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

beam projectiles [1/s]

106 107 108 109 1010 1011 1012 1013 1014 1015 1016 1017 1018

targ

et n

ucl

ei [1

/cm

2]

1010

1012

1014

1016

1018

1020

1022

1024

1026

1028

1030

COMPASS

CB-ELSA E155

E154,3He

HERMES 3He

HERMES H,D

1030

1032

1034

1028

L = 1036

cm-2s-1

< 100nA

< 30A

< 50mA

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 indilution fridges: up to 1 nA

Continuous Mode in4He- evaporators: up to 100 nA

Gas Targets:

Compressed 3He for

external experiments: up to 30 A

H, D storage cells forinternal 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: tot

N N

N

Physics asymmetry for a pure target:1

t

AP

H-Butanol:

H H H H

H - C – C – C – C –OH

H H H H

f=10/74~13.5%Dilution factor:

0(1 )A

AA A

ff f

f f

= fraction of polarizable nucleons

Physics asymmetry for a dilute target:1 1

t

Af P

Absolute error of A:

2 2 21 1 1 1 1 1 1t

t t t t

P fA A A

f P P f f P f P T L

small

Measuring time for A = const :

2 22 2

1 1:

t t

Tf P L A FoM I A

Target Figure of Merit:

22 2target thickness [1 / ]tt t t cmnFoM f P n

H-Butanol 13.5 90 0.985 0.62 1

14NH317.6 90 0.853 0.58 1.4

7LiH 25 (?) 90 (?) 0.82 0.55 2.5

D-Butanol 23.8 45 / 90 (!) 1.12 0.62 1 / 4

14ND330 30 - 40 1.00 0.58 0.6 – 1.05

6LiD 50 55 0.82 0.55 4.3

Material fA[%] P[%] [g/cm3] (pack.f.)

fA2·Pt

2··

Typical FoM‘s (continuous polarization at B = 2.5 T, COMPASS like dilution fridge)

incr

easi

ng r

adia

tion h

ard

ness

incr

easi

ng d

iluti

on f

act

ors

Magnet: 2 7 T

Cryogenics: 1 K 100 mK

Microwaves: 50 200 GHz

NMR: 10 200 MHz

DAQ

Refrigerator

The Basic Concept of The Basic Concept of Dynamic Nuclear PolarizationDynamic Nuclear Polarization

~PT

B

k

B / T Pp[%] Pd [%] Pe [%]

2.5 T / 1 K 0.25 0.05 93

15 T / 10mK

91 30 100

Doping and transferof polarization

DNP in the Picture of Spin TemperatureDNP in the Picture of Spin Temperature

( ) L

E

kTN E e

( ) SS

E

kTN E e

( ) L

E

kTN E e

~PT

B

k

DNP in the Picture of Spin TemperatureDNP in the Picture of Spin Temperature

min| | LSSZ

TET

E

SS

PT

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 The special problem of low nuclei (e.g. deuterons) nuclei (e.g. deuterons)

E

Part I: Material Developments

Example 1: Electron irradiation of Example 1: Electron irradiation of 66LiDLiD

• 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 6LiD(2000 Bochum) Pmax = 55 % at 2.5 T

7Li (large ) impurity has considerable influence on Pmax

F-Center:

• s-wave electron• no g-anisotropy• weak HF interaction

+

B

Li

D

20 MeV atT = 185 K

Example 2: Electron irradiated deuterated ButanolExample 2: Electron irradiated deuterated Butanol

Trityl

Example 3: Trityl doped deuterated alcohols and diolsExample 3: Trityl doped deuterated alcohols and diols

@ B = 2.5 T

@ B = 2.5 T

Part II: Magnet Developments

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

Disadvantages of the frozen spin mode:

1) Polarization decays while data taking

2) Pmax (frozen) ~ 0.8 · Pmax (cont.)

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

Peff (frozen) ~ 0.7 · Pmax (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 < 10-4) in a volume comparable to the field volume

Already realized as internal holding magnets sincemiddle of 1990 (GDH @ 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 << 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:

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