Upload
constance-hood
View
214
Download
0
Embed Size (px)
Citation preview
Making Gold : Nuclear Alchemy
Prof. Paddy Regan
Department of Physics
University of Surrey
Guildford, GU2 7XH
Medieval alchemist…trying to turn base metals into gold…Mistake…to try and use ‘chemistry’…needed nuclear physics!!
‘Elemental my dear…..’
Mendeleyev
Moseley’s Law….evidence forAtomic numbers….
‘Characteristic’ X-rays…with a chemical (Z) dependence
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-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-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
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
How it all starts….Hydrogen (Z=1) to Helium (Z=2)
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
Spectral Maps of the Galaxy
Ref http://adc.gsfc.nasa.gov/mw/mmw_images.html
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
SN1987a before and after !!
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…
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)
Nuclear reactions in Red giant stars create‘spare’ neutrons
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’
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
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).
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
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.