1. General Physics (PHY 2140) Lecture 39 Modern Physics
Nuclear and Particle Physics
Nuclear Energy
Elementary particles
Chapter 30
9/22/2018

2. Lightning Review Last lecture: Nuclear physics Nuclear reactions
Review Problem: A beam of particles passes undeflected through crossed electric and magnetic fields. When the electric field is switched off, the beam splits up in several beams. This splitting is due to the particles in the beam having different
A. masses.
B. velocities.
C. charges.
D. some combination of the above
E. none of the above
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3. Processes of Nuclear Energy
Fission
A nucleus of large mass number splits into two smaller nuclei
Fusion
Two light nuclei fuse to form a heavier nucleus
Large amounts of energy are released in either case
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4. Nuclear Fission A heavy nucleus splits into two smaller nuclei
The total mass of the products is less than the original mass of the heavy nucleus
First observed in 1939 by Otto Hahn and Fritz Strassman following basic studies by Fermi
Lisa Meitner and Otto Frisch soon explained what had happened
Fission of 235U by a slow (low energy) neutron
236U* is an intermediate, short-lived state
X and Y are called fission fragments
Many combinations of X and Y satisfy the requirements of conservation of energy and charge
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5. Sequence of Events in Fission
The 235U nucleus captures a thermal (slow-moving) neutron
This capture results in the formation of 236U*, and the excess energy of this nucleus causes it to undergo violent oscillations
The 236U* nucleus becomes highly elongated, and the force of repulsion between the protons tends to increase the distortion
The nucleus splits into two fragments, emitting several neutrons in the process
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6. Energy in a Fission Process
Binding energy for heavy nuclei is about 7.2 MeV per nucleon
Binding energy for intermediate nuclei is about 8.2 MeV per nucleon
Therefore, the fission fragments have less mass than the nucleons in the original nuclei
This decrease in mass per nucleon appears as released energy in the fission event
An estimate of the energy released
Assume a total of 240 nucleons
Releases about 1 MeV per nucleon
8.2 MeV – 7.2 MeV
Total energy released is about 240 Mev
This is very large compared to the amount of energy released in chemical processes
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7. QUICK Problem
In the first atomic bomb, the energy released was equivalent to about 30 kilotons of TNT, where a ton of TNT releases an energy of 4.0 × 109 J. The amount of mass converted into energy in this event is nearest to: (a) 1 g, (b) 1 mg, (c) 1 g, (d) 1 kg, (e) 20 kilotons
(c). The total energy released was E = (30 ×103 ton)(4.0 × 109 J/ton) = 1.2 × 1014 J. The mass equivalent of this quantity of energy is:
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8. Chain Reaction Neutrons are emitted when 235U undergoes fission
These neutrons are then available to trigger fission in other nuclei
This process is called a chain reaction
If uncontrolled, a violent explosion can occur
The principle behind the nuclear bomb, where 1 g of U can release energy equal to about tons of TNT
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9. Nuclear Reactor
A nuclear reactor is a system designed to maintain a self-sustained chain reaction
The reproduction constant, K, is defined as the average number of neutrons from each fission event that will cause another fission event
The maximum value of K from uranium fission is 2.5
In practice, K is less than this
A self-sustained reaction has K = 1
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10. Basic Reactor Design Fuel elements consist of enriched uranium
The moderator material helps to slow down the neutrons
The control rods absorb neutrons
When K = 1, the reactor is said to be critical
The chain reaction is self-sustaining
When K < 1, the reactor is said to be subcritical
The reaction dies out
When K > 1, the reactor is said to be supercritical
A run-away chain reaction occurs
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11. Elementary Particles

12. 1. The Big Question of Particle Physics…
How did we get from here… … to here?
And what does it have to
do with heavy quarks?

13. Time
Seems like…

14. Just after the Big Bang: symmetric Universe
equal number of particles and antiparticles
Now:
asymmetric Universe
planets, stars, galaxies, Wayne State, …
Note: macroscopic laws of Nature do not distinguish matter and antimatter

15. A 10,000,000.00 Swedish Kronor question: Where did all the antimatter go?
The “Onion paradigm”:
identify degrees of freedom
see if the problem has a solution
if not, dig deeper…

16. What are the right degrees of freedom?
Fire
Water
Earth
Air
… that is, according
to the Greeks!

17. What would be the modern picture?
Imagine that we have a very powerful microscope…

18. Modern understanding: the ``onion’’ picture
Atom
Let’s see what’s inside!

19. Modern understanding: the ``onion’’ picture
Nucleus
Let’s see what’s inside!

20. Modern understanding: the ``onion’’ picture
Protons and
neutrons
Let’s see what’s inside!

21. Modern understanding: the ``onion’’ picture
Collective name for particles containing 3 quarks
Mesons and baryons
Collective name for particles containing quark and antiquark
Let’s see what’s inside!

22. Modern understanding: the ``onion’’ picture
Collective name for particles containing 3 quarks (such as proton and neutron)
Mesons and baryons
Collective name for particles containing quark and antiquark
Let’s see what’s inside!
Note: apparent excess of matter over antimatter can be traced to excess of the number of baryons over antibaryons. Thus our Big Problem is called Problem of Baryon Asymmetry of the Universe.

23. Modern understanding: the ``onion’’ picture
Quarks and gluons
Let’s see what’s inside!

24. Modern understanding: the ``onion’’ picture?
… so the answer depends on the energy scale!

25. … same thing about the interactions

26. Unification of forces

27. The Standard Model of particle physics

28. The Standard Model of Elementary Particle Physics
``Periodic table’’ of matter
Interactions: electromagnetic, weak, strong, (gravity)…
Contains 26 parameters: needs experimental input
+ Higgs particle

29. Conditions for baryon asymmetry
Matter-antimatter imbalance in the Universe
A.D. Sakharov
Baryon (and lepton) number - violating processes
to generate asymmetry
Universe that evolves out of thermal equilibrium
to keep asymmetry from being washed out
Matter interactions differ from antimatter interactions (“Microscopic CP-violation”)
to keep asymmetry from being compensated in the “anti-world”

30. Can Standard Model explain baryon asymmetry?
does it have “the right stuff”?
what are the conditions for the baryon asymmetry?
does it have enough of “the right stuff”?

31. Experimental methods
video

32. Experimental methods

33. Experimental Facilities I
Cornell University
SLAC

34. Experimental Facilities II
KEK (Japan)
Fermilab (Batavia, IL)

35. What do physics PhD’s do?
Science route
Research in physics (national lab, research university)
Teaching and research (college)
Industry route
Computing/engineering jobs in companies
Finance industry (problem solving)
Scientific Publishing route

36. A couple of review problems and notes to remember…
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37. Remember: Electricity: Magnetism: Special relativity
Electric field and electric potential are different things
Moreover, field is a vector while the potential is a scalar
Remember the difference between parallel and series connections
Remember that formulas for capacitors and resistors are “reversed”
Magnetism:
Use right hand rule properly
Special relativity
If the problem involves speeds close to the speed of light, use relativistic formulas for momentum, energy, addition of velocities
In particular, KE=mv2/2 is a NONRELATIVISTIC expression for KE
Atomic and nuclear physics
In a way of handling, nuclear reactions are very similar to chemical reactions
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38. Example : Proton moving in uniform magnetic field
A proton is moving in a circular orbit of radius 14 cm in a uniform magnetic field of magnitude 0.35 T, directed perpendicular to the velocity of the proton. Find the orbital speed of the proton.
Given:
r = 0.14 m
B = 0.35 T
m = 1.67x10-27 kg
q = 1.6 x C
Recall that the proton’s radius would be
Thus
Find:
v = ?
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