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Chapter 9
The Nucleus, Radioactivity, and
Nuclear Medicine

9.1 Natural Radioactivity
• Radioactivity
- process by which atoms emit energetic particles or rays
• Radiation
- the particles or rays emitted
– comes from the nucleus
• Nuclear symbols
- what we use to designate the nucleus
–
Atomic symbol
–
Atomic number
–
Mass number

Nuclear Symbols mass number number of protons and neutrons
11
B
5 atomic symbol atomic number number of protons

Writing Nuclear Symbols
11
B
5
•
This defines an isotope of boron
•
In nuclear chemistry, often called a nuclide
•
This is not the only isotope of boron
– boron-10 also exists
–
How many protons and neutrons does boron-10 have?
•
5 protons, 5 neutrons

Three Isotopes of Carbon
• Each nucleus contains the same number of protons
•
Only the number of neutrons is different
• With different numbers of neutrons the mass of each isotope is different

Unstable Isotopes
•
Some isotopes are stable
•
The unstable isotopes are the ones that produce radioactivity
•
To write nuclear equations we need to be able to write the symbols for the isotopes and the following:
– alpha particles
– beta particles
– gamma rays

Alpha Particles
• Alpha particle
( a
) - 2 protons, 2 neutrons
•
Same as He nucleus (He 2+ )
•
Slow moving, and stopped by small barriers
•
Symbolized in the following ways:
4
2
He
2

4
2
He
α
2
4 α

Beta Particles
• Beta particles
( b
) - fast-moving electron
•
Emitted from the nucleus as a neutron, is converted to a proton
•
Higher speed particles, more penetrating than alpha particles
•
Symbolized in the following ways:

0
1
0
1

Gamma Rays
• Gamma rays
( g
) - pure energy
(electromagnetic radiation)
• Highly energetic
• The most penetrating form of radiation
• Symbol is simply… g

Properties of Alpha, Beta, and
Gamma Radiation
• Ionizing radiation
- produces a trail of ions throughout the material that it penetrates
• The penetrating power of the radiation determines the ionizing damage that can be caused
• Alpha particle < beta particle < gamma rays

9.2 Writing a Balanced
Nuclear Equation
• Nuclear equation
- used to represent nuclear change
•
In a nuclear equation, you do not balance the elements, instead...
– the total mass on each side of the reaction arrow must be identical
– the sum of the atomic numbers on each side of the reaction arrow must be identical

Alpha Decay
238
92
U
 234
90
Th

2
4
He
238 = 234 + 4 mass number
92 = 90 + 2 atomic number

Beta Decay
•
Upon decomposition, nitrogen-16 produces oxygen-16 and a beta particle
•
In beta decay, one neutron in nitrogen-
16 is converted to a proton and the electron, the beta particle is released
16
7
N
 16
8
O
 0
1 e

Gamma Production
•
Gamma radiation occurs to increase the stability of an isotope
–
The energetically unstable isotope is called a metastable isotope
•
The atomic mass and number do not change
•
Usually gamma rays are emitted along with alpha or beta particles
99 m
43
Tc
 99
43
Tc
 g

Predicting Products of Nuclear
Decay
•
To predict the product, simply remember that the mass number and atomic number are conserved
239
U
92

X
 0
1 e
•
What is the identity of X?
239
93
Np

9.3 Properties of Radioisotopes
Nuclear Structure and Stability
• Binding energy
- the energy that holds the protons, neutrons, and other particles together in the nucleus
•
Binding energy is very large
•
When isotopes decay (forming more stable isotopes) binding energy is released

Stable Radioisotopes
Important factors for stable isotopes
–
Ratio of neutrons to protons
–
Nuclei with large number of protons (84 or more) tend to be unstable
– The “magic numbers” of 2, 8, 20, 50, 82, or 126 help determine stability – these numbers of protons or neutrons are stable
–
Even numbers of protons or neutrons are generally more stable than those with odd numbers
–
All isotopes (except 1 H) with more protons than neutrons are unstable

Half-Life
• Half-life
(t
1/2
) - the time required for one-half of a given quantity of a substance to undergo change
•
Each radioactive isotope has its own half-life
– Ranges from a fraction of a second to a billion years
– The shorter the half-life, the more unstable the isotope

Half-Lives of Selected
Radioisotopes

Decay Curve for the Medically
Useful Radioisotope Tc-99m

Predicting the Extent of
Radioactive Decay
A patient receives 10.0 ng of a radioisotope with a half-life of 12 hours. How much will remain in the body after 2.0 days, assuming radioactive decay is the only path for removal of the isotope from the body?
•
Calculate n , the number of half-lives elapsed using the half-life as the conversion factor n = 2.0 days x 1 half-life / 0.5 days = 4 half lives
• Calculate the amount remaining
10.0 ng 5.0 ng 2.5 ng 1.3 ng 0.63 ng
1 st half-life 2 nd half-life 3 rd half-life
• 0.63 ng remain after 4 half-lives
4 th half-life

9.4 Nuclear Power
Energy Production
E = mc 2
•
Equation by Albert Einstein shows the connection between energy ( E ) and mass ( m )
• c is the speed of light
• The equation shows that a very large amount of kinetic energy can be formed from a small amount of matter
– Release this kinetic energy to convert liquid water into steam
–
The steam drives an electrical generator producing electricity

Nuclear Fission
•
Fission (splitting) - occurs when a heavy nuclear particle is split into smaller nuclei by a smaller nuclear particle
0
1 n
 235
U

92
236
U

92
92
36
Kr
 141
Ba
56

3
0
1 n
 energy
•
Accompanied by a large amount of energy
•
Is self-perpetuating
•
Can be used to generate steam

Fission of Uranium-235
•
Chain reaction - the reaction sustains itself by producing more neutrons

Representation of the “Energy
Zones” of a Nuclear Reactor
•
A nuclear power plant uses a fissionable material as fuel
– Energy released by the fission heats water
–
Produces steam
–
Drives a generator or turbine
–
Converts heat to electrical energy

Nuclear Fusion
• Fusion (to join together) - combination of two small nuclei to form a larger nucleus
• Large amounts of energy is released
•
Best example is the sun
•
An Example: 2
1
H
 3
1
H
 4
2
He

0
1 n
 energy
•
No commercially successful plant exists in U.S.

Breeder Reactors
•
Breeder reactor - fission reactor that manufactures its own fuel
•
Uranium-238 (non-fissionable) is converted to plutonium-239 (fissionable)
•
Plutonium-239 undergoes fission to produce energy

9.5 Radiocarbon Dating
• Radiocarbon dating
- the estimation of the age of objects through measurement of isotopic ratios of carbon
– Ratio of carbon-14 and carbon-12
•
Basis for dating:
–
Carbon-14 (a radioactive isotope) is constantly being produced by neutrons from the sun
14
7
N

0
1 n
 14
6
C
 1
1
H

Radiocarbon Dating
•
Living systems are continually taking in carbon
–
The ratio of carbon-14 to carbon-12 stays constant during its lifetime
•
Once the living system dies, it quits taking in the carbon-14
–
The amount of carbon-14 decreases according to the reaction: 14
6
C
 14
7
N
 0
1 e
• The half-life of carbon-14 is 5730 years
–
This information is used to calculate the age

9.6 Medical Applications of
Radioactivity
•
Modern medical care uses the following:
–
Radiation in the treatment of cancer
– Nuclear medicine
- the use of radioisotopes in the diagnosis of medical conditions

Cancer Therapy Using Radiation
•
Based on the fact that high-energy gamma rays cause damage to biological molecules
•
Tumor cells are more susceptible than normal cells
•
Example: cobalt-60
•
Gamma radiation can cure cancer, but can also cause cancer

Nuclear Medicine
•
The use of isotopes in diagnosis
• Tracers
small amounts of radioactive substances used as probes to study internal organs
• Nuclear imaging
- medical techniques involving tracers
•
Example:
–
Iodine concentrates in the thyroid gland
–
Using radioactive 131 I and 125 I will allow the study of how the thyroid gland is taking in iodine

Tracer Studies
• Isotopes with short half-lives are preferred for tracer studies. Why?
– They give a more concentrated burst
–
They are removed more quickly from the body
•
Examples of imaging procedures:
– Bone disease and injury using technetium-99m
– Cardiovascular disease using thallium-201
–
Pulmonary disease using xenon-133

Making Isotopes for Medical
Applications
• Artificial radioactivity - a normally stable, nonradioactive nucleus is made radioactive
• Made in two ways:
•
In core of a nuclear reactor
• In particle accelerators – small nuclear particles are accelerated to speeds approaching the speed of light and slammed into another nucleus

Examples of Artificial
Radioactivity
197
79
Au

0
1 n
 198
Au
79
•
Tracer in the liver
• Used in the diagnosis of Hodgkin’s disease
66
30
Zn
 1
1 p
 67
31
Ga

Preparation of Technetium-99m
•
Some isotopes used in nuclear medicine have such a short half-life that they need to be generated on site
• 99 m Tc has a half-life of only 6 hours
99
42
Mo
 99m
43
Tc
 0
1 e

9.7 Biological Effects of
Radiation
Radiation Exposure and Safety
The Magnitude of the Half-Life
•
Isotopes with short half-lives have one major disadvantage and one major advantage
–
Disadvantage: larger amount of radioactivity per unit time
–
Advantage: if accident occurs, reaches background radiation levels more rapidly

Radiation Exposure and Safety
Shielding
•
Alpha and beta particles need a low level of shielding: lab coat and gloves
•
Lead, concrete or both are required for gamma rays
Distance from the Radioactive Source
•
Doubling the distance from the source decreases the intensity by a factor of 4

Radiation Exposure and Safety
Time of Exposure
•
Effects are cumulative
Types of Radiation Emitted
•
Alpha and beta emitters are generally less hazardous then gamma emitters
Waste Disposal
•
Disposal sites are considered temporary

9.8 Measurement of Radiation
Nuclear Imaging
•
Isotope is administered
•
Isotope begins to concentrate in the organ
•
Photographs (nuclear images) are taken at periodic intervals
•
Emission of radioactive isotope creates the image

Computer Imaging
•
Computers and television are coupled
•
Gives a continuous and instantaneous record of the voyage of the isotope throughout the body
– Gives increased sensitivity
–
CT scanner is an example

The Geiger Counter
• Detects ionizing radiation
• Has largely been replaced by more sophisticated devices

Film Badges
• A piece of photographic film that is sensitive to energies corresponding to radioactive emissions
• The darker the film, when developed, the longer the worker has been exposed

Units of Radiation Measurement
The Curie
•
The amount of radioactive material that produces 3.7 x 10 10 atomic disintegrations per second
•
Independent of the nature of the radiation

Units of Radiation Measurement
The Roentgen
•
The amount of radiation needed to produce 2 x 10 9 ion pairs when passing through one cm 3 of air at 0 o C
•
Used for very high energy ionizing radiation only

Units of Radiation Measurement
Rad Radiation absorbed dosage
•
The dosage of radiation able to transfer
2.4 x 10 -3 cal of energy to one kg of matter
•
This takes into account the nature of the absorbing material

Units of Radiation Measurement
The Rem
• Roentgen Equivalent for Man
• Obtained by multiplication of the rad by a factor called the relative biological effect (RBE)
• RBE = 10 for alpha particles
• RBE = 1 for beta particles
• Lethal dose (LD
50
) - the acute dosage of radiation that would be fatal for 50% of the exposed population
– LD
50
= 500 rems