Nuclear Physics
Reminders:
HW11 due Monday 4/25, 10pm D2L
Presentations this week!
Phys 1020, Lecture 22:
Paper due on Friday, 10pm on D2L
Nuclear Weapons Blmfld 16.1
Review lecture(s) next week. Topics?
Final, Wed May 4th, 1.30-4pm, in here (G125).
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Nuclear physics so far:
• Structure of atom
– Atomic number
– Mass number
• Potential energy curves between 2 particles
– Force is gradient of the curve
• Forces in the nucleus
– Electrostatic repulsion
– Nuclear binding force
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1
So how can a bunch of positively charged protons
stick together?
Two different types of forces involved
- Electrostatic forces
- Repulsive
- Only affects charged particles (protons)
- Long range (each protons repels all other protons in nucleus)
- Nuclear force
- Attractive
- Affects protons and neutrons (nucleons) in same way
- Short range – only bonds nucleons that are touching
- Overwhelms electrical force when protons and/or neutrons (nucleons) are
REALLY close together
Hopper toy analogy
Spring legs - like electrical force, pushes man away from base over large distance.
Suction cup - like nuclear force, bonds man to base but only strong when in contact
(Or nuclear force like double-sided tape - only works if in objects in close contact.)
Why is the nuclear force so different? That is the way nature is!
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Complete potential energy curve for proton approaching nucleus
Electrostatic
repulsion
+
Nuclear energy scale GIGANTIC
compared to chemical energy.
Why? Simple coulomb’s law:
PE
PE
F= k (charge 1)(charge 2)
r2
And associated electric potential energy:
r
r
E= k (charge 1)(charge 2)
r
Nuclear
attraction
Complete
nuclear
potential
Chemistry:
- Forces and stored energy between
electrons and protons on distance scale of
atomic size (~ 10-10 m).
PE
Nuclear forces:
- Forces and stored energy between
nucleons on distance scale of nuclear size
(~10fm = 10-14m)
~ 10,000 times smaller than atom
r
- Lots more potential energy stored!!!
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2
Binding energy per nucleon
Binding energy - Energy required to pull a nucleus apart into its
constituent neutrons and protons
Small
nuclei
Large
nuclei
Binding energy per nucleon
Small
nuclei
Why is binding energy
increasing with mass
number for small nuclei?
Large
nuclei
Why is binding energy
decreasing with mass
number for large nuclei?
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Small nuclei
Consider how forces in nucleus change when we go from 4He (2
protons) to 7Lithium (3 protons)
4He
7Li
Electrostatic repulsion:
- Protons feel repulsion of 1 more proton
- Electrostatic repulsion increases
Nuclear attraction:
- All nucleons have more nucleons in contact
- Nuclear binding increases A LOT
Average effect – More strongly bound nucleus
Large nuclei
Consider how forces in nucleus change when we go from 235U (92
protons) to 237Np (93 protons)
Electrostatic repulsion:
- All 90+ protons feel repulsion of 1 more proton
- Electrostatic repulsion increases ALOT
Nuclear attraction:
- Only a couple of nucleons at edge increase number in contact
- Nuclear binding increases only a little
Average effect – Less strongly bound nucleus
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What is this curve useful for?
Max binding energy for
medium nuclei
Small
nuclei
Large
nuclei
There are 2 types of nuclear bombs – fission and fusion.
Which elements should I choose for each type of bomb (be ready to explain why)?
a) Small nuclei for both
b) Medium sized nuclei for both
c) Large nuclei for both
d) Fission – small nuclei, fusion – large nuclei
e) Fusion – small nuclei, fission large nuclei
Fission and fusion
Fission
Fusion
• Energy is conserved.
• For bomb we must RELEASE energy stored in nucleus
• Each nucleon in product(s) must be more tightly bound (in bigger potential
energy hole) than in original nucleus(i)
• E = mc2. Mass of product(s) less than mass of original nucleus(i)
Fuse small nuclei
Fission (break) large nuclei
To release energy and create explosion
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Nuclear Physics
• Structure of an atom and nucleus
• Nuclear forces and stored energy
• Nuclear fission
- Alpha decay (spontaneous)
- Fission bomb (neutron induced fission)
• Nuclear Fusion
• Radioactivity
- Alpha, beta and gamma radiation
- Why its bad for you
• Other interesting stuff that we won’t have time for
- Nuclear power
- Nuclear medicine
Sadly, (or perhaps much to your relief), demos will be limited……
Fission of large nuclei
• Spontaneous
- Radioactive (nuclear) decay
• Neutron induced
- Fission bomb
- Nuclear power
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Spontaneous fission – alpha decay
Alpha particle (2p + 2n) spontaneously breaks away from nucleus
Nucleus is changed from one element to another
Quantum mechanical tunneling:
• Small particles not localised (fuzzy)
• Occasionally a outside range of
nuclear binding force
• Has lots of electrostatic PE
• Converts PE to KE and runs for it
2P-2N
- a ‘tunnels’ out of potential well
- Never had enough energy to climb out
Potential energy curve for alpha
particle and product nucleus
13
238U
222Rn
4.5 billion years
3.8 days
234Th
+a
218Po
+a
No. of radon atoms
Radon – example of alpha decay
T1/2
Time
Half – life: Time for half of nuclei in sample to decay
Decay time of any particular nucleus is unknown and random
Radon exposure:
• Radon is a gas
• Biggest cause of public exposure to nuclear radiation
• Occurs where lots of 238U in soil (like round here)
• Accumulates in basements – pump it out
• 2nd biggest cause of lung cancer
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A useful simulation
alpha decay
http://phet.colorado.edu/simulations/sims.php?sim=Alpha_Decay
15
Potential curves for alpha decay from different nuclei
Tunneling difficulty = width x depth of tunnel
E
E
E
r
r
1. Hard
- takes long time,
- billions of years!
2. Medium
r
3. Easy
Takes millionths of
a second!
How much energy released?
a. 1 most, 2 second, 3 least
b. 2 most, 1, 3 least
c. 3 most, 2, 1 least
d. 3 most, 1, 2 least
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