Nuclear reactions

1
Nuclear reactions
Using the strong nuclear force to produce useful
energy

• Micro-world Macro-world
• Lecture 17

2
Strong Nuclear Force

• It is very strong
• It overcomes the electrical repulsion between
  positively charged protons that are only 10-15m
  apart.
• It acts over a very short range
• It is not felt by nucleons when they are more
  than 10-15m apart.
• It is selective
• It is felt by neutrons protons, but not by
  electrons

3
Nuclear bullets
Protons are repelled by electrical the repulsion
force of the positively nucleus. Only protons
with KE of a few MeV or more can get within the
range of the strong nuclear force produce
nuclear reactions







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Producing nuclear reactions with protons (or any
other charged nuclei) is a challenge



4
Neutron induced nuclear reactions
Neutrons dont feel the electrical force so even
very slow, low-energy neutrons can strike the
nucleus produce nuclear reactions




v













Low energy neutrons are effective nuclear
bullets


5
Nuclear fission
235 142 92
n 92U ? 56Ba 36Kr 2n
6
Energy balance in a fission reaction
141Ba 92Kr 2n
200 MeV ? KE ? heat
235U n
7
Chain reaction
Use the neutrons produced by one fission to
initiate another fission
Enrico Fermi
8
Requirements for A-bomb

• Fissionable material 235U or 239Pu
• Critical mass
• Mechanism

9
Critical Mass
Mcrit
Enriched 235U 50kg 239Pu 10kg
10
Fissionable Material

• Fortunately, only certain nuclear isotopes
  undergo the fission process
• 235U only 0.7 of naturally occurring U
• (99.3 is 238U, which doesnt fission)
• 239Pu doesnt occur naturally, but is produced
• in nuclear reactors
• . There are other fissionable isotopes, e.g.
  233U
• 232Th, but they are very rare

11
Little boy (235U)
(doughnut-like)
12
Fat man (239Pu)
13
Devastation
Hiroshima Aug 6 1945 815AM 80,000 people killed
immediately 100,000 people were exposed to
lethal radiation died painful slow deaths
14
Hiroshima aftermath
15
Devastation
Nagasaki Aug 9 1945 1045AM 39,000 people killed
immediately 70,000 people were exposed to
lethal radiation died painful slow deaths
16
Nagasaki aftermath
17
Nuclear fusion
Here the nuclei have to start out with large
energy in order to overcome the electrical
repulsion
Two light nuclei fuse together to form a heavier
one
2H 3H ? 4He n
18
Energy balance in a fusion reaction
4Hen
12.3 MeV ? KE ? heat
2H 3H
19
Need to overcome electric repulsion
Protons need 2MeV energy to get within 10-15 m
of each other (where strong nuclear force can be
felt)


This requires super-high temperatures
(several Million degrees). Such high temperatures
exist in the core of the Sun or in an
Atomic-Bomb explosion
20
H-bomb powered by nuclear fusion
Nuclear fusion bomb
Nuclear fission bomb detonator produces the
high temperature required to initiate fusion
processes
21
Brighter than 1000 suns
1000 times the power of an A-bomb!!
22
Dangers of teaching nuclear physics
Oh, and I suppose it was me who said what harm
could it be to give the chickens a book on
nuclear physics?
23
Fusion in the Sun
The core temperature is 14 million degrees
Here a tiny fraction of the protons have enough
thermal energy to undergo fusion
24
Solar fusion processes
1.4 MeV
5.5 MeV
12.9 MeV
25
pp-cycle
6 protons ? 4He 2 protons 2 positrons
2neutrinos
26
Energy balance in the pp-cycle
4He
25 MeV ? KE ? heat
4 protons
2 neutrinos
27
How do we know what goes oninside the Sun?
28
Superkamiokande
29
Superkamiokande
30
Direction of neutrinosdetected in Superkamiokande
31
Sun as seen by a neutrino detector
32
Neutrinos come directly from solar core
33
Neutrinos are everywhere
test
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