Nuclear Transmutations (“Nucleosynthesis”)
“Nucleosynthesis” reactions in the interior of stars have produced nearly all
of the naturally occurring elements on earth. In a nuclear transmutation a
nucleus is struck by a neutron or another nucleus, inducing a nuclear
reaction. This is how heavier elements are made from lighter elements.
Rutherford was the first to observe “transmutation” of an element by
alpha bombardment. N was transmuted into O.
14
4
17
1
7 N + 2 α → 8 O+1 p
A shorthand notation used to represent such induced transmutations is:
target nucleus ( bombarding particle, ejected particle(s)) product
nucleus
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7
•
•
N(α , p) 178 O
Cyclotron Accelerator
To do “nucleosynthesis” on earth, we must overcome LARGE energy
barriers by simulating the very high energy conditions found inside a star.
Two ways to do this: (1) Heat to VERY high temperatures (stars) (2) Use
particle accelerators like the cyclotron shown above.
Today, the use of particle accelerators and neutrons as the bombarding
particle have greatly extended nuclear reaction chemistry. All the elements
past U are man-made through this process of acceleration or neutron
bombardment.
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Nuclear Chemistry
Accelerator Facilities
ll
Foothi
CERN - Geneva, Switzerland
Nobel Prizes in Physics
Nobel Prizes in Physics
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•
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The
The
The
The
Nobel
Nobel
Nobel
Nobel
Prize
Prize
Prize
Prize
in
in
in
in
Physics
Physics
Physics
Physics
SLAC - Stanford Linear Accelerator
1976
1984
1988
1992
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1976 - Charm: The 4th Quark
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1990 - Quarks Revealed: Structure Inside Protons and Neutrons
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1995 - Tau: The Third Electron-Like Particle
Nobel Prize in Chemistry
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Nuclear Chemistry
2006 - Molecular basis of Eukaryotic Transcription
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Transuranium Element Synthesis
All of the elements past uranium are man-made in the laboratory. The synthesis of these transuranium elements is one
of the most important achievements of particle physicists during the 1940’s and 1950’s. Research continues today with
over 25 elements synthesized.
Beta emission after neutron bombardment is the key to many synthesis. Remember, beta emission increases the
atomic number by one.
Below are a few of the reactions used to create these man-made elements.
238
1
239
239
92
0
92
93
U + n→
1940-Neptunium, Np:
239
93
1940-Plutonium, Pu:
1944-Americum, Am:
Np →
239
94
239
94
U→
Np + −10 β
Pu + −10 β
Pu + 2 01n →
241
94
Pu →
241
95
Am + −10 β
Heavier nuclei like He and C can also be accelerated and used in bombardment:
1949-Berkelium, Bk:
241
95
Am + 24α →
1969-Rutherfordium, Rf:
249
98
243
97
Bk + 2 01n
257
Cf + 126 C → 104
Rf + 4 01n
Nuclear Chemistry
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Using Neutrons for Element Transmutations
1.
2.
3.
4.
Nuclear transmutation reactions using neutrons as the bombarding particle do not require the neutrons to be
accelerated to high speeds. Why not?
In fact neutrons cannot be accelerated. Why not?
The neutrons needed for this are produced by the reactions that occur in nuclear reactors.
Neutrons are used in the medical industry to create unstable nuclei that are gamma and/or beta emitters. These
isotopes are used for targeted cancer radiation therapy or as “tracers” within the body.
Example of nuclear chemistry in medicine is the formation of cobalt-60.
60Co is used for γ-radiation “gamma knife” treatment of cancer.
The steps in the synthesis are shown below. Write the appropriate equations.
1.
Neutron bombardment of iron-58 to produce iron-59:
2.
Beta emission from iron-59:
3.
Neutron bombardment of cobalt-59 to produce cobalt-60:
Write the “net” synthesis:
Nuclear Chemistry
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Rates (Kinetics) of Radioactive Decay
The rate is not affected by temperature, pressure or chemical environment.
For radioactive nuclides it is impossible to say when a particular nucleus will decay. What we can measure is the time it
takes for a certain percentage of nuclei in a sample to decay. The number of nuclei that will decay over a given time
period is determined by Poisson statistics.
The rate of nuclear decay is measured by the activity, A, of the sample, often expressed as the number of
disintegrations observed per unit time. This is analogous to the Rate in chemical kinetics.
Rate of decay = Activity = A = –dN/dt = kN. (units are disintegrations/time)
Where k is the decay constant, and N is the number of radioactive nuclei in the sample.
The becquerel (Bq), the SI unit for activity, is defined as one nuclear disintegration per second (dps).
This is an extremely low activity and not often used.
1 Bq = 1 dps
The Curie (Ci) is defined as 3.7 x 1010 Bq = 3.7 x 1010 dps
(Originally defined as the activity of 1.0 g of Radium-226.)
The Curie an older but still common unit of reporting activity.
A “hot” sample has a high activity (dps) (danger!), while a “cold” sample has a low activity (dps).
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Nuclear Chemistry
Kinetics of Radioactive Decay
−
dN
= A = kN
dt
The decay process of radioactive nuclei always follows first order kinetics. For radioactive decay, concentration is
replaced in the integrated rate law equation by the number of radioactive nuclei, N. We can also use the mass of
radioactive nuclei, m, or the activity of the nuclei, A.
We can derive the following formulas from first order kinetics:
N t = N 0 e− kt
mt = m0 e− kt
At = A0 e− kt
Nt
= e− kt
N0
mt
= e− kt
m0
At
= e− kt
A0
⎛N ⎞
ln ⎜ t ⎟ = −kt
⎝ N0 ⎠
⎛m ⎞
ln ⎜ t ⎟ = −kt
⎝ m0 ⎠
⎛A ⎞
ln ⎜ t ⎟ = −kt
⎝ A0 ⎠
ln ( N t ) = −kt + ln(N 0 )
ln ( mt ) = −kt + ln(m0 )
ln ( At ) = −kt + ln(A0 )
m = MASS of radioactive nuclei
A = ACTIVITY of radioactive nuclei
N = # of radioactive nuclei
Over “short” time periods ∆t:
ΔN = (−A)Δt = (−kN )Δt
Nuclear Chemistry
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Half-Lives of Radioactive Decay
The rates of decay of nuclei are commonly expressed in terms of their half- lives. Each isotope has its own
characteristic half-life.
Strontium-90 occurs in the fall-out after a nuclear bomb test or an
accidental release of radioactive materials in the air from a
nuclear power plant (Japan, 2011). It is chemically similar to
calcium, and can be easily incorporated into bones making
exposure very dangerous.
Decay of a 10.0 g sample of
strontium-90.
Nuclear Chemistry
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Kinetics of Radioactive Decay-Example Problems
1.
Tritium (hydrogen-3, 3.01605 g/mol) has a decay constant of 0.0564/yr. Calculate the following for a 10.7 mg
sample of tritium:
1.1. The half-life of the decay process.
1.2. The initial activity of the sample in Bq and Ci.
1.3. The average number of nuclei that will decay in one day.
1.4. How many mg of tritium remain after 25 years.
1.5. How long it will take to decrease the activity to 1.0 Curies (Ci).
Nuclear Chemistry
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Kinetics of Radioactive Decay -Example Problems
2.
Cobalt-60 has a half-life of 5.26 yr. The cobalt-60 in a radiotherapy unit must be replaced when its radioactivity
falls to 75% of the original sample. If the original sample was purchased in August 2012, when will it be necessary
to replace the cobalt-60?
Nuclear Chemistry
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Radiometric Dating with C-14
Under the right conditions the age of a sample can be determined by measuring the activity of a radioactive isotope.
The best known example is C-14 dating of organic based material. C-14 is a beta emitter with a half-life of 5715 years.
It is formed in the upper atmosphere and is found in about 1 in every 1012 carbon atoms.
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14
1
7
0
6
1
Production:
N + n→ C+ p
Living organism
decay rate:
15.2 d/min*g C
Dead organism
decay rate:
<15.2 d/min*g C
Decay: 146 C → 147 N + −10 e
Nuclear Chemistry
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How has radiocarbon dating changed the way
scientists are able to interpret and understand history?

Before the 1950’s there was no way of
knowing the precise age of an artifact or site;
we had to depend on historical records and
context.

Revolutionized the approach to dating organic
archeological objects almost overnight.

C-14 dating was one of the most critical
discoveries of 20th century science: The 1960
Nobel Prize in chemistry was given to Willard
Libby for his work with radiocarbon dating.
•
This dating technique has limitations:
 Assumes constant C-14 amounts in the
atmosphere over thousands of years. This
is not true, the C-14 has varied with
variation in cosmic ray activity. For the
best accuracy, data needs to be corrected
for fluctuations in atmospheric C-14.
Carbon-14 dating of tree rings can be used
to determine the variation of C-14 in the
atmosphere over a period of time.
 Precision: About ±100 yr
(“Recent” human activities have impacted
dating for more recent objects.)
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Nuclear Chemistry
 TIme Limit ≈ 50,000 yr (about 10 half-lives)
Famous things that have been radiocarbon dated...
Shroud of Turin
The proposed burial cloth
of Jesus


Samples were sent to 3 labs -Tucson (USA), Oxford (England) and Zurich (Switzerland).
Results all very consistent- between AD 1260 and AD 1390.

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Fits closely with first appearance in history (mid 14th century).
Strongly suggests that the artifact is from the Middle Ages, rather than a genuine 2000 year old burial cloth.
The Dead Sea Scrolls
Nuclear Chemistry

Radiocarbon date 100BC - 100AD.


Close to dates written on them.
Close to dates estimated based on writing
style.
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Radiocarbon Dating
The charred bones of a sloth in a cave in Chile represent the earliest evidence of the human presence in the
southern tip of South America. A sample of bone from the sloth has a 14C activity of 5.22 disintegrations per
minute per gram of carbon (d/min*g C) If a living organism has an activity of 15.2 d/min*g C, how old are the
bones?
Nuclear Chemistry
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Nuclear Chemistry
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