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General Physics (PHY 2140)
Lecture 38
 Modern Physics
Nuclear and Particle Physics
Radioactivity
Nuclear reactions
Nuclear Energy
Elementary particles
http://www.physics.wayne.edu/~apetrov/PHY2140/
Chapter 29-30
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1
Lightning Review
Last lecture:
1. Nuclear physics
 Radiation
A
Z
X
r  r0 A1/ 3
Review Problem: Isotopes of a given element have many different
properties, such as mass, but the same chemical properties. Why is
this?
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Alpha Decay
When a nucleus emits an alpha particle it loses two
protons and two neutrons



N decreases by 2
Z decreases by 2
A decreases by 4
Symbolically
A
Z


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X
A 4
Z 2
Y He
4
2
X is called the parent nucleus
Y is called the daughter nucleus
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Beta Decay
During beta decay, the daughter nucleus has the same
number of nucleons as the parent, but the atomic
number is one less
In addition, an electron (positron) was observed
The emission of the electron is from the nucleus



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The nucleus contains protons and neutrons
The process occurs when a neutron is transformed into a proton
and an electron
Energy must be conserved
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Beta Decay – Electron Energy
The energy released in the decay
process should almost all go to
kinetic energy of the electron
Experiments showed that few
electrons had this amount of
kinetic energy
To account for this “missing”
energy, in 1930 Pauli proposed the
existence of another particle
Enrico Fermi later named this
particle the neutrino
Properties of the neutrino

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
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Zero electrical charge
Mass much smaller than the
electron, probably not zero
Spin of ½
Very weak interaction with matter
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Beta Decay
Symbolically


A
Z
XZA1Y  e   
A
Z
XZA1Y  e   
 is the symbol for the neutrino
 is the symbol for the antineutrino
To summarize, in beta decay, the following pairs of
particles are emitted


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An electron and an antineutrino
A positron and a neutrino
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Gamma Decay
Gamma rays are given off when an excited nucleus “falls” to a lower
energy state

Similar to the process of electron “jumps” to lower energy states and giving off
photons
The excited nuclear states result from “jumps” made by a proton or neutron
The excited nuclear states may be the result of violent collision or more
likely of an alpha or beta emission
Example of a decay sequence




The first decay is a beta emission
The second step is a gamma emission

12
5
B C *  e  
12
6
C*126 C  
12
6
The C* indicates the Carbon nucleus is in an excited state
Gamma emission doesn’t change either A or Z
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Uses of Radioactivity
Carbon Dating


Beta decay of 14C is used to date organic samples
The ratio of 14C to 12C is used
Smoke detectors
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Ionization type smoke detectors use a radioactive source to ionize the
air in a chamber
A voltage and current are maintained
When smoke enters the chamber, the current is decreased and the
alarm sounds
Radon pollution
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Radon is an inert, gaseous element associated with the decay of radium
It is present in uranium mines and in certain types of rocks, bricks, etc
that may be used in home building
May also come from the ground itself
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29.5 Natural Radioactivity
Classification of nuclei

Unstable nuclei found in nature
Give rise to natural radioactivity

Nuclei produced in the laboratory through nuclear reactions
Exhibit artificial radioactivity
Three series of natural radioactivity exist
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Uranium
Actinium
Thorium
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Decay Series
of 232Th
Series starts with
232Th
Processes through
a series of alpha
and beta decays
Ends with a stable
isotope of lead,
208Pb
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29.6 Nuclear Reactions
Structure of nuclei can be changed by bombarding them
with energetic particles

The changes are called nuclear reactions
As with nuclear decays, the atomic numbers and mass
numbers must balance on both sides of the equation
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Problem
Which of the following are possible reactions?
(a) and (b). Reactions (a) and (b) both conserve total charge and total
mass number as required. Reaction (c) violates conservation of mass
number with the sum of the mass numbers being 240 before reaction
and being only 223 after reaction.
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Q Values
Energy must also be conserved in nuclear reactions
The energy required to balance a nuclear reaction is
called the Q value of the reaction

An exothermic reaction
There is a mass “loss” in the reaction
There is a release of energy
Q is positive

An endothermic reaction
There is a “gain” of mass in the reaction
Energy is needed, in the form of kinetic energy of the incoming
particles
Q is negative
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Problem: nuclear reactions
Determine the product of the reaction 3 Li  2 He  ?  n
What is the Q value of the reaction?
7
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4
14
7
4
X
Determine the product of the reaction 3 Li  2 He  Y ?  n
What is the Q value of the reaction?
Given:
reaction
In order to balance the reaction, the total amount of
nucleons (sum of A-numbers) must be the same on
both sides. Same for the Z-number.
Number of nucleons (A):
Number of protons (Z):
Thus, it is B, i.e.
7
3
Find:
Q=?
7  4  X  1  X  10
3 2  Y  0  Y  5
Li  24 He  105 B  01n
The Q-value is then


Q   m  c 2  m 7 Li  m 4 He  m10 B  mn c 2  2.79MeV
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Threshold Energy
To conserve both momentum and energy, incoming particles must
have a minimum amount of kinetic energy, called the threshold
energy
KEmin


 m
 1   Q
 M
m is the mass of the incoming particle
M is the mass of the target particle
If the energy is less than this amount, the reaction cannot occur
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QUICK QUIZ
If the Q value of an endothermic reaction is -2.17 MeV,
the minimum kinetic energy needed in the reactant
nuclei if the reaction is to occur must be (a) equal to
2.17 MeV, (b) greater than 2.17 MeV, (c) less than 2.17
MeV, or (d) precisely half of 2.17 MeV.
(b). In an endothermic reaction, the threshold energy exceeds the
magnitude of the Q value by a factor of (1+ m/M), where m is the
mass of the incident particle and M is the mass of the target nucleus.
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Nuclear Energy
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
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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

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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 QUIZ
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