Nuclear forces and
Radioactivity
The nucleus is a competition between
opposing forces
Radioactivity is a result of imbalance
between the forces
Learning objectives
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Describe the basic forces and particles involved in
nuclear structure
Describe principles behind nuclear decay and
radioactivity
Describe the particles emitted in nuclear decay
Define half-life and apply the concept to simple
problems
Describe the relationship between energy and matter
Identify the differences between nuclear fission and
fusion and their importance in generation of nuclear
power
Forces act in opposing directions
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Electrostatic repulsion: pushes protons apart
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Strong nuclear force: pulls protons together
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Nuclear force is much shorter range: protons must be close together
Neutrons only experience the strong
nuclear force
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Proton pair experiences both forces
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Neutrons experience only the strong nuclear force
But: neutrons alone are unstable
Neutrons act like nuclear glue
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Helium nucleus contains 2 protons and 2
neutrons – increase attractive forces
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Overall nucleus is stable
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As nuclear size increases,
electrostatic repulsion builds up
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There are electrostatic repulsions between
protons that don’t have attractive forces
Long range
repulsive force
with no
compensation
from attraction
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More neutrons required
Neutron to proton ratio increases
with atomic number
Upper
limit of
stability
4
U 234
90Th  2 He
238
92
Upper limit to nuclear stability
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Beyond atomic number 83, all nuclei are
unstable and decay via radioactivity
Radioactive decay (Transmutation) –
Alpha
formation of new element
particle
Mass
number
U  Th  He  
238
92
234
90
Atomic
number
Atomic
number
decreases
4
2
emitted
Stability is not achieved in one step:
products also decay
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Atomic number (Z) increases
Neutron:proton ratio decreases
Th  Pa  e  
234
90
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0
1
Beta particle emission occurs with neutron-excess nuclei
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234
91
Larger M/Z
Alpha particle emission occurs with proton-heavy nuclei
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Smaller M/Z
Beta
particle
emitted
Summary of types of radiation
Radioactive series are complex
• The decay series from
thorium-232 to lead-208
• Each intermediate nuclide is
radioactive and undergoes
nuclear decay
• Left-pointing longer arrows
(red) are alpha emissions
• M and Z decrease
• Right-pointing shorter
arrows (blue) are beta
emissions
• M constant, Z increases
Half-life measures rate of decay
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Concentration of
nuclide is halved after
the same time interval
regardless of the initial
amount – Half-life
Can range from
fractions of a second to
billions of years
Fission and fusion:
Radical nuclear engineering
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Attempts to grow larger
nuclei by bombardment
with neutrons yielded
smaller atoms instead.
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Distorting the nucleus causes the
repulsive forces to overwhelm the
attractive
The foundation of
nuclear energy and the
atomic bomb
Nuclear fission
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Nuclear fission produces nuclei with lower nucleon
mass
1
0
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n U  Kr  Ba 3 n
235
92
91
36
142
56
1
0
One neutron produces three: the basis for a chain
reaction – explosive potential
Neutrons must be obtained from other nuclear
processes such as bombardment of aluminum with
alpha particles
Chain reactions require rapid multiplication
of species
Chain reactions have the potential
for nuclear explosions
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Bomb requires creation of high rate of collisions in
small volume
How to achieve this at the desired time in a controlled
manner?
The importance of U-235
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U-235 is less than 1 % of naturally occurring
uranium, but undergoes fission with much
greater efficiency than U-238
1
0
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n U  Kr  Ba 3 n
235
92
91
36
142
56
1
0
Fission can follow many paths: over 200
different isotopes have been observed
Each process produces more neutrons than it
consumes 1
235
90
137
1
0
n 92 U 40 Zr  52Te 20 n
Enrichment of uranium
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Weapons grade uranium
requires a high
concentration of U-235
This is achieved by
isotope separation
The lighter U-235
diffuses more rapidly
than the heavier U-238
in the gas phase as UF6
Why is fission so energetic?
Conversion of mass into energy
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Binding energy measures
nuclear stability
Higher binding energy
particles have lower mass
Mass converted to energy
E = mc2 (Einstein’s relation)
Fission: combined mass of
smaller nuclei less than
original nucleus
AB+C
MA > M B + M C
Loss in mass = energy
released:
Comparison of nuclear and chemical
energy sources
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Conversion of nuclear
matter into energy
produces masses:
1 gram  1014 J
Chemical process:
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1 gram fuel produces
103 J
Nuclear process:
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1 gram uranium at 0.08
% produces 1011 J
Nuclear fusion: opposite of fission
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Fusion: combine small
nuclei to make larger ones
Nuclear mass converted to
energy
+E
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Example is the deuterium
– tritium reaction
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About 0.7 % mass is converted
into energy
So why is there still hydrogen?
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The sun is a helium factory
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The sun’s energy derives from the fusion of hydrogen
atoms to give helium
4 H  He  2 e  2 e
0
0
1 e  1 e  2
1
1
4
2
0
1
0
1
So what’s the catch?
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The benefits:
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The problem:
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High energy output (10 x more
output than fission)
Clean products – no long-lived
radioactive waste or toxic heavy
metals
Positive charges repel
Reproduce the center of the sun
in the lab
Fusion is demonstrated but
currently consumes more
energy than it produces