AstroPhysics Notes Nuclear Physics Dr. Bill Pezzaglia A. Nuclear Structure Nuclear & Particle Physics B. Nuclear Decay C. Nuclear Reactions D. Particle Physics Updated: 03Aug9 (for physics 700) A. Nuclear Structure 3. Parts of Atom 4. Parts of the Atom. Isotopes Electrons (negative charge) orbital diameter approximately 0-0 m 3. Nuclide Table Nucleus size 0-5 m Nucleus made of Protons (+ charge) Neutrons (neutral) b. The Electron 5 c. The Proton (98) 897 Thomson discovers the electron. Three experiments on cathode rays ) deflected by magnetic field ) Deflected by electric field 3) Measures e/m 909 Millikan s Oil drop experiment determines value of charge e 886 Goldstein discovers canal rays which move in opposite direction as cathode rays 98 Rutherford s experiment demonstrates small size of Hydrogen nucleus, which is 800x more massive than electron Rutherford calls it the proton (greek word protos for first )
d. The Neutron 7 d. Antimatter 8 90 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 + protons) 93 Chadwick discovers neutron (Nobel prize!) Slightly heavier than proton; spin ½ like proton, even though it is neutral, it has a significant magnetic moment! Every particle has an antiparticle, which is analogous to the particle moving backwards in time 97 Paul Dirac predicts anti-electron 93 Anderson finds it ( positron ) 955 Segre & Chamberlain discover the antiproton (at UCB!) 956 the anti-neutron is discovered at UCB!. Isotopes 9 b. Nomenclature 0 H H 3 H Isotopes have same atomic number (number of protons) 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 has 6 protons, because its carbon. A X Z C c. Atomic Mass 3. Nuclide Table G. Seaborg 940 AMU: Atomic Mass Unit Carbon is exactly amu Atomic number is on vertical axis, Neutron number on the horizontal Or mole of C is grams [ mole=6.00 3 atoms] Isotopes: same Z C, C 3 Naturally occurring carbon 98.9% C (.00000 amu).% C 3 (3.00335 amu) Average:. 0(3.00335).989().0 amu Isotones: same N C, N 5, O 6 Isobars: same A C, N, O
Nuclide Table (Small Z) 3 Nuclide Table (BIG Z) 97 96 95 94 93 9 9 B. Nuclear Decay 5. Radioactivity 6. Activity. Decay Law 3. Modes (Alpha, Beta, Gamma) 4. Dosage (a) Phenomena 898 Term coined by Pierre & Marie Curie (radiation-active) 896 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: Ci = 3.7 0 0 Bq (activity of gram of radium 6) (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 (85-908), 903 Nobel Prize for discovery of radioactivity. Decay Law 7 3. Decay Modes 8 90 Rutherford & Soddy realized that all radioactive decays obeyed the same exponential decay law N( t) N 0 e Half Life: time for half of sample to decay. It is related to decay constant : t / Ln() t This emination law showed radioactive decay was not deterministic, but statistical (indeterminant) in nature. Rutherford (897) clarifies that there are two types of Becquerel Rays, alpha (which he identifies as a Helium nucleus), and beta which is 00x more penetrating. By emitting any of these, the element undergoes transmutation into another element. 3
3. Decay Modes 9 3a. Alpha Decay Alpha particle is a Helium Nucleus 4 He 0 Example: Pu 4 38 4 94 U 9 Most alpha emitters are heavy nuclei 3b. Beta Decay 3b. More Beta Decay Beta particle is actually an electron, identified in 897 by Thomson. Beta decay involves a neutrino (described by Enrico Fermi in 930s) 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 decay Example: Neutron decays to proton (plus beta and neutrino) with min halflife n p C 6 N 7 3b. Inverse Beta Decay 3 3c. Gamma Decay 4 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). p n Hydrogen fusion in sun changes protons into neutrons to make helium: 4 4( H ) He Gamma Rays discovered 900 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 4
4 Dosage 5 4b How much is bad? 6 Dose (energy damage) o Rad=0.0 Joule/kg o New unit: Gray= Joule/kg=00 Rad Dose Equivalent o Rem=RadRBE o New unit: Sievert:=GrayRBE=00 rem RBE: Relative Biological Effectiveness o for X-ray, Gamma, Beta o 0 for Alpha (more damage!) Disasters 0 Fukushima Daiichi nuclear disaster msv 986 Chernobyl: much worse Hiroshima: 4.5 Gray ( km from blast) Exposures: 500 rem 360 mrem 30 mrem 35 mrem 50/50 chance of death average annual dose one X-ray annual dose from inside body C. Nuclear Reactions. Stability 7. Nuclear Stability (a) Binding Energy: the energy required to remove one nucleon from the nucleus 8. Fission & Fusion 3. Efficiency The mass of an atom is LESS than the sum of its parts due to negative potential energy of nuclear force. Mass Defect: m=(zm p +Nm n -m atom ) Binding Energy: BE=m(93.49 MeV/u) b. Binding energy per nucleon 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 9 c. 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 30 5
b Fusion 3 c Fission 3 Combine two (or more) small nuclei to make a bigger, more stable, nuclei Large (bigger than iron), unstable nucleus is split into two (or more) smaller, more stable nuclei Fusion of 4 Hydrogen to Helium is how sun produces energy Fission can be induced by tossing a slow neutron at a nucleus. Fusion of 3 Helium to Carbon is how red giants create energy During fission, often or more neutrons are released, which can create more fissions (chain reaction) All elements up to iron in the universe were made this way inside of stars ( nucleosynthesis ). Nuclear reactors generate power from fission of U 35. 3. Efficiency 33 D. Particle Physics 34 The reaction that the sun uses to generate energy is to fuse (four) hydrogen into helium.. Quark model The mass of 4 Hydrogen s (protons) is: 4(.00785) = 4.03300 amu This is more than the Helium (4.0060), so there was a small amount of mass converted to energy m=(4.03300-4.0060)=0.087 amu. Four fundamental forces 3. The standard model Converted to a percentage: 0.087/4.03300 = 0.007 or 0.7% of the mass was converted to energy. This is called the efficiency. a. Quark model of Baryons 35 b. The Neutron 36 All Baryons made of 3 quarks (one of each color, so that they can all three be in the same s orbital and not violate pauli exclusion principle) Proton is an uud, which adds up to plus charge 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 6
c More Quarks 968 s Strange Quark 974 c Charmed Quark 977 b Bottom (beauty) quark 37 Charmed Baryons: Spin / 38 C=+ C=+ 995 t Top (truth) quark Murry Gell-Mann 969 Nobel Prize Quark Model C=0. Fundamental Forces 39 3. The Standard Model 40 ) Gravity. Three generations ) Electromagnetism 3 isospin doublets of quarks 3) Strong (Nuclear) Force Matches 3 generations of lepton doublets 4) Weak Force (beta decay) Matches (?) 4 fundamental forces? Fundamental Particles and Interactions 4 References/Notes 4 Physics Today, Feb (996) -6, The Discovery of Radioactivity 7