CPO Science
Foundations of Physics
Unit 9, Chapter 30
Unit 9: The Atom
Chapter 30 Nuclear Reactions and Radiation
 30.1 Radioactivity
 30.2 Radiation
 30.3 Nuclear Reactions and Energy
Chapter 30 Objectives
1. Describe the cause and types of radioactivity.
2. Explain why radioactivity occurs in terms of energy.
3. Use the concept of half-life to predict the decay of a
radioactive isotope.
4. Write the equation for a simple nuclear reaction.
5. Describe the processes of fission and fusion.
6. Describe the difference between ionizing and
nonionizing radiation.
7. Use the graph of energy versus atomic number to
determine whether a nuclear reaction uses or
releases energy.
Chapter 30 Vocabulary Terms
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radioactive
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alpha decay
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beta decay
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gamma decay
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radiation
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isotope
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radioactive decay
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energy barrier intensity 
inverse square law
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shielding
fission reaction
CAT scan
ionizing
nonionizing
ultraviolet
fusion reaction
Geiger counter
rem
nuclear waste
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neutron
antimatter
x-ray
neutrino
background
radiation
dose
fallout
detector
half-life
30.1 Radioactivity
Key Question:
How do we model radioactivity?
*Students read Section 30.1 AFTER Investigation 30.1
30.1 Radioactivity
 The word radioactivity was first
used by Marie Curie in 1898.
 She used the word radioactivity
to describe the property of
certain substances to give off
invisible “radiations” that could
be detected by films.
30.1 Radioactivity
 Scientists quickly learned that
there were three different kinds
of radiation given off by
radioactive materials.
— Alpha rays
— Beta rays
— Gamma rays
 The scientists called them “rays”
because the radiation carried
energy and moved in straight
lines, like light rays.
30.1 Radioactivity
 We now know that
radioactivity comes from the
nucleus of the atom.
 If the nucleus has too many
neutrons, or is unstable for
any other reason, the atom
undergoes radioactive decay.
 The word decay means to
"break down."
30.1 Radioactivity
 In alpha decay, the nucleus ejects two protons and two
neutrons.
 Beta decay occurs when a neutron in the nucleus splits
into a proton and an electron.
 Gamma decay is not truly a decay reaction in the sense
that the nucleus becomes something different.
30.1 Radioactivity
 Radioactive decay gives off energy.
 The energy comes from the conversion of mass
into energy.
 Because the speed of light (c) is such a large
number, a tiny bit of mass generates a huge
amount of energy.
 Radioactivity occurs because everything in nature
tends to move toward lower energy.
30.1 Radioactivity
 If you started with one kilogram of C-14 it would decay into
0.999988 kg of N-14.
 The difference of 0.012 grams is converted directly into
energy via Einstein’s formula E = mc2.
30.1 Radioactivity
 Systems move from higher energy to lower energy over
time.
 A ball rolls downhill to the lowest point or a hot cup of
coffee cools down.
 A radioactive nucleus decays because the neutrons and
protons have lower overall energy in the final nucleus than
they had in the original nucleus.
30.1 Radioactivity
 The radioactive decay of C-14 does not happen
immediately because it takes a small input of energy to
start the transformation from C-14 to N-14.
 The energy needed to start the reaction is called an energy
barrier.
 The lower the energy barrier, the more likely the atom is to
decay quickly.
30.1 Radioactivity
 Radioactive decay depends on chance.
 It is possible to predict the average behavior of
lots of atoms, but impossible to predict when any
one atom will decay.
 One very useful prediction we can make is the
half-life.
 The half-life is the time it takes for one half of the
atoms in any sample to decay.
30.1 Half-life
 The half-life of carbon-14
is about 5,700 years.
 If you start out with 200
grams of C-14, 5,700
years later only 100
grams will still be C-14.
 The rest will have
decayed to nitrogen-14.
30.1 Half-life
 Most radioactive materials
decay in a series of
reactions.
 Radon gas comes from
the decay of uranium in
the soil.
 Uranium (U-238) decays
to radon-222 (Ra-222).
30.1 Applications of radioactivity
 Many satellites use radioactive decay from
isotopes with long half-lives for power because
energy can be produced for a long time without
refueling.
 Isotopes with a short half-life give off lots of
energy in a short time and are useful in medical
imaging, but can be extremely dangerous.
 The isotope carbon-14 is used by archeologists to
determine age.
30.1 Carbon dating
 Living things contain a large amount of carbon.
 When a living organism dies it stops exchanging
carbon with the environment.
 As the fixed amount of carbon-14 decays, the ratio of
C-14 to C-12 slowly gets smaller with age.
30.1 Calculating with isotopes
 A sample of 1,000 grams of
the isotope C-14 is created.
 The half-life of C-14 is
5,700 years.
 How much C-14 remains
after 28,500 years?
30.2 Radiation
Key Question:
What are some types
and sources of
radiation?
*Students read Section 30.2
AFTER Investigation 30.2
30.2 Radiation
 The word radiation means the flow of energy
through space.
 There are many forms of radiation.
 Light, radio waves, microwaves, and x-rays are
forms of electromagnetic radiation.
 Many people mistakenly think of radiation as only
associated with nuclear reactions.
30.2 Radiation
 The intensity of radiation measures how much power
flows per unit of area.
 When radiation comes from a single point, the
intensity decreases inversely as the square of the
distance.
 This is called the inverse square law and it applies to
all forms of radiation.
30.1 Intensity
Intensity
(W/m2)
Area (m2)
I=P
A
Intensity = 7.96 W/m2
Power (watt)
Intensity = 1.99 W/m2
30.2 Harmful radiation
 Radiation becomes
harmful when it has
enough energy to remove
electrons from atoms.
 The process of removing
an electron from an atom
is called ionization.
 Visible light is an
example of nonionizing
radiation.
 UV light is an example of
ionizing radiation.
30.2 Harmful radiation
 Ionizing radiation absorbed by people is measured
in a unit called the rem.
 The total amount of radiation received by a person
is called a dose, just like a dose of medicine.
 It is wise to limit your exposure to ionizing radiation
whenever possible.
 Use shielding materials, such as lead, and do your
work efficiently and quickly.
 Distance also reduces exposure.
30.2 Sources of radiation
 Ionizing radiation is a natural part of our
environment.
 There are two chief sources of radiation you will
probably be exposed to:
— background radiation.
— radiation from medical procedures such as x-rays.
 Background radiation results in an average dose of
0.3 rem per year for someone living in the United
States.
30.2 Background radiation
 Background radiation
levels can vary widely
from place to place.
— Cosmic rays are high
energy particles that come
from outside our solar
system.
— Radioactive material from
nuclear weapons is called
fallout.
— Radioactive radon gas is
present in basements and
the atmosphere.
30.2 X-ray machines
 X-rays are photons, like
visible light photons only
with much more energy.
 Diagnostic x-rays are used
to produce images of
bones and teeth on x-ray
film.
 Xray film turns black when
exposed to x-rays.
30.2 X-ray machines
 Therapeutic x-rays are used
to destroy diseased tissue,
such as cancer cells.
 Low levels of x-rays do not
destroy cells, but high levels
do.
 The beams are made to
overlap at the place where
the doctor wants to destroy
diseased cells.
30.2 CAT scan
 The advent of powerful
computers has made it possible
to produce three-dimensional
images of bones and other
structures within the body.
 To produce a CAT scan,
computerized axial tomography,
a computer controls an x-ray
machine as it takes pictures of
the body from different angles.
30.2 CAT scan
 People who work with radiation
use radiation detectors to tell
when radiation is present and to
measure its intensity.
 The Geiger counter is a type of
radiation detector invented to
measure x-rays and other
ionizing radiation, since they are
invisible to the naked eye.
30.3 Nuclear Reactions and Energy
Key Question:
How do we describe
nuclear reactions?
*Students read Section 30.3
AFTER Investigation 30.3
30.3 Nuclear Reactions and Energy
 A nuclear reaction is any process that changes
the nucleus of an atom.
 Radioactive decay is one form of nuclear reaction.
30.3 Nuclear Reactions and Energy
 If you could take apart a nucleus and separate all of
its protons and neutrons, the separated protons and
neutrons would have more mass than the nucleus
did.
 The mass of a nucleus is reduced by the energy
that is released when the nucleus comes together.
 Nuclear reactions can convert mass into energy.
30.3 Nuclear Reactions and Energy
 When separate protons and
neutrons come together in a
nucleus, energy is released.
 The more energy that is
released, the lower the
energy of the final nucleus.
 The energy of the nucleus
depends on the mass and
atomic number.
30.3 Fusion reactions
 A fusion reaction is a
nuclear reaction that
combines, or fuses, two
smaller nuclei into a larger
nucleus.
 It is difficult to make fusion
reactions occur because
positively charged nuclei
repel each other.
30.3 Fusion reactions
 A fusion reaction is a nuclear reaction that combines, or fuses,
two smaller nuclei into a larger nucleus.
30.3 Fission reactions
 A fission reaction splits
up a large nucleus into
smaller pieces.
 A fission reaction typically
happens when a neutron
hits a nucleus with
enough energy to make
the nucleus unstable.
30.3 Fission reactions
 The average energy of the nucleus for a combination of
molybdenum-99 (Mo-99) and tin-135 (Sn-135) is 25 TJ/kg.
 The fission of a kilogram of uranium into Mo-99 and Sn-135 releases
the difference in energies, or 98 trillion joules.
30.3 Rules for nuclear reactions
 Nuclear reactions obey conservation laws.
 Energy stored as mass must be included in order
to apply the law of conservation of energy to a
nuclear reaction.
 Nuclear reactions must conserve electric charge.
 The total baryon number before and after the
reaction must be the same.
 The total lepton number must stay the same
before and after the reaction.
30.3 Conservation Laws
 There are conservation laws that apply to the type of
particles before and after a nuclear reaction.
— Protons and neutrons belong to a family of particles called
baryons.
— Electrons come from a family of particles called leptons.
30.3 Calculating nuclear reactions
 The nuclear reaction above is proposed for
combining two atoms of silver to make an atom of
gold.
 This reaction cannot actually happen because it
breaks the rules for nuclear reactions.
 List two rules that are broken by the reaction.
30.3 Antimatter, neutrinos and others
particles
 The matter you meet in the world ordinarily
contains protons, neutrons, and electrons.
 Cosmic rays contain particles called muons and
pions.
 Thousands of particles called neutrinos from the
sun pass through you every second and you
cannot feel them.
30.3 Antimatter, neutrinos and others
particles
 Every particle of matter has an antimatter twin.
 Antimatter is the same as regular matter except
properties like electric charge are reversed.
— An antiproton is just like a normal proton except
it has a negative charge.
— An antielectron (also called a positron) is like an
ordinary electron except that it has positive
charge.
30.3 Neutrinos
 When beta decay was first discovered, physicists
were greatly disturbed to find that the energy of
the resulting proton and electron was less than
the energy of the disintegrating neutron.
 The famous Austrian physicist Wolfgang Pauli
proposed that there must be a very light,
previously undetected neutral particle that was
carrying away the missing energy.
 We now know the missing particle is a type of
neutrino.
30.3 Neutrinos
 Despite the difficulty of
detection, several carefully
constructed neutrino
experiments have
detected neutrinos coming
from nuclear reactions in
the sun.
Application: Nuclear Power