Making Gold : Nuclear Alchemy

Prof. Paddy Regan

Department of Physics

University of Surrey

Guildford, GU2 7XH

p.regan@surrey.ac.uk

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.

Z=43Tc Z=61

Pm Z=84Po

Elemental composition of the Solar Nebula

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.

Mendeleyev

Moseley’s Law….evidence forAtomic numbers….

‘Characteristic’ X-rays…with a chemical (Z) dependence

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.

Radioactive 26Al around the Galaxy….Diehl et al., Astron. Astrophys 97, 181 (1993)

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

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.

A=N+Z = fixed

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 + - +

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 !!

36

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=125 can be from

Z=52, N=73 →125Te73 ; or

Z=51, N=74 → 125Sb74…

Mass Parabolas and Radioactive Decays

A = constant

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

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…

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).

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