1. Polarized Proton Solid Target for RI beam experiments 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 Developed at CNS, University of Tokyo Takashi Wakui CYRIC, Tohoku University Experiments with radioactive 6 He 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 Polarized target using thin foil Polarized target in a lower B and at a higher T [P. Hautle] (< 0.3 T) (> 100 K) Polarize nuclei of interest Optical pumping in superfluid helium Collinear optical pumping technique Projectile-fragmentation reaction Tilted-foil technique Pick-up reaction Polarized target + RI beam [T. Furukawa] [T. Shimoda] [H. Ueno] [G. Goldring] [M. Mihara]

5. Target material Polarizable protons 6.3% by weight Density cm -3 Concentration 0.01 mol% Target size 1 mm x 14 mm  Target material a crystal of aromatic molecules pentacene (C 22 H 14 ) Host material Guest material naphthalene (C 10 H 8 ) Polarizing process 1.Optical excitation (Laser) Electron alignment 2.Cross polarization (Microwave) Electron alignment Proton polarization 3.Diffusion of polarization p in guest p in host

6. Optical excitation Energy levels of pentacene (guest molecule) Decay to T 1 state (intersystem crossing) Electron alignment depend on the angle between H and x-axis 100  s

7. Polarization transfer Cross polarization Adiabatic Fast Passage of ESR Effective Larmor frequency in the rotating frame All spin packets can contribute to polarization transfer  R =  I ) Microwave

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 ground state is diamagnetic long relaxation time Repeating 1 4Protons are polarized 100  s

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

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

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

15. Polarization Reversal To reduce systematic uncertainties Waste of time : 10 hours polarization reversal by pulsed NMR method  =  t  H 1 t = 2.2  s  =180 [July 2003] July 2003 July 2005 Experiment can go on without interruption for buildup

16. Relaxation Rate Proton Polarization during Buildup A : Buildup rate  : Relaxation rate P e : Average Population difference  I : Intrinsic (paramagnetic impurities)  T : pentacene on photo-excited triplet state (Laser ON)  L : damage due to Laser irradiation ( power time : 0.0011(5) h -1 /W ・ h)  B : radiation damage Relaxation rate during experiment

17. Radiation Damage before experiment (  I +  L )  = 0.060(1) h -1  = 0.132(2) h -1 after experiment (  I +  L’ +  B ) 4.1 10 8 /mm 2 p+ 6 He experiment (July 2003) before experiment  = 0.127(6) h -1  = 0.295(4) h -1 after experiment 1.1 10 9 /mm 2   = 0.060 (10) h -1   = 0.132 (12) h -1 p+ 6 He experiment (July 2005)

18. Relaxation Rate during Experiment  B =0.0130 (4) h -1 /10 8 mm -2 Laser power 200 mW Beam intensity 2 x 10 5 /s Beam spot size 10 mm   I +  T +  L @7 days BB contribution of each source

19. Annealing For a higher beam intensity  B should be reduced periodically by changing the target crystal by annealing Relaxation rate clearly decreased at 200 K Effect of Annealing 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 6 He beam at RIKEN Radiation damage Protons were polarized in 90 mT at 100 K Analyzing power (A y ) for p+ 6 He elastic scattering Polarization reversal by pulsed NMR method Average value of p was 13.5% (July 2005)  B =0.0130 (4) h -1 /10 8 mm -2