richmond nuclear regan day2
TRANSCRIPT
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Making Gold : Nuclear AlchemyProf. Paddy Regan
Department of Physics
University of Surrey
Guildford, GU2 7XH [email protected]
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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.
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Z=43
Tc Z=61
Pm Z=84
Po
Elemental composition of the Solar Nebula
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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).
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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.
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Mendeleyev
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Moseley’s Law….evidence for
Atomic numbers….
‘Characteristic’
X-rays…with a
chemical (Z) dependence
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X-rays come from
atomic ‘vacancies’
i.e. holes in the
electron shells
around the atom.
Quantum mechanics
means that theelectron orbits are
fixed in energy….
X-rays come from an
electron ‘dropping’
from one energy level
to a lower one
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X-rays come from
atomic ‘vacancies’
i.e. holes in the
electron shells
around the atom.
Quantum mechanics
means that theelectron orbits are
fixed in energy….
X-rays come from an
electron ‘dropping’
from one energy level
to a lower one
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X-rays come from
atomic ‘vacancies’
i.e. holes in the
electron shells
around the atom.
Quantum mechanics
means that theelectron orbits are
fixed in energy….
X-rays come from an
electron ‘dropping’
from one energy level
to a lower one
X-ray
emitted
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Spectral Maps of the Galaxy
Ref http://adc.gsfc.nasa.gov/mw/mmw_images.html
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Full-sky Comptel
map of 1.8 MeV
gamma rays in
26Mg following 26Al -decay.
Radioactive 26Al around the Galaxy….
Diehl et al., Astron. Astrophys
97, 181 (1993)
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Z=43
Tc Z=61
Pm Z=84
Po
Elemental composition of the Solar Nebula
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How it all starts….Hydrogen (Z=1) to Helium (Z=2)
The Proton-Proton Chain
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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 star
and create elements up to A~56
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B ~1.5 MeV
per A
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Once you have 56Fe what next?
•
Top of the binding energy per nucleoncurve reached at A~56…fusion above thiscosts energy…bad news for the star -
supernova
•
BUT elements from 27-92 exist in nature–
how are these made ?
•
Neutron Capture –
neutrons have nocharge –
no electrostatic repulsion.
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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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SN1987a before and after !!
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A=N+Z = fixed
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Mass Parabolas and Radioactive Decays
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Proton Drip Line
Neutron Drip Line
Super Heavies
Fewer than 300 nuclei
For a give fixed A (isobar), we have
different combinations of Z and N.
e.g., A=Z+N=137 can be from
Z=56, N=81 →137Ba81
; or
Z=55, N=82 → 137Cs82 (see later)…
Mass Parabolas and Radioactive Decays
A = constant
Example of a mass parabola
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Example of a mass parabola
Mass energy
(mc2
)
A=N+Z=125
p →
n + +
+
125Xe : Z=54; N=71
125Cs : Z=55; N=70
125I : Z=53; N=72
125Ba : Z=56; N=69
125Te : Z=52; N=73
STABLE ISOBAR
FOR A=125
125In : Z=49
125Sn : Z=50;
125Sb : Z=51;
n →
p + -
+
238
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Q210Pb) = 5.41 MeVE
= 5.30 MeV
E(206Pb) = 0.11 MeV
T1/2 = 138 days.
‘218Po
=Radium A’
‘218 At
=Radium B’
C
D
E
210Po
=Radium ‘F’ Radon
=‘Emanation’
‘Radium’
C’
C’’
The Natural Decay Chain for 238U
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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 barrierto nuclear fusion….
all you need are enough
Neutrons…
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Nuclear reactions in
Red giant stars create
‘spare’
neutrons
Stellar neutron sources in themiddle of Red Giant stars
(e.g., Betelguese)
13C+4He→17O*→16O + n22 Ne+4He→26Mg*→25Mg + n
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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
196
Pt ( Pt = Z=78, N=196-78=118)
•
Reaction is 196Pt + neutron to make 197Pt
•
197Pt is radioactive and ‘beta-decays’
to make197Au (i.e., normal ‘stable’
gold).
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S-process makes
209
Bi from
208
Pb+n→209
Pb (T1/2
=3.2hr)→209
Bi
Neutron capture on stable 209Bi → 210Bi (T1/2
=5 days) → 210Po.
210Po →
+ 206Pb (stable nucleus, as is 207Pb and 208Pb).
Summary
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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, 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 spitsout material for future neutron capture in 2nd
/ 3rd
generation star
•
(b) can get up to
209Bi (210Po end-point) by slow neutroncapture
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Tomorrow’s Lectures
•
Nuclear Weapons and Security (Dr. Gilfoyle
–
U. Richmond)
•
The Polonium-210 Story (Dr. Regan –
U. Surrey UK)