Making Gold : Nuclear Alchemy

Dr. 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!!

‘Elemental my dear…..’

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

Inside a nucleus

Neutron, changes mass,No charge.

Proton, changes massand charge, atomic number(Z) = number of protonsin atomic nucleus

3 protons +4 neutrons =7Li

2 protons +2 neutrons =4He

~10-15m

How it all starts….Hydrogen (Z=1) to Helium (Z=2)

PP-I

Qeff= 26.20 MeV

proton-proton chain

p + p d + e+ + p + d 3He +

3He + 3He 4He + 2p

86% 14%

3He + 4He 7Be +

2 4He

7Be + e- 7Li + 7Li + p 2 4He

7Be + p 8B + 8B 8Be + e+ +

99.7% 0.3%

PP-II

Qeff= 25.66 MeV PP-III

Qeff= 19.17 MeV

net result: 4p 4He + 2e+ + 2 + Qeff

proton-proton chain

Spectral Maps of the Galaxy

Ref http://adc.gsfc.nasa.gov/mw/mmw_images.html

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

SN1987a before and after !!

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

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’

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

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).

M.S. Smith and K.E. Rehm,Ann. Rev. Nucl. Part. Sci, 51 (2001) 91-130

Overview of main astrophysical processes

the vast majority of reactions encountered in these processes involve UNSTABLE species

hence the need for Radioactive Ion Beams

Summary

What’s made where and how ?– Hydrogen to Helium (in the sun, p-p chain

CNO cycles).– Helium to Carbon (a story for another day)– Carbon to iron: nuclear fusion reactions– Iron to Uranium

• Up to 209Bi (210Po end-point) slow neutron capture• Heavier than Bismuth (Z=83), rapid neutron

capture, supernovae.