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