
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Mushroom Clouds
Cooler,
Denser Air
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Natural Mushroom Clouds
Mount Redoubt Eruption - Alaska
Hot, Less
Dense Air
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Hydrogen Bomb (Thermonuclear)
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Nuclear Physics
Castle Bravo Cloud – Bikini Island 1954
Hydrogen Bomb Operation
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Castle Bravo Cloud – Crater Play Video
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Relative Bomb Strengths
1 Mile
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Tsar Bomba – The Biggest Play Movie
About 57 megatons – 14 miles high
Fireball Size
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Nuclear Physics
Fission Process
Fission Chain Reaction
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Energy Released
E = mc2
Energy
Change in Mass
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Speed of Light
3 x 108 m/s
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Castle Bravo Cloud – Bikini Island 1954
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