Download Origin of the Elements and their Isotopes

Geochemistry
DM Sherman, University of Bristol
2010/2011
Origin of the Elements
and their Isotopes
Geochemistry, DM Sherman
University of Bristol
The Big Bang..
NASA/WMAP
Page 1
Geochemistry
DM Sherman, University of Bristol
2010/2011
The Big Bang..
•  Cosmologists have dated the universe to be 13.7 Ga
old.
•  After the first second (T=1010 K) matter was present
as protons, neutrons and electrons.
The Big Bang..
•  After several minutes (T=109 K), protons and neutrons
combined to form light nuclei 2H, 3He and 4He and 7Li.
Page 2
Geochemistry
DM Sherman, University of Bristol
2010/2011
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 % 1H
100 ppm 2H
20 ppm 3He
25% 4He
0.5 ppb 7Li
are in close agreement with observed cosmic abundances.
Page 3
Geochemistry
DM Sherman, University of Bristol
2010/2011
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)
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Geochemistry
DM Sherman, University of Bristol
2010/2011
Nuclear Fusion
Nuclear fusion occurs between light nuclei when
subjected to extremely high pressures and temperatures:
Mass deficit (Δm) releases energy (E= mc2)
Stellar Nucleosynthesis via Fusion
4p → 4He
In a medium star’s interior,
temperatures and pressures
are high enough to cause
fusion of H atoms (protons)
into 2H, 3He, 4He nuclei.
Hydrogen burning:
1H
+ 1H → 2H + γ
2H + 1H → 3He + γ
3He + 3He → 4He + 21H + γ
Plus trace Li, Be and B
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Geochemistry
DM Sherman, University of Bristol
2010/2011
Steller Evolution: Red Giant Phase
When all of the hydrogen is used up the star enters the
Red Giant phase (T = 108K ρ = 104 g cm-3 ) with
reactions:
4He + 4He → 8Be + γ
8Be + 4He → 12C + γ
4He + 12C → 16O + γ
Stellar Nucleosynthesis in Large Stars
C and O burning:
12C
12C
+ 12C → 20Ne + 4He + γ
+ 16O → 24Mg + 4He + γ
Ne burning:
20Ne
+ 12C → 28Si + 4He + γ
Si burning:
28Si
+ 4He → 32S + γ
32S + 4He → 36Ar + γ
also produces 40Ca, 44Ca, 48Ti, 52Cr, 56Fe
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Geochemistry
DM Sherman, University of Bristol
2010/2011
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 (4He)
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Geochemistry
DM Sherman, University of Bristol
2010/2011
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.
4He
2
16O 40Ca
8
20
48Ca 208Pb
20
82
This motivates the shell model of the nucleus in
analogy with electronic configurations.
Chart of the Nuclides
Z+1
N-1
β-
Z
N
Z
β+
Z-1
N+1
Protons
Z-2
N-2
N
Neutrons
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Geochemistry
DM Sherman, University of Bristol
2010/2011
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:
56Fe
+ n → 57Fe
57Fe
+ n → 58Fe
58Fe
+ n → 59Fe
But 59Fe nucleus is unstable and decays to 59Co by
converting neutron to proton and emitting radiation:
59Fe
→ 59Co + γ
β-Decay
Neutron transforms to a proton by emitting and electron:
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Geochemistry
DM Sherman, University of Bristol
2010/2011
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.
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Geochemistry
DM Sherman, University of Bristol
2010/2011
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 7Li formed in the big bang; other Li isotopes along
with Be and B formed by spallation processes.
Element Origin Summary
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Geochemistry
DM Sherman, University of Bristol
2010/2011
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
Geochemistry
DM Sherman, University of Bristol
2010/2011
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
(http://www.geo.cornell.edu/geology/classes/Chapters/Chapter10.pdf)
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