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Polarized 11 Li beam at TRIUMF and its application

Polarized 11 Li beam at TRIUMF and its application for spectroscopic study of the daughter nucleus 11 Be. T. Shimoda 1 , Y. Hirayama 1,2 , H. Izumi 1 , H. Hatakeyama 3 , K.P. Jackson 4 , C.D.P. Levy 4 , H. Miyatake 2 , M. Yagi 1 , H. Yano 1.

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Polarized 11 Li beam at TRIUMF and its application

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  1. Polarized 11Li beam at TRIUMF and its application for spectroscopic study of the daughter nucleus 11Be T. Shimoda1, Y. Hirayama1,2, H. Izumi1, H. Hatakeyama3, K.P. Jackson4, C.D.P. Levy4, H. Miyatake2, M. Yagi1, H. Yano1 1: Osaka Univ., 2: KEK. 3: Univ. of Tokyo, 4:TRIUMF CONTENTS 1. Physics motivation newb-delayed decay spectroscopy using polarized nucleus 2. Polarizer for RNB at TRIUMF ISAC polarization by collinear optical pumping special cares to achieve high polarization 8Li: 80%, 9Li: 56%, 11LI: 55%, 20Na: 57%, 21Na: 56%, 26Na: 55%, 27Na: 51%, 28Na: 45% 3. Experimental results with polarized 11Li beam spin-parity assignments of levels in 11Be 4. Summary

  2. β-decay from a spin polarized nucleus b-decay angular distribution allowed transition A: asymmetry parameter of allowed b-decay P: polarization of the parent nucleus assignment polarized ～0 A takesvery different values depending on the final state spin. 11Li → 11Be a new method of β-delayed decay spectroscopy β-delayed decay spectroscopy

  3. Measurement of β-decay asymmetry R-detector L-detector : β-ray counts : β-ray counts polarization when spin orientation is reversed free from instrumental asymmetry Higher polarization is needed.

  4. Excited States of 11Be neutron halo T1/2 = 8.5 ms n 10Be 11Be S1n=504 keV ■ only a few spin-parity assignments ■ low level density at high energy region prevents comparison between experiment and theory level by level Most of the states decay by neutron emission. F. Ajzenberg-Selove, Nucl. Phys. A506 (1990) 1.

  5. TRIUMF ISAC ISOL-based RNB facility Isotope Separator / ACcelerator radioactive nuclear beams produced in target fragmentation induced by a 500 MeV 100 mA proton beam commissioned in Aug. 2001

  6. Alkali RI beam from ISOL A+1 beam at 10 – 60 keV neutralizer charge exchange in a Na vapor jet A+1 + Na → A0 + Na+1 : 90% efficiency optical pumping for fast neutral beam in collinear geometry two laser beams to pump the two ground-state hyperfine levels longitudinal polarization re-ionizer collision with a cold He gas target (12K) A0 → A+1 : 66% efficiency transversely nuclear-polarized ion beam bend

  7. Minamisono ANa moments, b-decay symmetry TRIUMF ISAC Polarized Beam Line Shimoda 11Li decay spectroscopy neutralizer re-ionizer Kiefl 8Li b-NMR condensed matter physics polarized 11Li+1 1.9 m unpolarized 11Li+1 B→ 10Gauss 30.48 keV pumping within 2.6ms beam velocity tuning C.D.P. Levy et al. Nucl. Instr. and Meth. B204 (2003) 689

  8. D1 673 nm 905 MHz n with hyperfine int. laser freq. without hyperfine int. pumping the two ground-state hyperfine levels in order to achieve high polarization

  9. Electro-Optic Modulator (EOM) driven at the hyperfine splitting frequency Only 1/3 laser power is used for each optical pumping.

  10. energy (Doppler) broadening of the neutralized beam >> laser line width ～ 1 MHz 6.3 eV multiple collisions with Na atoms in the neutralizer

  11. broadening the laser line width two EOMs in series laser beam EOM-2 19 MHz EOM-3 28 MHz 8Li P～20% P～70%

  12. simulation by rate equation = 0.80 ( q0 : opening angle of the b-detector) = 0.98 (t = 12.3 ms, T1 = 570 ms in Pt) 8Li -1.0 (3/2- → 1/2-) Polarization measurement known A = -1 AP (coin. with 0.32 MeV g) = －0.43 ± 0.005 11Li: P = 0.55 ± 0.007 We don't understand why the observed polarization is less than the expected one.

  13. 11Be*+b 11Ligs 10Be* + n b- n, b- g, b- n-g coincidence 10Begs + g b-ray telescope: ΔWb= 14.7% x 2, eb = 90% Ge detector: HPGe, 50 and 60 %, ΔWg eg = 3.2x10 @3 MeV -3 plastic scintillator: ΔWn= 1.8% x 6, en = 19%@2 MeV, En ≧500 keV Flight Length: 1.5 m Li-glass scintillator: ΔWn= 0.92% x 2, en =2.1%@15 keV, en = 1.3%@80 keV En ≧1 keV Flight Length:130 mm Detector Setup En = 1 keV – 9 MeV E903 at TRIUMF 30.48 keV Polarization was inverted in every 30 sec by changing the laser helicity.

  14. 11Be*+b 11Ligs 10Be* + n neutron TOF spectrum and coincident b-decay asymmetry high energy neutrons Asymmetry parameter is helpful to decompose the overlapping peaks. Y. Hirayama et al., Phys. Lett. B611 (2005) 239

  15. determined Ex, Ip ■ log ft ■ decay path ■ 11Li → 11Be spectroscopic factor ■ 11Be → 10Be + n New Level and Decay Schemes of 11Be New

  16. Y. Hirayama et al., Phys. Lett. B611 (2005) 239 F. Ajzenberg-Selove, Nucl. Phys. A506 (1990) 1.

  17. Summary ●Highly polarized (50 – 80%) radioactive nuclear beams of alkali ions (Li, Na) have been successfully produced at the collinear optical pumping system of TRIUMF ISAC. The success was due to (i) pumping of the two ground-state hyperfine levels and (ii) matching of the laser line width to the Doppler broadened absorption line of the beam. ●The highly polarized beam is a very powerful tool to explore the excited states of unstable nuclei by applying the new method of b-delayed decay spectroscopy. The b-decay asymmetry parameter is useful (i) to assign the peaks of the decaying particles and (ii) to assign the spin-parity of the daughter states.

  18. Laser system f8mm f12mm Ring dye laser Coherent 899-21 Dye: DCM SPECIAL/LC 6501 9W 673 nm cw circular polarized for 11Li frequency reference to actively stabilize the ring dye laser

  19. Achieved polarization 8Li: 80%, 9Li: 56%, 11LI: 55%, 20Na: 57%, 21Na: 56%, 26Na: 55%, 27Na: 51%, 28Na: 45% Pumping for 11Be+ beam is in progress.

  20. absorption line scanning velocity n beta-decay asymmetry Doppler-shift tuning 11Li deceleration bias (Na vapor cell) tuning to adjust ion beam velocity so as to meet the Doppler shift fixed n

  21. D1 transition frequency/wave number for Li atoms at rest 14904.41 cm-1 14903.30 cm-1 33.6 GHz Doppler shift 30.48 keV 11Li 1486.17 cm-1 Na vapor cell bias tuning 1 eV → 17.8 MHz

  22. Low energy neutrons thermal neutron R.E. Azuma et al., Phys. Rev. Lett. 43(1979)1652 3He ionization chamber 6Li-doped scintillators high energy neutrons spurious due to resonance in detector 72 keV 16 keV

  23. En = 72±5 keV • = 25±15 keV • = 12± 2.6 % A = -0.53±0.26 Ip= 3/2- • En = 16±1 keV • = 9±3 keV • = 5.1±1.5 % A = +0.72±0.10 Ip= 5/2- b-decay asymmetry laser helicity b-detector = L+, L-, R+, R-

  24. Low energy neutrons 12±2.6 % 5.1±1.5 %

  25. AP (peak B) = －0.377 ± 0.009 e(solid angle) =0.80 e(spin relaxation) =0.98 P = +0.48 ± 0.017 γ-rays b-g coincidence β-decay Doppler broadening due to neutron recoil 100±50 fs n-decay γ-decay

  26. spin-parity assignment b-n coincidence High Energy Neutrons mixed but one is dominant hidden peaks !

  27. β-n-γ-coincidence existence previously claimed by Aoi et al.

  28. β-n-γcoincidence 5 Γ=216±55 keV check the calculated neutron peak profiles 7 Γ=243±55 keV reproduction of spectra based on observed decays and assigned spins-parities neutron energy ✓ level width ✓ neutron detection efficiency curve ✓ detector time resolution ✓ → neutron peak profiles spin-parity ✓ → asymmetry spectrum

  29. from known levels from assumed levels reproduction of spectra by including unknown neutron decays from known levels and assumed levels further included neutron decays

  30. from known levels from assumed levels determined Ex, Ip ■ log ft ■ 11Li → 11Be spectroscopic factor ■ 11Be → 10Be + n New Level and Decay Schemes of 11Be previously unknown transitions

  31. neutron penetrability neutron spectroscopic factor overlapping between 11Be and 10Be 11Be → 10Be + n : partial decay width channel radius l–th order Hankel function of the first kind

  32. Y. Kanada-En’yo, H. Horiuchi, A. Dote, Nucl. Phys. A687 (2001) 146c Anti-symmetrized Molecular Dynamics Y. Kanada-En’yo and H. Horiuchi, Phys. ReV. C66 (2002) 024305 ab-initio fully microscopic theory single cluster state without any model assumptions such as mean field, clustering,・・・ ■ 2α-cluster states (α+α+3n) rotational bands ■ single-cluster state (α+5n+2p) Kp = 1/2－ 3n: p-shell (0hω) Kp = 3/2－ Kp = 3/2－ 2n: sd-shell (2hω) log-ft Kp =1/2+ Kp = 1/2－ 1n: sd-shell (1hω) Kp =1/2+

  33. new cluster states? ? ? Spectroscopic Factor Transitions with the largest spectroscopic factor are shown. assumed lowest possible L

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