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

Contents: Part I: Physics basics of the polarized solid target Luminosities of experiments with polarized targetsLuminosities of experiments with polarized targets The quality factor of a polarized target: The Figure of MeritThe quality factor of a polarized target: The Figure of Merit The polarized target: Concept and componentsThe polarized target: Concept and components The DNP processThe 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 preparationThree examples for an optimized target material preparation

Part III: HD, brute force and beyond ? Why HD ? Properties of H 2, D 2 and HD Why HD ? Properties of H 2, D 2 and HD Static polarization of HD Static polarization of HD The relaxation switch The relaxation switch Polarization values calculated & measured Polarization values calculated & measured Pro‘s and Con‘s of statically polarized HD Pro‘s and Con‘s of statically polarized HD The DNP option The DNP optionSummary

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

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

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

Measuring time for  A = const : Target Figure of Merit: H-Butanol NH LiH1/8+0.59/ (?) D-Butanol / 90 (!) / 4 14 ND – LiD Materialf A [%]P[%]  [g/cm 3 ]  (pack.f.) f A 2 ·P t 2 ·  ·  Typical FoM‘s (continuous polarization at B = 2.5 T, COMPASS like dilution fridge) increasing radiation hardness

Magnet: 2 7 T Cryogenics: 1 K 100 mK Microwaves: GHz NMR: MHz DAQ Refrigerator The basic concept of Dynamic Nuclear Polarization (DNP) B / T P p [%] P d [%] P e [%] 2.5 T / 1 K T / 10mK Doping and transfer of polarization

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

DNP in the picture of spin temperature

DNP in the Picture of Spin Temperature

Minimize  E while maintaining the thermal contact:  E ~ O( n ) Find a chemical radical with a narrow EPR line width Try radiation doping if only low  nuclei present The special problem of low  nuclei (e.g. deuterons) EE

Part II: Material developments

Example 1: Electron irradiated 6 LiD 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 1 liter for COMPASS: Synthesized from highly enriched 6 LiD (2000 Bochum)  P max = 55 % at 2.5 T 7 Li (larger  ) impurity broadenes the EPR line  lowers P max F-Center: s-wave electron no g-anisotropy weak HF interaction + B Li D 20 MeV at T = 185 K EE

Example 2: Electron irradiated deuterated Butanol Trityl

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

Part III: HD, brute force and beyond ?

Purest of all target materials: H 2 and its isotope variations 2 Fermions: Mol. wave function antisym. I=1:  (I) sym  J odd (=1) Ortho-H 2 I=0:  (I) antisym  J even (=0) Para-H 2 I=1:  (I) antisym  J odd (=1) 2 Bosons: Mol. wave function sym. I=0, 2:  (I) sym  J even (=0) Ortho-D 2 Para-D 2 Well distinguishable particels, no symmetries  p & d pol. independend I = 1/2, 3/2; J = 1 I = 1/2, 3/2; J = 0 H2H2 HD D2D2 172 k B K 128 k B K + 68 k B K Energy

2 Fermions: Mol. wave function antisym. I=1:  (I) sym  J odd (=1) Ortho-H 2 I=0:  (I) antisym  J even (=0) Para-H 2 H2H2 172 k B K Ground state not magnetic ! Purest of all target materials: H 2 and its isotope variations Converts from 75% initial abundance

1 : 5 Purest of all target materials: H 2 and its isotope variations I=1:  (I) antisym  J odd (=1) 2 Bosons: Mol. wave function sym. Ortho-D 2 Para-D 2 D2D k B K Converts from 33% initial abundance Problems with relaxation times due to residual para-D 2 (?) ? I=0, 2:  (I) sym  J even (=0)

J = 1 Ortho-H 2 J = 0 Para-H 2 J = 1 J = 0 Ortho-D 2 Para-D 2 I = 1/2, 3/2; J = 1 I = 1/2, 3/2; J = 0 H2H2 HD D2D2 172 k B K 128 k B K + 68 k B K fast decay

J = 1 Ortho-H 2 J = 0 Para-H 2 J = 1 J = 0 Ortho-D 2 Para-D 2 I = 1/2, 3/2; J = H 2 HD D k B K + 68 k B K spin spin interaction spin diffusion spin spin interaction spin diffusion I to J  J to lattice 6.3 d18.6 d T ~ 1 0.3K/1T ! ‚Relaxation Switch‘ via months of aging (W. N. Hardy (1966) A. Honig (1967) ) B ~ 15T T ~ 10mK I to J  J to lattice

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

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

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.

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 ! 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

Drawbacks of the static method  Very long production process, at least 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 ?

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 T 1H decreased from 30000s to 930 s after 8.5 · e/cm 2 = 280 s at 5 nA Radtke et al. (2005, Bochum): 2.3 MeV(max)  -radiation from 90 Sr source at T = 1K / B = 2.5 T: No degration of T 1H until several e/cm 2 = sev. hours at 5 nA  Dedicated measurements needed !

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 · O 2 to lower T 1e - Best result: P H = 3.75 % at 1.2 K / 1.24 T In situ EPR at 1.2 K 50.4 mT O2DO2D ~ 105 MHz Pol. enhancement at 1.24 T / 1.3 K „Pure Solid Effect“

These results are very promising having in mind the relatively high temperature of 1.2 K the relatively high temperature of 1.2 K the low magnetic field of 1.24 T 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: 1)‚In situ irradiation‘ must be possible at the experimental site or cold tranfer from the irradiation facility to the experiment ! 2)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 !

Possible solutions to 3): 1)Further development of internal (thin) polarization coils (!) Under investigation in Bonn since years (challenging !) 2)Detector ‚inside‘ the polarizing magnet (!) Planned for the Crystal Barrel detector at Bonn (1) or (2) allows continuous mode while maintaining a 4  geometry 3)Paramagnetic impurities of a different nature (?)  Polarization via spin temperature equilibrium 4)Combined 4 He evaporation + 3 He/ 4 He dilution refrigerator (?) Probably impossible in the usual horizontal configuration 3) 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 4  -detectors !

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: 1.Not only the H, also the D can be polarized nearly completely ! 2.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: 1.a high dynamic polarization of H and D in HD is technically possible 2.such a target really competes with / wins over conventional designs

But it may be the last big adventure in polarized target material research ! 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