Cosmic Alchemy: How Are We Made ? Prof. Paddy Regan FInstP Department of Physics University of...
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Transcript of Cosmic Alchemy: How Are We Made ? Prof. Paddy Regan FInstP Department of Physics University of...
Cosmic Alchemy: How Are We Made ?
Prof. Paddy Regan FInstP
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
3
Z = number of protonsN= number of neutrons
A = N+Z = mass numberneutral atom has Z electrons
m(nucleon) 2000 m(electron)
AXNZFor element X write
Atoms comprise nuclei and electrons - known since 1910Nuclei comprise protons and neutrons - known since 1932
the
neut
ron
Ernest RutherfordNobel Chemistry 1909
Founder of Nuclear Physicsth
e nu
cleu
s
Marie Curie (see later)Nobel Physics 1903
Nobel Chemistry 1911
radi
oact
ivity
J J ThomsonNobel Physics 1906
the
elec
tron
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 forAtomic numbers….
‘Characteristic’ X-rays…with a chemical (Z) dependence
Z=43Tc Z=61
Pm Z=84Po
Elemental composition of the Solar Nebula
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?
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
Spectral Maps of the Galaxy
Ref http://adc.gsfc.nasa.gov/mw/mmw_images.html
Full-sky Comptel map of 1.8 MeV gamma rays in 26Mg following 26Al -decay. Nuclear reactions are taking place continually around the galaxy.
Radioactive 26Al around the Galaxy….Diehl et al., Astron. Astrophys 97, 181 (1993)
Chart of the Nuclei
1H 2D
3He 4He
6Li 7Li
n
9Be
3T
6He
5Li
6Be 7Be8Li 9Li
10Be10Li 11Li
8He
11Be 12Be
10B 11B9B
14Be
12B 13B 14B 15B8B7B
12C 13C 14C 15C 16C 17C11C10C9C
Z =
No.
of
Pro
tons
0
1
2
3
4
5
6
N = No. of Neutrons
0 1 2 3 4 5 6 7 8 9
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).
23
Radiation in our Environment
We are all constantly subject to irradiation mainly from natural sources.
There 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
24
From NRPB-Average Radiation Dose in UK
NRPB is now HPA-RPD
25
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.
Z=43Tc Z=61
Pm Z=84Po
Elemental composition of the Solar Nebula
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 starand create elements up to A~56
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?
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).
B ~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 WorldFeb. 2002, page 33-38
Slow-neutron capture processallows formation of elementsfrom A~56 to A=209 (Bi)...
terminates at 209Bi...why?
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
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 209Bi → 210Bi (T1/2 =5 days) → 210Po.
210Po → + 206Pb (stable nucleus, as is 207Pb and 208Pb)
Polonium-210 ‘terminates’ the period table at Bi (via the s-process)
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
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
48
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
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 + - +
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?
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