nuclear physics and radioactivity. online introduction to nuclear physics ...
TRANSCRIPT
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Online Introduction to Nuclear Physics
• http://www.sciencejoywagon.com/physicszone/lesson/12nuclear/intronuc.htm
• Online lesson on nuclear decay http://207.10.97.102/chemzone/lessons/11nuclear/nuclear.htm
• Nuclear Fusion http://ippex.pppl.gov/ippex/About_fusion/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
7
15
8
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 teachingJob at Sorbonne-firstWoman to teach there in 650Years. Later she dies of anemia.
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Three Kinds of Radioactivity
• Alpha ()– Positively charged– Least penetrating. Paper stops it
• Beta ()– Negatively charged– ½ cm Aluminum stops it
• Gamma ()– uncharged– Most penetrating. Thick lead may not stop it.
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Which Way Will It Bend?
Magnetic field in x x x
Lead block
Radium source
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Alpha Decay
• Nucleus gives off particle - 2He4
• Z decreases by 2
• A decreases by 4
• 88Ra226 -->86Rn222 + 2He4
• Rn is different
elementGraphics courtesy of Centennial of Discovery of radioactivity http://web.ccr.jussieu.fr/radioactivite/english/accueil.htm
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Energy in Decay
• Energy released is (Mp – Md – mc
• (Mp – Md – m mass defect
• Mp is mass of parent 88Ra226
• Md is mass of daughter 86Rn222
• Energy appears as KE of particle and daughter (recoil energy) Compare the energy of the 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
Courtesy “How Stuff Works”
Ionization Chamber
Americium source inside
Smoke particles decrease flow of ionization current
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Beta Decay
• 6C14 --> 7N14 + -1e0 + (anti)neutrino
• -1e0 is electron (same as -)
• Z increases by 1; electron from nucleus
• A does not change
• Occurs for
neutron heavy
isotopes
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What is a Neutrino?
• 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 nu with bar over it - antineutrino
Wolfgang Pauli
*There is some evidence that the neutrino has a tiny non-zero mass
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Positron (Beta+) Decay
• 10Ne19 --> 9F19 + e+ +
• 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- --> 3Li7 +
• 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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Fermi’s 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 +
• Gamma and
x-ray are same,
high energy photon
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Courtesy Lawrence Berkeley Laboratory
What is true about the momenta of the daughter nucleus and the gamma ray?
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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 N = -Nt
• N = N0e-t by integration
• Decay constant is called rate of decay)• Number of decays per second also proportional to
e-t “exponential function”• Half life = time for half of original sample to
decay = 0.693/• Link for decay simulation
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Exponential Decay Curve N = N0e-t
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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
n
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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 --> 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 – PG&E
Courtesy Jim Zim
Power Output 1100 MW each
Domes are 215 feet high
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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
Fat Man
(uses implosion)
Little Boy
High explosive
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+ + 0.42 MeV
• 1H1 + 1H2 2He3 + eV
• 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
Courtesy Princeton Plasma Physics Laboratory
Extract Fuel from Water
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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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Tokamak – Magnetic Confinement in a Hollow Doughnut (Torus)
Courtesy Princeton Plasma Physics Laboratory