Modern Physics for Frommies IV The Universe - Small to Large Lecture 7

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Fromm Institute for Lifelong Learning University of San Francisco Modern Physics for Frommies IV The Universe - Small to Large Lecture 7 Agenda Administrative matters Corrections and/or

4 February 016 Modern Physics IV Lecture 7 Corrections and/or Clarifications to Lecture 6 Lec.

1 Fromm Institute for Lifelong Learning University of San Francisco Modern Physics for Frommies IV The Universe - Small to Large Lecture 7 Agenda Administrative matters Corrections and/or Clarifications to Lecture 6 Nuclear Physics adioactive Decay Nuclear eactions Interaction of adiation with Matter Astrophysics / Cosmology Dark Matter 4 February Modern Physics IV Lecture February 016 Modern Physics IV Lecture 7 Corrections and/or Clarifications to Lecture 6 Lec. 6 Slide 5: Yukawa potential energy 1 µ r U = g e r Where g coupling constant (analogous to charge in EM) r separation of two nucleons µ range parameter Lec. 6 Slides 33 and 34: Neutrino originally thought to have m = 0. Neutrino oscillations => ms 0 and different neutrinos have different masses. Very small, upper limits Σ < 0.7 ev 4 February 016 Modern Physics IV Lecture 7 3 Lec. 6 Slide 36: Free neutron mean life time 15 minutes (T 1/ 10 minutes) Binding energy considerations can extend lifetime or make neutron stable H He + e + ν T1/ 1 years He is stable Virtual Particles: Existence limited in time and space. Uncertainty principle allows violation of energy and momentum conservation over short times and distances E c = p c + m c 4 does not hold Virtual γ allowed to have mass from borrowed energy eal γ e + e - Cannot happen in free space Energy and momentum cannot be simultaneously conserved 4 February 016 Modern Physics IV Lecture 7 4 1

equires Coulomb interaction with a heavy nucleus like Pb real γ e + heavy nucleus virtual γ e - This can happen Virtual γ supplies mass borrowed from nucleus Energy and momentum are conserved overall

e γ. e + e - γγ is OK Fission 1938 Otto Hahn and Fritz Strassmann Lise Meitner and Otto Frisch 35 36 n U U N N + 9 9 + 1 + + neutrons + energy

2 equires Coulomb interaction with a heavy nucleus like Pb real γ e + heavy nucleus virtual γ e - This can happen Virtual γ supplies mass borrowed from nucleus Energy and momentum are conserved overall Similarly e + e - cannot annihilate to a single γ. e + e - γγ is OK Fission 1938 Otto Hahn and Fritz Strassmann Lise Meitner and Otto Frisch n U U N N neutrons + energy e.g. n + U Ba + Kr + 3n about 00 MeV Back to radioactive decay 4 February 016 Modern Physics IV Lecture February 016 Modern Physics IV Lecture 7 6 Fermi and others realized that the neutrons in the final state could be used to create a chain reaction Fermi s CP-1 at Stagg field, University of Chicago: (194) Problem: only 35 U fissions Natural Uranium is mostly 38 U with ~ 0.7 % 35 U 4 February 016 Modern Physics IV Lecture February 016 Modern Physics IV Lecture 7 8

Critical mass Dependent on: Fuel, moderator, enrichment Enrichment, separate out the 35 U Difficult, only 3 / 35 mass difference

Modern Physics IV Lecture 7 9 Breeding in a reactor: e.g Hanford, Washington Natural uranium containing mostly 38 U and about 0.

5 min): U Np + e + ν 39 39 9 93 Beta decay of 39 Np (T 1/ =.

3 Critical mass Dependent on: Fuel, moderator, enrichment Enrichment, separate out the 35 U Difficult, only 3 / 35 mass difference Electromagnetic separation Gaseous diffusion Oak idge, Tennessee (WWII) Cascade of ultracentrifuges (Present day) 4 February 016 Modern Physics IV Lecture 7 9 Breeding in a reactor: e.g Hanford, Washington Natural uranium containing mostly 38 U and about 0.7% 35 U. Fissioning 35 U supplies neutrons. Neutron absorption by 38 U n + 38 U 39 U Beta decay of 39 U ( T 1/ = 3.5 min): U Np + e + ν Beta decay of 39 Np (T 1/ =.35 days): Np Pu + e + ν February 016 Modern Physics IV Lecture 7 10 Little Boy uranium bomb: Fat Man plutonium bomb: 4 February 016 Modern Physics IV Lecture February 016 Modern Physics IV Lecture 7 1 3

4 Thermonuclear fusion Solar fusion: Proton-proton cycle 1 1 H + 11 H 1 H + e + + n 1 1 H + 1 H 3 He + γ 3 He + 3 He 4 He + 11 H + 11 H Net effect of this sequence: 4 11 H 4 He + e + + n + γ Q = 4.7 MeV + (1.0 MeV) e + annihilations = 6.7 MeV 4 February 016 Modern Physics IV Lecture 7 13 Thermonuclear weapons: Fusion requires high temperatures and densities. Stellar masses H-ignition via fission explosion 1 H + 13 H 4 He + n 4 February 016 Modern Physics IV Lecture 7 14 Uranium Fission: n + U Ba + Kr + 3 n + about 00 MeV Increase yield: add 38 U section. Fissionable with fast n Increases fallout 4 February 016 Modern Physics IV Lecture 7 15 Hydrogen Fusion: 4 11 H 4 He + e + + n + γ ΜeV Fission: Initial mass per reaction 36 amu Fusion: Initial mass per reaction 4 amu => Same fuel mass Fusion energy 36 Fusion energy per reaction 6.7 MeV = 59 Fission energy 4 Fission energy per reaction 00Mev = 7.9 Fusion gives more energy per mass of fuel by almost an order of magnitude 7 March 01 Modern Physics IV Lecture

5 Interaction of radiation with matter adiation damage: α, β, γ, X-rays, p,n etc. Ionizing radiation: Charged particles directly ionize material γ and X-rays produce electrons via PE and Compton effects which can then ionize material. γ above 1.0 Mev can produce e + e - pairs n interact with nuclei, breaking them up, giving charged particles and altering molecular structure.. 4 February 016 Modern Physics IV Lecture 7 17 Damage: Metals and other structural materials can become brittle and/or weakened. The materials can themselves become radioactive. Biological hazards: Cause malfunctions in chemical reactions in cells. Prompt damage: cell death nausea, fatigue, hair loss, hemorrhage and even death of the organism 4 February 016 Modern Physics IV Lecture 7 18 Delayed damage: Cells survive but are damaged, damage passed on to successive generations of cells. Cancer and/or sterility, mutations Somatic damage: any part of body other than reproductive organs. Includes cancer in any organ Genetic damage: reproductive cells mutations Dosimetry: Measurement of radiation exposure Source activity: 1 Curie (Ci) = 3.70 x Hz (1g of a) 1 Bequerel (Bq) = 1 Hz Absorbed dose: 1 rad (r) = 0.01 J/kg 1Gray (Gy) = 1J/kg = 100 r elative biological effectiveness or quality factor (QF): No. rads of X or γ producing same biol. damage as 1 rad of given radiation. 4 February 016 Modern Physics IV Lecture February 016 Modern Physics IV Lecture 7 0 5

6 Type X-ray and γ 1 QF β ~1 Fast p 1 Slow n ~3 Fast n Up to 10 α and heavy ions Up to 0 Astrophysics / Cosmology Effective dose: effective dose (rem) = dose (rad) x QF effective dose (Sv) = dose (Gy) x QF 4 February 016 Modern Physics IV Lecture February 016 Modern Physics IV Lecture 7 Dark Matter Discrepancy between mass of large astronomical objects determined from gravitational effects and the mass expected from the amount of luminous material. missing mass Jan Oort (193) Stars in Milky Way Fritz Zwicky (1933) Galaxies in clusters Estimated Inventory of the Components of Mass Energy in the Universe Why do we infer the existence of dark matter and dark energy? Gravitational lensing by galaxy clusters. Others Let s look at just one. otation curves for spiral galaxies. 4 February 016 Modern Physics IV Lecture February 016 Modern Physics IV Lecture 7 4 6

otation Curves and Dark Energy What is a rotation curve? igid disk rotating with angular velocity ω: v Tangential linear velocity v = ωr r v v slope, = ω r r Plot of tangential linear velocity vs.

3 M = (approximate orbit as circle) T π T = v 3 v M = = π 4π v 4π M M v = or v = π 4 February 016 Modern Physics IV Lecture 7 6 1 v Can we explain this?

7 otation Curves and Dark Energy What is a rotation curve? igid disk rotating with angular velocity ω: v Tangential linear velocity v = ωr r v v slope, = ω r r Plot of tangential linear velocity vs. radius 4 February 016 Modern Physics IV Lecture 7 5 Planets in the Solar System: Newton s mechanics applied to a central 1/r force Kepler s laws. 3 M = (approximate orbit as circle) T π T = v 3 v M = = π 4π v 4π M M v = or v = π 4 February 016 Modern Physics IV Lecture v Can we explain this? Newton's Law of Gravitation: m m F = G planet orbit The dominant mass in the solar system is the sun and is distributed spheroidally. From outside a spherical mass the gravitational field looks like that due to a point mass at the center. For a circular orbit m m planet v F = G = m planet = planet orbit v G v orbit planet 4 February 016 Modern Physics IV Lecture 7 7 orbit orbit m m 1 m v dist Keplerian Now, let s look at a spiral galaxy, e.g. NGC fall off as before 4 February 016 Modern Physics IV Lecture 7 8 Measure velocities from Doppler red and blue shifts. 7

4 February 016 Modern Physics IV Lecture 7 9 4 February 016 Modern Physics IV

8 4 February 016 Modern Physics IV Lecture February 016 Modern Physics IV Lecture7 30 Project CLEA Gettysburg College Mastering Astronomy Tutorials Pearson Education 4 February 016 Modern Physics IV Lecture February 016 Modern Physics IV Lecture 7 3 8

9 Dark Matter; what is this stuff? Accounts for a large part of the mass-energy of the universe 83% of matter and 3% of mass-energy of the universe Neither emits nor scatters electromagnetic radiation. Candidates: Massive Compact Halo Objects (MACHOs) Black holes, neutron stars, black dwarfs, brown dwarfs, etc. Such things as searches for microlensing, Hubble Space Telescope searches and Big Bang nucleosynthesis studies rule these out. 4 February 016 Modern Physics IV Lecture 7 33 Weakly Interacting Massive Particles (WIMPs) Axions, lightest supersymmetric (SUSY) particles (neutralinos). Non SUSY sterile neutrinos, other weirdos from beyond the Standard Model. Lightest SUSY particle is current favorite, People are looking at and for all of these things. Difficult experiments! Modified Gravity Laws Corrections to General elativity for different distance scales So far ruled out by observations Quantum Gravity Some theories => G varies over astrophysical scales. There are many theories but no experiments. 4 February 016 Modern Physics IV Lecture

Radioactivity. L 38 Modern Physics [4] Hazards of radiation. Nuclear Reactions and E = mc 2 Einstein: a little mass goes a long way

Radioactivity. L 38 Modern Physics [4] Hazards of radiation. Nuclear Reactions and E = mc 2 Einstein: a little mass goes a long way L 38 Modern Physics [4] Nuclear physics what s inside the nucleus and what holds it together what is radioactivity, halflife carbon dating Nuclear energy nuclear fission nuclear fusion nuclear reactors