Interactions. Laws. Evolution

Transcription

1 Lecture Origin of the Elements MODEL: Origin of the Elements or Nucleosynthesis Fundamental Particles quarks, gluons, leptons, photons, neutrinos + Basic Forces gravity, electromagnetic, nuclear Interactions Conservation Laws Universe: Elemental Abundances Sea of Stability OUTLINE: Primary Sources of the Elements () Big Bang Creation () Stellar Nucleosynthesis (3) Cosmic-Ray Interactions Evolution I. Cosmological Nucleosynthesis: The Big Bang (Gamow) A. Evidence. Red Shift: Doppler Effect Light from all distant galaxies is shifted to the RED λ is proportional to distance (Hubble) Implication: Everything is moving away from us ; a. Universe is EXPANDING b. Matter has a COMMON ORIGIN Age = 3 ± 9 years. Universal Black-Body Radiation Remnant of primordial Explosion T present =.7 K ρ 3 g/cm (Penzias & Wilson) This radiation is isotropic in space ; i.e., it's not from our galaxy

2 3. Abundances of the Light Elements esp. in old halo stars H dominates the universe ; H, 3 He, He (little variation) ; 7 Li Conclusion: Universe must have formed from the simplest particles.. STANDARD MODEL: Universe originated in a hot, dense explosion involving the simplest particles: THE BIG BANG B. Basic Assumptions of Standard Model (small subset). Only the known particles and forces are allowed GUTS = Grand Unification Theories Origin of Forces. Matter vs. Energy Dominance: E = Mc Energy drives expansion; gravity (mass) constrains expansion 3. T = f (density) Universe cools as it expands: <E> = (3/) kt ; k =.86 MeV/K C. Chronology of the Big Bang. Elementary Particle Phase: [time < 6 s; T > 3 K > M n ] a. Most of mass-energy of the Universe in the form of ENERGY. b. Only fundamental particles present ; complex particles dissolved in heat bath (including nucleons).. Hadron Phase: [ 6 s time sec; 3 K > T > K] a. quarks and gluons condense to form hadrons (p, n, π, ) b. No complex nuclei exist ; H is simplest and binding energy is. MeV (~ K) ; Universe is too hot for H to form. c. Equilibria: p + e n + v reaction rates determine p/n n + e + p + ν ratio: needs to be ~ : for H

3 3. Nucleosynthesis Phase: [time ~ 3 minutes ; T ~ 9 K, p ~. g/cm 3 a. First step in nucleosynthesis now possible H + n H + γ Li He EC 3 7 He + He Be 5d H + n H H + H H + He H + H He He n b. Chain stops at He, although a little 7 Li sneaks through ( ) Synthesis of heavier elements is inhibited due to nuclear shell structure and very short lifetimes for nuclei just beyond doublymagic He c. Theory predicts abundances of H, H, 3 He, He and 7 Li well.. Cooling Phase: [t > 3 min, T < 9 K] Expansion continues a. Neutron Decay: n H + - β + ν ; t/ =.8 min b. Nuclear reactions cease ; T too low to exceed Coulomb barrier for charged particles and neutrons are gone. c. Matter now dominates the Universe 5. Chemistry Phase: [t ~ 5 y ; T 5 K] Electrons now attach to H, He and Li ions to form neutral atoms FLASH! photon burst microwave background H + + e H + γ

4 II. Stellar Nucleosynthesis Neutral gas expands Local inhomogenities develop (consequence of Inflation at ~ s -3 ) Once matter is inhomogeneous, localized gravitational fields are created that set the stage for galaxy formation Result: () Density increases and reheats matter; esp. in core of field. () Stars form A. Main Sequence Stars (~ 9% of stars) Sun: M = 33 g. Primordial Proton ( + He & Li) Gas: conditions a. Gravitational pressure heats core; ionizes medium. b. Electrostatic repulsion inhibits nuclear reactions and contraction c. IF M star.5 M, gravity dominates and T increases d. At T core ~ 7 K, proton burning begins to occur in high energy tail of Maxwell-Boltzmann distribution: T N(E) T T > T N(E) E e E/kT E VCoul. Hydrogen Burning a. Core conditions T - 7 K Universe: ρ ~ 3 g/cm 3 Nucleus: ρ ~ g/cm 3 ρ g/cm 3 STP: ρ ~ g/cm 3

5 b. Fundamental Reaction H + H H + - β + + c. Solar Neutrino Experiment ν WEAK INTERACTION SLOW rate-determining step * Spectrum of solar neutrinos Homestake, SD: 7Cl + ν 8Ar + -e - ; 8Ar + e - Cl ν GALLEX (Italy) SAGE (Russia) 7 3 Ga + ν Ge + e Kamiokande (Japan) ν + e - v + e SNO (Canada/US) ν + H n + p + v ( 3 tons of D O) See also: GALLEX: The experimental procedure for GALLEX is as follows: 3.3 tons of gallium in form of a concentrated GaCl3-HCl solution are exposed to solar neutrinos. In GaCl3-HCl solution, the neutrino induced 7 Ge atoms (as well as the inactive Ge carrier atoms added to the solution at the beginning of a run) form the volatile compound GeCl, which at the end of an exposure is swept out of the solution by means of a gas stream (nitrogen). The nitrogen is then passed through a gas scrubber where the GeCl is absorbed in water (see figure ). The GeCl is finally converted to GeH, which together with xenon is introduced

6 into a proportional counter in order to determine the number of 7 Ge atoms by observing their radioactive decay. d. Result: Solar Neutrino Problem Observed Rate only 3-5% of expected rate 3. ppi chain FUNDAMENTAL ENERGY SOURCE FOR THE SOLAR SYSTEM H + H H + β + + ν - H + H He + γ 3 ALL EXOTHERMIC 3 He He + H NET: H He + β + + ν MeV nuclear fusion power Reactions hard to measure: kt ~ kev compared to Big Bang kt ~. MeV

7 . Other Chains ppii: ppiii: CNO: 7 Li catalyst 7 Be catalyst C catalyst All produce same net reaction, but each is important at different temperatures and elemental compositions 5. Stellar Structure: Star in Quasi Equilibrium a. Hydrogen burning adds a small amount of additional He to Universe. b. Nuclear burning (mass energy) counterbalances gravitational attraction. c. τ sun ~ y ; heavier stars burn faster (HR diagram) more abundant in early Universe. d. He in core cannot burn easily at 7 K due to Coulomb repulsion e. H envelope continues to burn as He accumulates in core. 6. Subsequent Evolution a. M 33 g White Dwarf (stellar graveyard calibration candle) b. M 33 g Red Giant (evolution continues) B. Helium Burning: Red Giant Stars (~%). Gravitation compresses He in core: ρ 5 g/cm 3 T 8 K. Nuclear Reactions a. LiBeB are highly unstable thermally ; burn up as soon as they are formed ; Accounts for inhibition of heavy element synthesis in Big Bang.

8 b. Hoyle: 3α Reaction EXOTHERMIC He + He 8 Be* ; τ ~ 6 s 8 Be* + He 6C + γ ; Eγ=7.65 MeV ( + ) c. Three-body nature of reaction results in low rate τ RedGiant 7 8 y ; still SLOW, like Main Sequence d. Sun will become a Red Giant in ~ 5 9 y and then become a White Dwarf. 3. Subsequent Reactions Richer Chemistry C + He O + γ O + He Ne + γ Q = +. Composition of the Universe at this stage: H, He, (Li), C, O and (Ne) 5. Synthesis of heavier elements in core inhibited by higher Coulomb barriers of O and Ne ; e.g., C + C Star in quasi-equilibrium again.