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Conceptual Physics
Fundamentals
Chapter 16:
THE ATOMIC NUCLEUS AND
RADIOACTIVITY
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
This lecture will help you
understand:
•
•
•
•
•
•
•
•
Radioactivity
Alpha, Beta, and Gamma Rays
The Atomic Nucleus and the Strong Force
Radioactive Half-Life
Transmutation of the Elements
Radiometric Dating
Nuclear Fission
Mass-Energy Equivalence—E = mc2
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The Atomic Nucleus and
Radioactivity
“The release of atomic energy has not
created a new problem. It has merely made
more urgent the necessity of solving an
existing one.”
—Albert Einstein
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Radioactivity
Radioactivity
• Radioactivity is the process of nuclear decay
(radioactive decay).
• Nothing new in the environment; it’s been going
on since time zero.
• It warms Earth’s interior, is in the air we breathe,
and is present in all rocks (some in trace
amounts).
• It is natural.
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Radioactivity
CHECK YOUR NEIGHBOR
The radioactive decay of nature’s elements occurs in the
A.
B.
C.
D.
soil we walk on.
air we breathe.
interior of Earth.
all of the above
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Radioactivity
CHECK YOUR ANSWER
The radioactive decay of nature’s elements occurs in the
A.
B.
C.
D.
soil we walk on.
air we breathe.
interior of Earth.
all of the above.
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Alpha, Beta, and Gamma Rays
Radioactive elements emit three distinct
types of radiation:
•
•
•
—alpha: positively charged (helium nuclei)
— beta: negatively charged (electrons)
—gamma (electromagnetic radiation)
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Alpha, Beta, and Gamma Rays
Relative penetrations
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Alpha Radiation
•
Alpha radiation is a heavy, very short-range
particle and is actually an ejected helium nucleus.
Some characteristics of alpha radiation are:
– Most alpha radiation is not able to penetrate human
skin or clothing.
– Alpha-emitting materials can be harmful to humans
if the materials are inhaled, swallowed, or absorbed
through open wounds.
– A thin-window Geiger-Mueller (GM) probe can
detect the presence of alpha radiation.
•
Examples of some alpha emitters: radium, radon,
uranium, thorium.
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Beta Radiation
•
Beta radiation is a light, short-range particle,and
is actually an ejected electron. Some
characteristics of beta radiation are:
– Beta radiation may travel several feet in air and is
moderately penetrating.
– Beta radiation can penetrate human skin to the
"germinal layer," where new skin cells are
produced. If high levels of beta-emitting
contaminants are allowed to remain on the skin for
a prolonged period of time, they may cause skin
injury.
– Beta-emitting contaminants may be harmful if
deposited internally.
– Clothing provides some protection against beta
radiation.
•
Examples of some pure beta emitters: strontium90, carbon-14, tritium, and sulfur-35.
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Gamma Radiation
•
•
Gamma radiation and x rays are highly penetrating
electromagnetic radiation. Some characteristics of
these radiations are:
–
Gamma radiation or x rays are able to travel many feet in
air and many inches in human tissue.
–
Gamma radiation and x rays are electromagnetic radiation
like visible light, radiowaves, and ultraviolet light.
–
Dense materials are needed for shielding from gamma
radiation. Clothing provides little shielding from penetrating
radiation, but will prevent contamination of the skin by
gamma-emitting materials.
–
Gamma radiation and/or characteristic x rays frequently
accompany the emission of alpha and beta radiation
during radioactive decay.
Examples of some gamma emitters: iodine-131,
cesium-137, cobalt-60, radium-226, and technetium99m.
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Gamma Radiation
What happens if you overdose on Gamma Radiation?
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Alpha, Beta, and Gamma Rays
CHECK YOUR NEIGHBOR
The origins of radioactivity go back to
A.
B.
C.
D.
military activities in the mid-20th century.
the industrial revolution two centuries ago.
the beginning of human error.
before humans emerged on Earth.
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Alpha, Beta, and Gamma Rays
CHECK YOUR ANSWER
The origins of radioactivity go back to
A.
B.
C.
D.
military activists in the mid-20th century.
the industrial revolution two centuries ago.
the beginning of human error.
before humans emerged on Earth.
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Alpha, Beta, and Gamma Rays
CHECK YOUR NEIGHBOR
Any atom that emits an alpha particle or beta particle
A.
B.
C.
D.
becomes an atom of a different element, always.
may become an atom of a different element.
becomes a different isotope of the same element.
increases its mass.
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Alpha, Beta, and Gamma Rays
CHECK YOUR ANSWER
Any atom that emits an alpha particle or beta particle
A.
B.
C.
D.
becomes an atom of a different element, always.
may become an atom of a different element.
becomes a different isotope of the same element.
increases its mass.
Explanation:
Contrary to the failures of alchemists of old to change elements
from one to another, this was going on all around them—
unnoticed.
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Alpha, Beta, and Gamma Rays
Food irradiation kills microbes
• doesn’t make the food radioactive
• there is no diarrhea with astronauts in space
(their food is first irradiated).
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Alpha, Beta, and Gamma Rays
CHECK YOUR NEIGHBOR
Which of these is the nucleus of the helium atom?
A.
B.
C.
D.
alpha
beta
gamma
all are different forms of helium
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Alpha, Beta, and Gamma Rays
CHECK YOUR ANSWER
Which of these is the nucleus of the helium atom?
A.
B.
C.
D.
alpha
beta
gamma
all are different forms of helium
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Alpha, Beta, and Gamma Rays
CHECK YOUR NEIGHBOR
Which of these is actually a high-speed electron?
A.
B.
C.
D.
alpha
beta
gamma
all are high speed
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Alpha, Beta, and Gamma Rays
CHECK YOUR ANSWER
Which of these is actually a high-speed electron?
A.
B.
C.
D.
alpha
beta
gamma
all are high speed
Explanation:
Choice D may be true, but doesn’t directly answer the question.
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Environmental Radiation
Radon, a common environmental hazard
• Most radiation from natural background
• About 1/5 from non-natural sources.
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Environmental Radiation
Units of radiation
Particle
alpha
beta
Radiation Dosage Factor
1 rad
10 =
10 rad
1 =
• Doses of radiation
– Lethal doses of radiation begin at 500 rems.
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Health effect
10 rems
10 rems
Environmental Radiation
Source received annually
Natural origin
Cosmic radiation
Ground
Air (Radon-222)
Human tissues (K-40; Ra-226)
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Typical dose (mrem)
26
33
198
35
Environmental Radiation
Doses of radiation
Typical dose (mrem)
Human origin
Medical procedures
Diagnostic X-rays
40
Nuclear diagnostics
15
TV tubes, other consumer products
11
Weapons-test fallout
1
Commercial fossil-fuel power plants
<1
Commercial nuclear power plants
<<1
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Radiation
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Environmental Radiation
Radioactive tracers
• Radioactive isotopes used to trace such
pathways are called tracers.
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The Atomic Nucleus and the
Strong Force
The strong force holds nucleons together.
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The Atomic Nucleus and the
Strong Force
The strong force is effective over a small
distance.
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The Atomic Nucleus and the
Strong Force
The strong force is more effective with
smaller nuclei.
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Strong Force vs number of nucleons
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The Atomic Nucleus and the
Strong Force
A lone neutron is radioactive, and
spontaneously transforms to a proton and
an electron.
• A neutron needs protons around to keep this from
happening.
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The Atomic Nucleus and the
Strong Force
Alpha emission
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The Atomic Nucleus and the Strong Force
CHECK YOUR NEIGHBOR
The strong force is a force in the
A.
B.
C.
D.
atom that holds electrons in orbit.
nucleus that holds nucleons together.
both A and B
neither A nor B
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The Atomic Nucleus and the Strong Force
CHECK YOUR ANSWER
The strong force is a force in the
A.
B.
C.
D.
atom that holds electrons in orbit.
nucleus that holds nucleons together.
both A and B
neither A nor B
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The Atomic Nucleus and the Strong Force
CHECK YOUR NEIGHBOR
In the nucleus of an atom, the strong force is a relatively
A.
B.
C.
D.
short-range force.
long-range force.
unstable force.
neutralizing force.
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The Atomic Nucleus and the Strong Force
CHECK YOUR ANSWER
In the nucleus of an atom, the strong force is a relatively
A.
B.
C.
D.
short-range force.
long-range force.
unstable force.
neutralizing force.
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Strong Force
Strong force
Binding energy and fission
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Radioactive Decay
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Radioactive Half-Life
The rate of decay for a radioactive isotope is
measured in terms of a characteristic time, the
half-life. The time for half of an original quantity of
an element to decay.
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Radioactive Half-Life
Uranium-238 to lead-206 through a series of alpha
and beta decays. In 4.5 billion years, half the
uranium presently in Earth will be lead.
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Radioactive Half-Life
Some radiation detectors
(a) a Geiger counter
(b) a scintillation counter
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Radioactive Half-Life
CHECK YOUR NEIGHBOR
A certain isotope has a half-life of 10 years. This means the
amount of that isotope remaining at the end of 10 years will
be
A.
B.
C.
D.
zero.
one quarter.
half.
the same.
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Radioactive Half-Life
CHECK YOUR ANSWER
A certain isotope has a half-life of 10 years. This means the
amount of that isotope remaining at the end of 10 years will
be
A.
B.
C.
D.
zero.
one quarter.
half.
the same.
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Radioactive Half-Life
A challenge…
Suppose the number of neutrons in a reactor that is starting
up doubles each minute, reaching 1 billion neutrons in 10
minutes. When did the number of neutrons reach half a
billion?
A.
B.
C.
D.
1 minute
2 minutes
5 minutes
9 minutes
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Radioactive Half-life
CHECK YOUR ANSWER
Suppose the number of neutrons in a reactor that is starting
up doubles each minute, reaching 1 billion neutrons in 10
minutes. When did the number of neutrons reach half a
billion?
A.
B.
C.
D.
1 minute
2 minutes
5 minutes
9 minutes
Explanation:
This question would be appropriate with Appendix D, Exponential Growth and
Doubling Time. Can you see that working backward, each minute has half the
number of neutrons?
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Transmutation of the Elements
• Nuclear transmutation is the
conversion of one chemical
element or isotope into another.
• This occurs either through
nuclear reactions (in which an
outside particle reacts with a
nucleus), or through radioactive
decay (where no outside particle
is needed).
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Transmutation of the Elements
With alpha or beta particle, a different element is
formed. This is transmutation, which occurs in
natural events, and also initiated artificially in the
laboratory.
Uranium naturally transmutes to thorium when an
alpha particle is emitted.
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Transmutation of the Elements
Natural transmutation
• Thorium naturally transmutes to protactinium when a
beta particle is emitted.
0
• An electron is - 1 e.
– Superscript 0 indicates electron’s mass is insignificant compared with
nucleons.
– Superscript -1 is the electric charge of the electron.
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Transmutation of the Elements
CHECK YOUR NEIGHBOR
When an element ejects an alpha particle and a beta
particle, the atomic number of the resulting element
A.
B.
C.
D.
reduces by 2.
reduces by 4.
increases by 2.
increases by 4.
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Transmutation of the Elements
CHECK YOUR ANSWER
When an element ejects an alpha particle, the atomic
number of the resulting element
A.
B.
C.
D.
reduces by 2.
reduces by 4.
increases by 2.
increases by 4.
Explanation:
An alpha particle (a helium nucleus) has atomic number 2. So
ejection of an alpha particle means a loss of 2 protons. So the
atomic number of the element is lowered by 2.
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Transmutation of the Elements
CHECK YOUR NEIGHBOR
When an element ejects an alpha particle and a beta
particle, the atomic number of that element
A.
B.
C.
D.
reduces by 1.
increases by 1.
reduces by 2.
increases by 2.
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Transmutation of the Elements
CHECK YOUR ANSWER
When an element ejects an alpha particle and a beta
particle, the atomic number of that element
A.
B.
C.
D.
reduces by 1.
increases by 1.
reduces by 2.
increases by 2.
Explanation:
Alpha emission reduces atomic number by 2, and beta emission
increases atomic number by 1, so net result is 1.
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Transmutation of the Element
Artificial transmutation
• an alpha particle fired at and impacting on a
nitrogen atom, which transmutes to oxygen and
hydrogen
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Transmutation of the Elements
CHECK YOUR NEIGHBOR
Atoms can transmute into completely different atoms in
A.
B.
C.
D.
nature.
laboratories.
both A and B
neither A nor B
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Transmutation of the Elements
CHECK YOUR ANSWER
Atoms can transmute into completely different atoms in
A.
B.
C.
D.
nature.
laboratories.
both A and B
neither A nor B
Explanation:
Atomic transmutation occurs in nature, in laboratories, and as far
as we know, throughout the cosmos.
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Transmutation of the Elements
CHECK YOUR NEIGHBOR
An element emits 1 beta particle, and its product then emits
1 alpha particle. The atomic number of the resulting
element is changed by
A.
B.
C.
D.
zero.
−1.
−2.
none of the above
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Transmutation of the Elements
CHECK YOUR ANSWER
An element emits 1 beta particle, and its product then emits
1 alpha particle. The atomic number of the resulting
element is changed by
A.
B.
C.
D.
zero.
−1.
−2.
none of the above
Explanation:
Beta emission increases atomic number by 1, then alpha
emission decreases atomic number by 2, so the net change is –1.
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Radiometric Dating
• Earth’s atmosphere is continuously bombarded
by cosmic rays, which causes many atoms in the
upper atmosphere to transmute. These
transmutations result in many protons.
• a nitrogen that captures a neutron and becomes
an isotope of carbon by emitting a proton:
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Radiometric Dating
• Carbon-14 is a beta emitter and decays back to
nitrogen.
• Because living plants take in carbon dioxide, any C-14
lost by decay is immediately replenished with fresh C-14
from the atmosphere.
• Dead plants continue emitting C-14 without
replenishment.
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Radiometric Dating
Relative amounts of C-12 to C-14 enable dating of
organic materials.
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Radiometric Dating
CHECK YOUR NEIGHBOR
The half-life of carbon-14 is about 5730 years, which
means that the present amount in your bones will reduce to
zero
A.
B.
C.
D.
when you die.
in about 5730 years.
in about twice 5730 years.
none of the above
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Radiometric Dating
CHECK YOUR ANSWER
The half-life of carbon-14 is about 5730 years, which
means that the present amount in your bones will reduce to
zero
A.
B.
C.
D.
when you die.
in about 5730 years.
in about twice 5730 years.
none of the above
Explanation:
In theory, the amount never reaches zero. In eons to come, trace
amounts of the carbon-14 in your bones, even if completely
dissolved, will still exist.
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Nuclear Fission
German scientists Otto Hahn and Fritz
Strassmann in 1938 accidentally discovered
nuclear fission.
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Nuclear Fission
A typical uranium fission reaction
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Nuclear Fission
Chain reaction—a self-sustaining reaction in which
the products of one reaction event stimulate further
reaction events.
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Nuclear Fission
CHECK YOUR NEIGHBOR
The greater the surface area of a piece of fission material,
the
A.
B.
C.
D.
less likely an explosion.
more likely an explosion.
neither A nor B; mass, rather than surface area is significant
none of the above
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Nuclear Fission
CHECK YOUR ANSWER
The greater the surface area of a piece of fission material,
the
A.
B.
C.
D.
less likely an explosion.
more likely an explosion.
neither, A nor B; mass, rather than surface area is significant
none of the above
Explanation:
When a chain reaction occurs, it fizzles out when neutrons escape
a surface. Therefore, the greater the surface area, the less likely
an explosion will occur.
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Mass per Nucleon
Mass per Nucleon
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Nuclear Fission
CHECK YOUR NEIGHBOR
Which of these nuclei has the greatest mass?
A.
B.
C.
D.
hydrogen
iron
lead
uranium
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Nuclear Fission
CHECK YOUR ANSWER
Which of these nuclei has the greatest mass?
A.
B.
C.
D.
hydrogen
iron
lead
uranium
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Nuclear Fission
CHECK YOUR NEIGHBOR
In which of these nuclei does the nucleon have the greatest
mass?
A.
B.
C.
D.
hydrogen
iron
lead
uranium
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Nuclear Fission
CHECK YOUR ANSWER
In which of these nuclei does the nucleon have the greatest
mass?
A.
B.
C.
D.
hydrogen
iron
lead
uranium
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Nuclear Fission
CHECK YOUR NEIGHBOR
In which of these nuclei does the nucleon have the least
mass?
A.
B.
C.
D.
hydrogen
iron
lead
uranium
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Nuclear Fission
CHECK YOUR ANSWER
In which of these nuclei does the nucleon have the least
mass?
A.
B.
C.
D.
hydrogen
iron
lead
uranium
Explanation:
Iron has the least mass per nucleon, but the strongest binding
energy.
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Nuclear Fission
When a uranium nucleus undergoes fission, the energy
released is primarily in the form of kinetic energy of
fission fragments
Kinetic energy of fragments is what becomes heat energy.
Interestingly, gamma-ray energy is tiny in comparison. Neutrons,
although important for the chain reaction, contribute a small part
of the energy release.
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Nuclear Fission
Fission bomb
• A bomb in which pieces of uranium are driven together is
a so-called “gun-type” weapon, as opposed to the now
more common “implosion weapon.”
• Constructing a fission bomb is a formidable task. The
difficulty is separating enough U-235 fuel.
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Nuclear Fission
Nuclear fission reactors
• About 20% of electric energy in the
United States is generated by nuclear
fission reactors.
• more in some other countries—about
75% in France
• Reactors are simply nuclear furnaces.
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Nuclear Fission
Diagram of a typical power plant.
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Nuclear Fission
• The benefits are plentiful electricity, conservation of
billions of tons of fossil fuels every year that are
converted to heat and smoke (which in the long run may
be far more precious as sources of organic molecules
than as sources of heat), and the elimination of
megatons of carbon dioxide, sulfur oxides, and other
deleterious substances put into the air each year by the
burning of fossil fuels.
• Drawbacks include risks of release of radioactive
isotopes into the atmosphere, by accident or by terrorist
activities. Radioactive waste disposal is a problem
(although not for some countries that monitor it for
potential use later).
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Nuclear Fission
Plutonium-239, like uranium-235, undergoes fission when it
captures a neutron.
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Nuclear Fission
The breeder reactor
• A breeder reactor breeds Pu-239 from U-238 while
“burning” U-235.
– occurs in all reactors to some extent.
– in few years can produce twice as much fissionable
fuel as it begins with.
– a more attractive alternative when U-235 reserves are
limited.
– fuel for a breeder may be today’s radioactive wastes
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Nuclear Fusion
•
•
•
•
nuclear fusion is the opposite of nuclear fission
fission, nuclei “fizz” apart
fusion, nuclei fuse together
Each releases energy in accord with the graph
in Figure 16.33.
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Nuclear Fusion
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Nuclear Fusion
Fission and fusion compared
• Less mass per nucleon occurs in both
processes.
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Nuclear Fusion
Typical fusion reactions
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Nuclear Fusion
CHECK YOUR NEIGHBOR
When a fusion reaction converts a pair of hydrogen
isotopes to an alpha particle and a neutron, most of the
energy released is in the form of
A.
B.
C.
D.
gamma radiation.
kinetic energy of the alpha particle.
kinetic energy of the neutron.
all of the above about equally
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Nuclear Fusion
CHECK YOUR ANSWER
When a fusion reaction converts a pair of hydrogen
isotopes to an alpha particle and a neutron, most of the
energy released is in the form of
A.
B.
C.
D.
gamma radiation.
kinetic energy of the alpha particle.
kinetic energy of the neutron.
all of the above about equally
Explanation:
By momentum conservation, the ejected neutrons have a high
speed compared with the alpha particle, and therefore much
kinetic energy. It is the kinetic energy of the neutrons that
becomes the heat needed for power. Gamma rays play a small
energy role, as they do in fission.
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Nuclear Fusion
Controlling fusion
• Carrying out fusion is more difficult than thought
when fission succeeded.
– plasma reactors have not been successful
– other schemes, including lasers, are being considered
– deuterium pellets rhythmically dropped into
synchronized laser crossfire; heat used to produce
steam
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Nuclear Fission
CHECK YOUR NEIGHBOR
In either a fission event or a fusion event, the quantity that
remains unchanged is
A.
B.
C.
D.
energy.
the mass of nucleons.
the number of nucleons.
none of the above
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Kinetic Energy
CHECK YOUR ANSWER
In either a fission event or a fusion event, the quantity that
remains unchanged is
A.
B.
C.
D.
energy.
the mass of nucleons.
the number of nucleons.
none of the above
Explanation:
This is a premise of reaction equations, whether nuclear or
chemical. Although energy and mass undergo changes, the
number of particles and amount of charge remains unchanged.
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Backup
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Mass-Energy Equivalence—
E = mc2
• Early in the early 1900s, Albert Einstein
discovered that mass is actually “congealed”
energy.
• Enormous work is required to pull nucleons from
a nucleus. This work is energy added to the
nucleon that is pulled out.
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Mass-Energy Equivalence—
E = mc2
Measurements of atomic mass are made with this
device.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Mass-Energy Equivalence—
E = mc2
• more explanation of the mass
spectrometer
• Electrically charged isotopes
directed into the semicircular
“drum” are forced into curved
paths by a strong magnetic
field. Lighter isotopes with less
inertia (mass) easily change
direction and follow curves of
smaller radii. Heavier isotopes
with greater inertia (mass)
follow larger curves. Mass of
an isotope ~ distance from
entrance slit.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Mass-Energy Equivalence—
E = mc2
The plot shows how nuclear mass increases
with increasing atomic number.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Mass-Energy Equivalence—
E = mc2
A very important graph results from the plot of
nuclear mass per nucleon from hydrogen through
uranium.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
Mass-Energy Equivalence—
E = mc2
The same graph, with emphasis on nuclear fission.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley