Recent Developments in Polarized Solid Targets H. Dutz, S. Goertz Physics Institute, University Bonn J. Heckmann, C. Hess, W. Meyer, E. Radke, G. Reicherz Institute for Experimental Physics, Ruhr-University Bochum

Contents: 1.Luminosities of experiments with polarized targets 2.The quality factor of a polarized target: The Figure of Merit 3.Polarized target Basics: Concept and components 4.The DNP process The idea of spin temperatures The role of the electron spin resonance line The problem of polarizing deuterons 5.Three examples for an optimized preparation 6.The special challange of a large solid angle experiment 7.Developments concerning internal superconducting magnets 8.Summary

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 LiH25 (?)90 (?) 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 increasing dilution factors

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

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 I: Material Developments

Example 1: Electron irradiation of 6 LiD Idea: A. Abragam 1980, Saclay 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 (large  ) impurity has considerable influence on Pmax F-Center: s-wave electron no g-anisotropy weak HF interaction + B Li D 20 MeV at T = 185 K

Example 2: Electron irradiated deuterated Butanol Trityl

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

Part II: Magnet Developments

Bonn: A 4  double polarization experiment in the frozen spin mode

Disadvantages of the frozen spin mode: 1) Polarization decays while data taking 2) P max (frozen) ~ 0.8 · P max (cont.) 3) Changing between polarization / measuring modes time consuming and dangerous ! P eff (frozen) ~ 0.7 · P max (cont.) Ways out: 1)Huge polarizing magnet enclosing the detector 2)Thin polarizing magnet as part of the refrigerator Challanges: High field (B > 2T) with only a few layers  120A current: HT superconductors ! Mechanical stability of the thin carrier structure  Stability of magnet operation Homogeneous magnetic field (  B/B < ) in a volume comparable to the field volume Already realized as internal holding magnets since middle of 1990 Mainz & Bonn, CB/ELSA) 120mm

Status of the project: Collaboration together with IKP FZ-Jülich and IAM Bonn Homogeneous volume can not be achieved just by correction coils !!! Result extremely sensitive to positioning errors of the individual wires But: Achieveable by a slightly non-cylindrical shape plus correction coils (  B/B << ?) Theoretical work successfully finished (patent application) Test coil to be manufactored in the workshops of the FZ-Jülich Internal magnet for transverse polarization: Saddle coil type with 7 layers B = A Only problem: Mechanical stability Order given to a company  Delivery forseen during 2008

Summary: Due to the limited luminosity a successfull polarization experiment demands an optimally working polarized target: 1.Choice of a suitable target material: Dilution factor Maximum polarization Long relaxation times (frozen spin) Sufficient radiation hardness (more intense beams) 2.Optimized operating conditions: Cryostat: Suitable design / high perfomance and reliability Magnet technology: Magnets enabling a continuous polarization mode Magnets for longitudinal AND transverse spin orientation