making gold : nuclear alchemy prof. paddy regan department of physics university of surrey...
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Making Gold : Nuclear Alchemy
Prof. Paddy Regan
Department of Physics
University of Surrey
Guildford, GU2 7XH
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Medieval alchemist…trying to turn base metals into gold…Mistake…to try and use ‘chemistry’…needed nuclear physics!!
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‘Elemental my dear…..’
Mendeleyev
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Moseley’s Law….evidence forAtomic numbers….
‘Characteristic’ X-rays…with a chemical (Z) dependence
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X-rays come fromatomic ‘vacancies’
i.e. holes in the electron shellsaround the atom.
Quantum mechanicsmeans that theelectron orbits arefixed in energy….
X-rays come from anelectron ‘dropping’ from one energy level to a lower one
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X-rays come fromatomic ‘vacancies’
i.e. holes in the electron shellsaround the atom.
Quantum mechanicsmeans that theelectron orbits arefixed in energy….
X-rays come from anelectron ‘dropping’ from one energy level to a lower one
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X-rays come fromatomic ‘vacancies’
i.e. holes in the electron shellsaround the atom.
Quantum mechanicsmeans that theelectron orbits arefixed in energy….
X-rays come from anelectron ‘dropping’ from one energy level to a lower one
X-ray emitted
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Inside a nucleus
Neutron, changes mass,No charge.
Proton, changes massand charge, atomic number(Z) = number of protonsin atomic nucleus
3 protons +4 neutrons =7Li
2 protons +2 neutrons =4He
~10-15m
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How it all starts….Hydrogen (Z=1) to Helium (Z=2)
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PP-I
Qeff= 26.20 MeV
proton-proton chain
p + p d + e+ + p + d 3He +
3He + 3He 4He + 2p
86% 14%
3He + 4He 7Be +
2 4He
7Be + e- 7Li + 7Li + p 2 4He
7Be + p 8B + 8B 8Be + e+ +
99.7% 0.3%
PP-II
Qeff= 25.66 MeV PP-III
Qeff= 19.17 MeV
net result: 4p 4He + 2e+ + 2 + Qeff
proton-proton chain
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Spectral Maps of the Galaxy
Ref http://adc.gsfc.nasa.gov/mw/mmw_images.html
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Nuclear Fusion creates energy up to A~56 (Z=26 = Iron)If the star is hot enough, nuclear fusion will fuel the starand create elements up to A~56
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SN1987a before and after !!
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Figure Wiescher, Regan & Aprahamian, Physics WorldFeb. 2002, page 33-38
Slow-neutron capture processallows formation of elementsfrom A~56 to A=209 (Bi)...
terminates at 209Bi...why?
Neutron capture…no electrostatic barrier to nuclear fusion….all you need are enoughNeutrons…
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Beta –radioactive decay, (consequence of E=mc2)2 types: (i) Beta- plus proton changes to a neutron (Z ->Z-1) (ii) Beta – minus neutron changes to a proton (Z -> Z+1)
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Nuclear reactions in Red giant stars create‘spare’ neutrons
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So, how do you make Gold ?
• Gold has 79 protons (i.e. Z=79)
• Start with Z=78 protons (i.e. Platinum)
• Specifically 196Pt ( Pt = Z=78, N=196-78=118)
• Reaction is 196Pt + neutron to make 197Pt
• 197Pt is radioactive and ‘beta-decays’ to make 197Au (i.e., normal ‘stable’ gold’
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Q210Pb) = 5.41 MeVE = 5.30 MeV E(206Pb) = 0.11 MeVT1/2 = 138 days.
‘218Po =Radium A’
‘218At =Radium B’
C
D
E
210Po=Radium ‘F’ Radon
=‘Emanation’
‘Radium’
C’
C’’
The Natural Decay Chain for 238U
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S-process makes 209Bi from 208Pb+n → 209Pb (T1/2=3.2hr) → 209Bi
Neutron capture on stable 209Bi → 210Bi (T1/2 =5 days) → 210Po.
210Po → + 206Pb (stable nucleus, as is 207Pb and 208Pb).
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M.S. Smith and K.E. Rehm,Ann. Rev. Nucl. Part. Sci, 51 (2001) 91-130
Overview of main astrophysical processes
the vast majority of reactions encountered in these processes involve UNSTABLE species
hence the need for Radioactive Ion Beams
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Summary
What’s made where and how ?– Hydrogen to Helium (in the sun, p-p chain
CNO cycles).– Helium to Carbon (a story for another day)– Carbon to iron: nuclear fusion reactions– Iron to Uranium
• Up to 209Bi (210Po end-point) slow neutron capture• Heavier than Bismuth (Z=83), rapid neutron
capture, supernovae.