Origin of the Elements and their Isotopes

Transcription

1 Origin of the Elements and their Isotopes Geochemistry, DM Sherman University of Bristol The Big Bang.. NASA/WMAP Page 1

2 The Big Bang.. Cosmologists have dated the universe to be 13.7 Ga old. After the first second (T=10 10 K) matter was present as protons, neutrons and electrons. The Big Bang.. After several minutes (T=10 9 K), protons and neutrons combined to form light nuclei 2 H, 3 He and 4 He and 7 Li. Page 2

3 The Big Bang.. After 100,000 years (T= 5000 K) neutral atoms of H and He were able to form. Success of the Big Bang model.. The predicted relative abundances 75 % 1 H 100 ppm 2 H 20 ppm 3 He 25% 4 He 0.5 ppb 7 Li are in close agreement with observed cosmic abundances. Page 3

4 Atomic Structure Atoms consist of a nucleus (protons + neutrons) surrounded* by electrons. Number of protons determines the atomic number (element). Number of neutrons + protons determines atomic mass (isotope). Protons are positively charged, neutrons are neutral and electrons are negatively charged. *It s not really like this.. Star Formation by Condensation Stars form by the gravitational condensation of hydrogen clouds. High temperatures and pressures allow nuclear fusion of light element nuclei. Star formation in Orion Nebula (NASA, Hubble) Page 4

5 Nuclear Fusion Nuclear fusion occurs between light nuclei when subjected to extremely high pressures and temperatures: Mass deficit (Δm) releases energy (E= mc 2 ) Stellar Nucleosynthesis via Fusion 4p 4 He In a medium star s interior, temperatures and pressures are high enough to cause fusion of H atoms (protons) into 2 H, 3 He, 4 He nuclei. Hydrogen burning: 1 H + 1 H 2 H + γ 2 H + 1 H 3 He + γ 3 He + 3 He 4 He H + γ Plus trace Li, Be and B Page 5

6 Steller Evolution: Red Giant Phase When all of the hydrogen is used up the star enters the Red Giant phase (T = 10 8 K ρ = 10 4 g cm -3 ) with reactions: 4 He + 4 He 8 Be + γ 8 Be + 4 He 12 C + γ 4 He + 12 C 16 O + γ Stellar Nucleosynthesis in Large Stars C and O burning: 12 C + 12 C 20 Ne + 4 He + γ 12 C + 16 O 24 Mg + 4 He + γ Ne burning: 20 Ne + 12 C 28 Si + 4 He + γ Si burning: 28 Si + 4 He 32 S + γ 32 S + 4 He 36 Ar + γ also produces 40 Ca, 44 Ca, 48 Ti, 52 Cr, 56 Fe Page 6

7 The Curve of Nuclear Binding Energy Synthesis of elements with Z > 26 (Fe) is not favored by direct fusion. Fusion Fission α-decay Alpha decay is a type of nuclear fission that occurs by spontaneous emission of an alpha particle ( 4 He) Page 7

8 Magic Numbers and Stable Nuclei Nuclei with even numbers of protons and neutrons are more stable than those with odd numbers. In particular, nuclei with 2, 8, 20, 28, 50, 82 and 126 nucleons (protons or neutrons) are especially stable. 4 He 16 O 40 Ca 48 Ca 208 Pb This motivates the shell model of the nucleus in analogy with electronic configurations. Chart of the Nuclides Z+1 N-1 β- Protons Z Z-2 N-2 N Z N β+ Z-1 N+1 Neutrons Page 8

9 The S-Process (Slow Neutron Capture) In Red giant stars, O and Si burning reactions produce a high flux of neutrons. These can be captured by nuclei to generate new isotopes beyond 56: 56 Fe + n 57 Fe 57 Fe + n 58 Fe 58 Fe + n 59 Fe But 59 Fe nucleus is unstable and decays to 59 Co by converting neutron to proton and emitting radiation: 59 Fe 59 Co + γ β-decay Neutron transforms to a proton by emitting and electron: Page 9

10 Example: S-process from Ag to Sb The R-Process (Rapid Neutron Capture) After all the material in the star s core is converted to Fe, the star can no longer produce energy. The star collapses; the heat released from gravitation causes a massive explosion known as a supernova. The explosion yields a large flux of neutrons. Heavy elements (Z > 26) are synthesised by neutron capture. Crab Nebula: a remnant of a supernova explosion. Page 10

11 Formation of Li, Be and B Li, Be and B have low binding energies Li, Be and B are destroyed in steller interiors. Although some 7 Li formed in the big bang; other Li isotopes along with Be and B formed by spallation processes. Element Origin Summary Page 11

12 Solar ( Cosmic ) Abundance of Elements This is determined from spectroscopic measurements of the sun s photosphere Solar ( Cosmic ) Abundance of Elements Big Bang Steller Nucleosynthesis Spallation Large Stars & Supernovae Page 12

13 Summary Big Bang nucleosynthesis predicts correct abundance of light nuclei. Steller nucleosynthesis: Fusion by H, C, O, Si burning S-process (neutron capture + beta decay) Spallation Explosive nucleosynthesis (supernovae): R-process (neutron capture + beta decay) Reading: White, Chapter 10 ( Page 13