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Lesson 13

Lesson 13. Nuclear Astrophysics. Elemental and Isotopic Abundances. Elemental and Isotopic Abundances (cont.). Elemental and Isotopic Abundances. Overview of the Sun and the Nucleosynthetic Processes Involved. Primordial Nucleosynthesis.

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Lesson 13

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  1. Lesson 13 Nuclear Astrophysics

  2. Elemental and Isotopic Abundances

  3. Elemental and Isotopic Abundances (cont.)

  4. Elemental and Isotopic Abundances

  5. Overview of the Sun and the Nucleosynthetic Processes Involved

  6. Primordial Nucleosynthesis • Age of the universe 10-20 billion years with best estimate being 14  1 x 109 y. • Universe started with the Big Bang. • Evidence for the Big Bang: 2.7 K microwave radiation. • Photon density ~ 400/cm3

  7. Early history of the Universe

  8. Evolution of the Universe • 10-43 s Planck time 1032 K, vol=10-31 volcurrent • kBT(eV)=8.5x10-5 T(K) • Matter is QGP, all particle present. • 10-6 s, T ~1013K, hadronic matter condenses out. • Matter is nucleons, mesons, neutrinos, photons, electrons.

  9. Evolution of the Universe • 10-2 s. T~ 1011 K, ~ 4 x 106 kg/m3

  10. Evolution of the Universe • At T=1012K, n/p ~ 1, at T=1011 K n/p ~ 0.86, etc. At T = 1011K, no complex nuclei were formed because the temperature was too high to allow deuterons to form. When the temperature fell to T= 1010K (t~ 1 s), the creation of e+/e- pairs (pair production) ceased because kT < 1.02 MeV and the neutron/proton ratio was ~ 17/83. At a time of 225s, this ratio was 13/87, the temperature was T ~ 109K, then density was ~ 2 x 104kg/m3, and the first nucleosynthetic reactions occurred.

  11. First Nucleosynthesis • Hydrogen burning • n + p  d +  • p + d 3He +  • n + d 3H +  • He formation • 3H + p 4He +  • 3He + n 4He +  • 3H + d 4He + n • d + d 4He + 

  12. Big Bang Nucleosynthesis After about 30 m, nucleosynthesis ceased. The temperature was ~ 3 x 108K and the density was ~ 30 kg/m3. Nuclear matter was 76% by mass protons, 24% alpha particles with traces of deuterium, 3He and 7Li. The /n/p ratio is 109/13/87. The relative ratio of p/4He/d/3He/7Li is a sensitive function of the baryon density of the Universe. Chemistry began about 106 years later, when the temperature had fallen to 2000K and the electrons and protons could combine to form atoms. Further nucleosynthesis continues to occur in the interiors of stars.

  13. Stellar Nucleosynthesis • All elements beyond H and He synthesized in stellar interiors • Stellar nucleosynthesis continues to date (2 x 105 y 99Tc lines in stars)

  14. Stellar Evolution • Population III stars (protostars)--H, He, short lifetimes, now extinct • Population II stars (H, He, 1% heavier elements) • Population I stars (H, He, 2-5% heavier elements) Includes our sun.

  15. Sun • Typical Population I star. • mass=2 x 1030 kg • radius=7 x 106 m • ~1.41 x 103 kg/m3 • surface T ~ 6000K • Luminosity ~ 3.83 x 1026 W • age ~ 4.5 x 109 y.

  16. Herzsprung-Russell Diagrams

  17. Stellar evolution and H-R diagrams

  18. Aside on our Sun • ~ 7 x 109 more years on main sequence • 1.1-1.5 x 109 years, luminosity will increase by ~10%, making Earth uninhabitable. • Terrestrial life has used up about 3/4 of its lifespan.

  19. Supernovas • Massive stellar explosions • ~1051 ergs released in a few seconds • 2-3/century, last observation was 1987. • Some supernovas lead to the formation of neutron stars.

  20. Thermonuclear reaction rates

  21. But these are charged particle reactions! • For p +p, CB ~ 550 keV. • kT~ 1.3 keVbarrier penetration problem

  22. Stellar Nucleosynthesis--A Scorecard • Big Bang 75% H, 25% He, trace 7Li • From ~ 106 years after the Big Bang to present, get nuclear fusion reactions in stars that synthesize the elements up to A ~ 60.

  23. Hydrogen Burning • First stage of a star; converts H into He. • First reaction(pp): p + p  d + e+ +e Q = 0.42 MeV • Weak branch (pep)p + e- + p  d + e Q = 1.42 MeV • Next Reaction d + p 3He +  Q = 5.49 MeV. • 86% Branch 3He + 3He 4He + 2p Q = 12.96 MeV • Net reaction • 4p 4He + 2e+ + 2e Q = 26.7 MeV

  24. Hydrogen Burning(ppI chain)

  25. Side reaction • 3He + 4He 7Be + e • e- + 7Be 7Li + e Q = 0.86 MeV • p + 7Li  2 4He • This side branch along with the p+p, d+p is called the ppII process

  26. Another side branch • 7Be + p 8B +  • 8B 8Be* + e+ + e • 8Be*  2 4He • This sequence along with the p+p, d+p, etc is called the PPIII chain.

  27. 12C + p 13N + 13N 13C + e+ + e13C + p 14N + 14N +p 15O + 15O 15N + e+ + e15N + p 12C + 4He CNO Cycle Net effect is 4p 4He + 2e+ + 2e

  28. CNO Cycle

  29. He burning • Eventually the H fuel will be exhausted, get gravitational collapse, further heating and red giant formation. Then He burning will commence. The reaction is the 3 process.

  30. 12C + 12C 20Ne + 4He12C + 12C 23Na + p12C + 12C 23Mg + n12C + 12C 24Mg + 16O+ 16O 24Mg + 2 4He16O+ 16O 28Si + 4He16O+ 16O 31P + p16O+ 16O 31S + n16O+ 16O 32S + 

  31. Synthesis of A > 60 • Use neutron capture • There are two types of n-capture reactions. one on a slow time scale, the s-process and one on a rapid time scale, the r process. • In s process reactions, - decay intervenes between n captures while in the r process, it does not.

  32. s-Process • Example • 56Fe + n 57Fe (stable) +  • 57Fe + n 58Fe (stable) +  • 58Fe + n 59Fe (t1/2 = 44.5 d) +  • 59Fe 59Co (stable) + - + • Process terminates at 209Bi

  33. r-process ( in supernovas)

  34. p process • makes proton-rich nuclei • most reactions are photonuclear reactions like (,p), (, n), (, ) • probably occurs in supernovas.

  35. rp process

  36. Solar Neutrinos Sun emits 1.8 x 1038 neutrinos/s with the flux hitting the Earth being 6.4 x 1010 neutrinos/s/cm2.

  37. Fluxes and Detectors

  38. Solar Neutrino Detectors • The most famous detector is the Chlorine detector of Ray Davis. • Contained 100,000 gal C2Cl4 in a cavern 1600 m below the earth’s surface in the Homestake kine. • Reaction used: • Ar nuclei collected as gas, detect 2.8 keV e from • Auger from EC • Produce ~3 atoms/wk in volume of 1030 atoms.

  39. Gallex detector • e + 71Ga 71Ge + e- • Collect Ge gas product • threshold = 0.232 MeV

  40. Super K • Detect Cerenkov radiation from •  + e- + e- • threshold = 8 MeV

  41. SNO •  + e- + e- • e + d  2p + e- •  + d  n + p +  • Sensitive to all types of neutrinos

  42. Results • Davis : measured 2.1 +- 0.3 SNU expect 7.9 +- 2.4 SNU 1 SNU = 10-36 neutrino captures/s/target atom • Gallex: measured 77 +- 10 SNU expect 127 SNU

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