Transcript nuc_alchemy_talk-fgs-dec07

Making Gold : Nuclear Alchemy
Dr. Paddy Regan
Department of Physics
University of Surrey
Guildford, GU2 7XH
[email protected]
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 for
Atomic numbers….
‘Characteristic’ X-rays…with a
chemical (Z) dependence
X-rays come from
atomic ‘vacancies’
i.e. holes in the
electron shells
around the atom.
Quantum mechanics
means that the
electron orbits are
fixed in energy….
X-rays come from an
electron ‘dropping’
from one energy level
to a lower one
X-rays come from
atomic ‘vacancies’
i.e. holes in the
electron shells
around the atom.
Quantum mechanics
means that the
electron orbits are
fixed in energy….
X-rays come from an
electron ‘dropping’
from one energy level
to a lower one
X-rays come from
atomic ‘vacancies’
i.e. holes in the
electron shells
around the atom.
Quantum mechanics
means that the
electron orbits are
fixed in energy….
X-rays come from an
electron ‘dropping’
from one energy level
to a lower one
X-ray
emitted
Inside a nucleus
2 protons +
2 neutrons =
4He
Neutron, changes mass,
No charge.
~10-15m
3 protons +
4 neutrons =
7Li
Proton, changes mass
and charge, atomic number
(Z) = number of protons
in atomic nucleus
How it all starts….Hydrogen (Z=1) to Helium (Z=2)
proton-proton chain
proton-proton chain
p + p  d + e+ + n
p + d  3He + g
86%
3He
14%
+ 3He  4He + 2p
3He
99.7%
PP-I
Qeff= 26.20 MeV
+ 4He  7Be + g
+ e-  7Li + n
7Li + p  2 4He
7Be
0.3%
+ p  8B + g
8B  8Be + e+ + n
7Be
PP-II
2 4He
Qeff= 25.66 MeV
PP-III
Qeff= 19.17 MeV
net result:
4p  4He + 2e+ + 2n + Qeff
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 star
and create elements up to A~56
SN1987a before and after !!
Figure Wiescher, Regan &
Aprahamian, Physics World
Feb. 2002, page 33-38
Slow-neutron capture process
allows formation of elements
from A~56 to A=209 (Bi)...
terminates at 209Bi...why?
Neutron capture…
no electrostatic barrier
to nuclear fusion….
all you need are enough
Neutrons…
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’
The Natural Decay Chain for 238U
Qa(210Pb) = 5.41 MeV
Ea = 5.30 MeV
E(206Pb) = 0.11 MeV
T1/2 = 138 days.
‘Radium’
‘218At
=Radium B’
210Po
Radon
=‘Emanation’
=Radium ‘F’
C’
E
D
C
C’’
‘218Po
=Radium A’
S-process makes 209Bi from 208Pb+n → 209Pb (T1/2=3.2hr) → 209Bi
Neutron capture on stable
210Po
209Bi
→ 210Bi (T1/2 =5 days) → 210Po.
→ a + 206Pb (stable nucleus, as is 207Pb and 208Pb).
Overview of main astrophysical processes
M.S. Smith and K.E. Rehm,
Ann. Rev. Nucl. Part. Sci, 51 (2001) 91-130
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.