Download Chapter 9 Nuclear Radiation

Chapter 9 Nuclear Radiation
1
9.1
Natural Radioactivity
Stable and Radioactive Isotopes
A radioactive isotope
• has an unstable nucleus
• emits radiation to become more stable, changing the
composition of the nucleus
• is identified by the mass number of the isotope
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Nuclear Radiation
Nuclear radiation is the radiation emitted by an unstable
atom and takes the form of alpha particles, neutrons,
beta particles, positrons, or gamma rays.
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Types of Radiation
Alpha () particle is two protons and two neutrons.
4 He
2
Beta () particle is a high-energy electron.
0e
1
Positron (+) is a positive electron.
0e
1
Gamma rays are the high-energy radiation
released from a nucleus when it decays.
0
0
4
Learning Check
Give the name and symbol for the following types
of radiation.
A. alpha radiation
B. radiation with a mass number of 0 and atomic number
of +1
C. radiation that represents high-energy radiation
released from the nucleus
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Solution
Give the name and symbol for the following types
of radiation.
4 He
A. alpha radiation
2
B. radiation with a mass number of 0 and
atomic number of +1
C. radiation that represents high-energy
radiation released from the nucleus
0e
1
0
0
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Radiation Protection
Radiation protection requires
• paper and clothing for alpha particles
• a lab coat or gloves for beta particles
• a lead shield or a thick concrete wall for gamma rays
• limiting the amount of time spent near a radioactive
source
• increasing distance from the source
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Shielding for Radiation Protection
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Learning Check
What type of radiation will be blocked by
A. lead shield
B. paper, clothing
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Solution
What type of radiation will be blocked by
A. lead shield
B. paper, clothing
gamma radiation
alpha radiation
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Chapter 9 Nuclear Radiation
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9.2
Nuclear Equations
Balancing Nuclear Equations
In a balanced nuclear equation, particles are emitted
from the nucleus, and
• the mass number and atomic number may change
• the sum of the mass numbers and the sum of the atomic
numbers are equal for the reactants and the products
Mass number sum:
Atomic number sum:
251 =
251Cf 
98
251
247Cm +
96
98
98
=
4He
2
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Guide to Balancing Nuclear Equations
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Alpha Decay
When a radioactive nucleus emits an alpha particle, a
new nucleus forms with a mass number that is
decreased by 4 and an atomic number that is decreased
by 2.
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Equation for Alpha Emission 24He
Write an equation for the alpha decay of Rn-222.
Step 1 Write the incomplete nuclear equation.
222Rn  ?  4He
86
2
Step 2 Determine the missing mass number.
222 – 4 = 218
Step 3 Determine the missing atomic number.
86 – 2 = 84
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Equation for Alpha Emission
Write an equation for the alpha decay of Rn-222.
Step 4 Determine the symbol of the new nucleus.
84 = Po
Step 5 Complete the nuclear equation.
222Rn  218Po  4He
86
84
2
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Beta Emission
A beta particle is an electron emitted from the nucleus
when a neutron in the nucleus breaks down, increasing
the atomic number by 1.
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Equation for Beta Emission
Write an equation for the decay of 42K, a beta emitter.
Step 1 Write the incomplete nuclear equation.
42K  ?  0e
19
1
Step 2 Determine the missing mass number.
42 – 0 = 42
Step 3 Determine the missing atomic number.
19  (1) = 20
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Equation for Beta Emission
Write an equation for the decay of 42K, a beta emitter.
Step 4 Determine the symbol of the new nucleus.
20 = Ca
Step 5 Complete the nuclear equation.
42K  42Ca  0e
19
20
1
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Learning Check
Write the nuclear equation for the beta decay of 60Co.
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Solution
Write the nuclear equation for the beta decay of
Step 1 Write the incomplete nuclear equation.
60Co.
60Co  ?  0e
27
1
Step 2 Determine the missing mass number.
60 – 0 = 60
Step 3 Determine the missing atomic number.
27  (1) = 28
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Solution
Write the nuclear equation for the beta decay of
60Co.
Step 4 Determine the symbol of the new nucleus.
28 = Ni
Step 5 Complete the nuclear equation.
60Co  60Ni  0e
27
28
1
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Positron Emission
0e
1
In positron emission,
• a proton is converted to a neutron and a positron.
1 p  1n  0e
1
0
1
• the mass number of the new nucleus is the same, but
the atomic number decreases by 1.
49Mn  49Cr  0e
25
24
1
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Gamma Radiation
0
0
In gamma radiation,
• energy is emitted from an unstable nucleus, indicated by
m following the mass number
• the mass number and the atomic number of the new
nucleus are the same
99mTc  99Tc  0
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43
0
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Summary of Types of Radiation
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Producing Radioactive Isotopes
Radioactive isotopes are produced
• when a stable nucleus is converted to a radioactive
nucleus by bombarding it with a small particle
• in a process called transmutation
When nonradioactive B-10 is bombarded by an alpha particle, the products
are radioactive N-13 and a neutron.
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Learning Check
What radioactive isotope is produced when a neutron
bombards 98Tc, releasing an alpha particle?
98Tc  1n  ?  4He
27
0
2
27
Solution
What radioactive isotope is produced when a neutron
bombards 98Tc, releasing an alpha particle?
Step 1 Write the incomplete nuclear equation.
98Tc  1n  ?  4He
27
0
2
Step 2 Determine the missing mass number.
98 + 1  4 = 95
Step 3 Determine the missing atomic number.
27  2 = 25
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Solution
What radioactive isotope is produced when a neutron
bombards 98Tc, releasing an alpha particle?
Step 4 Determine the symbol of the new nucleus.
25 = Mn
Step 5 Complete the nuclear equation.
98Tc  1n  95Mn  4He
27
0
25
2
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Chapter 9 Nuclear Radiation
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9.3
Radiation Measurement
Geiger Counter
A Geiger counter
• detects beta and gamma radiation
• uses ions produced by radiation to create an electrical
current
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Units for Measuring Radiation
Units for measuring radiation activity include
• Curie – measures activity as the number of atoms that
decay in 1 second
• rad (radiation absorbed dose) – measures the radiation
absorbed by the tissues of the body
• rem (radiation equivalent) – measures the biological
damage caused by different types of radiation
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Measuring Radiation Activity
Often the measurement for an equivalent dose is in
millirems (mrem). One rem is equal to 1000 mrem. The
SI unit is the sievert (Sv). One sievert is equal to 100 rem.
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Radiation Exposure
Exposure to radiation occurs from
• naturally occurring
radioisotopes
• medical and dental procedures
• air travel, radon, and smoking
cigarettes
• cosmic rays
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Radiation Sickness
LD50
• is the amount of radiation
to the whole body that is
the lethal dose for one-half
the population
• varies for different
life-forms, as
Table 9.6 shows
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Learning Check
A typical intravenous dose of I-125 for a thyroid
diagnostic test is 100 Ci. What is this dosage in
megabecquerels (MBq) (3.7  1010 Bq = 1 Ci)?
A. 3.7 MBq
B. 3.7  106 MBq
C. 2.7  102 MBq
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Solution
A typical intravenous dose of I-125 for a thyroid diagnostic test
is 100 Ci. What is this dosage in megabecquerels (MBq)
(3.7  1010 Bq = 1 Ci)?
100 Ci  1 Ci
 3.7  1010 Bq  1 MBq x
1 x 106 Ci
1 Ci
1  106 Bq
= 3.7 MBq The answer is A.
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Chapter 9 Nuclear Radiation
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9.4
Half-Life of a Radioisotope
Half-Life and Decay Curves
The half-life of a radioisotope is the time for the radiation
level to decrease (decay) to one-half of the original value.
The decay curve for I-131 shows that one-half the sample
decays every 8 days.
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Half-Lives of Some Radioisotopes
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Guide to Using Half-Lives
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Half-Life Calculations
The radioisotope strontium-90 has a half-life of 38.1 years. If a
sample contains 36 mg of Sr-90, how many milligrams will
remain after 152.4 years?
Step 1 State the given and needed quantities.
Given: 36 mg, 152.4 years, half-life of 38.1 years
Needed: mg Sr-90 remaining
Step 2 Write a plan to calculate unknown quantity.
Half-life
152.4 years
number of half-lives
Number of half-lives
36 mg Sr-90
mg Sr-90 remaining
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Half-Life Calculations
The radioisotope strontium-90 has a half-life of 38.1 years. If a
sample contains 36 mg of Sr-90, how many milligrams will
remain after 152.4 years?
Step 3 Write the half-life equality and conversion factors.
1 half-life = 38.1 years
38.1 years
1 half-life
and
1 half-life
38.1 years
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Half-Life Calculations
The radioisotope strontium-90 has a half-life of 38.1 years. If a
sample contains 36 mg of Sr-90, how many milligrams will
remain after 152.4 years?
Step 4 Set up the problem to calculate amount of active
radioisotope.
Number of half-lives
4 years x 1 half-life = 4 half-lives
38.1 years
1 half-life
2 half-lives 3 half-lives 4 half-lives
36 mg  18 mg  9 mg  4.5 mg  2.3 mg Sr-90
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Learning Check
Carbon-14 was used to
determine the age of the Dead
Sea Scrolls. If the Dead Sea
Scrolls were determined to be
2000 years old and the half-life
of carbon-14 is 5730 years,
what fraction of this half-life
has passed?
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Solution
Carbon-14 was used to determine the age of the Dead Sea
Scrolls. If the Dead Sea Scrolls were determined to be
2000 years old and the half-life of carbon-14 is 5730 years,
what fraction of this half-life has passed?
2000 years x 1 half-life = 0.35 half-life
5730 years
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Learning Check
The half-life of I-123 is 13 h. How much of a 64-mg
sample of I-123 is left after 26 hours?
A. 32 mg
B. 16 mg
C. 8 mg
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Solution
The half-life of I-123 is 13 h. How much of a 64-mg sample of
I-123 is left after 26 hours?
Step 1 State the given and needed quantities.
Given: 64 mg, 26 hours, half-life of 13 hours
Needed: mg I-123 remaining
Step 2 Write a plan to calculate unknown quantity.
Half-life
26 hours
number of half-lives
Number of half-lives
64 mg I-123
mg I-123 remaining
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Solution
The half-life of I-123 is 13 h. How much of a 64-mg sample of
I-123 is left after 26 hours?
Step 3 Write the half-life equality and conversion factors.
1 half-life = 13 hours
13 hours and
1 half-life
1 half-life
13 hours
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Solution
The half-life of I-123 is 13 h. How much of a 64-mg sample of
I-123 is left after 26 hours?
Step 4 Set up the problem to calculate amount of active
radioisotope.
Number of half-lives
26 hours x 1 half-life = 2 half-lives
13 hours
1 half-life
2 half-lives
64 mg  32 mg  16 mg I-123
The answer is B, 16 mg of I-123 remain.
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Chapter 9 Nuclear Radiation
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9.5
Medical Applications
Using Radioactivity
Medical Applications
Radioisotopes with short half-lives are used in nuclear
medicine because
• they have the same chemistry in the body as the
nonradioactive atoms
• in the organs of the body, they give off radiation that
exposes a photographic plate (scan), giving an image
of an organ
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Medical Applications of Radioisotopes
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Scans with Radioisotopes
After a radioisotope is ingested by the patient,
• the radiologist determines the level and location of
the radioactivity emitted by the radioisotope
• the scanner moves slowly over the organ where the
radioisotope is absorbed
• the gamma rays emitted from the radioisotope can be
used to expose a photographic plate, giving a scan of
the organ
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Scans with Radioisotopes
(a) A scanner is used to detect radiation from a
radioisotope that has accumulated in an organ.
(b) A scan of the thyroid shows the accumulation of
radioactive iodine-131 in the thyroid.
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Positron Emission Tomography (PET)
Positron emitters with short half-lives
• can be used to study brain function, metabolism, and
blood flow
• include carbon-11, oxygen-15, nitrogen-13, and fluorine-18
18
18
0
F

O

• combine
with
after emission to produce gamma rays,
9
8 electrons
1e
which are then detected by computers, creating a 3-D image of the
organ
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Positron Emission Tomography (PET)
These PET scans of the brain show a normal brain on the left and a
brain affected by Alzheimer’s disease on the right.
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Computed Tomography (CT)
A computer monitors the absorption of 30 000 X-ray
beams directed at the brain in successive layers.
Differences in absorption
based on tissue densities
and fluids provide images
of the brain.
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Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging
• is an imaging technique that does not involve
X-ray radiation
• is the least invasive imaging method available
• is based on absorption of energy when protons in
hydrogen atoms are excited by a strong magnetic field
• works because the energy absorbed is converted to color
images of the body
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Magnetic Resonance Imaging
An MRI scan of the heart and lungs, with the left ventricle
shown in red.
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Learning Check
Which of the following radioisotopes are most likely to be
used in nuclear medicine?
A.
40K
half-life 1.3 x 109 years
B.
42K
half-life 12 hours
C.
131I
half-life 8 days
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Solution
Which of the following radioisotopes are most likely to be used
in nuclear medicine?
Radioisotopes with short half-lives are used in nuclear medicine.
A.
40K
half-life 1.3  109 years Not likely: half-life is too long.
B.
42K
half-life 12 hours
Short half-life, likely used.
C.
131I
half-life 8 days
Short half-life, likely used.
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Chapter 9 Nuclear Radiation
9.6
Nuclear Fission and Fusion
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Nuclear Fission
In nuclear fission,
• a large nucleus is bombarded with a small particle
• the nucleus splits into smaller nuclei and several
neutrons
• large amounts of energy are released
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Nuclear Fission
When a neutron bombards 235U,
•
•
•
•
an unstable nucleus of 236U undergoes fission (splits)
the nucleus splits to release large amounts of energy
smaller nuclei are produced, such as Kr-91 and Ba-142
neutrons are also released to bombard more 235U nuclei
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Nuclear Fission Diagram
1n
0
+ 235U
92
236U
92
91Kr
36
+
142Ba
56
+ 3 1n + energy
0
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Nuclear Fission Chain Reaction
In a nuclear chain reaction,
the fission of each U-235
atom produces three
neutrons that cause the
nuclear
fission of more and more
uranium-235 atoms.
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Energy and Nuclear Fission
The total mass of the products in this reaction is slightly less
than the mass of the starting materials. The missing mass
has been converted into energy, consistent with the famous
equation derived by Albert Einstein, E = mc2.
E is the energy released, m is the mass lost, and c is the
speed of light, 3.0  108 m/s. Using this equation, a small
amount of mass is multiplied by the speed of light squared,
resulting in a large amount of energy.
Fission of 1 g U-235 produces same energy as 3 tons of coal.
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Learning Check
Supply the missing atomic symbol to complete the equation
for the following nuclear fission reaction.
1n
0
69
+
235U
92
137Te
52
+ ??X + 2 01n + energy
Solution
Supply the missing atomic symbol to complete the equation
for the following nuclear fission reaction.
1n
0
70
+
235U
92
137Te
52
+
97Zr
40
+ 2 01n + energy
Nuclear Power Plants
In nuclear power plants,
• fission is used to produce energy
• control rods in the reactor absorb neutrons to slow
and control the chain reactions of fission
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Nuclear Fusion
Fusion
• occurs at extremely high temperatures (100 000 000 C)
• combines small nuclei into larger nuclei
• releases large amounts of energy
• occurs continuously in the sun and stars
Hydrogen atoms combine in a fusion reactor at very high temperatures.
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Learning Check
Indicate whether each of the following describes nuclear
fission or nuclear fusion.
___ A.
___ B.
___ C.
___ D.
___ E.
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A nucleus splits.
Large amounts of energy are released.
Small nuclei form larger nuclei.
Hydrogen nuclei react.
Several neutrons are released.
Solution
Indicate whether each of the following is nuclear fission or
nuclear fusion.
nuclear fission
A. A nucleus splits.
nuclear fission and fusion B. Large amounts of energy
are released.
nuclear fusion
C. Small nuclei form larger nuclei.
nuclear fusion
D. Hydrogen nuclei react.
nuclear fission
E. Several neutrons are
released.
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Nuclear Radiation – Concept Map
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