recent developments in polarized solid targets h. dutz, s. goertz physics institute, university bonn...
TRANSCRIPT
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