General Physics 3 - PH213, Lecture notes of Physics

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Chapter 45

Applications of nuclear physic

45.1 Interactions Involving Neutrons 45.2 Nuclear Fission 45.3 Nuclear Reactors 45.4 Nuclear Fusion 45.5 Radiation Damage 45.6 Uses of Radiation

Processes of Nuclear Energy

Fission ◼ A nucleus of large mass number splits into two smaller nuclei Fusion ◼ Two light nuclei fuse to form a heavier nucleus Large amounts of energy are released in both cases

45.1 Interactions Involving Neutrons

Fast Neutrons

A fast neutron has energy greater than approximately 1 MeV During its many collisions when traveling through matter, the neutron gives up some of its kinetic energy to a nucleus For fast neutrons in some materials, elastic collisions dominate ◼ These materials are called moderators since they moderate the originally energetic neutrons very efficiently

45.1 Interactions Involving Neutrons

Thermal Neutrons

Most neutrons bombarding a moderator will become thermal neutrons They are in thermal equilibrium with the moderator material Their average kinetic energy at room temperature is about 0.04 eV This corresponds to a neutron root-mean-square speed of about 2 800 m/s ◼ Thermal neutrons have a distribution of speeds

45.2. Nuclear Fission

A heavy nucleus splits into two smaller nuclei Fission is initiated when a heavy nucleus captures a thermal neutron The total mass of the daughter nuclei is less than the original mass of the parent nucleus ◼ This difference in mass is called the mass defect

45.2. Nuclear Fission

Short History of Fission

First observed in 1938 by Otto Hahn and Fritz Strassman following basic studies by Fermi ◼ Bombarding uranium with neutrons produced barium and lanthanum Lise Meitner and Otto Frisch soon explained what had happened ◼ After absorbing a neutron, the uranium nucleus had split into two nearly equal fragments ◼ About 200 MeV of energy was released

45.2. Nuclear Fission

Fission Example:

235

U

A typical fission reaction for uranium is

45.2. Nuclear Fission

Distribution of Fission Products

The most probable products have mass numbers A ~ 140 and A ~ 95 There are also an average of 2.5 neutrons released per event

Quick Quiz 45.

When a nucleus undergoes fission, the two daughter nuclei are generally radioactive. By which process are they most likely to decay? (a) alpha decay (b) beta decay (e

• ) (c) beta decay (e

) ➢ According to Figure 44.4, the ratio N / Z increases with increasing Z. ➢ As a result, when a heavy nucleus fissions to two lighter nuclei, the lighter nuclei tend to have too many neutrons for the nucleus to be stable. ➢ Beta decay in which electrons are ejected decreases the number of neutrons and increases the number of protons in order to stabilize the nucleus.

Quick Quiz 45.

Which of the following are possible fission reactions?

Example 45.1 The Energy Released in the

Fission of

235

U

Calculate the energy released when 1.00 kg of 235 U fissions, taking the disintegration energy per event to be Q = 208 MeV

Example 45.1 The Energy Released in the Fission of 235U Calculate the energy released when 1.00 kg of 235 U fissions, taking the disintegration energy per event to be Q = 208 MeV ➢ if released slowly, is enough energy to keep a 100-W lightbulb operating for 30 000 years! If the available fission energy in 1 kg of 235 U were suddenly released, it would be equivalent to detonating about 20 000 tons of TNT.

45.3 Nuclear Reactors

Chain Reaction

Neutrons are emitted when 235U undergoes fission ◼ An average of 2.5 neutrons These neutrons are then available to trigger fission in other nuclei This process is called a chain reaction ◼ If uncontrolled, a violent explosion can occur ◼ When controlled, the energy can be put to constructive use Chain Reaction – Diagram

45.3 Nuclear Reactors

Enrico Fermi

1901 – 1954 Italian physicist Nobel Prize in 1938 for producing transuranic elements by neutron irradiation Other contributions include theory of beta decay, free- electron theory of metal, development of world’s first fission reactor (1942)