Nuclear Fission

1
Nuclear Fission

• Historic Dates
• Fission Mechanism
• Binding Energy
• Liquid-Drop Model
• Classification of Heavy Nuclides
• Prompt Neutrons
• Delayed Neutrons

2
Historic Introduction

• 1932 The neutron is discovered byJ.Chadwick ?
  experiments with neutrons.
• 1939 Bombardment of U gives medium-heavy atoms
  (Ba), O.Hahn and F.Strassman L.Meitner explains
  E.Fermi and co-workers did not recognize fission.
• 1940 Spontaneous Fission (SF) is discovered by
  G.N.Flerov and K.A.Petrzhak.
• 1947 Ternary fission is discovered by Qian
  Sabqiang and He Zehui.

3
Basic Notation

• Z is the number of protons, charge number or
  atomic number
• N is the number of neutrons
• A Z N is the number of nucleons,A is called
  the mass number
• is a specific nucleus, X chem. sym.
• is in an excited state

4
Nuclides and Isotopes

• Isotope atoms with same Z and A
• Nuclide atoms/nuclei with same Z and A and
  being in a specific energy state
• Isomer long-lived excited states,

5
Nuclear Energy Production
6
Fission Process
Ternary 1/400
??0.0065?0.7
7
Compound Nucleus
8
Binding Energy
9
Energy Release
10
Fission Energy
11
Liquid-Drop Model
Nuclear force
Surface force
Coulomb repulsion
Stability maximum
Spin factor
12
Fissionable Ratio
13
Excitation Energy
AX
A1X
14
Classification of Heavy Nuclides
15
Examples of Fission
16
Fission Product Yield
17
Kinetic Energy Distribution
18
Energy Released in Fission
19
Prompt Neutrons
239Pu
235U
233U
E 1
20
Prompt Neutron Energy
21
Delayed Neutron Precursors
22
Delayed Neutron Groups
23
Delayed Neutron Spectra
24
Microscopic Cross Section
Background material
detector
?x
Reaction rate R /(sm2 sec)
?
Monoenergetic beam of neutrons I n/(sm2 sec)
x
detector
cross section s
Microscopic cross section s is defined by
R s I
NB /(sm2 sec) sm2 /(sm2 sec)/sm2
s/A Probability per nucleus that a
neutron in the beam will interact with it
25
Fission Cross Section
26
Resonances
27
235U vs. 238U
28
The END