NUCLEAR PHYSICS Chapter 29

Chapter 29 Objectives Students will understand the significance of the mass number and charge of nuclei Students will understand the nature of the strong nuclear force Students will understand nuclear fission

Chapter 29 Objectives Students will understand the significance of half-life radioactive decay Students will understand the relationship between mass and energy

Modern Physics Virtual Concept Map

Properties of Nuclei Atomic # (Z) Neutron # (N) Mass # ( A ) A Z X

Properties of Nuclei Nuclei of all atoms of a particular element must contain the same number protons. Nucleons Isotopes

Properties of Nuclei Charge and Mass Electron m e = 9.11 X kg Proton m p = X kg Neutron m n = X kg Unified Mass Unit 1 u = X kg

Properties of Nuclei Size of Nuclei Average radius = r 0 A 1/3 r 0 = 1.2 X m All nuclei have nearly the same density

Properties of Nuclei Nuclear Stability Nuclear Force ( Strong Force ) Coulomb Force ( Electro-magnetic ) When Z = 83 the repulsive forces between protons cannot be compensated by the addition of more neutrons

Binding Energy The total energy of the nucleus is less than the combined energy of the separated nucleons. Binding Energy E b = [(m p + m n ) – m element )] c 2 The total energy required to break up a nucleus into its constituent protons and neutrons can be calculated from E =  mc 2, called nuclear binding energy

Binding Energy

Radioactivity

Decay Constant and Half-Life Decay rate = N N = N 0 e - T = ln 2 T 1/2

Radioactive Decay

Methods of Radioactive Decay Periodic Table of Elements

Decay Processes Decay Rules The sum of the mass numbers A must be the same on both sides of the equation The sum of the atomic numbers Z must be the same on both sides of the equation

Decay Processes Alpha Decay Parent nucleus Daughter nucleus Spontaneous decay

Decay Processes Beta Decay The daughter nucleus has the same # of nucleons as the parent nucleus, but the atomic # is changed by 1 A neutron is transformed into an electron, proton, and neutrino.

Positron Decay Something inside the nucleus of an atom breaks down, which causes a proton to become a neutron. It emits a positron and a neutrino which go zooming off into space. The atomic number goes DOWN by one and mass number remains unchanged.

Electron Capture An electron from the closest energy level falls into the nucleus, which causes a proton to become a neutron. A neutrino is emitted from the nucleus. Another electron falls into the empty energy level and so on causing a cascade of electrons falling. One free electron, moving about in space, falls into the outermost empty level. (Incidently, this cascade of electrons falling creates a characteristic cascade of lines, mostly (I think) in the X-ray portion of the spectrum. This is the fingerprint of electron capture.) The atomic number goes DOWN by one and mass number remains unchanged. 1)

Electron Capture

Decay Processes Gamma Decay Nucleus undergoes decay to achieve a lower energy state

Natural Radioactivity Radioactive Nuclei Unstable nuclei found in nature (Natural) Nuclei produced in the laboratory (Artificial) Decay series

Works Cited Writing-Alpha-Beta.html Writing-Alpha-Beta.html /radioactivity.html /radioactivity.html astr.gsu.edu/hbase/nuclear/halfli2.html astr.gsu.edu/hbase/nuclear/halfli2.html

CHAPTER 30 Nuclear Energy and Elementary Particles

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 either case

Nuclear Fission A heavy nucleus splits into two smaller nuclei The total mass of the products is less than the original mass of the heavy nucleus First observed in 1939 by Otto Hahn and Fritz Strassman following basic studies by Fermi Lisa Meitner and Otto Frisch soon explained what had happened

Fission Equation Fission of 235 U by a slow (low energy) neutron 236 U* is an intermediate, short-lived state X and Y are called fission fragments Many combinations of X and Y satisfy the requirements of conservation of energy and charge

Sequence of Events in Fission The 235 U nucleus captures a thermal (slow- moving) neutron This capture results in the formation of 236 U*, and the excess energy of this nucleus causes it to undergo violent oscillations The 236 U* nucleus becomes highly elongated, and the force of repulsion between the protons tends to increase the distortion The nucleus splits into two fragments, emitting several neutrons in the process

Sequence of Events in Fission – Diagram

Energy in a Fission Process Binding energy for heavy nuclei is about 7.2 MeV per nucleon Binding energy for intermediate nuclei is about 8.2 MeV per nucleon Therefore, the fission fragments have less mass than the nucleons in the original nuclei This decrease in mass per nucleon appears as released energy in the fission event

Energy, cont An estimate of the energy released Assume a total of 240 nucleons Releases about 1 MeV per nucleon 8.2 MeV – 7.2 MeV Total energy released is about 240 Mev This is very large compared to the amount of energy released in chemical processes

QUICK QUIZ 30.1 In the first atomic bomb, the energy released was equivalent to about 30 kilotons of TNT, where a ton of TNT releases an energy of 4.0 × 10 9 J. The amount of mass converted into energy in this event is nearest to: (a) 1  g, (b) 1 mg, (c) 1 g, (d) 1 kg, (e) 20 kilotons

QUICK QUIZ 30.1 ANSWER (c). The total energy released was E = (30 ×10 3 ton)(4.0 × 10 9 J/ton) = 1.2 × J. The mass equivalent of this quantity of energy is:

Chain Reaction Neutrons are emitted when 235 U undergoes fission 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 The principle behind the nuclear bomb, where 1 g of U can release energy equal to about tons of TNT

Chain Reaction – Diagram