An attempt toward dynamic nuclear polarization for liquid 3 He

1. 17/Nov./2005 @ PST2005 An attempt toward dynamic nuclear polarization for liquid 3He Takahiro Iwata Yamagata University 1. Motivation of the study 2．Polarizing 3He in dense form 3. DNP for liquid He3 3．Doping with free radical 4. ESR signals 5. Summary

2. Motivation of the study • Polarized 3He targets have been employed in various scattering experiments • Only neuron is polarized in 3He • Good target for the study of neutron characteristics • Realized by the optical pumping technique • applied only for gas • Due to its gaseous form, the density is limited. • Its application is also limited • Polarized 3He in dense form will open a door to extended applications. • not only in particle physics, but also in other fields ( e.g. medical applications, condensed matter physics, chemistry, …)

3. Possible ways for polarizing3He in dense form • Brute force method • 55% polarization obtained in solid at 6.6T, 6mK and 30 bar, G.Bonfait et al., Phys.Rev.Lett. 53 (1984) 1092 • Polarized liquid is also obtained by quickly melting the polarized solid. • However, its application is limited due to the extreme condition. • Dynamic Nuclear Polarization (DNP) • Direct coupling from electron system to 3He, Delheiji et al. in 1990 • diluted paramagnetic centers in liquid He3 • no polarization enhancement obtained • Coupling between 3He and polarized material with large surface area

4. Coupling between He3 and polarized material • Nuclei polarized by DNP in material with large surface area • Polarization transfer to 3He in solid or liquid on the surface • A.Shuhl et al.,Phys.Rev.Lett. 54 (1985) 1952, Coupling to19F in Teflon beads(d=2000A) polarized by DNP, • originally existing paramagnetic centerｓ in Teflon • enhancement factor 2.0 for 3He • L.W.Engel and K.Deconde Phys.Rev. 33 (1986) 2035, DNP of Liquid 3He in powdered charcoal • originally existing paramagnetic centers in charcoal • enhancement factor 1.18 for 3He • B.van den Brandt et al. (PSI-group), NIM A 356 (1995) 138-141 • beads of Polyethylene, Teflon,Zeolite • Doping of free radical (TEMPO) • small polarization (P=2.5%) obtained with Teflon • 3He NMR signal changed (PE case) • Doping was not successful for Teflon and Zeolite

5. Development of DNP for polarizing He3 in liquid • Our idea: • Direct coupling between a free radical and 3He • The free radical is embedded into porous material. • The porous material is filled with Liquid 3He • Coupling between the free radical and the 3He is induced by microwave. • Diffusion of 3He in the material would help the spin diffusion of 3He.

6. The key issue • One of the key issues: • Embedding a free radical in porous materials • The free radical molecules • should be firmly trapped • should be well dispersed • Matching the cavity size of the porous material to the free radical molecule. • NaY type zeolite with a combination of TEMPO free radical.

7. H H H H H H H3C CH3 H3C CH3 N O Zeolite and TEMPO • NaY type zeoltie • Cavity(supercage): • 13A(max. dia.) • 7.4A(window dia.) • 4.7x1019 cavities/g • porosity: ~6% • TEMPO (2,2,6,6-tetramethyl-piperidinyl-1-oxyle) • Melting point: 36 oC. • Boiling point: 67 oC • Molecule size: 6~8A • 3He • atomic radius : 1.5A NaY zeolite (Ｎａ５６) ＡＬ５６Ｓｉ１３９Ｏ３８４ supercage Si or Al double T6-ring sodalite cage TEMPO

8. Doping process The amounts to give 1.6 x 1019 spins/cc, ¼ of super cages occupied with TEMPO zeolite(7510mg) activated at 500 oC for 8 hours TEMPO(24.8mg) n-pentane(300ml) boiling point: 36 oC evaporating n-pentane in draft chamber stirred for 8 hours stirrer This method is used in studies of unstable radicals

9. ESR signals TEMPO inzeolite • ESR signal of TEMPO in zeolite • a little broader than that in ethanol • peaks still separated • TEMPO molecules are dispersed at some level TEMPO in ethanol

10. Stability of TEMPO in zeolite Measurements of intensity variation of the ESR signal Intensity of the ESR signal in the air at room temperature Intensity of the ESR signal in vacuum at room temperature in the air in vacuum TEMPO is firmly trapped in zeolite

11. Stability of TEMPO in PE • PE foil(0.1 mm thick) doped with TEMPO by diffusion • The ESR intensity decreases with a time constant of ~5 hours • Reasons • TEMPO trapped in the amorphous part • PE molecule movable at room temp. (Tg=205K) •  enhance diffusion • evaporation from surface • Zeolite case • TEMPO trapped in the supercage which is a part of firm structure. Intensity of TEMPO in PE in the air at room temperature

12. Next step • Filling the zeolite with liuid 3He and trying to do DNP • @2.5T, 0.6K • jobs • installing the sample cell into the cryostat (final assembly needed) • setting the 3He gas handling system (almost ready) • NMR system for 3He (tuning required) • system for DNP (ready)

13. Summary • DNP for 3He in dense form will open the door to various applications. • DNP for liquid 3He is pursued through the direct interaction between 3He and a free radical molecule embedded in cavities of zeolite. • We have prepared the zeolite doped with TEMPO free radical. • Being well dispersed, the TEMPO molecules are firmly trapped in zeolite. • We are ready to make DNP for liquid 3He in the zeolite.

14. Backup Slides

15. pump system detector system holdign magnet beam pol. magnet target cell microwave zeolite doped with free radical polarized 3He circulation scheme

16. Characteristics of the zeolite for the test • 320NAA • Cation type: Na • SiO2/ALO3(mol/mol):5.5 • Na2O(wt%):12.5 • U.C.C. by ASTM: 24.63 • NH3-TPD(mmol/g): - • Surface Area (BET,m2/g) 700 • Crystal Size: 0.3 micro meter • Mean Particle Size: 6 micro meter • density: 1.38g/cc • lattice constant: 24.2-25.1 A • supercage density: 6.6x1019 cages/cc