RDCH 702: Nucleosynthesis - UNLV Radiochemistry
Transcript of RDCH 702: Nucleosynthesis - UNLV Radiochemistry
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RDCH 702: Nucleosynthesis
• Readings:
Modern Nuclear Chemistry: Chapter 12
Nuclear Astrophysics, Chapter 2 Nuclear
Properties
• Formation processes
Role of nuclear reactions
• Relationship between nuclear properties and
chemical abundance
• Electron orbitals
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Natural Element Production • Nuclear Astrophysics
fundamental information on the properties of nuclei and their reactions to the
perceived properties of astrological objects
processes that occur in space
• Universe is composed of a large variety of massive objects
distributed in an enormous volume
Most of the volume is very empty (< 1x10-18 kg/m3) and cold (~ 3 K)
Massive objects very dense
(sun's core ~ 2x105 kg/m3) and very hot (sun's core~16x106 K)
• At temperatures and densities
light elements are ionized and have high enough thermal velocities to induce a nuclear reaction
heavier elements were created by a variety of nuclear processes in massive stellar systems
• systems must explode to disperse the heavy elements
distribution of isotopes here on earth
• underlying information on the elemental abundances
• nuclear processes to produce the primordial elements
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Timeline
• Big bang 15E9 years ago
• Temperature 1E9 K
• Upon cooling influence of forces felt
2 hours
H (89 %) and He (11 %)
Strong force for nucleus
Electromagnetic force for electrons
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Subatomic particles
• A number of subatomic particles have relevance to radiochemistry
Electron
Proton
Z, atomic number
Neutron
isotopes
Photon
Neutrino
Positron
a particle
Is actually a nucleus
b particle
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Stable Nuclei
N even odd even odd
Z even even odd odd
Number 160 53 49 4
• As Z increases the line of stability moves from N=Z to N/Z ~ 1.5
Influence of the Coulomb force
For odd A nuclei only one stable isobar is found
for even A nuclei multiple stable nuclei are possible
no stable heavier odd-odd nuclei
Find the stable odd-odd nuclei
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Origin of element
• Initial H and He
• Others formed from nuclear reactions
H and He still most abundant
• Noted difference in trends with Z
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Abundances
• General logarithmic decline in the elemental abundance with atomic number
a large dip at beryllium (Z=4)
peaks at carbon and oxygen (Z=6-8), iron (Z ~ 26) and the platinum (Z=78) to lead (Z=82) region
a strong odd-even staggering
• All the even Z elements with Z>6 are more abundant than their odd atomic number neighbors
nuclear stability
nearly all radioactive decay will have taken place since production
the stable remains and extremely long lived
isotopic abundances
strong staggering and gaps
lightest nuclei mass numbers multiple of 4 have highest abundances
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Elemental Trends • Trends are based on isotopes rather than elements
Isotope described the nucleus composition
Number of protons and neutrons
Stability driven by combination of nucleons
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Abundances
• Earth predominantly
oxygen, silicon, aluminum, iron and calcium
more than 90% of the earth’s crust
• Solar system is mostly hydrogen
some helium
Based on mass of sun
• Geophysical and geochemical material processing
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Origin of Elements • Gravitational coalescence of H and He into clouds
• Increase in temperature to fusion
• Proton reaction
1H + n → 2H + g
2H + 1H → 3He
2H + n → 3H
3H + 1H → 4He + g
3He + n → 4He + g
3H + 2H → 4He + n
2H + 2H → 4He + g
4He + 3H → 7Li + g
3He+4He → 7Be + g
7Be short lived
Initial nucleosynthesis lasted 30 minutes
* Consider neutron reaction and free neutron half life
• Further nucleosynthesis in stars
No EC process in stars
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Stellar Nucleosynthesis
• He burning
4He + 4He ↔ 8Be + γ - 91.78 keV
Too short lived
3 4He → 12C + γ + 7.367 MeV
12C + 4He →16O
16O + 4He →20Ne
• CNO cycle
12C + 1H →13N + g
13N →13C + e++ νe
13C + 1H →14N + γ
14N + 1H →15O + γ
15O →15N + e+ + νe
15N + 1H →12C + 4He
Net result is conversion of 4 protons to alpha particle
4 1H → 4He +2 e++ 2 νe +3 γ
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Origin of elements Neutron Capture and proton
emission
14N + n →14C +1H; 14N(n,1H)14C
• Alpha Cluster Based on behavior of
particles composed of alphas
• Stability nuclear stability related to abundance Even-even, even A
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Formation of elements A>60 Neutron Capture; S-process
A>60
68Zn(n, γ) 69Zn, 69Zn → 69Ga + b- + n
mean times of neutron capture reactions longer than beta decay half-life
Isotope can beta decay before another capture
Up to Bi
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Nucleosynthesis: R process • Neutron capture time scale very much less than b- decay lifetimes • Neutron density 1028/m3
Extremely high flux capture times of the order of fractions of a second Unstable neutron rich nuclei
• rapidly decay to form stable neutron rich nuclei • all A<209 and peaks at N=50,82, 126 (magic numbers)
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P process • Formation of proton rich nuclei • Proton capture process • 70<A<200 • Photonuclear process, at higher Z (around 40)
(g, p), (g,a), (g, n)
190Pt and 168Yb from p process • Also associated with proton capture process (p,g) • Variation on description in the literature
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rp process (rapid proton
capture)
• Proton-rich nuclei with Z = 7-26
• (p,g) and b+ decays that populate the p-rich nuclei
Also associated with rapid proton capture process
• Initiates as a side chain of the CNO cycle
21Na and 19Ne
• Forms a small number of nuclei with A< 100
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Origin of elements
• Binding energy
Difference between energy of nucleus and nucleons
Related to mass excess
Dm=mnucleons-mnucleus
Ebind=Dmc2
* Related to nuclear models
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Periodic property of element
• Common properties of
elements
• Modern period table
develop
Actinides added in
1940s by Seaborg
s, p, d, f blocks
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Bohr Atom
• Models of atoms
Plum pudding
Bohr atom
Inclusion of quantum states
Based on Rutherford atom
• Bohr atom for 1 electron system
Etotal =1/2mev2+q1q2/4peor
q2=-e
* Include proton and electron
1/2mev2-Ze2/4peor
12 d
Electron position described by wavefunction
x, y, z, and time Probability of finding electron in a space proportional to 2
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Bohr Atom
• Net force on the electron is zero
0=Fdynamic+Fcoulombic
1/2mev2/r+q1q2/4peor2
Force is 1/r2
Energy 1/r
1/2mev2/r-Ze2/4peor2
Z is charge on nucleus
• Quantize energy through angular momentum
mvr=nh/2p, n=1,2,3….
Can solve for r, E, v
• R=(eoh2/pmee2)(n2/Z)
Radius is quantized and goes at n2
R=0.529 Å for Z=1, n=1
Ao (Bohr radius)
FdrE
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Orbitals
• Wavefunctions specified by quantum numbers
n=1,2,3,4
Principal quantum number
l=0 to n-1
Orbital angular momentum
Electron orbitals
* s,p,d,f
ml= +l
Spin=+-1/2
Energy related to Z and n
* DEtrans=-
kZ2D(1/n2)
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Many Electron Atoms
• Electron configuration
Based on quantum
numbers
Pauli exclusion principle
Aufbau principle and
Hund’s rule
Degenerate orbitals
have same spin
Maximize unfilled
orbitals
* 1s 2s 2p 3s 3p 4s
3d 4p 5s 4d 5p 6s
4f 5d 6p 7s 5f
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Many electron orbitals
• Electron configuration of
Zr and Zr4+
[Kr]4d25s2 and [Kr]
• For Fe, Fe2+, and Fe3+
[Ar]4s23d6,
[Ar]4s23d4,
[Ar]4s23d3
• Effective nuclear charge
Zeff=Z-s
Related to
electron
penetration
towards nucleus
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Atomic Radii
• Increase down a group
• Decrease across a period
Lanthanide and actinide contraction for ionic
radius
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Topic review
• Routes and reactions in nucleosynthesis
• Influence of reaction rate and particles
on nucleosynthesis
• Relationships between nuclear and
chemical properties
• Electron orbitals and interactions
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Study Questions
• How are actinides made in nucleosynthesis?
• What is the s-process?
• What elements were produced in the big bang?
• Which isotopes are produced by photonuclear
reactions?
• What do binding energetic predict about
abundance and energy release?
• What are the stable odd-odd isotopes?