Making Gold : Nuclear Alchemy Dr. Paddy Regan 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!!

‘Elemental my dear…..’ Mendeleyev

Moseley’s Law….evidence for Atomic numbers…. ‘Characteristic’ X-rays…with a chemical (Z) dependence

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

Inside a nucleus Neutron, changes mass, No charge. Proton, changes mass and charge, atomic number (Z) = number of protons in atomic nucleus 3 protons + 4 neutrons = 7 Li 2 protons + 2 neutrons = 4 He ~ m

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

PP-I Q eff = MeV proton-proton chain p + p  d + e + + p + d  3 He +  3 He + 3 He  4 He + 2p 86%14% 3 He + 4 He  7 Be +  2 4 He 7 Be + e -  7 Li + 7 Li + p  2 4 He 7 Be + p  8 B +  8 B  8 Be + e %0.3% PP-II Q eff = MeV PP-III Q eff = MeV net result: 4p  4 He + 2e Q eff proton-proton chain

Spectral Maps of the Galaxy Ref

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

SN1987a before and after !!

Figure Wiescher, Regan & Aprahamian, Physics World Feb. 2002, page Slow-neutron capture process allows formation of elements from A~56 to A=209 (Bi)... terminates at 209 Bi...why? Neutron capture… no electrostatic barrier to nuclear fusion…. all you need are enough Neutrons…

Beta –radioactive decay, (consequence of E=mc 2 ) 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 196 Pt ( Pt = Z=78, N=196-78=118) Reaction is 196 Pt + neutron to make 197 Pt 197 Pt is radioactive and ‘beta-decays’ to make 197 Au (i.e., normal ‘stable’ gold’

Q  210 Pb) = 5.41 MeV E  = 5.30 MeV E( 206 Pb) = 0.11 MeV T1/2 = 138 days. ‘ 218 Po =Radium A’ ‘ 218 At =Radium B’ C D E 210 Po =Radium ‘F’ Radon =‘Emanation’ ‘Radium’ C’ C’’ The Natural Decay Chain for 238 U

S-process makes 209 Bi from 208 Pb+n → 209 Pb (T 1/2 =3.2hr) → 209 Bi Neutron capture on stable 209 Bi → 210 Bi (T 1/2 =5 days) → 210 Po. 210 Po →  Pb (stable nucleus, as is 207 Pb and 208 Pb).

M.S. Smith and K.E. Rehm, Ann. Rev. Nucl. Part. Sci, 51 (2001) 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 209 Bi ( 210 Po end-point) slow neutron capture Heavier than Bismuth (Z=83), rapid neutron capture, supernovae.