Nuclear Physics and Radioactivity

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Nuclear Physics and Radioactivity
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Online Introduction to Nuclear Physics

• http//www.sciencejoywagon.com/physicszone/lesson/
  12nuclear/intronuc.htm
• Online lesson on nuclear decay http//207.10.97.10
  2/chemzone/lessons/11nuclear/nuclear.htm
• Nuclear Fusion http//ippex.pppl.gov/ippex/About_f
  usion/INDEX.HTML

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Protons and Neutrons

• Atomic nuclei are made of protons and neutrons
• Proton is positive mp 1.6726 x 10-27 kg
• Neutron is neutral mn 1.6749 x 10-27 kg
• Both called nucleons

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Courtesy Lawrence Berkeley Laboratory
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Different types of Nuclei Are Called Nuclides

• Protons and neutrons are nucleons
• Atomic number Z is
• number of protons
• Atomic mass number A is
• protons plus neutrons
• Neutron number N A Z
• Nuclide symbol ZXA

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Usually A and Z are on the left
Courtesy Lawrence Berkeley Laboratory
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What is 7N15 ?

• Chemical element?
• Atomic number?
• Atomic mass number?
• Neutron number?
• Pronounced?

Nitrogen
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15
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Nitrogen Fifteen
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Properties

• Atomic properties determined by number of
  electrons
• Nuclei with certain atomic number but different
  neutron number are called
• Most elements have many isotopes

isotopes
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Nuclear Masses

• 6C12 has mass 12.000000 u
• Neutron 1.008665 u
• Proton 1.007276 u
• Neutral hydrogen atom 1.007825 u
• By E mc2 1 u 1.6605 x 10-27 kg
• 931.5 MeV/c2

Try this yourself
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Rest Masses in MeV/c2

• Electron 0.51100
• Proton 938.27
• Neutron 939.57
• 1H1 atom 938.78
• Is hydrogen more or less massive than proton and
  electron together?
• How can you explain this?

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Binding Energy

• Energy holding the nucleus together
• Stable Nucleus called a bound state
• Mass of stable nucleus less than sum of masses of
  protons and neutrons in it
• It takes energy to break it apart
• Binding energy is negative

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Example of 2He4

• 2 x mn 2(1.008665 u) 2.017330u
• 2 x 1H1 2(1.007825 u) 2.015650u
• Sum 4.032980u
• Measured 2He4 mass 4.002602u
• (With electrons)
• Difference 0.030378u
• Must use 1H1 instead of p to balance electrons

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2He4 continued

• 0.030378u x 931.5 MeV/c2/u 28.3 MeV
• Total binding energy of nucleus
• Energy that must go into nucleus to split it into
  separate nucleons

Comparison binding energy of electron in
hydrogen atom is 13.6 eV. What does that tell
you?
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Average Binding Energy per Nucleon
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Four Forces of Nature(in order of decreasing
strength)

• Strong
• Electromagnetic
• Weak
• Gravity

The strong force holds the nucleus together. It
is very short range compared to electric and
gravity
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Radioactivity

• Some nuclei change disintegrate into pieces whose
  total mass is less than mass of nucleus
• Called radioactive decay
• Discovered by Bequerel in 1896 (U)
• Curies found Ra and Po

Pitchblende sample
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Marie and Pierre Curie

• She coined term
• radioactivity
• Both won Nobel prize
• Pierre killed crossing
• street
• Marie gets his teaching
• Job at Sorbonne-first
• Woman to teach there in 650
• Years. Later she dies of anemia.

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Three Kinds of Radioactivity

• Alpha (a)
• Positively charged
• Least penetrating. Paper stops it
• Beta (b)
• Negatively charged
• ½ cm Aluminum stops it
• Gamma (g)
• Uncharged, released as photons of light
• Most penetrating. Thick lead may not stop it.

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Neutron Emission

• A fourth type of Radioactivity- emits no charged
  particles, just releases a neutron from the
  nucleus.
• By far the most damaging of any type, takes 3 ft
  of lead or a massive amount of concrete to stop
  it.

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Which Way Will It Bend?
Magnetic field in x x x
Radium source
Lead block
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Alpha Decay

• Nucleus gives off a particle - 2He4
• Z decreases by 2
• A decreases by 4
• 88Ra226 --gt86Rn222 2He4
• Rn is different
• element

Graphics courtesy of Centennial of Discovery of
radioactivity http//web.ccr.jussieu.fr/radioactiv
ite/english/accueil.htm
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Energy in a Decay

• Energy released is (Mp Md ma) c2
• (Mp Md ma) mass defect
• Mp is mass of parent 88Ra226
• Md is mass of daughter 86Rn222
• Energy appears as KE of a particle and daughter
  (recoil energy)

Compare the energy of the a particle with that of
the recoiling daughter.
What is true about their momenta and directions?
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Conservation Laws in Nuclear Processes

• Total energy is conserved
• Momentum is conserved
• Charge is conserved
• Angular momentum is conserved
• Number of nucleons (plus anti-nucleons) is
  conserved

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You Find Out

• What does Americium 241 decays into
• Use your periodic table at back of text

Answer 93Np237 Neptunium

• Application
• 95Am241is used in smoke detectors

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Smoke Detector
Ionization Chamber
Americium source inside
Smoke particles decrease flow of ionization
current
Courtesy How Stuff Works
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Beta Decay

• 6C14 --gt 7N14 -1e0 (anti)neutrino
• -1e0 is electron (same as b-)
• Z increases by 1 electron from nucleus
• A does not change
• Occurs for
• neutron heavy
• isotopes

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What is a Neutrino?
Wolfgang Pauli

• Massless, neutral particle that travels with the
  speed of light (hypothesized by Pauli in 1930)
• Incredibly penetrating - passes through Earth
• Required to be emitted in beta decay in order
  that momentum and energy be conserved(beta
  energies are not unique)
• Observed in 1956 by Reines and Cowan
• Symbol is n(nu) with bar over it - antineutrino

There is some evidence that the neutrino has a
tiny non-zero mass
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Positron (Beta) Decay

• 10Ne19 --gt 9F19 e n
• e is positron(anti-electron)
• Z of nucleus decreases by 1
• A does not change
• Occurs for
• neutron light
• isotopes

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Courtesy Stanford Linear Accelerator Lab
Question What is true about the directions of
the daughter nucleus, beta, and neutrino?
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Electron Capture

• Occurs when nucleus absorbs an orbital electron.
  Example
• 4Be7 e- --gt 3Li7 n
• Z of nucleus decreases by 1
• A does not change
• Electron disappears and one proton becomes a
  neutron
• X-rays are given off as electrons jump down

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Fermis Theory

• Explained beta decay and EC in terms of a new
  weak force
• Fermi was last double
• threat physicist great
• theorist and
• experimenter.

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Gamma Decay

• Emitted when excited nucleus jumps down to a
  lower energy state
• ZNA ? ZNA g
• Gamma and
• x-ray are same,
• high energy photon

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What is true about the momenta of the daughter
nucleus and the gamma ray?
Courtesy Lawrence Berkeley Laboratory
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Review

• There are stable nuclides (isotopes) and unstable
  (radioactive ones)
• Stable means mass of pieces is more than that of
  whole nucleus.
• Unstable means opposite
• MOST isotopes are NOT stable they undergo one
  form of decay or another

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Radioactive Decay Law

• Decay is random process
• no. decays in short time DN -lNDt
• N N0e-lt by integration
• Decay constant is called l (rate of decay)
• Number of decays per second also proportional to
• e-lt exponential
  function
• Half life time for half of original sample to
  decay 0.693/l
• Link for decay simulation

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Exponential Decay Curve N N0e-lt
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Question

• A sample contains about 1000 nuclei of a certain
  radioisotope. The half life is four minutes.
  About how many nuclei will remain after 16
  minutes?
• Hint make a table

Answer about 62 nuclei
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Decay Table
Number of Half Lives Fraction of Nuclei Remaining
1 1/2
2 1/4
3 1/8
4 1/16
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Randomness of Decay

• No way to tell which nucleus will decay when
• Actual number that decay varies around a most
  probable number
• Uncertainty is proportional to

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Decay Series

• A chain of successive decays
• Starting with U 238

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Radioactive Dating

• n 7N14 ? 6C14 p provides continual supply of
  carbon 14 at about rate of decay
• 6C14 --gt 7N14 -1e0 antineutrino
• When organism dies no more supply so ratio of
  carbon 14 to 12 decreases with 5730 yr half
  life
• Useful for dating objects up to 60,000 years old

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Nuclear Reactions

• Transformation of one element into another is
  called transmutation.
• Sought unsuccessfully by Alchemists
• Usually happens in collision
• Rutherford(1919) discovered in
• 2He4 7N14 ? 8O17 1H1

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Conservation Laws in Nuclear Reactions

• Momentum
• Energy
• Charge
• Nucleon(Baryon) Number heavy particles
• Lepton Number light particles

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Example slow neutron reaction

• 0n1 5B10 ? 3Li7 ?
• Answer 2He4 which is also called an
• Alpha particle
• Challenge Given speed of helium atom 9.30 x 106
  m/s find the
• Velocity and KE of the lithium atom
• Hint what is initial momentum of the system?

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Nuclear Fission and Fusion

• In fission a large nucleus breaks apart releasing
  energy
• In fusion light nuclei merge to form a heavier
  nucleus and energy is released.

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Nuclear Fission

• Uranium nucleus absorbs neutron and splits in two
• Easier to do with 92U235 than common 92U238
• Discovered Germany 1938
• Dangerous time

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Courtesy students at Illinois Math and Science
Academy
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Nuclear Chain Reaction


Courtesy Nuclear Energy/Nuclear Waste. Chelsea
House Publications New York, 1992.
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Above All, Fission Produces Heat
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Application Nuclear Power Plant
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How Control Rods Moderate Reaction
http//www.npp.hu/mukodes/anim/sta1-e.htm
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Diablo Canyon Nuclear Plant PGE
Power Output 1100 MW each
Domes are 215 feet high
Courtesy Jim Zim
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Ranch Seco Nuclear PlantNear Sacramento

• Shut down in 1989
• De-commissioning
• still underway
• Planned completion
• 2011

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Three Mile Island Nuclear Plant

• Partial meltdown, March 28, 1979
• 50 of reactor core destroyed or melted
• Hydrogen bubble forms inside containment
• Metropolitan Edison lies about radiation release
• Situation stabilized without injuries

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Meltdown SceneChernobyl Nuclear Plant Unit 4
Operating Power 3.2 GW Thermal,1 GW
electrical Estimated number of radiation victims
3.2 million
400 times more radioactivity was released than in
the explosion of the Hiroshima Atomic Bomb
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2001 Power Crisis Strikes California

• Nuclear Power plant proposed for Alameda Point,
  Alameda
• What do you think?

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Application Atomic BombExplodes When Critical
Mass Assembled
Little Boy
High explosive
Fat Man (uses implosion)
High explosive
Plutonium 239
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Fission Bombs

• Destructive Force about 20,000 tons of TNT
• 1945 Hiroshima and Nagasaki destroyed
• 100,000 civilians killed

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Nuclear Fusion

• Light nuclei come together (fuse) to form heavier
  nucleus
• Mass of product greater than sum of pieces
• Large energy release
• Powers the Sun
• Used to make H-bombs thermonuclear bombs

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Powering the Sun

• 1H1 1H1 ? 1H2 e n 0.42 MeV
• 1H1 1H2 ? 2He3 g 5.49 MeV
• 2He3 2He3 ? 2He4 1H1 1H1 12.86 MeV
• Proton-proton chain powers the sun
• Net effect 4 protons combine to form one helium
  nucleus

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Condition for Fusion

• Product needs more binding energy than reactants
• Reactants must be heated to millions of degrees
  to get close enough for nuclear reaction to be
  possible(very hot plasma)
• Overcome coulomb repulsion
• Nuclear forces very short range

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Reactions for Controlled Fusion

• 1H2 1H2 ? 1H3 1H1 4.03 MeV
• 1H2 1H2 ? 2He3 n 3.27 MeV
• 1H2 1H3 ? 2He4 n 17.59 MeV
• 1H1 is proton
• 1H2 is deuteron (deuterium - stable)
• 1H3 is triton (tritium, half life 12.3 years)

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Question

• How can you recognize a fusion reaction?

Makes lighter elements into heavier ones
Releases energy
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Fusion Reactors The Allure
Extract Fuel from Water
Courtesy Princeton Plasma Physics Laboratory
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Fusion Reactors The Challenge

• Need to create conditions at center of a star
• Need to contain bulk amounts of plasma at temps
  above 20 million degrees
• Need to get more energy out than you put in
• Need to demonstrate on commercial scale

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Possible Design
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Tokamak Magnetic Confinement in a Hollow
Doughnut (Torus)
Courtesy Princeton Plasma Physics Laboratory