Polarized Proton Solid Target for RI beam experiments

1. Polarized Proton Solid TargetforRI beam experiments Developed at CNS, University of Tokyo Takashi Wakui CYRIC, Tohoku University M. Hatano University of Tokyo H. Sakai University of Tokyo T. Uesaka CNS, University of Tokyo S. Sakaguchi CNS, University of Tokyo T. Kawahara Toho University A. Tamii RCNP, Osaka University Experiments with radioactive 6He beam at RIKEN

2. Outline Polarizing method Optical excitation Cross polarization Polarized proton target system Laser, Microwave, NMR Target chamber Target performance during an experiment Polarization history during the experiment Polarization reversal Radiation damage

3. Introduction Nuclear physics has been established for nuclei close to the stability line RI beam technique Extend experimental nuclear physics to nuclei far from the stability line Spin polarization Structure study of unstable nuclei A key technical ingredient Production spin polarization

4. Structure study of unstable nuclei Polarize nuclei of interest Optical pumping in superfluid helium [T. Furukawa] Collinear optical pumping technique [T. Shimoda] Projectile-fragmentation reaction [H. Ueno] Tilted-foil technique [G. Goldring] Pick-up reaction [M. Mihara] Polarized target + RI beam Polarized target using thin foil [P. Hautle] Polarized target in a lower B and at a higher T (> 100 K) (< 0.3 T)

5. Target material Target material acrystal of aromatic molecules Host material naphthalene (C10H8) Guest material pentacene (C22H14) Polarizable protons 6.3% by weight Density cm-3 Concentration 0.01 mol% Target size 1 mm x 14 mmf Polarizing process • Optical excitation (Laser) • Electron alignment • Cross polarization (Microwave) • Electron alignment Proton polarization • Diffusion of polarization • p in guest p in host

6. Optical excitation Energy levels of pentacene (guest molecule) 100 ms Decay to T1 state (intersystem crossing) Electron alignment depend on the angle between H and x-axis

7. Polarization transfer Cross polarization Adiabatic Fast Passage of ESR Microwave Effective Larmor frequency in the rotating frame (wR = wI) All spin packets can contribute to polarization transfer

8. Polarizing process 1 Optical excitation electron alignment 2 Cross polarization polarization transfer 3 Decay to the ground state 4 Diffuse the polarization to protons in host molecules by dipolar interaction 100 ms ground state is diamagnetic long relaxation time Repeating 1 4 Protons are polarized

9. Polarized Proton Target

10. Polarizing System

11. Target Chamber Target Crystal Naphthalene doped with pentacene Concentration 0.01 mol% Thickness 1 mm Diameter 14 mm 100 K

12. Microwave Resonator Copper film loop-gap resonator (LGR) [B. T. Ghim et al., J. Magn. Reson A120 (1996) 72.] r=8 mm z=20 mm Resonance frequency: 3.4 GHz Thin film resonator Recoiled protons can reach to detectors

13. Analyzing power (Ay) measurement in p+6He at 71 MeV Experiments with Polarized Target Experiments with radioactive 6He beam at RIKEN (July 2003, July 2005) [S. Sakaguchi: poster session]

14. p+4He elastic scattering Polarization during Experiment [July 2005] Magnetic field : 90 mT Temperature : 100 K Relative polarization pulsed NMR Polarization calibration Polarization reversal to reduce systematic uncertainties Pmax = 19.7 (56)% Pav = 13.5 (39)% pulsed NMR Radiation damage

15. Polarization Reversal To reduce systematic uncertainties [July 2003] Waste of time : 10 hours polarization reversal by pulsed NMR method q = g tH1 July 2005 July 2003 t = 2.2 ms q=180 Experiment can go on without interruption for buildup

16. Relaxation Rate Proton Polarization during Buildup • A : Buildup rate • : Relaxation rate Pe : Average Population difference Relaxation rate during experiment GI : Intrinsic (paramagnetic impurities) GT : pentacene on photo-excited triplet state (Laser ON) GL : damage due to Laser irradiation ( power time : 0.0011(5) h-1/W・h) GB : radiation damage

17. Radiation Damage p+6He experiment (July 2003) before experiment (GI+GL) G = 0.060(1) h-1 4.1 108 /mm2 after experiment (GI+GL’+GB) G = 0.132(2) h-1 GB = 0.060 (10) h-1 p+6He experiment (July 2005) before experiment G = 0.127(6) h-1 1.1 109 /mm2 after experiment G = 0.295(4) h-1 GB = 0.132 (12) h-1

18. GB GI+GT+GL @7 days Relaxation Rate during Experiment GB=0.0130 (4) h-1/108 mm-2 contribution of each source Laser power 200 mW Beam intensity 2x105 /s Beam spot size 10 mmf

19. Annealing For a higher beam intensity GB should be reduced periodically by changing the target crystal by annealing Effect of Annealing Relaxation rate clearly decreased at 200 K Polarization decreases Crystal should be changed

20. Summary Polarized proton target for RI beam experiments developed at CNS, University of Tokyo The target was used in the experiments with radioactive 6He beam at RIKEN Analyzing power (Ay) for p+6He elastic scattering Protons were polarized in 90 mT at 100 K Average value of p was 13.5% (July 2005) Polarization reversal by pulsed NMR method Radiation damage GB=0.0130 (4) h-1/108 mm-2