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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: Luminosities of experiments with polarized targets

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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:

  • Luminosities of experiments with polarized targets

  • The quality factor of a polarized target: The Figure of Merit

  • Polarized target Basics: Concept and components

  • The DNP process

    • The idea of spin temperatures

    • The role of the electron spin resonance line

    • The problem of polarizing deuterons

  • Three examples for an optimized preparation

  • The special challange of a large solid angle experiment

  • Developments concerning internal superconducting magnets

  • Summary


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


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


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

increasing dilution factors


The Basic Concept of

Dynamic Nuclear Polarization

Doping and transfer

of polarization

Cryogenics: 1 K 100 mK

NMR: 10 200 MHz

Refrigerator

Microwaves: 50 200 GHz

Magnet: 2 7 T

DAQ




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



D

  • F-Center:

  • s-wave electron

  • no g-anisotropy

  • weak HF interaction

+

Li

20 MeV at

T = 185 K

B

7Li (large m) impurity has considerable influence on Pmax

1 liter for COMPASS: Synthesized from highly enriched 6LiD

(2000 Bochum)  Pmax = 55 % at 2.5 T





CB/ELSA @ Bonn: A 4p double polarization experiment in the frozen spin mode


  • Disadvantages of the frozen spin mode:

  • Polarization decays while data taking

  • Pmax (frozen) ~ 0.8 · Pmax (cont.)

  • 3) Changing between polarization / measuring modes time consuming and dangerous !

Peff (frozen) ~ 0.7 · Pmax (cont.)

  • Ways out:

  • Huge polarizing magnet enclosing the detector

  • Thin polarizing magnet as part of the refrigerator

Already realized as internal holding magnets since

middle of 1990 (GDH @ Mainz & Bonn, CB/ELSA)

120mm

  • 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 (DB/B < 10-4) in a volume comparable to the field volume


  • 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 (DB/B << 10-4 ?)

  • 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 = 0.5 T @ 30 A

  • Only problem: Mechanical stability

  • Order given to a company

  •  Delivery forseen during 2008


  • Summary: FZ-Jülich and IAM Bonn

  • Due to the limited luminosity a successfull polarization experiment

  • demands an optimally working polarized target:

  • Choice of a suitable target material:

    • Dilution factor

    • Maximum polarization

    • Long relaxation times (frozen spin)

    • Sufficient radiation hardness (more intense beams)

  • 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


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