Nuclear Polarization, Recent Results and New Applications

1. Nuclear Polarization, Recent Results and Yet Another Discussion of HD Nuclear Polarization, Recent Results and New Applications S. Goertz Physics Institute, University Bonn

2. Contents: • Part I: Physics basics of the polarized solid target • Luminosities of experiments with polarized targets • The quality factor of a polarized target: The Figure of Merit • The polarized target: Concept and components • The DNP process • - The Solid State Effect (SSE) • - The idea of Equal Spin Temperatures(EST) • - The role of the electron spin resonance line • - The problem of polarizing deuterons • Part II: Material developments • Three examples for an optimized target material preparation

3. Part III: HD, brute force and beyond ? • Why HD ? Properties of H2, D2 and HD • Static polarization of HD • The relaxation switch • Polarization values calculated & measured • Pro‘s and Con‘s of statically polarized HD • The DNP option • Summary

4. Part I: Physics basics of the polarized solid target

5. Polarized Luminosities in Different Beams Lunpol = 1036 – 1037 cm-2s-1 Polarized Solid Targets: Frozen Spin Mode in dilution fridges: up to 107 1/s Continuous Mode in dilution fridges: up to 1 nA Continuous Mode in 4He- evaporators: up to 100 nA Gas Targets: Compressed 3He for external experiments: up to 30 mA H, D storage cells for internal experiments: up to 50 mA COMPASS E155 CB-ELSA < 100nA E154,3He L = 1036 cm-2s-1 < 30mA 1034 1032 1030 1028 HERMES 3He HERMES H,D < 50mA

6. The Figure of Merit in Asymmetry Experiments - transverse target asymmetry in the case of spin-1/2 - H-Butanol: H H H H     H - C – C – C – C –OH     H H H H f=10/74~13.5% Measured counting rate asymmetry: Physics asymmetry for a pure target: Dilution factor: = fraction of polarizable nucleons Physics asymmetry for a dilute target: Absolute error of A: small

7. Measuring time for DA = const : Target Figure of Merit: Typical FoM‘s (continuous polarization at B = 2.5 T, COMPASS like dilution fridge) increasing radiation hardness

8. The basic concept of Dynamic Nuclear Polarization (DNP) Doping and transfer of polarization Cryogenics: 1 K 100 mK NMR: 10 200 MHz Refrigerator Microwaves: 50 200 GHz Magnet: 2 7 T DAQ

9. The DNP process via the Solid State Effect (SSE) The DNP process uses the high TE-polarization of electrons by transferring it to the nucleons via off-resonance microwave transitions fast electronic relaxation

10. DNP in the picture of spin temperature

11. DNP in the Picture of Spin Temperature

12. The special problem of low m nuclei (e.g. deuterons) DE • Minimize DE while maintaining the thermal contact: DE ~ O(nn) • Find a chemical radical with a narrow EPR line width • Try radiation doping if only low m nuclei present

13. Part II: Material developments

14. Example 1: Electron irradiated 6LiD • Idea: A. Abragam 1980, Pioneer work at Saclay during 80‘s - 90‘s • Refinement of preparation: • Since 1991 in Bonn,from 1995 in Bochum  COMPASS D • F-Center: • s-wave electron • no g-anisotropy • weak HF interaction + DE Li 20 MeV at T = 185 K B 7Li (larger m) impurity broadenes the EPR line  lowers Pmax 1 liter for COMPASS: Synthesized from highly enriched 6LiD (2000 Bochum)  Pmax = 55 % at 2.5 T

15. Example 2: Electron irradiated deuterated Butanol Trityl

16. Example 3: Trityl doped deuterated alcohols and diols @ B = 2.5 T

17. Part III: HD, brute force and beyond ?

18. Purest of all target materials: H2 and its isotope variations Well distinguishable particels, no symmetries  p & d pol. independend 2 Bosons: Mol. wave function sym. 2 Fermions: Mol. wave function antisym. I=1: c(I) sym  J odd (=1) Ortho-H2 172 kBK I=1: c(I) antisym  J odd (=1) I = 1/2, 3/2; J = 1 Energy 128 kBK Para-D2 68 kBK I=0, 2: c(I) sym  J even (=0) I=0: c(I) antisym  J even (=0) + I = 1/2, 3/2; J = 0 Ortho-D2 Para-H2 D2 H2 HD

19. Purest of all target materials: H2 and its isotope variations 2 Fermions: Mol. wave function antisym. I=1: c(I) sym  J odd (=1) Ortho-H2 172 kBK Converts from 75% initial abundance I=0: c(I) antisym  J even (=0) Ground state not magnetic ! Para-H2 H2

20. Purest of all target materials: H2 and its isotope variations 2 Bosons: Mol. wave function sym. I=1: c(I) antisym  J odd (=1) Para-D2 Converts from 33% initial abundance 68 kBK ? I=0, 2: c(I) sym  J even (=0) Problems with relaxation times due to residual para-D2 (?) + 1 : 5 Ortho-D2 D2

21. J = 1 Ortho-H2 J = 1 172 kBK I = 1/2, 3/2; J = 1 128 kBK Para-D2 fast decay 68 kBK J = 0 J = 0 + I = 1/2, 3/2; J = 0 Ortho-D2 Para-H2 D2 H2 HD

22. ‚Relaxation Switch‘ via months of aging (W. N. Hardy (1966) A. Honig (1967) ) T ~ 10mK J = 1 B ~ 15T I to J  J to lattice I to J  J to lattice Ortho-H2 J = 1 172 kBK spin spin interaction spin diffusion T ~ 1 a @ 0.3K/1T ! Para-D2 6.3 d 18.6 d spin spin interaction spin diffusion 68 kBK J = 0 J = 0 + I = 1/2, 3/2; J = 0 Ortho-D2 Para-H2 10-4 D2 10-3 H2 HD

23. Evolution of proton relaxation times with aging time (J.-P. Didelez, PST04 Bad Honnef)

24. Static polarization results Measured after month of polarizing / aging: PH ~ 60 – 70 % PD ~ 15 – 20 % (static) Goal: Equilibrium polarization predicted by the Brillouin function Polarization transfer by forbidden AFP (FAFP), max 2/3 of PH

25. LEGS results presented at PST05 (T. Kageya) In Beam Cryostat (IBC) : 250 mK / 1T Production Dewar (PD) : 2-4 K / O(T) 20 mass-% pure aluminum wire to remove heat during ortho-para-conv.

26. Advantages of HD as target material: Very low density (~ 0.13 g/cm3):  Much less radiative background compared to other mat.  Reduces luminosity and thus the FoM • Target nuclei are only H and D: •  H and D may be polarized independently •  Almost no dilution (20 % aluminum + target cont.) •  High value of the FoM: • FoM(HD, 70%/20%) ~ 2 · FoM(H-but, 90%) • FoM is highly dependent on the degree of polarization !

27. Drawbacks of the static method • Very long production process, at least 3 - 4 months • Very sophisticated handling: At least 4 cryostats needed: PD, TC, DR, IBC Production and NMR calibration in the PD Transfer to the dilution refrigerator: T(sample) = ? Transfer back to the PD for NMR measurement Transfer to the IBC for physics measurement Transfer back to the PD for final NMR meas. • Accidents during the processes: Cryostat blockages, Power cuts, … • Reliability of the NMR measurements: Long term stability ?

28. Radiation Resistance:  Ok for real photons, shown in experiments in the past  Unclear for charged particles of nA currents Mano & Honig (1975): 10 GeV e-beam at T = 4.2K / B = 0.28 T T1H decreased from 30000s to 930 s after 8.5 · 1012e/cm2 = 280 s at 5 nA Radtke et al. (2005, Bochum): 2.3 MeV(max) b-radiation from 90Sr source at T = 1K / B = 2.5 T: No degration of T1H until several 1014 e/cm2 = sev. hours at 5 nA  Dedicated measurements needed !

29. Dynamic Polarization: Situation unclear either • First attempts bei J.C. Solem (1973) • T = 4.2 K and 1.2 K, B = 0.83 T and 1.24 T • Creation of paramagnetic impurities (atomic H) • by irradiation (60 MeV Bremsstrahlung) • Admixture of 3 · 10-4 O2 to lower T1e • Best result: PH = 3.75 % at 1.2 K / 1.24 T In situ EPR at 1.2 K ~ 105 MHz O2D Pol. enhancement at 1.24 T / 1.3 K „Pure Solid Effect“ 50.4 mT

30. These results are very promising having in mind • the relatively high temperature of 1.2 K • the low magnetic field of 1.24 T • One should be able to do a better job nowadays ! • But: Unclear weather a real DNP-HD target is feasible at all ! • Problems: • ‚In situ irradiation‘ must be possible at the experimental site • or cold tranfer from the irradiation facility to the experiment ! • What happens with the D-polarization during / after DNP • Spin temperature equilibrium between H and D ? • If no: How to polarize 2/3 of the material ? (SSE ?) • If yes: Same relation as in the static case • Cross relaxation when paramagnetic impurities present ? • If yes:Independent polarization of H and D lost !

31. DNP via SSE requires high microwave power levels ! • These are acceptable only by evaporation cryostats ! • on the other hand: • Paramagnetic centers require conventional frozen spin techniques • to maintain the polarization in combination with 4p-detectors ! • Possible solutions to 3): • Further development of internal (thin) polarization coils (!) • Under investigation in Bonn since years (challenging !) • Detector ‚inside‘ the polarizing magnet (!) • Planned for the Crystal Barrel detector at Bonn • or (2) allows continuous mode while maintaining a 4p geometry • Paramagnetic impurities of a different nature (?) •  Polarization via spin temperature equilibrium • Combined 4He evaporation + 3He/4He dilution refrigerator (?) • Probably impossible in the usual horizontal configuration

32. Summary: • In 1957 Abragam & Jeffries proposed the ‚Solid Effect‘ as a possible • mechanism to produce enhanced nuclear spin polarization in a solid. • After half a century of continuous research: • Not only the H, also the D can be polarized nearly completely ! • A polarized solid target can be adapted to nearly every • experimental set-up ! • Left: The dream of a pure AND highly polarized solid target • This dream may be dreamed in form of dynamically polarized HD • But it‘s still unclear whether: • a high dynamic polarization of H and D in HD is technically possible • such a target really competes with / wins over conventional designs

33. The bare necesseties: • New and strong efforts needed to clarify these questions ! • Not possible for one ‚curious professor‘ and his PhD student • Experienced working group needed dedicated only to this • research field But it may be the last big adventure in polarized target material research !