Nuclear Spectroscopy: From Natural Radioactivity to Studies of the Most Exotic Isotopes.

1. Nuclear Spectroscopy: From Natural Radioactivity to Studies of the Most Exotic Isotopes. Prof. Paddy Regan Department of Physics University of Surrey, Guildford, & Radioactivity Group, National Physical Laboratory, Teddington p.regan@surrey.ac.uk

2. Outline of talk • Elements, Isotopes and Isotones • Alpha, beta and gamma decay • Primordial radionuclides…..why so long ? • Internal structures, gamma rays and shells. • How big is the nuclear chart ? • What could this tell us about nucleosynthesis?

4. Darmstadtium Copernicium Roentgenium

5. The Microscopic World… •ATOMS ~ 10-10 m • •NUCLEI ~ 10-14 m •NUCLEONS-10-15 m •QUARKS ~?

7. Mass Spectrograph (Francis Aston 1919) Atoms of a given element are ionized. The charged ions go into a velocity selector which has orthogonal electric (E) and magnetic fields (B) set to exert equal and opposite forces on ions of a particular velocity → (v/B) = cont. The magnet then separates the ions according to mass since the bending radius is r = (A/Q) x (v/B)Q = charge of ion & A is the mass of the isotope Results for natural terrestrial krypton 0.4% 2.3 11.6 11.5 57.0 17.3 Nuclear Isotopes Not all atoms of the same chemical element have the same mass (A) Frederick Soddy (1911) gave the nameisotopes. (iso = same ; topos = place). Krypton, Z=36 N = 42 44 46 47 48 50

8. Nuclear chart

9. Atomic Masses and Nuclear Binding Energies M(Z,A) = mass of neutral atom of element Z and isotope A The binding energy is the energy needed to take a nucleus of Z protons and N neutrons apart into A separate nucleons M(Z,A) m ( 11H ) + Nmn - Bnuclear Mass of Z protons + Z electrons + N neutrons (N=A-Z) energy = binding energy (nuclear + atomic) Mass of neutral atom  MeV  eV

10. increasing Z → increasing Z → A=125, odd-A even-Z, odd-N or odd-Z, even N A=128, even-A even-Z, even-N or odd-Z, odd- N increasing binding energy = smaller mass 125Sn, Z=50, N=75 125Xe, Z=54, N=71 ISOBARS have different combinations of protons (Z) and neutrons (N) but same total nucleon number, A → A = N + Z. (Beta) decays occur along ISOBARIC CHAINS to reach the most energetically favoured Z,N combination. This is the ‘stable’ isobar. This (usually) gives the stable element for this isobaric chain. A=125, stable isobar is 125Te (Z=52, N=73); Even-A usually have 2 long-lived.

11. 137Xe83 137Ba81 137Cs82 A=137 Mass Parabola Mass (atomic mass units) Nucleus can be left in an excited configuration. Excess energy released by Gamma-ray emission. b - decay: 2 types: 1) Neutron-rich nuclei (fission frags) n → p + b- + n 2)Neutron-deficient nuclei (18F PET) p → n + b+ + n

12. Some current nuclear physics questions • 286 combinations of protons and neutrons are either stable or have decay half-lives of more than 500 million years. • What are the limits of nuclear existence…i.e. how many different nuclear species can exist? • N/Z ratio changes for stable nuclei from ~1:1 for light nuclei (e.g., 16O, 40Ca) to ~1.5 for 208Pb (126/82 ~ 1.5) • How does nuclear structure change when the N/Z ratio differs from stable nuclear matter?

13. Accelerator facility at GSI-Darmstadt • The Accelerators: • UNILAC(injector) E=11.4 MeV/n • SIS 18Tmcorr. U 1 GeV/n • Beam Currents: • 238U - 108 pps • some medium mass nuclei- 109 pps • (A~130) • FRS provides secondary radioactive ion beams: • fragmentation or fission of primary beams • high secondary beam energies: 100 – 700 MeV/u • fully stripped ions

14. Reaction products travelling at Relativistic Energies Beam at Relativistic Energy ~0.5-1 GeV/A FIREBALL Formation of an exotic compound nucleus Target Nucleus Ablation Abrasion An Efficient Way to Make Exotic Nuclei:Projectile Fragmentation Reaction Process

18. A few physics examples….

19. b+ decay/ec b- decay

21. How are the heavy elements made ? Is it via the Rapid Neutron Capture (R-) Process ? T1/2 = 10.4 s 205Au126 K-electrons L-electrons 202Pt Many of the nuclei which lie on the r-process predicted path have yet to be studied. Do these radioactive nuclei act as we expect ?

22. SN1987a before and after !!

24. A (big!) problem, can’t reproduce the observed elemental abundances. • We can ‘fix’ the result by changing the shell structure (i.e. changing • the magic numbers)….but is this scientifically valid ? N=82 N=126 • Need to look at N=82 and 126 ‘exotic’ nuclei in detail….

25. Excitation energy (keV) • ~2 D • = ‘pair gap’ 2+ 0+ Ground state (Ex=0) config has Ip=0+ ; Even-Even Nuclei Excited states spin/parities depend on the nucleon configurations. i.e., which specific orbits the protons and neutrons occupy. Result is a complex energy ‘level scheme’. First excited state in (most) even-N AND even-Z has Ip=2+

26. Excitation energy (keV) 2+ 0+ Ground state Configuration. Spin/parity Ip=0+ ; Ex = 0 keV PHR, Physics World, Nov. 2011, p37

27. exp. pronounced shell gap shell structure quenched Is there evidence for a N=82 shell quenching ? r-process abundances mass number A Assumption of a N=82 shell quenching leads to a considerable improvement in the global abundance fit in r-process calculations !

29. Search for the 8+ (g9/2)-2 seniority isomer in 130Cd (structure should look lots like 98Cd…apart from size?) two proton holes in the g9/2 orbit g9/2 M. Górska et al., Phys. Rev. Lett. 79 (1997)

30. Evidence for nuclear shell structure…..energy of 1st excited state in even-even nuclei….E(2+).

31. Facility for Anti-Proton and Ion Research (FAIR) To be constructed at the current GSI site, near Darmstadt, Germany Will bring currently ‘theoretical nuclear species’ into experimental reach for the first time.

32. Summary • Radionuclides (e.g. 235U, 238U, 232Th, 40K) are everywhere. • Radioactive decays arise from energy conservation and other (quantum) conservation laws. • Characteristic gamma ray energies tell us structural info. • The limits for proton-richness in nuclei has been reached. • Neutron-rich nuclei are harder to make at the extremes, but we are starting to be able to reach r-process radionuclides. • Does the nuclear shell model remain valid for nuclei with ‘diffuse neutron skins’ ? • FAIR will increase dramatically our reach of nuclear species for experimental study