Synthesis of Elements Sophia Heinz GSI Helmholtzzentrum Darmstadt, Germany and Justus-Liebig-University Giessen

The Origin of Matter in the Universe Big Bang 10–32 s End of inflationary expansion 10–5 s Nucleons are formed 100 s Synthesis of light nuclei with Z ≤ 3 109 y First stars; formation of nuclei with Z > 3 5 · 108 y The first galaxies are formed 14 · 109 y Today

Evolution of the Early Universe ▪ A description of the universe in frame of our familiar laws of nature is only possible for the period starting after the Planck time: G: Gravitational Constant Planck length: ▪ for t < tPlanck the laws loose their validity → in this regime, the quantisation of space and time is required ▪ The evolution of the universe can already be well described for the time t > 10–10 s after the Big Bang; this time corresponds to a temperature of T ≈ 1015 K and energy E = kBT ≈ 100 GeV → energy region is accessible experimentally with particle accelerators (e.g. LHC at CERN)

The first 10 μs: Neutrons and Protons are formed t ≈ (10–43 - 10–5) s; T ≈ (1032 - 1013) K ▪ plasma-like state composed of elementary particles and radiation (quark-gluon plasma, electrons, neutrinos and photons) → all constituents are in thermal equilibrium; the universe Is dominated by electromagnetic radiation; distinction bet- ween matter and radiation is hardly possible (E = mc2) t ≈ 10–5 s (10 μs); T ≈ 1013 K ▪ formation of the first nucleons („Baryogenesis“) ▪ thermal energy: Etherm ≈ 1 GeV ≈ mp, mn ▪ for T < 1 GeV, the created nucleons can no longer be destroyed by photons

t ≈ 1 s: Beginning of Nucleosynthesis t ≈ 1 s: T ≈ 1010 K; E ≈ 1 MeV ▪ The first deuterons are synthesized in fusion reactions: n + p → d + γ (Eγ = EB = 2.23 MeV) → the break-up of deuterons can be induced by photons with E ≥ 2.23 MeV ▪ due to expansion and cooling of the universe, the number of photons with E > 2 MeV is now sufficiently small to stop the break-up of deuterons → the expansion rate of the early universe is a very critical parameter for the first nucleosynthesis processes („primordial nucleosynthesis“) ▪ When the number of high-energy photons is small enough, the synthesis of nuclei heavier than A = 2 can proceed in fusion reactions t ≈ (1 – 400) s; T ~ 109 K; E ~ 0.1 MeV

t ≈ 5 minutes: End of Primordial Nucleosynthesis Z Result of primordial nucleosynthesis: ▪ composition of matter: ~ 25% He-4, ~ 75% protons; heavier elements only in traces ▪ endpoint nucleus of all reaction paths: He-4 (→ high binding energy) N

t > ~109 years: Synthesis of Heavier Elements Natural nucleosynthesis reactions ▪ Fusion ▪ n-capture, p-capture s – Process • slow neutron capture • time scale: years • in red giant stars (end point at 209Bi) Sp=0 Sn=0 rp – Process rapid proton capture r – Process • rapid neutron capture • time scale: milliseconds to seconds • in supernova explosions • > 1020 neutrons/(cm2s) → end point unknown (A ≈ 270 ?) Fusion ▪ up to Z ~ 26 (Fe) (largest binding energy per nucleon)

Nucleosynthesis in the Laboratory 1) Fragmentation, spallation und fission → at relativistic energies: vprojektile ≈ 0.9 c, E ≈ (300 – 1000) MeV/n figure: CERN

Nucleosynthesis in the Laboratory 2) Fusion reactions → beam energies at the Coulombbarriere: vbeam ≈ 0.1 c, E ≈ 5 MeV / nucleon Excited compound nuclus Evaporation residue → presently the only and most effective way to produce nuclei heavier than uranium (Z > 92) → The elements with Z > 100 were synthesized in fusion reactions

The Fusion Process in Heavy Systems Nuclear Molecule FUSION Compound Nucleus (CN) TRANSFER, QUASI-FISSION FUSION-FISSION Fission Fragments Evaporation Residue (ER) Evaporation residue cross-section:

Nucleosynthesis in the Laboratory Fusion, Fragmentation and Fission Fusion Projektil-Fragmentation Projektil-Spaltung

1935 1958 2015 Proton number Giorgio Fea, 1935 Karlsruher Nuklidkarte 126 2 8 20 28 50 82 114 184 1958 Proton number 126 2 8 20 28 50 82 114 184 2015 Giorgio Fea, 1935 Karlsruher Nuklidkarte Neutron number

Physics at the boarders of the Chart of Nuclides new decay modes: proton emission very exotic nuclear structure Halo nuclei Astrophysical nucleosynthesis Superheavy Nuclei 118 known elements ca. 3000 known isotopes ca. 4000 still unknown isotopes

What is a „Superheavy“ Nucleus? Binding energy of a nucleus in the liquid drop model (Weizsäcker formula): Condensation energy Surface energy ES Coulomb- energy EC Asymmetry energy ES EC deformation V fission barrier Bf for Z2 / A > 50 → Z > 100 Superheavy nuclei: Bf = 0

Fission Barriers of Superheavy Nuclei Shell corrections and pairing 268106162 298114184 Quadrupole deformation β2 LD Potential energy / MeV LD + shell 25098152 Microscopic corrections to the binding energy The fission barriers of superheavy nuclei are determined by the shell correction energy and the pairing term: Bf = B(N,Z)micro = Eshell + Epair

Fission Barriers of Superheavy Nuclei Model calculations of shell correction energies of superheavy nuclei N = 184 Z = 114 A. Sobiczewski et al., 1995 Enhanced stability is expected for nuclei in the following regions: ► Z = 108, N = 162 (deformed nuclei) → experimentally confirmed ► Z = 114, 120 or 126, N = 184 (spherical nuclei) → exp. confirmation still missing

The Search for New Elements ● 1932: Discovery of the neutron by J. Chadwick ● 1940 - 1952: Synthesis of the elements Z = 93 - 100 by irradiation of uranium nuclei with neutrons (Berkeley) → application of chemical identification methods 1951: Nobel prize for Chemistry for G.T. Seaborg and E.M. McMillan for their „discovery in the chemistry of the transuranium elements“ ● 1958 - 1974: Synthesis of the elements Z = 101 – 106 in Berkeley and Dubna in fusion reactions ● since 1980: Investigation of nucleon transfer reactions (e.g. in collisions of U+U or Ca+U) for the synthesis of new elements → successful up to Z = 101 ● since 1980: Investigation of fusion reactions → turned out to be the more successful method − Synthesis of the elements Z = 107 – 112 at GSI (SHIP) in fusion reactions with Pb and Bi- targets at energies below the Coulomb barrier (P. Armbruster, S. Hofmann, G. Münzenberg et al., 1981 - 1996); 2004: Synthesis of Z = 113 at RIKEN, Japan (K. Morita et al.) → Identification of the nuclei via their alpha decay chains − Synthesis of the elements Z = 113 – 118 in Dubna in fusion reactions with actinide targets (U, Pu, Cm, ...)

„Cold“ and „Hot“ Fusion Reactions Cold Fusion → doubly magic target nuclei: Pb, Bi; E*(CN) = 10 – 20 MeV; evaporation of 1 – 2 neutrons; up to now successful for Z ≤ 113 Hot Fusion → actinide targets (U, Cm, …) and 48Ca projectiles; E*(CN) = 30 – 40 MeV; evaporation of 3 – 4 neutrons; up to now successful for Z ≤ 118

Excitation Functions Characteristics: σER = σcapture x PCN x Psurvival • Maxima of cold fusion excitation functions are located below the Coulomb barrier • Location of the maxima corresponds to an internuclear distance of d > R1 + R2 → σcapture increases with beam energy but PCN and Psurvival decrease Cold Fusion σER = σcapture x PCN x Psurvival Compound nucleus excitation energy: Hot Fusion E* = Ecm + Q (Q ≈ −200 MeV)

Production Cross-sections and Rates kalte Fusion (X + Pb, Bi) Heiße Fusion (48Ca + X) 1 pb; corresponds to 1 nucleus / week → requires very sensitive separation and detection techniques !

Decay Channels and Half-lives figure: A. Sobiczewski, S. Hofmann → Nuclei on and around the island of stability have long fission half-lives and decay predominantly via alpha decay

Experimental Techniques: Separators 1) Separation according to magnetic rigidity Bρ Magnetic dipole field Gas-filled dipole magnet vacuum (A/q)3 (A/q)2 (A/q)1 (A/Z1/3)1 (A/Z1/3)2 H or He gas q ≈ vion/vo Z1/3

Experimental Techniques: Separators Example:: The Dubna gas-filled recoil separator (DGFRS)

+ Experimental Techniques: Separators + 2) Separation according to velocity Principle of a velocity filter (Wien filter): crossed electric und magnetic fields Fel particle can pass the aperture if: Fel = FL → qE = qvoB + vo = E / B + velocity filter aperture FL

Isotope identification Separation and Detection Example: the velocity filter SHIP (Separator for Heavy Ion reaction Products) at GSI Ge Detectors Si ToF 7.5° Magnet Quadrupoles Magnets Beam stop Electric field Target wheel Ion beam Ndetector ≈ 100 / s Isotope identification via alpha decay chains 1 2 sf 3 E, T1/2 v ~ E/B Nbeam ≈ 5·1012 / s

(silicon „stop detector“) Particle Identification via Alpha Decays focal plane detector (silicon „stop detector“) 293116 89 ms α 10.54 289114 3.9 s α 9.81 281110 16.5 s sf 285112 66 s α 9.11 35 mm 80 mm 300 μm ER α-particle → recording of the decays during beam-off periods for background suppression

Present State of SHE Research Z = 120, 126 ? Z = 114 ? ▪ Island of spherical shell closures is not yet reached ▪ bottleneck: available projectile / target combinations have not enough neutrons to reach N = 184

Heavy Element Research Worldwide Physics and Chemistry with Single Ions Mass measurements Dubna GANIL Berkeley GSI Lanzhou RIKEN Spectroscopy Search for Z = 119 and 120 54Cr 248Cm 302120*

Summary ▪ The first „natural elements“ were synthesized in fusion reactions in the first 5 min. after the Big Bang („primordial nucleosynthesis“) → mainly He-4 ▪ After a „break“ of ~1 Billion years, nucleosynthesis continued, also to elements with Z > 2 with the upcoming of the first stars ▪ Nature uses fusion reactions and n or p – capture reactions for nucleosynthesis ▪ Laboratory nucleosynthesis applies mainly fusion, fragmentation and fission ▪ The synthesis of new elements with Z > 100 is performed in fusion reactions ▪ For synthesis and detection of new elements highly senstiive exp. techniques are required (small production rates) → efficient separation techniques and single event identification ▪ Present main focus: attempt to produce Z=119 and 120; at Z=120 a possible spherical shell closre is expected