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Dr. Bill Pezzaglia Nuclear & Particle Physics
1 AstroPhysics Notes Dr. Bill Pezzaglia Nuclear & Particle Physics Updated: 2013Aug29 (for physics 1700)
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Nuclear Physics A. Nuclear Structure B. Nuclear Decay
2 Nuclear Physics A. Nuclear Structure B. Nuclear Decay C. Nuclear Reactions D. Particle Physics
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3 A. Nuclear Structure Parts of the Atom Isotopes Nuclide Table
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1. Parts of Atom 4 Electrons (negative charge) orbital diameter approximately m Nucleus size m Nucleus made of Protons (+ charge) Neutrons (neutral)
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1b. The Electron 5 1897 Thomson discovers the electron. Three experiments on “cathode rays” deflected by magnetic field Deflected by electric field Measures e/m 1909 Millikan’s Oil drop experiment determines value of charge “e”
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1c. The Proton (1918) 1886 Goldstein discovers “canal rays” which move in opposite direction as “cathode rays” 1918 Rutherford’s experiment demonstrates small size of Hydrogen nucleus, which is 1800x more massive than electron Rutherford calls it the “proton” (greek word “protos” for “first”)
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1d. The Neutron 7 1920 Rutherford proposes neutral particle in nucleus (thought it was a proton combined with electron) to explain nuclear masses (e.g. helium is mass of 4, but only has charge of +2 protons) 1932 Chadwick discovers neutron (Nobel prize!) Slightly heavier than proton; spin ½ like proton, even though it is neutral, it has a significant magnetic moment!
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8 1d. Antimatter Every particle has an “antiparticle”, which is analogous to the particle moving backwards in time 1927 Paul Dirac predicts “anti-electron” 1931 Anderson finds it (“positron”) 1955 Segre & Chamberlain discover the “antiproton” (at UCB !) 1956 the “anti-neutron” is discovered at UCB !
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2. Isotopes 9 Isotopes have same atomic number (number of protons)
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2b. Nomenclature 10 Z: Atomic Number Number of Protons Tells what is chemical “X” N: Neutron Number Number of Neutrons A: Mass Number Number of Nucleons A=Z+N Don’t really need “Z”: You know Carbon 14 has 6 protons, because its carbon.
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2c. Atomic Mass 11 AMU: Atomic Mass Unit Carbon 12 is exactly 12 amu
Or 1 mole of C12 is 12 grams [1 mole=6.021023 atoms] Naturally occurring carbon 98.9% C12 ( amu) 1.1% C13 ( amu) Average:
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3. Nuclide Table 12 Isotopes: same Z C12, C13
G. Seaborg 1940 Atomic number is on vertical axis, Neutron number on the horizontal Isotopes: same Z C12, C13 Isotones: same N C14, N15, O16 Isobars: same A C14, N14, O14
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Nuclide Table (Small Z)
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Nuclide Table (BIG Z) 14 97 96 95 94 93 92 91
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B. Nuclear Decay Activity Decay Law Modes (Alpha, Beta, Gamma) Dosage
15 Activity Decay Law Modes (Alpha, Beta, Gamma) Dosage
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1. Radioactivity 16 (a) Phenomena
1898 Term coined by Pierre & Marie Curie (radiation-active) 1896 Becquerel discovers radioactive emissions (“Becquerel Rays”) of uranium salts (using photographic plates) (b) Units Activity: decays per second (emissions per second) new SI unit Bq=becquerels= decays per second Old Unit: Curie: 1 Ci = 3.7×1010 Bq (activity of 1 gram of radium 226) (c) Decay Constant Activity is proportional to number of nuclei present “N” Activity = N Decay Constant “” is probability of decay per second. Antoine Henri Becquerel ( ), 1903 Nobel Prize for discovery of radioactivity
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17 2. Decay Law 1902 Rutherford & Soddy realized that all radioactive decays obeyed the same exponential decay law Half Life: time for half of sample to decay. It is related to decay constant : This “emination law” showed radioactive decay was not deterministic, but statistical (indeterminant) in nature.
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3. Decay Modes 18 Rutherford (1897) clarifies that there are two types of “Becquerel Rays”, alpha (which he identifies as a Helium nucleus), and beta which is 100x more penetrating. By emitting any of these, the element undergoes “transmutation” into another element.
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3. Decay Modes 19
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3a. Alpha Decay Example: Most alpha emitters are heavy nuclei
20 3a. Alpha Decay Alpha particle is a Helium Nucleus Example: Most alpha emitters are heavy nuclei
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21 3b. Beta Decay Beta particle is actually an electron, identified in 1897 by Thomson. Beta decay involves a “neutrino” (described by Enrico Fermi in 1930s) Example: Neutron decays to proton (plus beta and neutrino) with 12 min halflife
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22 3b. More Beta Decay Nuclei with neutron excess will change a neutron into a proton by beta decay, emitting an “anti-neutrino” and beta minus (aka an “electron”) Example: Carbon 14 decay
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23 3b. Inverse Beta Decay Nuclei with proton excess will change a proton into a neutron by inverse beta decay emitting a neutrino and beta plus (aka positron or anti-electron). Hydrogen fusion in sun changes 2 protons into neutrons to make helium:
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24 3c. Gamma Decay “Gamma Rays” discovered 1900 by Villard (later identified as high energy photons, which were what Becquerel originally saw) For example: If a - (electron) combines with its “antimatter” particle, the + (positron), they will annihilate, creating two gamma rays
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4 Dosage Dose (energy damage) Dose Equivalent
25 Dose (energy damage) Rad=0.01 Joule/kg New unit: Gray=1 Joule/kg=100 Rad Dose Equivalent Rem=RadRBE New unit: Sievert:=GrayRBE=100 rem RBE: Relative Biological Effectiveness 1 for X-ray, Gamma, Beta 10 for Alpha (more damage!)
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4b How much is bad? Disasters
26 Disasters 2011 Fukushima Daiichi nuclear disaster 1 mSv 1986 Chernobyl: much worse Hiroshima: 4.5 Gray (1 km from blast) Exposures: 500 rem 50/50 chance of death 360 mrem average annual dose 30 mrem one X-ray 35 mrem annual dose from inside body
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27 C. Nuclear Reactions Stability Fission & Fusion Efficiency
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1. Nuclear Stability 28 (a) Binding Energy: the energy required to remove one nucleon from the nucleus The mass of an atom is LESS than the sum of its parts due to negative potential energy of nuclear force. Mass Defect: m=(Zmp+Nmn-matom) Binding Energy: BE=m( MeV/u)
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1b. Binding energy per nucleon
29 Low Z: more nucleons means more nuclear force, hence more stable High Z: nuclear force is short range, big nuclei unstable Iron is most stable nuclei
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30 1c. Nuclear Force Aka “strong force”. This is what holds the protons together in a nucleus Nucleons attract each other Force is short range, hence big nuclei are unstable
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2b Fusion 31 Combine two (or more) small nuclei to make a bigger, more stable, nuclei Fusion of 4 Hydrogen to Helium is how sun produces energy Fusion of 3 Helium to Carbon is how “red giants” create energy All elements up to iron in the universe were made this way inside of stars (“nucleosynthesis”).
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2c Fission 32 Large (bigger than iron), unstable nucleus is split into two (or more) smaller, more stable nuclei Fission can be induced by tossing a slow neutron at a nucleus. During fission, often 2 or more neutrons are released, which can create more fissions (chain reaction) Nuclear reactors generate power from fission of U235.
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33 3. Efficiency The reaction that the sun uses to generate energy is to fuse (four) hydrogen into helium. The mass of 4 Hydrogen’s (protons) is: 4( ) = amu This is more than the Helium ( ), so there was a small amount of mass converted to energy m=( )= amu Converted to a percentage: / = or 0.7% of the mass was converted to energy. This is called the efficiency.
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D. Particle Physics Quark model Four fundamental forces
34 D. Particle Physics Quark model Four fundamental forces The standard model
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1a. Quark model of Baryons
35 All Baryons made of 3 quarks (one of each “color”, so that they can all three be in the same “1s” orbital and not violate pauli exclusion principle) Proton is an “uud”, which adds up to plus charge
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1b. The Neutron 36 The neutron is a “udd” combination, which has net zero charge. The “beta” decay of a neutron into a proton is hence due to one of the “d” quarks decaying into an “u” quark
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1c More Quarks 1968 “s” Strange Quark 1974 “c” Charmed Quark
37 1968 “s” Strange Quark 1974 “c” Charmed Quark 1977 “b” Bottom (beauty) quark 1995 “t” Top (truth) quark Murry Gell-Mann 1969 Nobel Prize Quark Model
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Charmed Baryons: Spin 1/2
38 C=+2 C=+1 C=0
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2. Fundamental Forces Gravity Electromagnetism Strong (Nuclear) Force
39 2. Fundamental Forces Gravity Electromagnetism Strong (Nuclear) Force Weak Force (beta decay)
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3. The Standard Model 40 1. Three generations
3 isospin doublets of quarks Matches 3 generations of lepton doublets Matches (?) 4 fundamental forces?
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Fundamental Particles and Interactions
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References/Notes 42 Physics Today, Feb (1996) 21-26, “The Discovery of Radioactivity”
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