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Nuclear Reactionsand

Radioactivity

Part II

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Chemical vs. Nuclear Reactions

Chemical Reactions - atoms are rearranged to form new substance(s).

• Involve changes in electrons

Nuclear Reactions - nuclei of atoms change to form a new element.

• Involve changes in protons and neutrons• Nuclear Transmutation

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Nuclear Transmutation -Transformations

The induced conversion of one nucleus into another (change of one element into another).

Usually accomplished using a particle accelerator.

1327

24

1530

01A

lHe P n+ +

98249

818

106263

014C

fO X n+ +

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A Schematic Diagram of a Cyclotron Accelerator

Used to accelerate particles to produce:

•radioisotopes used in medical applications

•synthesis of the transuranium elements

•studying the fundamental nature of matter.

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A Schematic Diagram of a Linear Accelerator

The voltage of each section is alternated, such that a (+) particle is repelled from the section it is leaving and attracted to the section it is entering causing the particle to accelerate in speed.

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Sample Problem: Transmutations

Mass(A) 239 + 4 = ? + 1

243 = 242 + 1

243 = 243

Charge (Z)94 + 2 = ? + 0 96 = 96 + 0 96 = 96

?= XAZ

Cm24296

?=

94

239

0

1Pu n ?+ He4

2

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Nuclear Binding Energy

The amount of energy required to separate the nucleus (1 mole of nuclei) into protons and neutrons.

Nucleus + Binding --> Protons + NeutronsEnergy

The greater the binding energy for a nucleus, the greater the stability

Stability of Nucleus

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According to Einstein's mass energy equation:

E=mc2 where m = mass defect

This expression indicates that the mass defect (m) which is lost in the formation of stable nucleus is converted into energy. This amount of energy must be released when nucleons are combined to form a stable nucleus.

This is the binding energy that holds the nucleons in a nucleus so that despite strong repulsive forces between protons they are forced to unite in the nucleus.

Stability of Nucleus:Nuclear Binding Energy

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Binding energy per nucleus as a function of mass

number Stability of Nucleus:

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Thermodynamic Stability of Nucleus:The Mass Defect:

Mass Defect - The difference between the mass of a nucleus and the sum of the masses of its constituent nucleons.

Careful measurements have shown that the mass of a particular atom is always slightly less than the sum of the masses of the individual neutrons and protons of which the nucleus of the atom consists. The difference between the mass of the atom and the sum of the masses of its parts is called the Mass Defect (m).

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The Mass Defect:A much larger mass change accompanies a nuclear

process due to the enormous energy required to break the nucleus apart or bind the nucleus together.

Energy change = E = mxc2

2c

Em

Thermodynamic Stability of Nucleus:

(recall that m = mass defect)

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The Mass Defect

Law of Conservation of Mass and Energy: The total quantity of mass-energy in the universe is constant.When a reaction releases or absorbs energy, a loss or gain in mass must accompany the reaction.

Thermodynamic Stability of Nucleus:

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Sample Problem: Mass Defect

Thermodynamic Stability of Nucleus

Consider the example of deuteron (1H2) which contains: Number of protons = 1 Number of neutrons = 1

Mass of deuteron (1H2) = 3.3431 x 10-27 kg

Mass of proton = 1.6727 x 10-27 kgMass of neutron = 1.6748 x 10-27 kg

Determine the MASS DEFECT.

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Problem Solution: Mass Defect

Thermodynamic Stability of Nucleus

For deuteron (1H2):

Sum of the nuclide masses: = 1.6727 x 10-27 kg + 1.6748 x 10-27

kg= 3.3472 x 10-27 kg

Difference of mass:= 3.3472 x 10-27 kg - 3.3431 x 10-27

kg m = 0.004 x 10-27 kg

This difference in mass is the MASS DEFECT

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Fission

The splitting a heavy nucleus into two nuclei with smaller mass numbers accompanied by a large release of energy.

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Fission

Neutrons are also produced during fission which makes it possible to have a self-sustaining fission process, i.e., a chain reaction.

If exactly one neutron from each fission event causes another fission event, the process will be self-sustaining.

A critical mass of fissionable material is required to obtain the proper number of neutron to be self-sustaining.

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Fission - Subcritical Mass (A fission event that does

not sustain)

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Fission - Supercritical Mass (A fission event that is

sustained)

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Neutrons produced during fission make it possible to have a self-sustaining fission process, i.e., a chain reaction, if exactly one neutron from each fission event causes another fission event, the process will be self-sustaining.

Fission - Supercritical Mass (A fission event that is

sustained)

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Fusion

Fusion is the combining of two light nuclei to form a more stable nucleus accompanied by a large release of energy.

Energy He H H 3

2

2

1

1

1

21

Fusion

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Both fission and fusion produce more stable nuclides

Stability of Nucleus:

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Applications of Fission and Fusion

FISSION:Controlled: Nuclear Energy Reactors

Uncontrolled: Atomic Bomb

FUSION:The hydrogen bombDevelopment of fusion as a direct energy source

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Schematic Diagram (Nuclear Power

Generation)

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Schematic Diagram (Conventional Power

Generation)

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Schematic Diagram of the Reactor

Core

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Fusion as an Energy Source

Difficulties:• Requires enormous heat energy to

give the (+) nuclei enough kinetic energy to force together

• Gaseous plasma results at these high temperatures making containment very difficult.

nHe HH 1

042

31

21 ++