Transcript regan-kent-25nov09a

Cosmic Alchemy:
How Are We Made ?
Prof. Paddy Regan FInstP
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
the neutron
radioactivity
the nucleus
Atoms comprise nuclei and electrons - known since 1910
Nuclei comprise protons and neutrons - known since 1932
Marie Curie (see later)
the electron
Nobel Physics 1903
Nobel Chemistry 1911
J J Thomson
Nobel Physics 1906
Ernest Rutherford
Nobel Chemistry 1909
Founder of Nuclear Physics
Z = number of protons
N= number of neutrons
A = N+Z = mass number
neutral atom has Z electrons
m(nucleon)  2000  m(electron)
A
For element X write
Z XN
3
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.
Mendeleyev
Moseley’s Law….evidence for
Atomic numbers….
‘Characteristic’ X-rays…with a
chemical (Z) dependence
Elemental composition of the Solar Nebula
Z=43
Tc
Z=61
Pm
Z=84
Po
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?
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. Nuclear reactions are
taking place continually around the galaxy.
Chart of the Nuclei
Z = No. of Protons
6
9C
10C
11C
12C
13C
14C
15C
16C
8B
9B
10B
11B
12B
13B
14B
15B
5
7B
4
6Be
7Be
3
5Li
6Li
2
3He 4He
1
1H
0
2D
9Be 10Be 11Be 12Be
7Li
8Li
9Li
14Be
11Li
10Li
8He
6He
3T
n
0
1
2
3
4
5
6
7
N = No. of Neutrons
8
9
17C
Chart
of the
Nuclei
The Landscape
~300 stable
~ 7000 unstable … radioactive.
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).
Radiation in our Environment
are all constantly subject to irradiation mainly from natural sources.
ere are three main sources of such radiation.
 a) Primordial -around since the creation of the Earth ( 4.5 x 109years)
 235,8U ( and daughters including 210Po), 232Th or 40K (+ 87Rb, 138La and others....)
 b) Cosmogenic – from interaction of Cosmic rays with Earth and atmosphere.
 14C, 7Be formed from cosmic ray interactions. Cosmic rays are mostly protons.
 c) Produced or enhanced by human activity.
 Medical or dental X-rays;
137Cs (product from nuclear fission, 239Pu,
 241Am, 239Pu from weapons fallout
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From NRPB-Average Radiation Dose in UK
24
NRPB is now HPA-RPD
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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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
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?
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.
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?
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
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)
Polonium-210 ‘terminates’ the period table at Bi (via the s-process)
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
‘218Po
=Radium A’
C’’
BUT: Evidently, heavier (radioactive) elements like Th (Z=90) ; U (Z=92) exist ?
How are they made?
= 214Pb
= 214Bi
SN1987a before and after !!
A=N+Z = fixed
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=137 can be from
Proton Drip Line
Z=56, N=81 →137Ba81 ; or
Z=55, N=82 → 137Cs82 (see later)…
A = constant
Neutron Drip Line
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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
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?
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