Nuclear Chemistry

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Nuclear Chemistry

• Chapter 21

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Radioactivity

• Much of our understanding of atomic structure
  came from studies of radioactive elements.
• Radioactivity - The process by which atoms
  spontaneously emit high energy particles or rays
  from their nucleus.
• First observed by Henri Becquerel in 1896.

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Discovery of Radiation

• William Roentgen Nov, 1895 - X-RAYS
• Henri Becqueral, Feb 1896

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Antoine Henri BecquerelFrance (1852-1908)
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Each nucleus contains Z protons and N neutrons.
Shown here in schematic fashion are the nuclei
of 42He, 126C, and 6329Cu.
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The common types ofradioactive emissions

• While there are many modes of radioactive
  emission, the three most common are
• ?, ? and ?.
• These types of emission are the most common and
  most often used when developing analytical
  methods.
• Lets review each.

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Ernest Rutherford - 1899
Target
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Radiation
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Atomic number (Z) number of protons in nucleus
Mass number (A) number of protons number of
neutrons
atomic number (Z) number of neutrons
A
1
1
0
0
4
Z
1
0
-1
1
2
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Balancing Nuclear Equations

1. Conserve mass number (A).

The sum of protons plus neutrons in the products
must equal the sum of protons plus neutrons in
the reactants.
235 1 138 96 2x1

1. Conserve atomic number (Z) or nuclear charge.

The sum of nuclear charges in the products must
equal the sum of nuclear charges in the reactants.
92 0 55 37 2x0
23.1
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212Po decays by alpha emission. Write the
balanced nuclear equation for the decay of 212Po.
212 4 A
A 208
84 2 Z
Z 82
23.1
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Natural Decay
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Fig 21-8
Pg 1012
Electron capture can be observed only indirectly.
The signature of electron capture is an X-ray
emitted when an electron undergoes a transition
from an outer to an inner orbital.
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Electron Capture

• electron surrounding the nucleus
• proton electron gt neutron

1s
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Positron emission

• Antimatter equivalent of an electron
• positively charged electron
• proton gives off charge - no mass

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Fig 21-7
Pg 1011
Positron emission can be observed only
indirectly. The signature of positron emission
is two g-rays generated by the annihilation of a
positron and an electron.
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Annihilation
Positron meets an Electron
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Nuclear Stability and Radioactive Decay
Beta decay
Decrease of neutrons by 1
Increase of protons by 1
Positron decay
Increase of neutrons by 1
Decrease of protons by 1
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Nuclear Stability and Radioactive Decay
Electron capture decay
Increase of neutrons by 1
Decrease of protons by 1
Alpha decay
Decrease of neutrons by 2
Decrease of protons by 2
Spontaneous fission
23.2
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Nuclear Stability

• Certain numbers of neutrons and protons are extra
  stable
• n or p 2, 8, 20, 50, 82 and 126
• Like extra stable numbers of electrons in noble
  gases (e- 2, 10, 18, 36, 54 and 86)
• Nuclei with even numbers of both protons and
  neutrons are more stable than those with odd
  numbers of neutron and protons
• All isotopes of the elements with atomic numbers
  higher than 83 are radioactive

Protons Neutrons Number even even
154 even odd 53 odd even 50 odd odd 5
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• Ratio increases with Atomic
• No stable nuclides with Atomic gt 83
• Above the band are neutron- rich
• Below are neutron-poor
• Above 83 Heavy
• What can they do to become stable?

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Fig 21-6 Pg 1011
A detailed view of one portion of the N vs. Z
plot of nuclides, illustrating the modes of
nuclear decay for nuclides on either side of the
belt of stability.
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n/p too large
beta decay
n/p too small
positron decay or electron capture
23.2
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Binding Energy
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Energy changesin nuclear reactions.

• Binding energy
• Measure of stability gained when protons and
  neutrons get together to form a nucleus.
• The equation that shows the relationship between
  mass and energy is
• E mc2
• We can use this relationship to determine how
  much energy is produced by a decrease in mass.

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Binding energy

• A more useful version of the equation is
• ?E ?mc2
• where
• ?E the binding energy
• ?m mass difference between the nucleus and
  the separate nucleons.
• c2 speed of light squared (watch units pg
  1003)

C2 (2.998 x 108 m/s)2 x 10-3 kg/g)(10-3kJ/J)
(8.988 x 1010 kJ/g)
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Nuclear binding energy (BE) is the energy
required to break up a nucleus into its component
protons and neutrons.
E mc2
BE 9 x (p mass) 10 x (n mass) 19F mass
BE (amu) 9 x 1.007276 10 x 1.008665 18.9984
BE 0.1537 g/mol x (8.988 x 1010 kJ/g)
BE 1.381 x 1010 kJ/mol
HUGE!!!!!
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Nuclear binding energy (BE) is the energy
required to break up a nucleus into its component
protons and neutrons.
E mc2
BE 1.381 x 1010 kJ/mol x 1/6.022 x 1023 F/mol
2.29 x 10-14 kJ/F
1 amu 1.49 x 10-10 J
1.25 x 10-15 kJ
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Example

• Determine the binding energy of 16O.
• We have accurate measurements of the masses for
  stable nuclides that can be used.
• 16O 15.9949146 g/mol
• n 1.008665 g/mol
• p 1.007276 g/mol
• e 0.0005486 g/mol

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Example

• To determine the binding energy, we simply need
  to look at the mass of the nucleon and the
  particles if taken separately.
• 16O - 8 protons and 8 neutrons
• 8 n 8 x 1.008665 8.06932
• 8 p 8 x 1.007276 8.05821
• 8e 8 x 0.0005486 0.00114949
• Total 16.128679 g/mol

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Example

• Finally, calculate the binding energy based on
  the mass difference.
• ?m 16.128679 - 15.9949146
• 0. 1337649 g/mol
• BE 0.1337649 g/mol (8.988 x 1010 kJ/g)
• 12.02279 x 109 kJ/mol

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Fig 21-3 Pg
1005
Plot of the binding energy per nucleon vs. mass
number A. The most stable nuclides lie in the
region around A60.
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Patterns of Nuclear Stability
Neutron-to-Proton Ratio
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Nuclear power

• Power can be obtained two ways.
• Fission Splitting atoms
• Get energy if the nucleus is big.
• The smaller ones are more stable.
• What we do in nuclear reactors.
• Fusion Joining atoms
• Get energy if the nuclei are small.
• The larger one is more stable.
• This is how the sun works.

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Nuclear Fission
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Nuclear Fission
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Chain reaction
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Fig 21-13 Pg 1024
Schematic view of the start of a fission chain
reaction. The first neutron causes fission,
which generates additional neutrons. They cause
more fission, and the chain continues to grow if
more than one neutron, on average, is captured
for every fission event.
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Nuclear Fission
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Chain reactions

• Critical Reaction
• When just enough fissions occur to keep the chain
  reaction going.
• (neutrons formed neutrons used)
• - nuclear power
• Supercritical Reaction
• When excess neutrons are produced and the rate of
  fission keeps increasing.
• - nuclear bombs

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Nuclear Fission
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Nuclear Fission
When enough material is present for a chain
reaction, we have critical mass. Below critical
mass (subcritical mass) the neutrons escape and
no chain reaction occurs. At critical mass, the
chain reaction accelerates. Anything over
critical mass is called supercritical
mass. Critical mass for 235U is about 1 kg. We
now look at the design of a nuclear bomb.
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Nuclear Fission
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Nuclear bombs
A conventional explosive is used to drive two
sections of U-235 together.
This creates a supercritical mass.
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Pg 1025
Bombing of Nagasaki, August 9, 1945.
Courtesy U.S. Department of Defense.
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Energy from fission

• Uranium-235 is used as a fuel in a reactor.
• One common reaction is
• n U Kr Ba 3 n
  energy
• 100 grams of 235U could produce as much energy as
  80 trillion tons of TNT.

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Nuclear Fission
Nuclear Reactors Use fission as a power
source. Use a subcritical mass of 235U (enrich
238U with about 3 235U). Enriched 235UO2 pellets
are encased in Zr or stainless steel
rods. Control rods are composed of Cd or B, which
absorb neutrons. Moderators are inserted to slow
down the neutrons. Heat produced in the reactor
core is removed by a cooling fluid to a steam
generator and the steam drives an electric
generator.
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1954 President Eisenhower on September 6
activated an automated power shovel to begin
construction on the first nuclear reactor in the
United States.
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Energy from fusion

• When you join small atoms together, you can also
  get energy.
• The Sun fuses hydrogen to make helium.
• Were currently trying to fuse two isotopes of
  hydrogen - its easier.
• H H He n
  1.7x109 kJ/mol
• It will be great when it works. Fuel would come
  from the oceans - almost free.

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Nuclear Transmutations
Using Charged Particles A cyclotron consists of
D-shaped electrodes (dees) with a large, circular
magnet above and below the chamber. Particles
enter the vacuum chamber and are accelerated by
making the dees alternatively positive and
negative. The magnets above and below the dees
keep the particles moving in a circular
path. When the particles are moving at sufficient
velocity they are allowed to escape the cyclotron
and strike the target.
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Finding Rate Laws

• Using graphical method
• based on integrated rate laws

zero order for A
first order for A
second order for A
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2H2O2(aq) ---gt 2H2O(l) O2(g)

• How can we find the rate of this reaction?
• Decrease of H2O2
• Increase of H2O or O2
• Evolution of a gas - moles/liter per unit time

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Zero Order?
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First Order?
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Second Order?
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First Order
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Finding rate laws

• Graphical method.
• Using integrated rate laws, one can produce
  straight line plots. The order for a reactant if
  verified if the data fits the plot.
• Rate integrated Graph
  Slope
• Order law rate law
  vs. time
• 0 rate k At -kt A0
  At -k
• 1 rate kA lnAt -kt lnA0
  lnAt -k
• 2 ratekA2 kt
  k

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First order reactions

• Reactions that are first order with respect to a
  reactant are of great importance.
• Describe how many drugs pass into the blood
  stream or used by the body.
• Often useful in geochemistry
• Radioactive decay
• Half-life (t1/2)
• The time required for one-half of the quantity
  of reactant originally present to react.

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1st order - Half-Life
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1st order - Half-Life
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Using the Integrated rate law

• Find value of k from the slope
• Given the rate constant you can calculate
• time to reach a certain concentration
• concentration reached in a given time
• the initial concentration, given some conc. with
  time
• Half-Life time for reactant to decrease by
  1/2

0
693
.

t
first order
1
2
/
k
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Use of 1/2 Life

• C4H8(g) --gt 2C2H4 (g) First Order Reaction
• At 1000C k87 s-1
• What is the 1/2 life?

• So 50 is gone after 8ms
• After 24ms how much is gone?

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Use of 1/2 Life
of t1/2s 0 1 2 3 4
Percent Left 100 50 25 12.5 6.25
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Rates of Radioactive Decay
90Sr has a half-life of 28.8 yr. If 10 g of
sample is present at t 0, then 5.0 g is present
after 28.8 years, 2.5 g after 57.6 years, etc.
90Sr decays as follows 9038Sr ? 9039Y 0-1e Each
isotope has a characteristic half-life. Half-lives
are not affected by temperature, pressure or
chemical composition. Natural radioisotopes tend
to have longer half-lives than synthetic
radioisotopes.
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Rates of Radioactive Decay
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Rates of Radioactive Decay
Half-lives can range from fractions of a second
to millions of years. Naturally occurring
radioisotopes can be used to determine how old a
sample is. This process is radioactive dating.
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Rates of Radioactive Decay
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Rates of Radioactive Decay
Dating Carbon-14 is used to determine the ages of
organic compounds because half-lives are
constant. We assume the ratio of 12C to 14C has
been constant over time. For us to detect 14C the
object must be less than 50,000 years old. The
half-life of 14C is 5,730 years. It undergoes
decay to 14N via ?-emission 146C? 147N 0-1e
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Rates of Radioactive Decay
Calculations Based on Half-Life Radioactive decay
is a first order process Rate kN In
radioactive decay the constant, k, is called the
decay constant. The rate of decay is called
activity (disintegrations per unit time). If N0
is the initial number of nuclei and Nt is the
number of nuclei at time t, then
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Rates of Radioactive Decay
Calculations Based on Half-Life With the
definition of half-life (the time taken for Nt
½N0), we obtain
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Detection of Radioactivity
Matter is ionized by radiation. Geiger counter
determines the amount of ionization by detecting
an electric current. A thin window is penetrated
by the radiation and causes the ionization of Ar
gas. The ionized gas carried a charge and so
current is produced. The current pulse generated
when the radiation enters is amplified and
counted.
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Detection of Radioactivity
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Radiation exposureand safety

• Factors that influence the degree of exposure
• Magnitude of the half-life
• Shorter half-life materials decay faster and can
  result in greater damage.
• Shielding
• Provides protection by blocking radiation.
• Type of radiation
• Some types are worse than others.
• Area of exposure
• Hand exposure not as bad as ovaries.

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Other factors

• Distance from radiation source
• Intensity decreases with increased distance.
• Intensity ? 1/d2
• Time of exposure
• Effects are cumulative.

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Biological Effects of Radiation
The penetrating power of radiation is a function
of mass. Therefore, ?-radiation (zero mass)
penetrates much further than ?-radiation, which
penetrates much further than ?-radiation. Radiatio
n absorbed by tissue causes excitation
(nonionizing radiation) or ionization (ionizing
radiation). Ionizing radiation is much more
harmful than nonionizing radiation.
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Fig 21-21 Pg 1038
Different types of radiation penetrate matter to
different degrees. Although a-particles are the
most damaging radiation, they lose their energy
after traveling a very short distance. Gamma
rays, on the other hand, are very dangerous
because they travel long distances before
shedding all their energy.
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Biological Effects of Radiation
The penetrating power of radiation is a function
of mass. Therefore, ?-radiation (zero mass)
penetrates much further than ?-radiation, which
penetrates much further than ?-radiation. Radiatio
n absorbed by tissue causes excitation
(nonionizing radiation) or ionization (ionizing
radiation). Ionizing radiation is much more
harmful than nonionizing radiation.
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Biological Effects of Radiation
Radiation Doses The SI unit for radiation is the
becquerel (Bq). 1 Bq is one disintegration per
second. The curie (Ci) is 3.7 ? 1010
disintegrations per second. (Rate of decay of 1
g of Ra.) Absorbed radiation is measured in the
gray (1 Gy is the absorption of 1 J of energy per
kg of tissue) or the radiation absorbed dose (1
rad is the absorption of 10-2 J of radiation per
kg of tissue).
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Units of radiation

• curie The amount of radioactive materials
  that produces 3.7 x 1010 disintegrations/secon
  d.
• rad radiation absorbed dosage, accounts for
  the type of radiation.
• rem rad equivalent for man used to describe
  biological damage.

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Biological Effects of Radiation
Radiation Doses
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Acute radiation syndrome

• LD50 for people is 400-500 REM.
• Level, REM Reported Effect
• below 100 No definite sign of syndrome.
• 100-500 50 survival rate.
• Loss of hair.
• Altered blood chemistry.

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Acute radiation syndrome

• Level, REM Reported Effect
• 500-700 All earlier effects plus
• Destruction of bone marrow.
• Loss of red blood cells, white blood
  cells platelets.
• Death within 36 days.
• 900-2000 Vomiting, diarrhea, infections.
• Death within 10 days.

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Acute radiation syndrome

• Level, REM Reported Effect
• 5000 Death within 3 days.
• All earlier effects.
• Third degree burns.
• 12000 Death within 36 hours.
• Severe bleeding and
• fluid loss.

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Biological Effects of Radiation
Radon The nucleus 22286Rn is a product of
23892U. Radon exposure accounts for more than
half the 360 mrem annual exposure to ionizing
radiation. Rn is a noble gas so is extremely
stable. Therefore, it is inhaled and exhaled
without any chemical reactions occurring. The
half-life of is 3.82 days. It decays as
follows 22286Rn ? 21884Po 42He
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Biological Effects of Radiation
Radon The ?-particles produced have a high
rem. Therefore, inhaled Rn is thought to cause
lung cancer. The picture is complicated by
realizing that 218Po has a short half-life (3.11
min) also 21884Po ? 21482Pb 42He The 218Po
gets trapped in the lungs where it continually
produces ?-particles. The EPA recommends 222Rn
levels in homes to be kept below 4 pCi per liter
of air.
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Uses of radionuclides

• Our ability to measure radioactivity is very
  sensitive. As a result, radioisotopes have a
  number of uses.
• In addition, its interaction with living matter
  can also be exploited.
• Uses includes
• Dating techniques
• Cancer treatment
• Tracers
• Imaging
• Testing methods

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Carbon dating

• Carbon-14 is constantly being produced in the
  upper atmosphere by cosmic rays at an almost
  constant rate.
• N n C H
• It rapidly combines with oxygen in the air to
  make CO2.
• 14C O2 14CO2

14 7
1 0
14 6
1 1
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Carbon dating

• Plants and algae use the carbon dioxide when
  making sugars and protein. They get eaten and
  so on . . .

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Carbon dating

• After death, the carbon-14 decays with a
  half-life of 5730 years.
• We can tell how old things are based on the
  amount of carbon-14 that remains.
• The method is pretty good in the 1,000-20,000
  year range. (50,000 year limit)
• Ideal tool for dating the artifacts of man - or
  at least it was. Materials like plastics are
  14C depleted.

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Medical applications

• Cancer therapy
• Radiation both causes and can treat cancer
• Radiation causes molecules in the cell to break
  apart - ionization
• Most significant damage is when DNA is destroyed.
• Effect is greatest in rapidly growing cells.

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Examples of radiation treatment

• External bombardment
• 60Co - expose area to gamma rays.
• Implants
• 182Ta - used as wire, treatment of eye.
• 137Cs - use in a balloon catheter for
  bladder.
• Interstitial therapy
• 198Au - inject directly into tumor.
• Internal irradiation
• 131I - Ingest solution, goes to thyroid.

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Nuclear tracers

• Our ability to measure radiation is VERY
  sensitive.
• Can introduce a small amount of a radioactive
  material and see where it goes in the body -
  Tracers
• Can be used to measure small amounts of chemicals
  in the body like hormones - Radioimmunoassay

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Example page 1045

• Iodine-127 is used to trace thyroid function.

Normal Benign
Cancer Thyroid tumor
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Fig 21-24 Pg
1044
Radioactive tracers can show flow pathways that
cannot be detected using other methods of
detection. Tritiated water added to a holding
tank results in radioactivity in the water from
wells that draw from the ground water supply
into which the holding tank drains.
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Siltation measurement

• The movement of mud and sand in river estuaries.
• Tracer to use.
• Since the study is relatively short term, use a
  short half-life tracer.
• Use an insoluble form of the tracer
• ( 140BaSO4 - t1/2 12.8 d)
• Due to the volume, several curies may be
  required.

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Siltation measurement

• Grind precipitate to a size range that is
  representative of river silt.
• Dump tracer in the river at a known point.
• Send a boat into the estuary with a suspended
  detector to map the silt patterns.

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Siltation measurement
river
to ocean
This type of study can help predict how
often commercial waterways will need to be
dredged.