Polarized radioactive ion beams

1. Polarized radioactive ion beams XIIth International Workshop on Polarized Sources, Targets & Polarimetry September 2007 Phil Levy, TRIUMF, Vancouver

2. Methods of Production

3. Radioactive ion beam (RIB) production Projectile fragmentation: Separator High energy stable beam High energy RIB Moderate thickness target

4. Radioactive ion beam (RIB) production ISOL – isotope separation on-line: Thick target p+ beam 500 MeV Diffusion and effusion of reaction products Ion source “dirty” ion beam Mass separator “pure” RIB 10 – 60 keV

5. Radioactive ion beam (RIB) production Combined: Post acceleration electrodes High energy RIB Low energy RIB Low emittance No chemical selectivity Fast Helium gas stopper and ion guide

6. In-beam High energy Low energy Optical pumping Tilted foils Recoil angle and energy selection Atomic beams Transverse Collinear Methods of polarization

7. Motivation

8. Angular distribution W(q) of emitted beta rays W(q) = 1 + (v/c) AasymP cos q Aasym= -1/3 P = 1 Usefulness of polarized RIBs • Experimenters want beta emitters • Can monitor/control relaxation and precession of polarization of • nuclei implanted in crystals. • Decay asymmetry depends on the transition

9. Geometry of a β-NMR Experiment 8Li Backward H0 t½= 838 ms H1cos(ωt) Forward

10. Physics with polarized b-emitters • b-NMR and b-NQR • condensed matter physics – ultrathin films, superconductivity [Kiefl et al, • NIMB 204 (2003) 682] • nuclear magnetic dipole and electric quadrupole moments • High resolution atomic spectroscopy • b-detected optical pumping of low intensity beams • • hyperfine structure, nuclear moments, isotope shifts, nuclear charge radii • [Kluge and Nörtershäuser, Spectrochimica Acta B 58 (2003) 1031] • Fundamental symmetries • spin manipulation with applied rf • comparison of symmetry between b-decays in mirror nuclei (G-parity) • [Minamisono et al, Hyperfine Interactions 159 (2004) 265] • Nuclear structure • b-delayed nuclear spectroscopy • b-decay asymmetries measured in coincidence with delayed radiations • to obtain spin and parity of daughter product excited levels • [Miyatake et al, Phys. Rev. C 67 (2003) 014306]

11. High-field βNMR Nuclear structure • βNQR and low field βNMR • Fundamental symmetries Polarizer Collinear spectroscopy

12. Collinear polarization method

13. Collinear Optical Pumping at TRIUMF

14. Collinear beam line at TRIUMF

15. Li 671 nm Na 590 K 770 Rb 795 Cs 894 Fr 817 3/2 Optical pumping of alkali metals Valence electron absorbs light g tranfers polarization to nucleus via hyperfine coupling Typical polarization = 60 – 70% mF -5/2 -3/2 -1/2 1/2 3/2 5/2 F =5/2 2P1/2 3/2 + s 8Li I = 2 5/2 381 MHz 2S1/2

16. Advantages of collinear geometry • Allows long laser interaction time (microseconds) with fast beams • Doppler width of beam is small g high resolution • g low saturation powers • [Anton et al., PRL 40 (1978) 642] For beam energy spreadDE = 1 eV:

17. 500 MHz Other energy broadening mechanisms • Collisions in neutralizer cell • Li+(beam) + Na(vapour)g Li + Na+ • Changes in internal energy change the kinetic energy of the forward scattered lithium atom. This broadens the energy spread of the beam and limits the thickness of Na vapour to less than optimum for neutralization • Typically ~40% neutralization efficiency is used • Collisions have negligible effect on transverse emittance 7Li fluorescence 435 deg 80% 385 deg 30% • Zeeman splitting of hyperfine magnetic substates (unimportant for guide fields < 10 gauss) • Ion beam and laser divergences (unimportant for typical ~2 mrad)

18. Relative power MHz MHz Matching laser to absorption Key to high polarization is matching laser profile to absorptiong single-mode laser broadened by electro-optic modulators Single mode laser To collinear beam line 28 MHz EOM 19 MHz EOM ~300 mW 1 MHz bandwidth 1.0

19. Beam emittance Dashed line – Li+ beam with neutralizer switched off, deflectors off Solid line – “polarized” ion beam Shows scattering due to helium, no effect from sodium. 7Li+ @ 29 keV 60% reionization efficiency

20. Beyond the alkali metals

21. Alkaline earths • Ion has electronic structure similar • to neutral alkalis. • Typical polarization = 20% ? 2P1/2 Repump 2D3/2 2S1/2 [W. Geithner et al., ISOLDE Collaboration, Phys. Rev. Lett. 83, 3792 (1999)]

22. Positive ion metastable state Ground state Rare gas Alkali Ground state Rare gases • Closed transition uses metastable • 1s5(J=2) atomic state as lower level • [Shimizu, Phys. Rev. A 39 (1989) 2758] • Rare gas+ + alkali g rare gas + alkali+ • Typical polarization = 20% ?

23. Other examples of closed transitions

24. Development of polarized 20F

25. J=1/2 3/2 3p4D 5/2 7/2 686 nm (polarizing) 678 nm (depopulating) 1/2 3s4P 3/2 metastable t= 7 ms 5/2 12.7 eV 1/2 2p52P ground state 3/2 Development of polarized 20F beam • Required for continuation of spin • alignment G-parity studies begun • with mirror nucleus 20Na • Questions • Metastable production efficiency • and survival • Hyperfine structure of 20F (I =2) • Methods • 20F is a precious commodity not • yet available, therefore use stable • 19F (I =1/2) • Laser induced fluorescence • 4P5/2 g 4D7/2 • Metastable depopulation pumping • 4P5/2 g 4D5/2 [C.D.P. Levy et al., doi.org/10.1016/j.nima.2007.07.013]

26. Polarizing transition 4P5/2g4D7/2 Laser induced fluorescence

27. Na vapour neutralizer g Depopulating transition 4P5/2g4D5/2

28. Metastable yield Metastable-derived ion beam fraction at low He flow = (R0-1)r 0.57R0(r-1) = 0.24 +0.16/-0.03 r = ionization cross section ratio = 3.4 [O.B. Firsov, Soviet Phys. JETP 36 (1959) 1076]

29. I = 1 at entrance to He cell Polarization and transmission efficiency • PMAX assumes 100% • polarization of metastables • P2I assumes I =1 at entrance • to He cell

30. Calculated 20F hyperfine structure • Electric quadrupole shifts • < 5 MHz (extrapolating • from other halogens) • 666 MHz EOM covers • 80% of population • g predicted beam polarization up to 14 - 17%

31. 101 V 100 V 100 V W W 30 keV 11Be+ Near term plans at TRIUMF • Species such as 20F+ and 11Be+ are paramagnetic. • Electron magnetic moment precesses rapidly in the Earth’s field and destroys nuclear polarization aExtend guide magnetic field to experiments • Doppler tuning of 11Be+ requires drift • region for ion beam in optical pumping • region. Avoid EOMs by applying ~1 eV • energy spread to beam.

32. Summary • Many elements can be polarized in a collinear beam line • - alkalis • - alkaline earths • - rare gases • - O, F, Si, Ge, Sn, Pb, ….. • Suited to low emittance, low energy beams of beta-emitters, • of course applicable to stable isotopes as well • Collinear beam lines are also useful more generally in high • resolution atomic spectroscopy. • Development can proceed with stable beams

33. Acknowledgements Students Thomas Cocolios Yoshi Hirayama Richard Labbe Rick Baartman John Behr Atsushi Hatakeyama Keerthi Jayamanna Matt Pearson Larry Root Geoff Wight Dick Yuan Anatoli Zelenski Users Rob Kiefl Andrew MacFarlane Kei Minamisono Tad Minamisono Tadashi Shimoda