Making Gold : Nuclear Alchemy Prof. 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

Atoms (‘indivisible’) …… ~ m, electrons (and their orbital structure) determine chemistry of the elements, e.g., NaCl Nuclei…..~ m 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 ~ m) have a substructure, three quarks in each nucleon (‘ups’ and ‘downs’)…but they don’t exists on their own.

Z=43 Tc Z=61 Pm Z=84 Po 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).

10 Radioactive species in the body Isotope Average amount by weightActivity U-Uranium 90μg 1.1Bq Th-Thorium 30 μg 0.11Bq 40 K 17mg 4.4 kBq Ra 31pg 1.1Bq 14 C 22ng 3.7kBq 3 H-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 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

Spectral Maps of the Galaxy Ref

Full-sky Comptel map of 1.8 MeV gamma rays in 26 Mg following 26 Al  -decay. Radioactive 26 Al around the Galaxy…. Diehl et al., Astron. Astrophys 97, 181 (1993)

Z=43 Tc Z=61 Pm Z=84 Po 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 star and create elements up to A~56

Once you have 4 He what next? 4 He + H → 5 Li not energetically allowed…neither is 4 He+ 4 He → 8 Be…we’re stuck with A=1,2,3 and 4…. BUT!!! 4 He can fuse with 2 other 4 He ( 8 Be) stuck together for a short time (~ sec) to make 12 C…complicated but understood. Once we have made 12 C (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 56 Fe 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 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 (mc 2 ) A=N+Z=125 p → n +  Xe : Z=54; N= Cs : Z=55; N= I : Z=53; N= Ba : Z=56; N= Te : Z=52; N=73 STABLE ISOBAR FOR A= In : Z= Sn : Z=50; 125 Sb : Z=51; n → p +  - +

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)

SN1987a before and after !!

36 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 → 125 Te 73 ; or Z=51, N=74 → 125 Sb 74 … Mass Parabolas and Radioactive Decays A = constant

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

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…

Nuclear reactions in Red giant stars create ‘spare’ neutrons Stellar neutron sources in the middle of Red Giant stars (e.g., Betelguese) 13 C+ 4 He→ 17 O*→ 16 O + n 22 Ne+ 4 He → 26 Mg*→ 25 Mg + n

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).

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).

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 56 Fe, (a) up to Z=92, 238 U, supernova, rapid neutron captures...also spits out material for future neutron capture in 2 nd / 3 rd generation star (b) can get up to 209 Bi ( 210 Po end-point) by slow neutron capture