Transcript regan-gas-10jul2010

Making Gold : Nuclear Alchemy
Prof. 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
Atoms (‘indivisible’) …… ~10-10 m, electrons (and their
orbital structure) determine chemistry of the elements, e.g., NaCl
Nuclei…..~10-14m across, protons determine the
chemical element (Z); neutron number (N) determines
the mass, (A = N+Z). > 99.9 % of the mass of the atom
is in the nucleus.
Nucleons (protons and neutrons ~10-15m) have a
substructure, three quarks in each nucleon
(‘ups’ and ‘downs’)…but they don’t exists on their
own.
Elemental composition of the Solar Nebula
Z=43
Tc
Z=61
Pm
Z=84
Po
What makes a nucleus ‘stable’?
• There is an ongoing interplay and competition between coulomb
repulsion and strong nuclear force interactions
• The result is that only certain combinations of Z and N give rise
to stable configurations (about 300 in total).
• Other non-stable types can ‘radioactively decay’ (about 7,000
predicted).
Radioactive species in the body
Isotope
Average amount by weight
Activity
U-Uranium
90μg
1.1Bq
Th-Thorium
30 μg
0.11Bq
40K
17mg
4.4 kBq
Ra
31pg
1.1Bq
14C
22ng
3.7kBq
3H-tritium
0.06pg
23Bq
Po-Polonium
0.2pg
37Bq
Some variation- for example smokers have 4-5 times more Po.
10
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
Spectral Maps of the Galaxy
Ref
http://adc.gsfc.nasa.gov/mw/mmw_images.html
Radioactive 26Al around the Galaxy….
Diehl et al., Astron. Astrophys 97, 181 (1993)
Full-sky Comptel map of 1.8 MeV gamma rays in
26Mg following 26Al b-decay.
Elemental composition of the Solar Nebula
Z=43
Tc
Z=61
Pm
Z=84
Po
How it all starts….Hydrogen (Z=1) to Helium (Z=2)
The Proton-Proton Chain
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
Once you have 4He what next?
•
4He + H→ 5Li not energetically allowed…neither is
4He+4He → 8Be…we’re stuck with A=1,2,3 and 4….
• BUT!!! 4He can fuse with 2 other 4He (8Be) stuck
together for a short time (~10-16sec) to make
12C…complicated but understood.
• Once we have made 12C (Z=6) nuclei can fuse together
and gain energy (if the star is hot and massive enough) to
make all elements up to Z=26 (Iron=Fe).
DB ~1.5 MeV per A
Once you have
56Fe
what next?
• Top of the binding energy per nucleon
curve reached at A~56…fusion above this
costs energy…bad news for the star supernova
• BUT elements from 27-92 exist in nature
– how are these made ?
• Neutron Capture – neutrons have no
charge – no electrostatic repulsion.
A=N+Z = fixed
Example of a mass parabola
125In
125Ba
: Z=56; N=69
: Z=49
p→
n→
A=N+Z=125
p + b- + n
125Sn
n + b+ + n
125Cs
: Z=50;
125Xe
Mass energy
(mc2)
: Z=55; N=70
125Sb
: Z=51;
125Te
125I
: Z=52; N=73
STABLE ISOBAR
FOR A=125
: Z=54; N=71
: Z=53; N=72
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)
SN1987a before and after !!
Mass Parabolas and Radioactive Decays
Super Heavies
For a give fixed A (isobar), we have
Fewer than 300 nuclei
different combinations of Z and N.
e.g., A=Z+N=125 can be from
Proton Drip Line
Z=52, N=73 →125Te73 ; or
Z=51, N=74 → 125Sb74…
A = constant
Neutron Drip Line
36
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’
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…
Nuclear reactions in
Red giant stars create
‘spare’ neutrons
Stellar neutron sources in the
middle of Red Giant stars
(e.g., Betelguese)
13C+4He→17O*→16O
+n
22Ne+4He →26Mg*→25Mg + n
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).
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).
Summary
What’s made where and how.
– Hydrogen to Helium (in the sun, p-p chain CNO cycles).
– Helium to Carbon, triple-alpha process, special fusion.
– Carbon to iron: nuclear fusion reactions, if hot enough.
– Above 56Fe,
• (a) up to Z=92, 238U, supernova, rapid neutron captures...also spits
out material for future neutron capture in 2nd / 3rd generation star
• (b) can get up to 209Bi (210Po end-point) by slow neutron
capture