Transcript MS PowerPoint - Catalysis Eprints database

Nuclear Reactions and Carbon Dating
Children’s club lecture
19.04.2010
M.Banu
The Nucleus
 Remember that the nucleus is comprised of the two
nucleons, protons and neutrons.
 The number of protons is the atomic number.
 The number of protons and neutrons together is
effectively the mass of the atom.
Isotopes
 Not all atoms of the same element have the same
mass due to different numbers of neutrons in those
atoms.
 There are three naturally occurring isotopes of
uranium:
 Uranium-234
 Uranium-235
 Uranium-238
Radioactivity
 It is not uncommon for some nuclides of an
element to be unstable, or radioactive.
 We refer to these as radionuclides
 There are several ways radionuclides can decay
into a different nuclide.
Types of Radioactive Decay
Alpha Decay
Loss of an -particle (a helium nucleus)
4
2
238
92
U
He
234

90
4
2
Th+ He
Types of Radioactive Decay
Beta Decay
Loss of a -particle (a high energy electron)
0
−1
131
53
I

0
or −1
131

54
e
Xe
+
0
−1
e
Types of Radioactive Decay
Positron Emission
Loss of a positron (a particle that has the same
mass as but opposite charge than an electron)
0
1
11
6
C
e
11

5
B
+
0
1
e
Types of Radioactive Decay
Gamma Emission
Loss of a -ray (high-energy radiation that
almost always accompanies the loss of a nuclear
particle)
0
0

Penetrating Power
Neutron-Proton Ratios
 Any element with more than one
proton
(i.e.,
anything
but
hydrogen) will have repulsions
between the protons in the
nucleus.
 A strong nuclear force helps
keep the nucleus from flying
apart.
 Neutrons play a key role
stabilizing the nucleus.
 Therefore,
the
ratio
of
neutrons to protons is an
important factor.
Neutron-Proton Ratios
For smaller nuclei (Z 
20) stable nuclei have a
neutron-to-proton ratio
close to 1:1.
Neutron-Proton Ratios
As nuclei get larger,
it takes a greater
number of neutrons
to
stabilize
the
nucleus.
Stable Nuclei
The shaded region in the
figure shows what nuclides
would be stable, the socalled belt of stability.
Stable Nuclei
 Nuclei above this
belt have too many
neutrons.
 They tend to decay
by emitting beta
particles.
Stable Nuclei
 Nuclei below the belt
have too many protons.
 They tend to become
more stable by positron
emission or electron
capture.
Stable Nuclei
 There are no stable nuclei with an atomic
number greater than 83.
 These nuclei
emission.
tend to decay by alpha
Radioactive Series
 Large
radioactive nuclei
cannot
stabilize
by
undergoing only one nuclear
transformation.
 They undergo a series of
decays until they form a
stable nuclide (often a
nuclide of lead).
Some Trends
Nuclei with
2, 8, 20, 28, 50, or 82 protons or
2, 8, 20, 28, 50, 82, or 126
neutrons tend to be more stable than nuclides with a
different number of nucleons.
Nuclei with an even
number of protons and
neutrons tend to be
more
stable
than
nuclides that have odd
numbers
of
these
nucleons.
Measuring Radioactivity
 One can use a device like this Geiger counter to
measure the amount of activity present in a radioactive
sample.
 The ionizing radiation creates ions, which conduct a
current that is detected by the instrument.
Energy in Nuclear Reactions
 There is a tremendous amount of energy stored in
nuclei.
 Einstein’s famous equation, E = mc2, relates directly to
the calculation of this energy.
 In chemical reactions the amount of mass converted to
energy is minimal.
 However, these energies are many thousands of times
greater in nuclear reactions.
Nuclear Fission
When atoms are bombarded with neutrons, their nuclei
splits into 2 parts which are roughly equal in size.
Nuclear fission in the process whereby a nucleus, with
a high mass number, splits into 2 nuclei which have
roughly equal smaller mass numbers.
During nuclear fission, neutrons are released.
There are 2 types of fission that exist:
1. Spontaneous Fission
2. Induced Fission
The Fission Process
A neutron travels at high speed towards a
uranium-235 nucleus.
1
0n
235
92U
1
0n
235
92U
1
0n
235
92U
The neutron strikes the nucleus which then
captures the neutron.
1
0n
235
92U
The nucleus changes from being uranium-235 to
uranium-236 as it has captured a neutron.
236
92U
The uranium-236 nucleus formed is very
unstable.
It transforms into an elongated shape for a
short time.
It then splits into 2 fission fragments and
releases neutrons.
1
0n
141
56Ba
1
0n
92
36Kr
1
0n
1
0n
141
56Ba
1
0n
92
36Kr
1
0n
1
0n
141
56Ba
1
0n
92
36Kr
1
0n
1
0n
141
56Ba
1
0n
92
36Kr
1
0n
Nuclear Fission Examples
235
1
141
235
92
1
0
138
55
U
n
+
92
0
U+ n
92
1
Ba
Kr
n
3
+
+
56
36
0
96
37
1
0
Cs+ Rb+ 2 n
Energy from Fission
Both the fission fragments and neutrons travel at high
speed.
The kinetic energy of the products of fission are far
greater than that of the bombarding neutron and target
atom.
EK before fission << EK after fission
Energy is being released as a result of the fission
reaction.
235
92
1
0
U+ n
138
55
96
37
Cs+ Rb+ 2 n
Element
Atomic Mass (kg)
235 U
92
3.9014 x 10-25
138 Cs
55
2.2895 x 10-25
96
37Rb
1.5925 x 10-25
1
1.6750 x 10-27
0n
1
0
Calculate the total mass before and after fission takes
place.
The total mass before fission (LHS of the equation)
:
3.9014 x 10-25 + 1.6750 x 10-27 = 3.91815 x 10-25 kg
The total mass after fission (RHS of the equation):
2.2895 x 10-25 + 1.5925 x 10-25 + (2 x 1.6750 x 10-27) = 3.9155 x 10-25 kg
The total mass before fission = 3.91815 x 10-25 kg
The total mass after fission
= 3 .91550 x 10-25 kg
total mass before fission > total mass after fission
mass difference, m = total mass before fission –
total mass after fission
m = 3.91815 x 10-25 – 3.91550 x 10-25
m = 2.65 x 10-28 kg
This reduction in mass results in the release of
energy.
Energy Released
The energy released can be calculated using the
equation:
E
E = mc2
m
c2
Where:
E = energy released (J)
m = mass difference (kg)
c = speed of light in a vacuum (3 x 108 ms-1)
Calculate the energy released from the following fission
reaction:
235
1
U
n
+
92
0
m = 2.65 x 10-28 kg
c = 3 x 108 ms-1
E=E
138
96
1
Cs
Rb
n
2
+
+
55
37
0
E = mc2
E = 2.65 x 10-28 x (3 x 108)2
E = 2.385 x 10-11 J
Nuclear Fusion
In nuclear fusion, two nuclei with low mass numbers
combine to produce a single nucleus with a higher mass
number.
2
1
3
4
1
2
H+ H
1
He+ n+Energy
0
2
1H
3
1H
2
1H
3
1H
2
1H
3
1H
2
1H
3
1H
1
0n
4
2 He
1
0n
4
2 He
1
0n
4
2 He
Energy from Fusion
2
3
H
H
+
1
1
Element
4
1
Energy
He
n
+
+
2
0
Atomic Mass (kg)
2
3.345 x 10-27
3
1H
5.008 x 10-27
4 He
2
6.647 x 10-27
1H
1
0n
1.6750 x 10-27
2
3
H
H
+
1
1
4
1
Energy
He
n
+
+
2
0
The total mass before fusion (LHS of the equation):
3.345 x 10-27 + 5.008 x 10-27 = 8.353 x 10-27 kg
The total mass after fission (RHS of the equation):
6.647 x 10-27 + 1.675 x 10-27 = 8.322 x 10-27 kg
m = total mass before fission – total mass after fission
m = 8.353 x 10-27 – 8.322 x 10-27
m = 3.1 x 10-29 kg
2
3
H
H
+
1
1
m = 3.1 x 10-29 kg
c = 3 x 108 ms-1
E=E
4
1
Energy
He
n
+
+
2
0
E = mc2
E = 3.1 x 10-29 x (3 x 108)2
E = 2.79 x 10-12 J
The energy released per fusion is 2.79 x 10-12 J.
Kinetics of Radioactive Decay
 Nuclear decay is a first-order process. The kinetics
of such a process obey this equation:
Nt
= -kt
ln
N0
• The half-life of such a process is:
0.693
= t1/2
k
• Comparing the amount of a radioactive nuclide present
at a given point in time with the amount normally
present, one can find the age of an object.
Cosmic Rays
(radiation)
Forms C-14
Collision with
atmosphere
(N14)
C-14 combines
with oxygen to
form carbon
dioxide (CO2)
 We can take a sample of air, count how many 12C
atoms there are for every 14C atom, and calculate the
14C/12C ratio.
 Because 14C is so well mixed up with 12C, we expect to
find that this ratio is the same if we sample a leaf
from a tree, or a part of your body.
 Once a plant or animal dies the clock starts.
 The plant or animal no longer takes in C-14.
 The C-14 present in the plant or animal begins to
decay.
No more
C-14
intake
C-14
continues to
decay
 Used only on organic material
 Cannot be used to date rocks
 Maximum age limit about 60,000 years
Radioisotopes in Medicine
•
24Na,
t½ = 14.8 hr, b emitter, blood-flow tracer
•
131I,
t½ = 14.8 hr, b emitter, thyroid gland activity
•
123I,
t½ = 13.3 hr, g-ray emitter, brain imaging
•
18F,
•
99mTc,
t½ = 1.8 hr, b+ emitter, positron emission
tomography
t½ = 6 hr, g-ray emitter, imaging agent
Brain images
with 123I-labeled
compound
Chemistry In Action: Food Irradiation
Dosage
Effect
Up to 100 kilorad
Inhibits sprouting of potatoes, onions, garlics.
Inactivates trichinae in pork. Kills or prevents insects
from reproducing in grains, fruits, and vegetables.
100 – 1000 kilorads
Delays spoilage of meat poultry and fish. Reduces
salmonella. Extends shelf life of some fruit.
1000 to 10,000 kilorads
Sterilizes meat, poultry and fish. Kills insects and
microorganisms in spices and seasoning.
Questions