Chapter 10
Lecture
Outline
Prepared by
Andrea D. Leonard
University of Louisiana at Lafayette
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
10.1 Introduction
A. Isotopes
atomic number (Z)
mass number (A)
mass number (A)
atomic number (Z)
=
the number of protons
=
the number of protons
+
the number of neutrons
12
6
C
number of protons
number of neutrons
6
12 – 6 = 6
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10.1 Introduction
A. Isotopes
Isotopes are atoms of the same element having a
different number of neutrons.
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10.1 Introduction
A. Isotopes
•A radioactive isotope, called a radioisotope, is
unstable and spontaneously emits energy to form
a more stable nucleus.
•Radioactivity is the nuclear radiation emitted by a
radioactive isotope.
•Of the known isotopes of all elements, 264 are
stable and 300 are naturally occurring but unstable.
•An even larger number of radioactive isotopes,
called artificial isotopes, have been produced in
the laboratory.
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10.1 Introduction
B. Types of Radiation
• Types of radiation: alpha particles, beta particles,
positrons, and gamma radiation.
• An alpha (α) particle is a high-energy particle that
contains 2 protons and 2 neutrons.
• It has a +2 charge and a mass number of 4.
alpha particle:
a
or
4
He
2
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10.1 Introduction
B. Types of Radiation
• Alpha particles are the heaviest of the radioactive
particles, so they can’t penetrate materials very far.
• Alpha particles can be stopped by a piece of paper
or your skin.
• Lab coats and gloves are sufficient protection from
alpha particles.
• Some uses: smoke detectors, antistatic devices
• Some sources: Radon-222, Uranium-238, Thorium232
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10.1 Introduction
B. Types of Radiation
•A beta (β) particle is a high-energy electron.
•It has a −1 charge and a negligible mass
compared to a proton.
0
e
−1
•A β particle is formed when a neutron (n) is
converted to a proton (p) and an electron (e).
β
beta particle:
1
n
0
1
p
1
neutron
proton
or
+
0
e
−1
 particle
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10.1 Introduction
B. Types of Radiation
• β particles are much lighter than alpha particles, and
can penetrate human tissue.
• Heavy lab coats and gloves must be worn when
working with substances that emit them.
•
β particles are widely used in radiation therapy
• Some isotopes – Carbon-14, Phosphorus-32, Iodine131
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10.1 Introduction
B. Types of Radiation
•A positron is called an antiparticle of a β particle.
•Their charges are opposite, but their masses are
the same (i.e., effectively zero).
•A positron has a +1 charge and is called a “positive
electron.”
0
e
+1
•A positron is formed when a proton is converted
to a neutron.
1
1
0
p
n
e
+
1
0
+1
proton
neutron
positron
positron:
β+ or
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10.1 Introduction
B. Types of Radiation
• Substances that emit positrons are used Positron
Emission Tomography (PET)
• Some isotopes – Carbon-11, Fluorine-18
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10.1 Introduction
B. Types of Radiation
•Gamma rays are high-energy radiation released
from a radioactive nucleus.
•They are a form of energy, so they have no mass
and no charge.
gamma ray:
g
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10.1 Introduction
B. Types of Radiation
• Gamma rays are sometimes emitted by themselves
• More often, gamma rays accompany alpha or beta
particles
• Gamma rays are useful in medical imaging and in
cancer treatment
• A thick lead shield is needed to stop gamma rays.
• Some isotopes: Cobalt-60, Technetium-99m
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10.1 Introduction
B. Types of Radiation
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10.1 Introduction
C. The Effects of Radioactivity
•Radioactivity cannot be detected by the senses,
yet it can have a powerful effect.
•Nuclear radiation will damage or kill rapidly
dividing cells such as bone marrow, skin, and the
reproductive and intestinal systems.
•Cancer cells divide rapidly as well, making
radiation an effective treatment for cancer.
•Food is irradiated, exposed to gamma radiation,
to kill any living organism in the food.
•Afterwards, the food is not radioactive, and has
a considerably longer shelf life.
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10.2 Nuclear Reactions
Radioactive decay is the process by which an
unstable radioactive nucleus emits radiation.
A nuclear equation can be written for this process:
original
nucleus
new
nucleus
+
radiation
emitted
The following must be equal on both sides of a
nuclear equation :
•The sum of the mass numbers (A)
•The sum of the atomic numbers (Z)
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10.2 Nuclear Reactions
A. Alpha Emission
Alpha emission (alpha decay) is the decay of a
nucleus by emitting an a particle.
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10.2 Nuclear Reactions
A. Alpha Emission
HOW TO Balance an Equation for a Nuclear Reaction
Example
Write a balanced nuclear equation showing
how americium-241 decays to form an
a particle.
Step [1]
Write an incomplete equation with the
original nucleus on the left and the particle
emitted on the right.
241
95 Am
4
2 He
+
?
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10.2 Nuclear Reactions
A. Alpha Emission
HOW TO Balance an Equation for a Nuclear Reaction
Calculate the mass number and atomic
number of the newly formed nucleus on
the right.
4
241
237
+
Np
He
Am
2
95
93
Step [2]
mass number
241 − 4 = 237
Step [3]
atomic number
95 − 2 = 93
Use the atomic number to identify the new
nucleus and complete the equation.
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10.2 Nuclear Reactions
B. Beta Emission
Beta emission is the decay of a nucleus by emitting
a β particle; 1 neutron is lost and 1 proton is gained.
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10.2 Nuclear Reactions
C. Positron Emission
Positron emission is the decay of a nucleus by emitting
a positron, β+; 1 proton is lost and 1 neutron is gained.
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10.2 Nuclear Reactions
D. Gamma Emission
Gamma emission is the decay of a nucleus by
emitting
g radiation.
•The g rays are a form of energy only.
•Their emission causes no change in the atomic
number or the mass number.
99m
43 Tc
99
43 Tc
+
g
•Technetium-99m is a metastable isotope; it
decays by gamma emission to the more stable
(but still radioactive) technetium-99.
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10.2 Nuclear Reactions
D. Gamma Emission
Commonly, g emission accompanies a or β emission.
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10.3 Half-Life
A. General Features
The half-life (t1/2) of a radioactive isotope is the time it
takes for one-half of the sample to decay.
The half-life of a radioactive isotope is a property of a
given isotope and is independent of the amount of
sample, temperature, and pressure.
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10.3 Half-Life
A. General Features
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10.3 Half-Life
A. General Features
HOW TO Use a Half-Life to Determine the Amount of
Radioisotope Present
Example If the half-life of iodine-131 is 8.0 days, how
much of a 100. mg sample remains after
32 days?
Step [1]
Determine how many half-lives occur in the
given amount of time.
32 days x 1 half-life =
8.0 days
4.0 half-lives
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10.3 Half-Life
A. General Features
HOW TO Use a Half-Life to Determine the Amount of
Radioisotope Present
Step [2]
100. mg
For each half-life, multiply the initial mass
by one-half to obtain the final mass.
x
1
2
x
1
2
x
1
2
x
1
2
=
6.25 mg
final mass
initial mass
The mass is halved four times.
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10.3 Half-Life
B. Archaeological Dating
•Radiocarbon dating uses the half-life of carbon-14
to determine the age of carbon-containing materials.
•The ratio of radioactive carbon-14 to stable
carbon-12 is a constant value in a living organism.
•Once the organism dies, the carbon-14 decays
without being replenished.
•By comparing the ratio of C-14 to C-12 in an
artifact to the same ratio present in organisms
today, the age of the artifact can be determined.
•The half-life of C-14 is 5,730 years.
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10.4 Detecting and Measuring
Radioactivity
Several units are used to measure the amount of
radiation absorbed by an organism.
•The rad—radiation absorbed dose—is the amount
of radiation absorbed by one gram of a substance.
•The rem—radiation equivalent for man—is the
amount of radiation that also factors in its energy
and potential to damage tissue.
•1 rem of any type of radiation produces the
same amount of tissue damage.
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10.4 Detecting and Measuring
Radioactivity
•The average radiation dose per year for a person
is about 0.27 rem.
•Generally, no detectable biological effects are
noticed for a radiation dose less than 25 rem.
•A single dose of 25–100 rem causes a temporary
decrease in white blood cell count.
•A dose of more than 100 rem causes radiation
sickness—nausea, vomiting, fatigue, etc.
•The LD50—the lethal dose that kills 50% of a
population—is 500 rem in humans, while 600
rem is fatal for an entire population.
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http://www.epa.gov/radiation/understand/perspective.html
10.5 Focus on Health and Medicine
A. Radioisotopes Used in Diagnosis
•Radioisotopes can be injected or ingested to
determine if an organ is functioning properly or
to detect the presence of a tumor.
•Technetium-99m is used to evaluate the gall
bladder and bile ducts and to detect internal
bleeding.
•Thallium-201 is used in stress tests to diagnose
coronary artery disease.
•Using a scan, normal organs are clearly visible,
while malfunctioning or obstructed organs are not.
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10.5 Focus on Health and Medicine
A. Radioisotopes Used in Diagnosis
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10.5 Focus on Health and Medicine
B. Radioisotopes Used in Treatment
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10.5 Focus on Health and Medicine
C. Positron Emission Tomography
•Positron emission tomography (PET) scans
use radioisotopes which emit positrons which
enable scanning of an organ.
•PET scans can detect tumors, coronary artery
disease, Alzheimer’s disease, and track the
progress of cancer.
•A PET scan is a noninvasive method of
monitoring cancer treatment.
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10.5 Focus on Health and Medicine
C. Positron Emission Tomography
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10.6 Nuclear Fission and Nuclear Fusion
A. Nuclear Fission
Nuclear fission is the splitting apart of a heavy
nucleus into lighter nuclei and neutrons. It can begin
when a neutron bombards a uranium-235 nucleus:
235
92 U
+
1
n
0
91
36 Kr
+
142
1
+
n
3
56 Ba
0
•The bombarded U-235 nucleus splits apart into
krypton-91, barium-142, and three high-energy
neutrons, while releasing a great deal of energy.
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10.6 Nuclear Fission and Nuclear Fusion
A. Nuclear Fission
Nuclear fission is the splitting apart of a heavy
nucleus into lighter nuclei and neutrons. It can begin
when a neutron bombards a uranium-235 nucleus:
235
92 U
+
1
n
0
91
36 Kr
+
142
1
+
n
3
56 Ba
0
•The released neutrons can then bombard other
uranium nuclei, creating a chain reaction.
•Critical mass: The minimum amount of U-235
needed to sustain a chain reaction.
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10.6 Nuclear Fission and Nuclear Fusion
A. Nuclear Fission
•A nuclear power plant uses the large amount of
energy released in fission.
•This energy is used to boil water and create steam,
which turns a turbine and generates electricity.
•The dangers of generating nuclear power are
possible radiation leaks and the disposal of
nuclear waste.
•Radiation leaks can be minimized by containment
facilities within the power plant itself.
•Nuclear waste is currently buried, but it is unclear
whether this is the best method.
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10.6 Nuclear Fission and Nuclear Fusion
B. Nuclear Fission
Nuclear fusion is the joining together of two light
nuclei to form a larger nucleus.
•Hydrogen-2 (deuterium) and hydrogen-3 (tritium)
undergo fusion to create a helium nucleus:
2
H
1
+
3
H
1
4
He
2
+
1
n
0
•A neutron and a large amount of energy are also
produced.
Fusion is not currently useable as an energy source
because it can only occur at extremely high
temperatures and pressures.
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