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General Physics (PHY 2140)
Lecture 39
 Modern Physics
Nuclear and Particle Physics
Nuclear Energy
Elementary particles
http://www.physics.wayne.edu/~apetrov/PHY2140/
Chapter 30
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1
Lightning Review
Last lecture:
1. Nuclear physics
 Nuclear reactions
A
Z
X
r  r0 A1/ 3
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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FE  FM
qE  qvB
E
v
B
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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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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
236
n 235
U

92
92 U*  X  Y  neutrons
1
0


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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Sequence of Events in Fission
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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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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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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:
E
1.2  1014 J
3
m 2 
 1.3  10 kg ~ 1g
8
2
c
(3.0  10 m/s)
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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


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If uncontrolled, a violent explosion can occur
The principle behind the nuclear bomb, where 1 g of U can release
energy equal to about 20000 tons of TNT
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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

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A self-sustained reaction has K = 1
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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 selfsustaining
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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Elementary Particles
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?
Time
Seems like…
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
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…
What are the right degrees of freedom?
•
•
•
•
Fire
Water
Earth
Air
… that is, according
to the Greeks!
What would be the modern picture?
Imagine that we have a very powerful microscope…
Modern understanding: the ``onion’’
picture
Atom
Let’s see what’s inside!
Modern understanding: the ``onion’’
picture
Nucleus
Let’s see what’s inside!
Modern understanding: the ``onion’’
picture
Protons and
neutrons
Let’s see what’s inside!
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!
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.
Modern understanding: the ``onion’’
picture
Quarks and gluons
Let’s see what’s inside!
Modern understanding: the ``onion’’
picture
… so the answer depends on the energy scale!
… same thing about the interactions
Unification of forces
The Standard Model of particle physics
The Standard Model of Elementary
Particle Physics
• ``Periodic table’’ of matter
• Interactions: electromagnetic, weak, strong,
(gravity)…
+ Higgs particle
Conditions for baryon
asymmetry
Matter-antimatter imbalance in the Universe
 Baryon (and lepton) number - violating processes
to generate asymmetry
A.D. Sakharov
 Universe that evolves out of thermal equilibrium
to keep asymmetry from being washed out
 Matter interactions differ from antimatter interactions (“Microscopic CPviolation”)
to keep asymmetry from being compensated in the “anti-world”
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”?
Experimental methods
video
Experimental methods
Experimental Facilities I
Cornell University
SLAC
Experimental Facilities II
KEK (Japan)
Fermilab (Batavia, IL)
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
A couple of review problems and notes to remember…
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Remember:
Electricity:

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

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In a way of handling, nuclear reactions are very similar to
chemical reactions
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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 10-19 C
Find:
v=?
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mv
r
qB
Recall that the proton’s radius would be
Thus
v
qBr
m
1.6 1019 C  0.35T  14 102 m





1.67 1027 kg


 4.7 106 m s
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