Lupei Zhu

1
Nuclear Physics and Bombs

• Lupei Zhu

2
Topics

• Elements and atoms
• Radioactive decay of atoms
• Fission and fusion
• Nuclear energy
• Nuclear bomb designs
• Nuclear explosion phenomena
• The Big-Bang theory

3
Basic Knowledge about Atom

• The smallest particle (10-10 m or 0.1 nm) of an
  element that still retains the characteristics of
  the element.
• An atom has a very small size (10-15 m) nucleus
  surrounded by negatively charged electrons.
• Nucleus consists of protons (positively charged)
  and neutrons.
• Proton and neutron have about the same mass that
  is very larger than electrons. So an atom's mass
  is essentially the mass of its nucleus.

4
Periodic Table

• The number of protons (the atomic number)
  determines which element it belongs to. Atoms
  with the same atomic number but different number
  of neutrons are called isotopes, e.g., 235U and
  238U.

5

• Our Universe
• 92 H 7 He
• Most abundant elements in the Earth O, Mg, Si,
  Fe
• Most other light elements have escaped.
• Peak at iron.

6
Radioactive Decay of Atoms

• Some elements will spontaneously turn into other
  elements. This is called radioactivity and was
  discovered in 1896.
• It happens randomly and the probability only
  depends on the structure of the nucleus
  (isotope).
• Scientists use half-life to describe the
  probability of decay.

7
Radioactivity Example

• Example 238U ? 206Pb 8 4He 6ß
• The half-life T1/2 of 238U is about 4.5 billion
  years.
• The graph shows the number of 238U and 206Pb at
  time t

N206(t) N0-N238(t)
N238(t) N0/2 t/T
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Radioactive Dating
206Pb/238U 2t/T - 1

• This can be used in the opposite way if we can
  count how much daughter isotope in the sample as
  compared to the parent isotope we can get the age
  of the sample t T log2 (206Pb/238U 1)
• This method is called radioactive dating.
• By using it, we find that Earth is 4.5 Ga old.

9
Fission Process
Heavy elements are split into lighter ones.
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Fusion Process
Light elements are combined into more heavy ones
11
Nuclear Energy
Nucleons in the nucleus are bound tightly
together by the so-called strong force. The
nuclear binding energy is the energy required to
break them apart. Therefore it is possible to
release large amount of energy by breaking an
nucleus to form a different nucleus that is bound
more tightly, i.e., has higher binding energy.
12
Nuclear Energy
The left figure shows the binding energy per
nucleon of different atoms. Iron is the most
stable element. Large amount of energy (yield)
is released by fusion of light elements into
heavier elements or by fission of heavy elements
into lighter elements.
13
Yield of a 50 kg Uranium Bomb?

• Fission of one 235U atom releases energy of 235
  1 MeV/nucleon 3.8 10-11 J.
• 235 gram of 235U has 6.0 1023 atoms (Avogadros
  number). A 50 kg U-bomb has a yield of
  50,000/2356.0 10233.8 10-11 4.8 1015 J.
• The yield of 1 kt of TNT is 4.2 1012 J. So the
  yield of the 50 kg nuclear bomb is 1100 kt.
• The first Uranium bomb, the Little Boy, used 64
  kg Uranium and its yield is 15 kt. The first
  generation A-bombs are not efficient (1-2).
• US electric power consumption is about 3 1018 J
  in 2000.

14
Chain Reaction

• For 235U and 239Pu, one fission takes one neutron
  and produces three neutrons on average. The
  produced neutrons can be used to split more 235U
  and start a chain reaction.

15
Nuclear Power Plants and Bombs

• To sustain the chain reaction, it is necessary to
  have many fissionable atoms around to catch
  neutrons. The smallest amount of fissile
  material is called critical mass.
• The critical mass depends on the density of the
  material and how easy for 235U to capture a
  neutron.
• Inside a nuclear power plant, the reaction is
  controlled by absorbing neutrons and moderating
  their speed. Slow neutrons can be captured more
  easily so nuclear power plants can use low-grade
  uranium (a few per cent in 235U) as fuel.
• In contrast, nuclear explosions are uncontrolled
  chain reaction. The fission material need to be
  supercritical and high grade (enriched).
• The minimum mass is 47 kg for 235U and 16 kg for
  239Pu in normal condition.
• The mass can be reduced substantially by using
  tamper and increasing material density (e.g. the
  Fat Man bomb only used 6 kg of 239Pu).

16
Fission Bomb Designs

• A fission device
• a subcritical system that can be made
  supercritical quickly
• a strong neutron source to initiate the
  supercritical system.

17
Fission Bomb Designs
18
Fission Bomb Designs

• Boosted weapon a small amount of
  deuterium-tritium mixture is placed in the center
  of the sphere of fissile material.
• When the primary explosion happens, it produces
  enormous pressure and temperature at the center.
  This causes the deuterium and tritium mixture to
  undergo fusion and releases lots of neutrons in
  the center of the fissile sphere, greatly
  increasing the overall fission energy release.
• Boosting can increase yields by a factor of ten
  (400 kt).

19
Fusion

• To achieve fusion enormous temperatures are
  required. It takes about 50,000,000 degrees to
  get deuterium (2H) to fuse with tritium (3H). The
  required temperatures are higher for heavier
  elements.
• At present, it appears that only a fission device
  is capable to initial fusion in a H-bomb. Other
  technology, such as laser, may be possible.
• Once started, the energy released by fusion will
  sustain the process if enough fusion material are
  available.

20
Thermonuclear Bomb
21
Material for Nuclear Bombs

• Usable materials are 235U, 239Pu, 2H, and 3H.
• 235U is less than 1 in natural uranium. It can
  be enriched by using gas-diffusion or
  gas-centrifuge.
• 239Pu is virtually nonexistent in nature and can
  be obtained by bombarding 238U with neutrons in
  nuclear reactors.
• 2H can be enriched by conventional methods.
• 3H can be produced by neutron irradiation of
  lithium.

22
Making a Nuclear Bomb

• Making a nuclear bomb requires a high degree of
  competence in various disciplinary.
• Given enough time and supply, any nation or group
  with competent people should be able to produce a
  crude, heavy nuclear device.
• Delivery the weapon on a sophisticated platform
  is not easy. Nuclear tests are likely to be
  needed in order to reduce the size and weaponize.
• Other possibilities are to steal or purchase
  existing nuclear weapons, technology, or experts.

23
Nuclear Explosion Phenomena

• The explosion happens in micro-seconds.
• Can you use the nuclear physics you've learned to
  explain what's happening here?

24
The forms of nuclear energy

• The energy released in the fission/fusion
  reactions are in the forms of kinetic energy of
  particles (neutrons and other produced elements)
  and high-energy gamma rays (photons).
• In the atmosphere, the particles collide with air
  molecules to raise the temperature to 10 million
  degrees (thermal energy)

25
Nuclear Explosion Phenomena

• Hot air radiates electro-magnetic waves in a wide
  spectrum from infra-red to visible and to X and
  gamma rays (thermal radiation and EM pulse).
• The EM waves travel at speed of light.
• The high temperature also causes the pressure of
  the air around the explosion to increase to a
  million atmospheric pressure (bar).
• The highly-pressured air expands at speeds larger
  than the sound speed and generates shockwave.
• For atmospheric explosions, shockwave takes about
  50 of the yield and thermal radiation 35.

26
Nuclear Explosion Phenomena

• The fireball rises through the atmosphere in the
  form of a mushroom cloud.
• Radioactive products of the nuclear explosions
  deposit around the area.
• Some enter the upper atmosphere with the plume
  and fall to the ground over a large area.

27
New Generation Weapons

• Neutron bomb enhance neutron radiation with
  minimum radioactive fallout and shockwaves.
• Reduced radiation weapons (RRW) maximize the
  electromagnetic pulse to destroy electronic
  equipment.
• doomsday bomb It has been hypothesized to
  produce a Cobalt bomb (coat the outside a
  thermonuclear device with a tamper of Cobalt).
  Neutrons produced in the fusion reaction will
  change Cobalt to Cobalt-60, which is a very
  radioactive atom with a half-life of 5.6 years.
  Such a bomb with enough Cobalt may kill all life
  by spreading dangerous radioactive material over
  the world.

28
Where Are Elements Made?

• The light elements hydrogen and helium were
  created during the Big Bang and the elements
  between hydrogen and iron can be fused together
  inside stars. Heavier elements form during
  massive stellar explosions called supernovae.

29
Fusion in Stars

• Stars like the Sun were mostly made of H
  initially and have a temperature of 107 K. So, H
  to He fusion reaction is going on in stars (main
  sequence).
• How did it get started in the first place?

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The Birth of a Star
As hydrogen clouds condense, pressure and
temperature at the center increase. This lead to
the ignition of H fusion.
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The Fate of a Star

• When most H is fused into He, fusion stops and
  and the star starts to collapse under gravity.
• For stars with mass less than the Sun, they
  become brown dwarf and eventually end up as cold,
  dead bodies in space.
• For stars like the Sun, the gravitational force
  can squeeze the center and make it hot enough to
  start fusion of He. Star starts to swell into a
  red-giant.
• Elements from Li up to Fe are produced.
• Eventually all fusion fuel are burnt out. It
  collapses to form a white dwarf.

32
Supernova

• More Massive stars tend to explode in a
  supernova.
• Elements heavier than Fe are produced in
  explosion.
• A small central core remains to form a neutron
  star.
• If the mass is is large enough, the core continue
  to shrink to form a black hole

The crab nebula
33
How Much Time Do We have?

• The Sun is radiating energy at a rate of 4 1026W.
• It has a mass of 2 1030 kg (1H).
• The main fusion process in the Sun is combining
  1H to 2H, which releases 1.4 MeV of energy. The
  amount of H in the Sun can keep the fusion
  process last for about 10336 1023 1.41.6
  10-13/4 10263 1017s10Ga.
• At a age of 4.5 Ga, the Sun is in its middle age
  (no crisis yet).