Here are the steps to solve a half-life problem: 1. Pick a starting mass (e.g. 30 g) 2. Choose a half-life (e.g. 5 years) 3. Plot the mass after each half-life on a graph with mass on the y-axis and time/half-lives on the x-axis 4. The graph will be exponential decay 5. Mass never reaches exactly zero 6. Radioactivity is no longer a problem after about 5-7 half-lives as the mass becomes very small 7. Exponential decay is described by the equation: M = M0 * (1/2)^(t/t1/2)

1. Nuclear Chemistry
P Squires
Basic Chemistry
2005-2006

2. Isotopes …
…have the same number of
protons and electrons but
different numbers of
neutrons.

3. Isotopes …
…have different mass
numbers but the same atomic
number.

4. Symbols for Isotopes
Mass number
Symbol
U
235
92 of
Element
Atomic
number

5. Symbols for Isotopes
Mass number
A is the
A Symbol
E
symbol for
mass of
number
Z Element
Z is the symbol for
Atomic
atomic number
number

6. Isotopes
Consider an atom of aluminum
with 13 protons and 15
neutrons. What is Z and A?
A = #p + #n
+
13 + 15 = 28

7. How are isotopes of the same
element alike and different?
Alike: Different:
1. Number of 1. Number of
protons and neutrons
electrons
2. Atomic number 2. Mass Number
3. Chemical 3. Atomic weight of
properties the isotopes

8. Isotopes
See Illustrated
Isotope example

9. Finding the number of Protons,
Neutrons, and Electrons
The atomic number is the
number of protons in the
nucleus.
The number of electrons in a
neutral atom equals the
number of protons.

10. Finding the number of Protons,
Neutrons, and Electrons
# of neutrons = A - Z

11. Finding the number of Protons,
Neutrons, and Electrons
Z = 92 U-235
protons = 92
electrons = 92 A = 235
protons + neutrons = 235

12. Finding the number of Protons,
Neutrons, and Electrons
Q. Find the number of neutrons in
the Ba-137 isotope.

13. Finding the number of Protons,
Neutrons, and Electrons
Element Symbol Z A #p #n #e
Zinc 66
In 68
85 38
82 210
Rn 136
35 47

14. Finding the number of Protons,
Neutrons, and Electrons
Element Symbol Z A #p #n #e
Zinc Zn 30 66 30 36 30
In 68
85 38
82 210
Rn 136
35 47

15. Finding the number of Protons,
Neutrons, and Electrons
Element Symbol Z A #p #n #e
Zinc Zn 30 66 30 36 30
Indium In 49 117 49 68 49
85 38
82 210
Rn 136
35 47

16. Finding the number of Protons,
Neutrons, and Electrons
Element Symbol Z A #p #n #e
Zinc Zn 30 66 30 36 30
Indium In 49 117 49 68 49
Strontium Sr 38 85 38 47 38
82 210
Rn 136
35 47

17. Finding the number of Protons,
Neutrons, and Electrons
Element Symbol Z A #p #n #e
Zinc Zn 30 66 30 36 30
Indium 49 117 49 68 49
Strontium Sr 38 85 38 47 38
Lead Pb 82 210 82 128 82
Rn 136

18. Finding the number of Protons,
Neutrons, and Electrons
Element Symbol Z A #p #n #e
Zinc Zn 30 66 30 36 30
Indium In 49 117 49 68 49
Strontium Sr 38 85 38 47 38
Lead Pb 82 210 82 128 82
Radon Rn 86 222 86 136 86
35 47

19. Finding the number of Protons,
Neutrons, and Electrons
Element Symbol Z A #p #n #e
Zinc Zn 30 66 30 36 30
Indium In 49 117 49 68 49
Strontium Sr 38 85 38 47 38
Lead Pb 82 210 82 128 82
Radon Rn 86 222 86 136 86
Bromine Br 35 82 35 47 35

20. Only one element has unique
names for its isotopes …
1
1 H = hydrogen
2
1 H = deuterium
3
1 H = tritium
Deuterium and tritium are used in
nuclear reactors and fusion research.

21. Some isotopes are radioactive
• Radioactive isotopes are called
radioisotopes.
• Radioisotopes can emit alpha, beta
or gamma radiation as they decay.
• Radioisotopes are important in a
number of research fields.

22. Protection from radiation
1. Shielding
2. Distance

23. Protection from radiation
How do you protect yourself from …
Alpha 2.5 cm of air, paper, skin
aluminum, lead, other
Beta metals, wood, plastic, etc.
Gamma up to a foot or two of lead,
many feet of concrete

24. Properties of Alpha Particles
• Alpha (α) particles are
the nuclei of helium
atoms and have the
symbol 2He 4

25. Properties of Beta Particles
Beta (β) particles are high
speed electrons ejected
from the nuclei of atoms
and have the symbol -1e .0

26. Properties of Gamma Rays
• Gamma (γ) rays are high energy
electromagnetic waves, not
particles.
• Gamma rays have short
wavelengths and high energies
and travel at the speed of light.

27. Alpha, Beta, Gamma
Electrically charged plates
+ + + + + + + +
What is the effect of electrically
charged plates on α, β, γ ?
- - - - - - - - -
Radioactive Source

28. Alpha, Beta, Gamma
Electrically charged plates
+ + + + + + + + β
γ
- - - - - - - - - α
Radioactive Source

29. Alpha, Beta, Gamma
Why do the α, β and
γ rays take the paths
that they do?

30. Alpha, Beta, Gamma
Paper Lead
α
Aluminum
Radioactive foil
Source

31. Alpha, Beta, Gamma
Paper Lead
β
α
Aluminum
Radioactive foil
Source

32. Alpha, Beta, Gamma
Paper Lead
β
γ
α
Aluminum
Radioactive foil
Source

33. Decay
equations

34. Alpha Decay
In alpha decay, an alpha
particle (2He4) is released
from the nucleus.

35. Alpha decay
decay product
92 U 238
 2He + 90Th
4 234
alpha particle

36. Alpha decay
Radon-220 decays by alpha emission.
What is the decay product?
86 Rn 220
 2He +
4
84 Po
???216

37. Alpha decay
Write the alpha decay equations for:
1. 95Am 241
2He + 93Np
4 237
2. 84Po
216
 2He + 82Pb
4 212
3. 88Ra
226
 2He + 86Rn
4 222

38. Beta Decay
Beta decay occurs because of
the instability of a neutron.
Neutrons are a little more massive
than protons; and neutrons are
neutral.

39. Beta decay
Decay of a neutron:
0n1  1H1 + -1e0
neutron proton electron
The electron ejected from the
nucleus is a beta particle.

40. Start with a
Li atom with
Beta decay
3 protons and Suddenly a
4 neutrons.
neutron
decays!
Now there A beta particle
are 4 protons goes zipping out of
and 3 neutrons. the nucleus.

41. Beta decay
decay product
6 C 14
 7N 14
+ -1e 0
beta particle

42. Beta decay
Zn-62 decays by beta emission.
What is the decay product?
30 Zn  -1e +
62 0
???
31Ga 62

43. Beta decay
Write the beta decay equations for:
1. 82Pb214
 -1 e + 83Bi
0 214
2. 27Co 
62
-1 e + 28Ni
0 62
3. 47??? 
Ag 113
-1 e + 48Cd
0 113

44. Gamma Decay
•Gamma rays are neutral
and thus no change is
made in the equation.
6 C 14
 6C 14
+¥

45. Half life

46. What is half life?
Half life is the time needed for
one half of a radioisotope to
decay.

47. Half Life
• Take 100 pennies and throw them on the
floor.
• Remove those that are heads up.
• Count remaining pennies.
• Continue until only one penny remains.
What can this tell us about Half Life?

48. What is half life?
• Suppose you start with 100.0
grams of a radioisotope that has
a half life of exactly 1 year.
How much will be left after 1 year?
How much will be left after 2 years?

49. Half life project
Questions:
1. A radioisotope has a half-life of 100
years. How long will it take for the
radiation to decrease to 1/16 of its
original value?
400 years

50. Half life project
Questions:
2. A radioisotope has an activity of 560 counts per
minute. After 16 hours the count rate has dropped
to 35 counts per minute. What is the half life of
the radioisotope?
4 hours

51. Nuclear Fission

52. Nuclear fission
Fission – the splitting of an atom
after being struck by a neutron.

53. Nuclear Fission

54. Nuclear fission
Fission fragment
U-235
U-235
Neutrons
Neutron
Fission fragment U-235

55. Nuclear fission
To picture a chain reaction, imagine
50 mousetraps in a wire cage.
And on each mousetrap are
two ping-pong balls.
Now imagine dropping one more
ping-pong ball into the cage …

56. Detail of
ping-pong
balls on
mousetraps.
http://www.physics.montana.edu/demonstrations/video/modern/demos/mousetrapchainreaction.html

57. http://www.physics.montana.edu/demonstrations/video/modern/demos/mousetrapchainreaction.html

58. http://www.physics.montana.edu/demonstrations/video/modern/demos/mousetrapchainreaction.html

59. Nuclear fission
This heat energy can be harnessed
to boil water,
creating steam,
that can turn a generator,
creating electricity.

60. Nuclear Fusion

61. Nuclear fusion
A day
without
sunshine is a
day without
fusion.

62. Nuclear Fusion

63. How do we detect
radiation?

64. Geiger-Mueller Tube
Counter 2435
Wire (+ side of circuit)
Metal shield (- side)
Low pressure Ar gas
Mica window (fragile)

65. GM Tube
Rays leave the source
Some hit the GM tube
Most do nothing
One ray may cause
a discharge…
Source and the detector clicks

66. GM Tube
Filled with low pressure argon gas
About 1% efficiency
About 1 in 100 rays causes an electric spark between
the case and the wire
Each spark registers as a count or click on the counter

67. Nuclear Fuel Cycle

69. Nuclear Fuel Cycle
• The Nuclear Fuel Cycle consists of
sequence of steps in which uranium ore is
mined, milled, enriched, and fabricated into
nuclear fuel and then irradiated in a reactor
for several years.
• The entire fuel cycle lifetime from mining
to discharge is about 8 years.

71. Where is Nuclear Waste Kept?
• After irradiation the fuel is cooled in the
spent fuel pit for several years and then
moved to dry cask storage on the reactor
site.

72. Spent Nuclear Fuel Pool
• Keep spent fuel rods under at
least 20 feet of water to provide
adequate shielding from the
radiation for anyone near the
pool
• Spent Fuel Pools were
designed as TEMPORARY
storage for fuel while short
lived isotopes decay (<1 yr)

73. NUCLEAR REACTOR CORE

75. US Nuclear Power Plants

76. Radioactive
Decay

77. The uranium decay
series
Uranium-238
decays through
many steps to
make stable
lead-206

78. Nuclide Half-Life Radiation *
U-238 4.468 · 109 years alpha
Th-234 24.1 days beta
Pa-234m 1.17 minutes beta
U-234 244,500 years alpha
Th-230 77,000 years alpha
Ra-226 1,600 years alpha
Rn-222 3.8235 days alpha
Po-218 3.05 minutes alpha
Pb-214 26.8 minutes beta
Bi-214 19.9 minutes beta
Po-214 63.7 microseconds alpha
Pb-210 22.26 years beta
Bi-210 5.013 days beta

79. The uranium decay
series
Radon is the
only gas
present in the
decay process.

80. Start
Symbol and U-238
Atomic number
Alpha
r e b m n ss a M
Beta
The uranium decay
u
End series in NC EOC
Pb-206 reference pages

81. Hazards from radon
• Radon gas works its way up
through the ground and into
your basements and crawl
spaces.
• You breathe radon into your
lungs.

82. Hazards from radon
• When a radon atom decays it
releases an alpha particle …
…which travels only a short
distance before it is absorbed by
your lungs, and transfers its
energy.

83. Hazards from radon
This ionizing radiation in your
lungs can cause lung cancer.
Smoking cigarettes and breathing
radon really increases your
chances of getting lung cancer.

84. Half Life Project

85. Half life project
1. Pick a mass between 10g and 50g.
2. Decide on a half life – any time.
3. Scale your graph – mass on y-axis
and at least six (6) half-lives on the x-
axis.
4. Plot the masses after intervals of one
half-life.

86. Half life project
5. What shape is the graph?
6. When will the mass of the
radioisotope fall to zero?
7. When is the radioactivity no longer a
problem?
8. What mathematical function
describes radioactive decay?